• Tess Saxton-Fox
University of Illinois at Urbana-Champaign

2.00pm Wednesday 22 June 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

• Pejman Tahmasebi
University of Wyoming

4.00pm Wednesday 15 June 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

• Josué Sznitman
Technion

5.00pm Wednesday 08 June 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

TU Delft

4.00pm Wednesday 01 June 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

• Susann Beier
UNSW

4.00pm Wednesday 25 May 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

• ## 2022 AFMSS: Earwax and a Wombat's Cube-Shaped Poo

David Hu
Georgia Institute of Technology

1.00pm Wednesday 18 May 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

In this talk, I will discuss two poorly understood biomaterials, earwax and wombat feces. For centuries, humans have invented a number of devices to remove accumulated earwax. We perform rheometer experiments and other assays that elucidate some of the adaptive properties of earwax in mammals. Earwax has low adhesion to the feet of ants, which reduces their ability to crawl into the ear, a problem for many people in the tropics. Earwax ages in the ear, leading to cracking and renewal on a regular basis. Wombat feces has a characteristic cube-shape, unique among mammals. We discuss the drying dynamics and peristalsis that lead to the generation of these shapes. The audience will learn how to design experiments on exotic biomaterials.

Dr. David Hu is a mechanical engineer who studies the interactions of animals with water. He has discovered how dogs shake dry, how insects walk on water, and how eyelashes protect the eyes from drying. Originally from Rockville, Maryland, he earned degrees in mathematics and mechanical engineering from M.I.T., and is now Professor of Mechanical Engineering and Biology and Adjunct Professor of Physics at Georgia Tech. He serves on the editorial boards for Proceedings of the Royal Society B: Biological Sciences, Scientific Reports, and Journal of Experimental Biology. His research has been featured in The Economist, The New York Times, Saturday Night Live, and Highlights for Children. He is a recipient of the National Science Foundation CAREER award for young scientists, the Ig Nobel Prize in Physics (twice), and the Pineapple Science Prize (the Ig Nobel of China). He is the author of the book "How to walk on water and climb up walls," which earned the American Institute of Physics Science Communication award and was finalist for the AAAS/Subaru Prize for Excellence in Young Adult Science Books. He lives with his wife and two children in Atlanta, Georgia.

• ## 2022 AFMSS: The Superharmonic Cascade of Internal Tides

Bruce Sutherland
University of Alberta

3.00pm Wednesday 11 May 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

Propagating in non-uniform stratification, the internal tide excites superharmonics with double the horizontal wavenumber of the internal tide and nearly double the frequency. Particularly in the tropics, where the Coriolis parameter is small, the superharmonics are nearly resonant with the internal tide, growing to large amplitude and themselves exciting super-harmonics. This work will present theory, in the form of coupled ordinary differential equations, which predict that the superharmonic cascade leads to the formation of a solitary wave-train. The results are in excellent agreement with fully nonlinear numerical simulations. For long waves in strong near-surface stratification, the results agree well with the prediction of shallow water theory including rotation through the Ostrovsky equation. The predictions of the Miyata-Choi-Camassa model including rotation are qualitatively similar, but less quantitatively accurate. Our theory thus provides new insight into the dynamics leading to the nonlinear steepening of the internal tide, and it provides an efficient algorithm for predicting the evolution of the waves going beyond the restrictions of shallow water theory.

Bruce Sutherland received his PhD in atmospheric science in the Department of Physics at the University of Toronto in 1994, then pursued postdoctoral training in the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge before taking up a position in 1997 as Assistant Professor in the Department of Mathematics at the University of Alberta. He is now a Professor jointly appointed in the Departments of Physics and of Earth & Atmospheric Sciences at the University of Alberta. His research combines theory, numerical simulations and laboratory experiments to examine phenomena occurring in stratified fluids. Main topics include interfacial and vertically propagating internal waves, the evolution of gravity currents and plumes in stratified fluids, and the transport and deposition of sediments and microplastics in geophysical flows.

• ## 2022 AFMSS: Quest for Equations Beyond the Navier-Stokes

Amit Agrawal
Indian Institute of Technology Bombay

4.00pm Wednesday 04 May 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

There is evidence in the literature as well as experimental data from our lab suggesting that the Navier-Stokes equations are inadequate to explain several observations with low-pressure gas flows. There seems to be no satisfactory alternative to theoretically describe the flow when the mean free path of the gas is of the order of the characteristic length scale. The two well established approaches of solving the Boltzmann equation yield the Burnett and Grad 13-moments equations, which are higher-order transport equations. However, several shortcomings of these equations are known by now. This motivated us to explore alternate ways to study and derive higher-order transport equations. techniques (e.g., riblets, compliant walls) as well as active feedback flow control (e.g., opposition control), and provides useful guidelines for the development of practicable control techniques. In this talk, I will first present experimental results on rarefied gas flow in a sudden expansion. This flow exhibits several unique features not seen with conventional flows; such as discontinuity in pressure gradient at the junction, and absence of flow separation yet an enhanced overall pressure drop. This and several other such flows cannot be modeled within the Navier-Stokes equations, and one can try invoking the higher-order transport equations. Solving higher-order transport equations is however not trivial. Using a novel iterative approach we could find the first ever analytical solution of the Burnett equations. In the second part of this talk, I will discuss our novel approach of employing distribution function consistent with Onsager's reciprocity principle to capture non-equilibrium thermodynamics effects, and the new equations derived in our group. I will present the attractive features of these newly derived OBurnett and O13 equations and some solutions of these equations. The talk therefore explains the conditions under which the celebrated Navier-Stokes equations fail, and the way to model the flow under such circumstances.

Prof. Amit Agrawal joined the Indian Institute of Technology (IIT) Bombay in 2004 and is currently an Institute Chair Professor in the Department of Mechanical Engineering, and Dean of International Relations, IIT Bombay. His research interests are in Micro-scale flows, Development of novel bio-microdevices, Theoretical fluid mechanics, and Turbulent flows. He has graduated 30 PhDs and advised several postdocs and Master's students on these and related topics. He has published more than 240 journal articles and filed for a dozen patents with his students. His primary contributions are in the development of a unique blood plasma separation microdevice and derivation of equations which (he believes) are more general than the Navier- Stokes equations. A technique for processing data in turbulent flows is sometimes referred to as Agrawal Decomposition. His insights and novel results on transmission of COVID-19 are documented as 8 Featured Articles in prestigious journal Physics of Fluids; these articles have been downloaded more than 112,000 times and covered by 200+ news outlets in more than 50 countries. Prof. Amit Agrawal has authored a well-received book entitled Microscale Flow and Heat Transfer: Mathematical Modelling and Flow Physics. The work from his lab has appeared on the cover page of prestigious journals such as Journal of Fluid Mechanics and Physics of Fluids. He serves as Editor-in-Chief of Transactions of INAE and Editor of ASME Journal of Heat Transfer, and other reputed journals. He is an elected Fellow of the prestigious Indian National Academy of Engineering (INAE), the National Academy of Sciences India, and Indian Academy of Sciences. He has been interviewed by All India Radio, Cooling India magazine, and web-series (such as 'The Saint in a Scientist'). He also features in a flipbook for kids 'They Made What? They Found What?'. He has been awarded the country's highest scientific honor - the Shanti Swarup Bhatnagar Prize for his seminal contributions.

• ## 2022 AFMSS: Modeling and prediction of high-speed turbulent boundary layers

University of Maryland

1.00pm Wednesday 27 April 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

The conversion of kinetic energy into internal energy in high-speed compressible boundary layers creates a non-uniform temperature field, and therefore also non-uniform density and viscosity fields. The variation in fluid properties invalidates many foundational theoretical models of wall-bounded turbulence flow, including the log-law for the mean velocity profile. Modeling and predictions of compressible turbulent boundary layers therefore requires a theoretical means of accounting for fluid property variations, the most famous of which is the so-called “Van Driest transformation” from 1951. The talk will describe recent developments in this area, with a focus on the new theoretical propositions during the last decade and applications of the theory to an engineering friction estimation method and the development of wall-models for large eddy simulations.

Johan Larsson is an Associate Professor at the University of Maryland where he works on multiple problems in the field of computational turbulence including wall-modeling for large eddy simulation, grid-adaptation for turbulence-resolving simulations, high-speed turbulent flows, and uncertainty quantification for turbulence problems. He earned his PhD at the University of Waterloo, Canada, in 2006, and then worked at the Center for Turbulence Research at Stanford University as a postdoctoral fellow and Research Associate for 6 years before joining the University of Maryland in 2012. He is an Associate Editor of the AIAA Journal.

• ## 2022 AFMSS: Numerical Simulation and Modelling of Turbulent, Plane Poiseuille-Couette Flow

D. I. Pullin
Caltech

4.00pm Wednesday 13 April 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

Numerical simulation and mean-flow modelling of plane Poiseuille-Couette (PPC) flows will be described. These flows exist in a parameter space $(Re,\theta)$ with $Re=({Re_c^2+Re_M^2})^{1/2}$, $\theta=\arctan(Re_M/Re_c)$ where $Re_c, Re_M$ are independent Reynolds numbers based on the plate speed $U_c$ and the volume flow respectively. The data base consists of direct numerical simulation at Re=4,000,6,000, wall-resolved large-eddy simulation at Re = 10,000, 20,000 and some wall-modeled LES up to Re=10^10. The transition from Couette-dominated (CD) to Poiseuille-dominated (PD) flow will be discussed. This is defined as the mean skin-friction Reynolds number $Re_{\tau,b}$ on the bottom wall changing sign at $\theta=\theta_c(Re)$. The mean-flow in the $(Re,\theta)$ plane is modelled with combinations of patched classical log-wake profiles. Several variants are constructed in both the CD and PD flow regions. Model calculations give predictions of $Re_{\tau,b}(Re,\theta), Re_{\tau,t}(Re,\theta)$ (the skin-friction Reynolds number on the top wall) and $\theta_c(Re)$. Both model and simulation indicate that, as $\theta$ is increased at fixed Re $Re_{\tau,b}(Re,\theta)$ increases monotonically. The flow laminarizes as $\theta$ passes through $\theta_c(Re)$ and then re-transitions to turbulence. This is accompanied by attenuation of streamwise roll structures present in pure Couette flow. The modelling enables exploration of the infinite Re limit.

D. I. Pullin is the Robert H. Goddard Professor of Aeronautics, California Institute of Technology. He completed his PhD at Imperial College. Current research interests include (1) development of large-eddy simulation for high-Reynolds number wall-bounded turbulent flow, (2) shock-driven flows in fluids and plasmas, and (3) vortex dynamics.

• ## 2022 AFMSS: Flow dynamics of vortex rings colliding with solid boundaries

Daniel T H New
NTU, Singapore

4.00pm Wednesday 06 April 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

Vortex rings colliding with solid boundaries often give rise to highly complex vortex structures and behaviour, due to the resulting vortex-vortex and vortex-boundary layer interactions. This had intrigued researchers for many years, due to fundamental curiosity over a seemingly “simple” flow scenario and a strong desire to understand the flow mechanisms underpinning the different collision behaviour associated with different solid boundaries used. At the same time, such flow scenarios are relevant towards engineering applications that make use of impinging jets for heat transfer or cold spray purposes. This talk will describe experimental and numerical work conducted on vortex rings colliding with round cylinders and V-walls, with particular attention paid towards how variations in key geometrical parameters could produce significantly different collision outcomes. Laser-induced fluorescence, particle image velocimetry and large-eddy simulation results will be used to showcase these flow differences and how the simulations are able to reproduce the basic resulting vortex structures and flow behaviour that agree well with the flow models.

Daniel T. H. New graduated from the National University of Singapore (NUS) with a PhD in Mechanical Engineering in 2004 after working on jet-in-crossflow phenomenon. He was with Temasek Laboratories, NUS (TL@NUS) as an Associate Scientist and later as a Research Scientist in 2004 specializing in flow control, before he ventured into fundamental high-speed combustion research at the University of Texas, Arlington, USA as a post-doctoral visiting researcher. He was a Lecturer at the University of Liverpool, UK between 2005-2010 and worked on jet-mixing enhancements, before joining Nanyang Technological University (NTU), Singapore in 2010. He is currently an Associate Professor at the School of Mechanical and Aerospace Engineering, NTU and his current research interests include experimental fluid dynamics and aerodynamics, vortex dynamics, flow physics and flow control in general.

• ## 2022 AFMSS: Hierarchy of counter-rotating pairs of vortex tubes in turbulence

Susumu Goto
Osaka University

4.00pm Wednesday 30 March 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

Developed turbulence is composed of coherent vortices at various length scales. For example, the below figure shows the scale-decomposed turbulence (DNS results, Re=5000) behind a cylinder. Here, we use a combination of the Gaussian filters in real space for the scale decomposition. The large (blue) vortices are the rolls shedding from the cylinder, and they form counter-rotating pairs, which are accompanied by a straining field around them. These large-scale shears stretch and create smaller vortices (the yellow rib vortices). The rib vortices also form counter-rotating pairs to stretch and create further smaller (black) vortex tubes in straining fields around pairs of them. This successive vortex stretching process is the energy cascade in the inertial range, which terminates at the scale (the scale of the black vortices in this case) when viscous effects get faster than the stretching. In this talk, we show the universality of this hierarchy of coherent vortices and its role in some transport phenomena by using examples of turbulence in a periodic cube, a turbulent boundary layer, turbulent channel flow, and solid particle transports in these turbulences.

Susumu Goto is a Professor of fluid mechanics in the Graduate School of Engineering Science at Osaka University. He received PhD in 1999 from the Graduate University for Advanced Studies (supervisor: Prof. Shigeo Kida). He then joined the National Institute for Fusion Science and he stayed at Imperial College London in 2003-2004, where he worked with Prof. J. Christos Vassilicos. After that, he studied at Kyoto and Okayama before joining Osaka University in 2012. He is theoretically, numerically, and experimentally studying the science and engineering applications of various complex flow phenomena.

• ## 2022 AFMSS: Model-based design of active and passive flow control for wall turbulence

Mitul Luhar
USC

4.00pm Wednesday 23 March 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

Control of wall-bounded turbulent flows has been an important area of research for several decades. However, the development of effective control techniques has been hindered by the limited availability of computationally-tractable models that can guide design and optimization. This talk describes extensions of the resolvent analysis formalism that seek to address this limitation. Under the resolvent formulation, the turbulent velocity field is expressed as a superposition of propagating modes (‘resolvent modes’), identified via a gain-based decomposition of the Navier-Stokes equations. Control is introduced into this framework via changes to the boundary conditions or through additional forcing terms in the governing equations. This alters the structure and gain of resolvent modes, whereby a reduction in gain indicates turbulence suppression. The modeling framework reproduces observations from previous experiments and simulations for passive techniques (e.g., riblets, compliant walls) as well as active feedback flow control (e.g., opposition control), and provides useful guidelines for the development of practicable control techniques.

Mitul Luhar is Associate Professor of Aerospace and Mechanical Engineering at the University of Southern California. Research in his group tackles control of wall-bounded turbulent flows and fluid-structure interactions; this work combinates laboratory experiments and reduced complexity modeling. Mitul is the recipient of the National Science Foundation CAREER Award and the Air Force Office of Scientific Research Young Investigator Award. Prior to joining USC, Mitul was a Postdoctoral Scholar in the Graduate Aerospace Laboratories at Caltech. He earned his Ph.D. in Civil and Environmental Engineering from MIT in 2012 and B.A. and M.Eng. degrees in Engineering from the University of Cambridge in 2007.

• ## 2022 AFMSS: The Spectral Link in Turbulent Frictional Drag

Pinaki Chakraborty
OIST

4.00pm Wednesday 16 March 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

I will review the spectral link in turbulent frictional drag, the missing link between two aspects of turbulent flows that have been the subject of extensive, but disjoint, research efforts: the frictional drag experienced by a turbulent flow over a wall and the turbulent spectrum. The standard theory of turbulent frictional drag, which is based on the pioneering work of Prandtl and von Karman, computes the frictional drag using an indirect approach and makes no contact with the spectrum. By contrast, the spectral link computes the frictional drag directly and expresses it as a functional of the turbulent spectrum. To illustrate the applications of the spectral link, I will obtain an analytical version of the arch-famous Nikuradse's diagram that is in minute qualitative agreement with the distinctive features in the diagram that have remained elusive to any theoretical elucidation. Thereafter, I will discuss unprecedented experimental measurements of frictional drag in turbulent soap-film flows over smooth and rough walls, and show how the results render the standard theory incomplete. This research is pursued in close collaboration with Gustavo Gioia (OIST); other collaborators include Nigel Goldenfeld (U. Illinois), Walter Goldburg (U. Pittsburgh), and Hamid Kellay (U. Bordeaux).

Pinaki Chakraborty is a Professor at the Okinawa Institute of Science & Technology (OIST), where he directs the Fluid Mechanics Unit. Using a combination of theory, experiments, and simulations, the Fluid Mechanics Unit works on turbulent flows, geological flows, atmospheric flows, and granular flows. Prior to joining OIST in 2012, Pinaki was at the University of Illinois at Urbana-Champaign, first at the Department of Theoretical and Applied Mechanics, where he received his Ph.D. in 2006, and then at the Department of Geology, where he was a postdoctoral fellow and later a Research Assistant Professor.

• ## 2022 AFMSS: Modeling the Fluid Physics of Wind Farms

Richard Stevens
University of Twente

4.00pm Wednesday 02 March 2022
Online Event (2022 Australasian Fluid Mechanics Seminar Series)

The performance of large wind farms depends on the development of turbulent wind turbine wakes and the interaction between these wakes. Turbulence also plays a crucial role in transporting kinetic energy from the large-scale geostrophic winds in the atmospheric boundary layer towards heights where wind farms can harvest this energy. High-resolution large-eddy simulations (LES) are ideal for understanding these flow phenomena. Much has been learned from wind farm simulations, which initially focused on 'idealized' situations. Nowadays, the community increasingly focuses on modeling more complex situations, such as the effect of complex terrain and different atmospheric stability conditions. As wind farms become larger, the need to improve their design and develop control strategies to mitigate wake effects increases. However, due to the large separation of length scales and the number of cases, it is unfeasible to use LES for wind farm design. Therefore, LES are used to develop computationally more tractable modeling approaches ranging from Reynolds Average Navier Stokes (RANS) models to analytical modeling approaches. In this presentation, we will give particular attention to the challenges of modeling wind farm dynamics in large-eddy simulations and emerging challenges to account for the effect of mesoscale flow phenomena in these simulations.

Dr. Richard Stevens is an Associate Professor in the Physics of Fluids group at the University of Twente. He has received an ERC-starting grant. His research interests include computational fluid dynamics and high-performance computing. His work is focused on the fundamental understanding of turbulent Rayleigh Benard convection and wind farm fluid mechanics.

• ## 2021 AFMSS: Large-Scale Coherent Structures in Turbulent Wakes

Georgios Rigas
Imperial College London

6.00pm Wednesday 1 December 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

This talk will give insight into the dynamics of turbulent wakes generated by canonical three-dimensional bluff-bodies and their control. Using wind-tunnel measurements (synced PIV and pressure) in the wake of an axisymmetric body at Reynolds number of ~200,000 – well into the turbulent regime – we will demonstrate that turbulent coherent structures manifest due to spatiotemporal symmetry break occurring at transitional regimes. Due to ergodicity, these structures restore statistically the broken symmetries at sufficiently high Reynolds numbers, which prevents their identification at turbulent regimes using existing modal decomposition techniques. A symmetry-aware Proper Orthogonal Decomposition framework has been developed and implemented based on autoencoders to extract the coherent symmetry-invariant structures. Finally, recent results for the control of coherent structures for road-vehicle drag-reduction applications will be demonstrated. Nonlinear dynamic feedback controllers are optimally tuned using a data-driven Reinforcement Learning framework, capable of stabilising the wake and substantially reducing the mean drag through synthetic jets or flaps placed at the rear trailing edges of the vehicle.

Dr Georgios Rigas is a Lecturer in the Department of Aeronautics at Imperial College London. Before joining IC in 2019, he was a Postdoctoral Scholar at Caltech and the University of Cambridge. He received his Ph.D. in Aeronautical Engineering from Imperial College in 2015. His work is focused on the fundamental understanding and control of transitional and turbulent flows.

• ## 2021 AFMSS: Stochastic and Statistical Dynamical Models of Geophysical Flows

Terry O'Kane
CSIRO

4.00pm Wednesday 24 November 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

The 2021 Nobel prize in physics, awarded to Manabe, Hasselmann and Parisi, focussed on understanding complexity in physical systems. In particular, the work of Hasselmann informed the general framework of stochastic models to understand the basic processes leading to the emergence of multiple length and time scales and the myriad causal interactions that determine climate variability. In this seminar, I will describe the application of multivariate stochastic models to modelling and predicting seasonal to decadal climate variability and coherence resonance effects including methods first developed by Hasselmann. Beyond assumptions of statistical stationarity, more general regularised approaches incorporating optimization and numerical methods from applied mathematics, have been developed to generate data driven stochastic models whose parameters are time dependent, thereby allowing for the identification of metastable states in nonstationary flows. Applying concepts from dynamical systems, such as hyperbolicity and local attractor dimension, these reduced order stochastic models are shown to provide a theoretical basis for the specification of forecast covariances and selection of initial forecast perturbations in operational weather prediction. Finally, the link between generalised Langevin equations and statistical closure theories of inhomogeneous turbulence are explored. Throughout, specific applications to problems in climate, atmospheric and ocean dynamics are described.

Terry O’Kane is principal research scientist and leader of the geophysical fluids team in the climate science centre at CSIRO Oceans and Atmosphere. Prior to that he was an Australian Research Council Future Fellow and in 2013 was awarded the JH Michell medal of the Australian Mathematics Society. His work focusses on aspects of nonlinear and stochastic climate dynamics including weather and climate prediction, data assimilation and statistical mechanics and dynamics.

• ## 2021 AFMSS: Underlying Subcritical Chaos and Spiral Turbulence in Taylor-Couette Flow

Alvaro Meseguer
UPC

7.00pm Wednesday 17 November 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

We investigate the local self-sustained process underlying spiral turbulence in counter-rotating Taylor-Couette flow using a periodic annular domain, shaped as a parallelogram, two of whose sides are aligned with the cylindrical helix described by the spiral pattern. The primal focus of the study is placed on the emergence of drifting-rotating waves (DRW) that capture, in a relatively small domain, the main features of coherent structures typically observed in developed turbulence. The transitional dynamics of the subcritical region, far below the first instability of the laminar circular Couette flow, is determined by the upper and lower branches of DRW solution originated at saddle-node bifurcations. The mechanism whereby these solutions self-sustain, and the chaotic dynamics they induce, are conspicuously reminiscent of other subcritical shear flows. Remarkably, the flow properties of DRW persist even as the Reynolds number is increased beyond the linear stability threshold of the base flow. From these DRW, two period-doubling cascades of relative periodic orbits emerge, leading to two strange attractors. We will describe the mechanisms that later on destroy the attractors through boundary crises. These mechanisms are similar to the ones recently reported in plane-Couette flow.

Alvaro Meseguer is an Associate Professor in the Department of Physics at Universitat Politechnica de Catalunya (UPC). He received his Ph.D. in Applied Physics from UPC and was an EPSRC Postdoctoral Research Officer at Oxford University. His research interests involve computational fluid dynamics, transition to turbulence, and dynamical systems theory. (Book recently published: Fundamentals on Numerical Mathematics for Physicists and Engineers, Wiley 2020).

• ## 2021 AFMSS: Mixing-Up the Climate? How the Mystery of Stratified Turbulence is Controlling All Our Futures

Colm-cille Caulfield
University of Cambridge

5.00pm Wednesday 10 November 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Feynman acknowledged that "turbulence is the most important unsolved problem of classical physics”, and it is always important to remember that he was referring to the simplest case of a fluid of constant density. An even more challenging class of problems arise when the turbulent fluid has a variable density, as turbulent mixing can then convert injected kinetic energy into both viscous dissipation and potential energy. Of course, the earth’s oceans are just such variable-density stratified fluids, and the larger scale effect of such stratified turbulence is one of the key areas of uncertainty in modelling the global climate system. As human activity strongly perturbs that system’s boundary conditions, it is critical to understand better how stratified turbulence is born, lives and dies within the world’s oceans. Fortunately, enormous advances in data availability from both observation and numerical simulation have led to breakthroughs in our fundamental understanding of turbulence in stratified fluids. In this talk I review some of these recent breakthroughs, and pose some of the fascinating open questions still requiring answers, highlighting how access to vast quantities of data is both a challenge and an exciting opportunity for the mathematically-minded (classical) physicist.

Colm-cille P. Caulfield is Professor of Environmental and Industrial Fluid Dynamics at the University of Cambridge, where he is a member of the BP Institute and the head of the Department of Applied Mathematics and Theoretical Physics. Before taking up his position in Cambridge in 2005, he also held faculty positions in Environmental Engineering at the University of California, San Diego, and in Mathematics at the University of Bristol. His research focuses on stability, transition, turbulence and mixing in environmental and industrial flows, particularly where density differences play a dynamically significant role. He is an associate editor of the Journal of Fluid Mechanics (Rapids).

• ## 2021 AFMSS: Bayesian Inference for Dynamical System Identification from Time-Series Data

Robert Niven
UNSW Canberra

4.00pm Wednesday 3 November 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Many inference problems including those of fluid mechanics involve a dynamical system, usually represented by $\mathbf{\dot{x}} = f(\mathbf{x})$ , where $\mathbf{x}$ is a state vector, $\mathbf{\dot{x}}$ is the derivative vector, and $f$ is the system model. Since the time of Newton, researchers have grappled with the problem of system identification: how should a user accurately and efficiently identify the model f, including its functional family and parameter values, from time-series data? Historically, a large assortment of machine learning methods have been used, including linear methods, computational intelligence (data-driven) methods, supervised or unsupervised learning, order reduction methods and statistical tools. However, few researchers have applied Bayesian inference, the fundamental method for the discovery of models from data. This seminar provides a Bayesian perspective on dynamical system identification. Firstly, Bayes’ theorem and its key constructs are introduced. The Bayesian maximum a posteriori (MAP) method is then applied to time-series data, and shown to be equivalent to a generalized Tikhonov regularization scheme, providing a rational justification for the choice of the residual and regularization terms respectively from the likelihood and prior distributions. The Bayesian viewpoint also provides access to the full Bayesian apparatus, enabling the ranking of models, the quantification of model uncertainties, and the exploration of the functional form of the posterior. Two Bayesian methods, joint maximum a posteriori (JMAP) and the variational Bayesian approximation (VBA), are compared to the popular SINDy algorithm for thresholded least-squares regression, by application to several dynamical systems with added noise. The case studies demonstrate several advantages of the Bayesian framework for model discovery from time-series data.

Robert Niven is an Associate Professor in the School of Engineering and Information Technology, UNSW Canberra. He conducts research in (i) probabilistic inference applied to a variety of natural and engineered systems, (ii) theoretical fluid mechanics, and (iii) environmental contaminant systems. A/Prof. Niven’s research has been recognised by several prestigious ARC and international grants and fellowships. He is a current Council member of the AFMS, and co-founder and current Chair of its ACT Chapter.

• ## 2021 AFMSS: Interactions between coherent structures in turbulent jets

Petrônio Nogueira
Monash University

6.00pm* Wednesday 27 October 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Coherent structures of several kinds have been observed in turbulent jets since the early 70’s. The most well-known is the Kelvin-Helmholtz (KH) wavepacket, which arises from a linear instability mechanism of the flow and plays a key role in sound generation. Recently, other energetic coherent structures were shown to exist in the turbulent field, all associated with different regions of the frequency-wavenumber spectrum. At low frequencies and high azimuthal wavenumbers, streaks arise as a result of the lift-up effect, where elongated streamwise vortices (or rolls) generate streamwise elongated structures in axial velocity. At low frequencies and low azimuthal wavenumbers, structures are amplified via the Orr mechanism. Finally, at zero frequency and zero azimuthal wavenumber, a shock-cell structure may be generated by the pressure mismatch at the nozzle in imperfectly expanded supersonic jets, which can also be predicted using linear stability tools. The objective of this talk is to analyse the interaction between some of these structures with the KH mode, which may lead to insight on their effects on sound generation. To this end, a combination of reduced order models will be applied to both jets and confined shear layers, which will allow us to understand how the KH instability mechanism is modified by them.

Petrônio Nogueira is a Research Fellow at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Monash University. He received his PhD from the Instituto Tecnológico de Aeronáutica (ITA - Brazil) in 2019, during which he spent six months as a visiting PhD student at the Royal Institute of Technology (KTH - Sweden). Most of his work is focused on the identification and analysis of the dynamics of coherent structures in turbulent flows, and he has been studying resonance phenomena and noise generation in shock containing jets since 2020.

• ## 2021 AFMSS: Hamiltonian Stability Theory for Fluids and Plasmas

Makoto Hirota
Tohoku University

6.00pm* Wednesday 20 October 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Linear stability of inviscid fluids and plasmas is generally discussed from the viewpoint of Hamiltonian dynamical system. The "canonical variable" for a fluid system is not the Eulerian velocity field, but the Lagrangian fluid particle orbits. In terms of the infinitesimal displacement field of fluid particles, the linearized dynamical system is reduced to a Newton's equation of motion including gyroscopic and potential forces (Friemann & Rotenberg 1960).The further reduced system for isovortical perturbation (Arnold 1966) is obtained under the topological constraint imposed by the vorticity conservation law. Various sufficient conditions for stability are systematically derived because of this framework. However, these sufficient conditions cannot achieve necessity in the presence of negative energy modes (which may be continuum mode in fluid system). If one can either eliminate or invert all negative energy modes successfully, necessary and sufficient condition becomes available. After reviewing this framework, its application to extended magnetohydrodynamics(or two-fluid plasma) is introduced. Reference: M. Hirota, Phys. Plasmas 28, 022106 (2021).

Makoto Hirota is an Associate Professor of Institute of Fluid Science, Tohoku University. He received his Ph.D. from The University of Tokyo and has worked as a postdoctoral fellow at Faculty of Mathematics, Kyushu University and Division of Advanced Plasma Research, Japan Atomic Energy Agency. He is a theoretical physicist studying hydrodynamic and magnetohydrodynamic stability problems that occur in boundary layer flows, vortices, magnetically confined plasmas, space plasmas and so on.

• ## 2021 AFMSS: On-the-fly Reduced Order Modeling with Time Dependent Bases

Hessam Babaee
University of Pittsburgh

4.00pm Wednesday 13 October 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Many important problems in fluid mechanics are described by high-dimensional partial differential equations (PDEs). The computational cost of solving these problems using classical discretization techniques increases exponentially with respect to the number of dimensions –– a fundamental challenge that is dubbed as the curse of dimensionality. On the other hand, many of these high-dimensional problems have a much lower intrinsic dimensionality, that if discovered, can mitigate the curse of dimensionality. This calls for techniques that extract and exploit correlated structures directly from the PDE. This approach is in direct contrast to classical discretization techniques that disregard multi-dimensional correlations and result in inefficient solutions for high-dimensional problems. While there are numerous data-driven dimension reduction techniques that can extract these correlated structures by solving the full-dimensional PDE, these techniques are only feasible for lower-dimensional PDEs (e.g., 2D/3D). This same workflow is impracticable for many high-dimensional PDEs as computing the solution of the full-dimensional PDE is the very problem we cannot afford to solve. To this end, we present a reduced order modeling framework, in which the correlated structures are extracted directly from the PDE –– bypassing the need to generate data. These structures are exploited by building on-the-fly reduced order models (ROM). The correlated structures are represented by a set of time-dependent orthonormal bases and their evolution is prescribed by the physics of the problem. We present several demonstration cases including reduced order modeling of reactive species transport equation in turbulent combustion as well as sensitivity analysis and uncertainty quantification in fluid dynamics problems.

Hessam Babaee is currently an Assistant Professor in the Department of Mechanical engineering and Materials Science at the University of Pittsburgh. He earned his PhD in Mechanical Engineering and Masters in Applied Mathematics both from Louisiana State University in 2013. He then joined the Mechanical Engineering Department at MIT for his postdoctoral research, where he worked with Prof. George Karniadakis. He then joined the University of Pittsburgh in January 2017. His research is focused on developing reduced order model and machine learning techniques for fluid mechanics problems. His research has been funded by many organizations including NASA, National Science Foundation (NSF), National Institute of Health (NIH) and Air Force Office of Scientific Research (AFOSR).

• ## 2021 AFMSS: Mean and turbulent characteristics of a bottom boundary layer forced by a strong surface tide and large amplitude internal waves

Nicole Jones
University of Western Australia

4.00pm Wednesday 06 October 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

The bottom boundary layer [BBL] is the region where the flow is directly influenced by the presence of the seabed. Despite the BBL often being thin with respect to the total ocean depth, the dynamics of this layer are of great physical, biological and engineering importance. I will present 15-days of detailed observations of a near well-mixed bottom boundary layer at a 250 m deep continental shelf site, where the ocean flow was characterised by strong tides and large internal waves. The boundary layer thickness varied at tidal frequencies, and was 10 m thick on average. In the lowermost 1 m of this layer, the flow conformed to conventional boundary layer. More than 1 m above the bed, however, the overlying internal waves modified both the nature of the turbulent flow and the time-averaged properties of the bottom mixing-layer. While the instantaneous BBL thickness is dominated by internal wave pumping, the strength of the ambient stratification and the frequency of the oscillation are important controls on the time averaged boundary-layer thickness.

Associate Professor Nicole Jones is a Physical Oceanographer at the University of Western Australia. She uses a combination of field observations and numerical modelling to study primarily relatively small-scale ocean dynamics, including turbulent mixing, internal waves and ocean eddies. Under-standing these processes is vital to quantify the transport of heat, pollutants and nutrients around the ocean. Nicole has extensive fieldwork and cruise experience and a particular interest in the development of novel field-observation techniques. Nicole has been an Editor for the Journal of Physical Oceanography since 2018. She represents the Western Australian marine science community by leading the Western Australia node of Australia’s Integrated Marine Observing System (IMOS). Since 2019 Nicole has served on the Research Advisory Committee of the Marine National Facility.

• ## 2021 AFMSS: Impact of surface and free-stream disturbances on boundary-layer transition

Xuesong Wu
Imperial College London

5.00pm* Wednesday 29 September 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Laminar-turbulent transition in boundary layer flows depends crucially on external perturbations that are present on the surface and in the free stream. Depending on their physical nature as well as locations and scales, these perturbations may influence transition route and position via different physical mechanisms including the familiar ones of receptivity, alteration of linear instability and the newly identified local scattering process. Establishing physics-based quantitative relations between the transition location and external perturbations is considered to be the ultimate aim of transition research. In this talk, I will report some of recent efforts made to achieve that goal. These include a particularly strong receptivity of supersonic boundary layers to free-stream sound waves in a small range of incident angle, a local scattering theory quantifying the impact of abrupt changes on instability, and a complete initial-boundary-value problem describing Gortler vortices induced by high-level free-stream vortical disturbances. The respective importance of theoretical concepts, mathematical analyses and full numerical computations in these efforts will be highlighted.

Professor Xuesong Wu is a Professor of Applied Mathematics in the Faculty of Natural Sciences, Department of Mathematics at Imperial College London.

• ## 2021 AFMSS: What can point-particle simulations tell us about turbulent dispersed flows?

Cristian Marchioli
University of Udine

4.00pm Wednesday 22 September 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Particle transport and mixing in turbulent flows are fundamental to science as well as to technology. Examples of open scientific issues include emissions reduction in combustion, rheological characterization of fibrous particle suspension, plankton population dynamics, convection of pollutants in the atmosphere, to name a few. The simplest numerical framework to study the dynamical and statistical features of turbulent particle dispersion is based on the assumption that particles can be modeled as point-like spheres brought about by the flow. In spite of its simplicity, this framework has led to significant advancements in the study of particles-turbulence interactions, allowing the precise identification of the coherent structures responsible for particle sedimentation and re-entrainment in turbulent boundary layers. In this talk we examine a possible source of bias in particle dispersion, which arises when particles are non-spherical (elongated), showing how this affects particle motion, preferential concentration and accumulation in turbulent boundary layer. We also discuss the possibility of using the point-particle approach to examine from a fundamental perspective particle interaction with a fluid interface, a problem of practical importance for scrubbing processes.

Cristian Marchioli is Associate Professor of Fluid Mechanics at the University of Udine, Editor of Acta Mechanica and former chairman of the COST Action “Fiber suspension flow modeling”. Currently, Prof. Marchioli is the Coordinator of the MSCA ITN Network COMETE “Next-Generation Computational Methods for Enhanced Multiphase Flow Processes” and sits in the scientific council of the International Center of Mechanical Sciences, where he coordinated several advanced schools on particles in turbulence. His research interests involve multiphase flow modeling, from small-scale particle-turbulence interactions to large-scale modeling of gas-solid/gas-liquid flows. Prof. Marchioli has published 60+ papers and 150+ conference proceedings. He has also delivered several invited and keynote lectures at international conferences.

• ## 2021 AFMSS: Active control of flow-induced vibration of a circular cylinder

Jisheng Zhao
Monash University

4.00pm Wednesday 15 September 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Flow-induced vibration (FIV) of structures is an important problem seen a large variety of engineering applications, e.g., from high-rise buildings in winds to offshore oil platforms and rigs and to energy-harvesting foils. In order to gain a deeper understanding of the fundamental characteristics and control of FIV, the present study experimentally investigated the FIVs of a rotating circular cylinder that was allowed to oscillate with one degree of freedom in the cross-flow direction. It was found that the imposed body rotation could significantly enhance and attenuate the cylinder vibration in a parameter space of the rotation rate and flow velocity. Moreover, we assessed the fluid-flow energy harvesting performance from FIV of rotating cylinder.

Jisheng Zhao is a Research Fellow (ARC DECRA Fellow) in the Department of Mechanical and Aerospace Engineering. He obtained his PhD degree from Monash University in 2012. His main research interests include fluid-structure interactions, flow-induced vibration of bluff bodies, structural vibration control, fluid-flow energy harvesting, and aerodynamics of flapping wings.

• ## 2021 AFMSS: Tracking streaks in the buffer layer of wall-bounded turbulence

Jane Bae
Caltech

4.00pm Wednesday 08 September 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Among the many organized structures observed in near-wall turbulent flows, streaks, defined as regions of slowly moving fluid elongated in the direction of the mean flow, are considered to be of major importance for their role in the regeneration of turbulent energy. Here, we identify and track individual streaks in time using time-resolved direct numerical simulation data of a low Reynolds number channel flow. The analysis of the streaks shows that there is a clear distinction between wall-attached and detached streaks, and that the former can be further categorized into streaks that are contained in the buffer layer and the ones that reach the outer region. The results reveal that streaks are born in the buffer layer, coalescing with each other to create larger streaks that are still attached to the wall. These large tall-attached streaks eventually split into wall-attached and wall-detached components, which are strongly related to ejections or burst events.

Jane Bae is an Assistant Professor in Aerospace at the Graduate Aerospace Laboratories at Caltech. She received her Ph.D. in Computational and Mathematical Engineering from Stanford University in 2018. She was a postdoctoral fellow in the Graduate Aerospace Laboratories at Caltech and the Institute for Applied Computational Science at Harvard University before joining the Caltech faculty. Her main research focuses on computational fluid mechanics, in particular on modeling and control of wall-bounded turbulence.

• ## 2021 AFMSS: Developments and Capabilities Of Lattice Boltzmann Method For non-Ideal Fluid Flows and Mixtures

Emilie Sauret
Queensland University of Technology

4.00pm Wednesday 01 September 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Non-ideal fluid flows and mixtures are ubiquitous in many engineering applications, from renewable thermodynamic cycles to lab-on-a-chip devices. Those fluids and mixtures exhibit significant inter and intra molecular interactions that affect their macroscopic behaviour. This talk will present the development and application of a high-order multi-component Lattice Boltzmann (LB) model. Expanding LB models to higher orders makes it possible to solve nonequilibrium flow quantities and fully recover the Navier-Stokes solution. Challenges in developing, validating and applying such high-order multi-component models will be discussed. The presented approach is a step towards an efficient, robust and accurate numerical method for simulating non-ideal fluid flows and mixtures and understanding complex physical phenomena.

Dr. Emilie Sauret is a Professor in the School of Mechanical, Medical and Process Engineering, Queensland University of Technology, and an elected council member of the Australasian Fluid Mechanics Society. She received a PhD in Turbulence Modelling from Pierre & Marie Curie University, Paris, France in 2004. Following some time in industry, she was awarded an ARC-DECRA in 2013 and a Future Fellowship in 2020. Dr. Sauret has extensive interdisciplinary research experience at the crossroads between mechanical engineering, applied mathematics, and applied physics. She specialised in developing advanced computational models to understand complex fluid phenomena for a variety of engineering applications.

• ## 2021 AFMSS: Measurements in Wall-Bounded Turbulence

Alexander J. Smits
Princeton University

9.00pm* Wednesday 18 August 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Our understanding of turbulent boundary layer scaling and structure has advanced greatly in the past 20 to 30 years. On the computational side, direct numerical simulations and large-eddy simulations have made extraordinary contributions as numerical methods and computational resources have advanced, while on the experimental side major advances in instrumentation have made available new imaging and quantitative techniques that provide unprecedented accuracy and detail. Here, I illustrate how our progress has been aided by the development of such experimental methods by reference to three questions on the scaling of turbulent boundary layers: (1) the similarity behavior in supersonic and hypersonic flows; (2) the structure of the outer layer in subsonic flows at high Reynolds number; and (3) the scaling of the turbulent stresses in the near-wall region.

Alexander J. Smits is the Eugene Higgins Professor Emeritus of Mechanical and Aerospace Engineering at Princeton University. His research interests are centered on fundamental, experimental research in turbulence and fluid mechanics. He has authored or co-authored three books and more than 450 journal and conference papers. In 2004, Dr. Smits received the American Institute of Aeronautics and Astronautics (AIAA) Fluid Dynamics Award. In 2007, he was awarded the American Society of Mechanical Engineers (ASME) Fluids Engineering Award, and in 2011 he received a degree Honoris Causa (D.Eng.) from the University of Melbourne; in 2014, he received the AIAA Aerodynamic Measurement Technology Award, and in 2019 he was awarded the Fluid Dynamics Prize of the American Physical Society (APS). In 2020, he received the George K. Batchelor Prize in Fluid Mechanics from the International Union of Theoretical and Applied Mechanics. He is a Member of the National Academy of Engineering and the American Academy of Arts and Sciences, and a Fellow of APS, AIAA, ASME, the American Association for the Advancement of Science, and the Australasian Fluid Mechanics Society. He served as the Editor-in-Chief of the AIAA Journal from 2015 to 2021.

• ## 2021 AFMSS: Seeing in the dark

Tamer A. Zaki
Johns Hopkins University

9.00pm* Wednesday 11 August 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Numerical simulations of turbulence provide non-intrusive access to flow quantities of interest, although they often invoke simplifying assumptions that can compromise realism such as truncated computational domains and idealized boundary conditions. Experiments, on the other hand, probe the true flow with less assumptions, but must nonetheless contend with limitations that arise due to sensor technology such as the tradeoff between field of view and spatio-temporal resolution. Assimilating observations directly in simulations can combine the advantages of both approaches and mitigate their respective deficiencies. The problem is expressed in variational form, where we seek the flow field that satisfies the Navier-Stokes equations and optimally reproduces available data. In this framework, observations are no longer a mere record of the instantaneous, local quantity, but rather an encoding of the antecedent flow events that we aim to decode using the governing equations. Chaos plays a central role in obfuscating the interpretation of the data: observations that are infinitesimally close may be due to entirely different earlier conditions. We will examine a number of state estimation problems that expose important aspects of wall turbulence and the fundamental difficulties of reconstructing the full state from limited observations.

Tamer Zaki is a professor in the Department of Mechanical Engineering at Johns Hopkins University (JHU). His research interests span high-fidelity simulations for the prediction of transitional and turbulent shear flows, in both Newtonian and complex fluids, and the development of nonlinear optimization techniques for data assimilation, interpretation of flow measurements and optimal sensing. Zaki has garnered several recognitions including the Office of Naval Research Young Investigator Award and the William H. Huggins Excellence in Teaching Award. He is member of the JHU Center for Environmental & Applied Fluid Mechanics, the Institute for Data Intensive Engineering & Science, and the American Physical Society. He serves on the Editorial Advisory Board of Flow, Turbulence and Combustion and the Editorial Board of Physical Review Fluids.

• ## 2021 AFMSS: Geometrical Aspects Of Biological Locomotion At Low Reynolds Number

Kenta Ishimoto
Kyoto University

4.00pm Wednesday 04 August 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Microorganisms live at low Reynolds number, where the steady Stokes equations well describe the biological fluid phenomena. In this talk, after an introduction to the hydrodynamics of swimming microorganisms, a couple of geometrical topics in the theory of microswimming will be presented. In particular, I will talk about a general theory of the hydrodynamic shape of a microscopic object, deriving a new formula for the motion of chiral particles with applications to biased locomotion of bacteria in a fluid flow. If time permits, I will discuss a control problem of microscopic particles using a geometrical theory.

Dr Kenta Ishimoto is an Associate Professor at Research Institute for Mathematical Sciences, Kyoto University. He received a PhD in Mathematical Science from Kyoto University in 2015, followed by research experiences at Kyoto University, University of Oxford and The University of Tokyo, before joining as a faculty at Kyoto University in 2019. He mainly studies biofluid mechanics at the cellular scale and related applied mathematics.

• ## 2021 AFMSS: Let's get high (order)!

Esteban Ferrer
ETSIAE-UPM - School of Aeronautics

4.00pm Wednesday 30 June 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

High order methods (order ≥ 2) have seen a rise in popularity during the last 10 years and are today relatively mature. They are characterised by low numerical errors and by their ability to perform mesh refinement and/or polynomial enrichment to achieve accurate solutions. Today, they are sufficiently mature to cover a wide range of applications and flow regimes. However, to industrialise these methods, robustness, physical model complexity and computational efficiency still need to be improved. In this talk, we present our last developments in high order methods that include solving a wide range of flow regimes/physics: compressible, incompressible, turbulent (LES) and multiphase flows (Navier-Stokes with Cahn-Hilliard). In addition, we will cover methods to improve stability (e.g. stable energy schemes) and new immersed boundary methods to facilitate meshing. Finally, we focus on the efficiency of high order solvers and discuss local p-adaption, implicit time-stepping and multigrid. Examples will include aeronautical applications, wind turbines, acoustics and stability/sensitivity analyses for flow control.

Dr. Esteban Ferrer is Associate Professor in Applied Mathematics at the School of Aeronautics ETSIAE-UPM (Madrid). He received his PhD from the University of Oxford and was awarded two Masters Degrees in Mechanical Engineering ETSEIB-UPC (Barcelona) and in Aeronautical Engineering from ISAE (Toulouse). Esteban worked in industry before becoming an academic and has more than 18 years’ experience in the fields of numerical simulation and stability analysis. His current research focusses in developing high order solvers and flow control techniques for wind turbines and aeronautical applications.

• ## 2021 AFMSS: Instabilities and coherent structures in swirling flows and their relevance for thermoacoustics

Kilian Oberleithner
Technische Universität Berlin

4.00pm Wednesday 23 June 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Swirling flows feature a remarkable richness in flow instabilities such as centrifugal instabilities, inertial waves and global helical modes. Likewise, swirling flows and their instabilities are of high technical relevance and need to be understood and controlled. E.g., swirling flows play a key role in stabilizing turbulent flames in modern lean-premixed combustion systems, they occur in hydro turbines where their instabilities cause severe pressure oscillations, and they contribute to the wake recovery of wind turbines. In this talk, we briefly review the main characteristics of swirling flows and their instabilities. We introduce the methodology of linear stability theory for turbulent flows and our approach to model coherent structures. We then dive into the field of lean-premixed combustion and discuss recent research on prediction and control of hydrodynamic instabilities in swirl flames and their interplay with combustion instabilities. We will put a focus on high-Reynolds-number, fully turbulent flows at industry relevant scales, but we will also make detours to DNS based fundamental studies.

Kilian Oberleithner is a Professor at the TU Berlin and head of the Laboratory for Flow Instabilities and Dynamics. He completed his PhD in 2012 under the joint supervision of Prof. Oliver Paschereit (TUB) and Prof. Israel Wygnanski (University of Arizona). He continued research as a post-doc in Prof. Julio Soria's group in Melbourne, Australia. After his return to Berlin in 2014 he established his own research group and was appointed as a Junior professor in 2018. His main research interests are focused on exploring physical mechanisms and developing flow control techniques for reacting and non-reacting turbulent shear flows, using linear stability theory and data-driven low-order modelling with the aim of bridging fundamental and applied research.

• ## 2021 AFMSS: Small-scale turbulence phenomenon: could have we got it wrong?

Lyazid Djenidi
The University of Newcastle

4.00pm Wednesday 16 June 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

At the 1961 International Colloquium on Turbulence, held in Marseille (France), Kolmogorov presented his response to Landau’s remark which states that the large-scale and slow variations of the energy dissipation prevent the universality of small-scale turbulence. The theory presented, which has been reproduced in a 1962 Journal of fluid Mechanics article (JFM 13, 82-85) and commonly known as K62, shaped the current dominant understanding of the behaviour of small-scale turbulence. However, among this consensus, are growing “dissident voices” (among which the present speaker) who do not share the view that K62 describes the correct behaviour of small-scale turbulence at very large, if not infinite, Reynolds number. In this seminar we will briefly highlight the major issues/inconsistencies raised from the past and present results and attempt to address these issues by examining the behaviour of the second order-moment of the velocity increments, S2, obtained via (i) a closed form of the Karman-Howarth equation and (ii) the 3D energy spectrum.

Lyazid Djenidi is Professor at the College of Engineering, Science and Environment (Mech. Eng Discipline) at the University of Newcastle (Australia). He completed his PhD (Fluid mechanics) at the Institute de Mechanique Statistique de la Turbulence (IMST, Marseille, France), the institute where (what an irony of fate) Kolmogorov presented his K62 theory. After completing his PhD studies, in 1989, he went to Cambridge University (UK) for a post-doctoral position, before joining the turbulence group at the University of Newcastle in 1991. His interest in turbulence encompasses experimental, numerical and theoretical work on turbulent boundary layers, turbulent channel flows, jet and wake flows and small-scale turbulence.

• ## 2021 AFMSS: Simulating urban flows and air pollution in the lab

Christina Vanderwel
University of Southampton

6.00pm* Wednesday 09 June 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

The aerodynamics around buildings and within cities are crucial for determining how urban air pollution spreads. This talk will discuss current approaches to urban flow modelling and the gap in knowledge when dealing with local scale phenomena. Two experimental cases studies are considered: a scale model of a Beijing neighbourhood and multi-scale fractal’ city models. Wind tunnel and water flume experiments are conducted measuring the drag, velocity, and concentration fields around the models using a force balance, particle image velocimetry (PIV), and planar laser-induced fluorescence (PLIF), respectively. Rough-wall boundary layer estimates based on assumptions of homogeneous surface morphology predict the general trends of the results; however, particular building arrangements and the presence of tall buildings appear to have distinct local effects on the flow structure and resulting scalar transport, highlighting the challenges for urban air quality modelling.

Christina Vanderwel is an Associate Professor in the Aerodynamics and Astronautics Department at the University of Southampton. She completed her PhD in Mechanical Engineering at the University of Ottawa, Canada, in 2014. She specialises in experimental fluid mechanics and turbulence, using laser-based diagnostics and wind tunnels to study the mechanisms of mixing in turbulent flows. She currently holds a UKRI Future Leaders Fellowship applying her research to study urban wind patterns and how air pollution spreads in cities.

• ## 2021 AFMSS: New perspectives on instability and transition of laminar separation bubbles

Daniel Rodríguez

5.00pm* Wednesday 02 June 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Decades of research have established the ubiquity of inflectional instability in laminar separation bubbles (LSBs) and its dominant role in the laminar-turbulent transition process. However, this instability alone cannot explain the plethora of different dynamics that are observed, including three-dimensionalization of the mean flow and self-excited vortex shedding and transition. To fill these gaps, global instability mechanisms of LSBs were proposed in the past. Partial confirmation of these mechanisms has been realized experimentally, but most low-turbulence wind tunnel experiments either did not find evidence of their presence, or attributed possible evidences to consequences of the inflectional instability alone. This talk will depart from the theoretically-predicted, 3D global instability of LSBs, that distorts the separated flow along the spanwise direction. New results for non-linear and secondary instabilities of 3D separated flows will be presented. They show that the spanwise distortion strongly enhances inflectional instability, potentially leading to their absolute instability and to the appearance of a self-excited global oscillator. This sequence triggers the laminar–turbulent transition without requiring external disturbances or actuation. The resulting LSBs are in good agreement with those reported for low-turbulence wind-tunnel experiments without explicit forcing. This indicates that the inherent dynamics described by the self-excited instability can have been present and disregarded in many experimental works.

Daniel Rodríguez is associate professor at the School of Aeronautics of Universidad Politécnica de Madrid (UPM). After completing his Aerospace Engineering (2007) and Ph.D. (2010) at UPM, he held a postdoctoral position at Caltech. He was awarded the EU's Marie Curie fellowship (2011-2014) and the Brazilian's "Attraction of Young Talent" fellowship (2014-2016), during which he was appointed at UPM, Universidade de Sao Paulo and PUC-Rio, Brazil. From 2016 to 2019 he held a faculty position at UFF, Brazil. In 2018 he received the "Young Researcher of Rio de Janeiro State" award, and in 2020 he was appointed as "Echegaray Professor", within the Program of Academic Excellence of the government of the Community of Madrid.

• ## 2021 AFMSS: A computational "rheometer" for turbulent flows

Ali Mani
Stanford University

4.00pm Wednesday 26 May 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

This study presents a numerical procedure, which we call the macroscopic forcing method (MFM), which reveals the differential operators acting upon the mean fields of quantities transported by underlying fluctuating flows. Specifically, MFM can reveal differential operators associated with turbulent transport of scalars and momentum. We present this methodology by considering canonical problems with increasing complexity. For spatially homogeneous and statistically stationary systems, we observe that eddy diffusivity can be approximated by an operator of the form D/√(1-l^2 ∇^2 ), where l is a length scale on the order of large-eddy size and D is a coefficient. A validation test shows that this operator leads to significant improvement in RANS prediction of axisymmetric turbulent jets. We show a cost-effective generalization of MFM for analysis of non-homogeneous and wall-bounded flows, where eddy diffusivity is found to be a convolution acting on the macroscopic gradient of transported quantities. We introduce MFM as an effective tool for quantitative understanding of non-Boussinesq effects in turbulence, particularly, the effects associated with anisotropy and non-locality of macroscopic mixing.

Ali Mani is an associate professor of Mechanical Engineering at Stanford University. He is a faculty affiliate of the Center for Turbulence Research and a member of Institute for Computational and Mathematical Engineering at Stanford. He received his PhD in Mechanical Engineering from Stanford in 2009. Prior to joining the faculty in 2011, he was a senior postdoctoral associate at Massachusetts Institute of Technology in the Department of Chemical Engineering. His research group builds and utilizes large-scale high-fidelity numerical simulations, as well as methods of applied mathematics, to develop quantitative understanding of transport processes that involve strong coupling with fluid flow and commonly involve turbulence or chaos. His teaching includes the undergraduate engineering math classes and graduate courses on fluid mechanics and numerical analysis. He is the recipient of an Office of Naval Research Young Investigator Award (2015), NSF Career Award (2016), and Tau Beta Pi Teaching Honor Roll (2019).

• ## 2021 AFMSS: Ubiquitous free surface flows: from kitchen to volcanoes

Mathieu Sellier
University of Canterbury

4.00pm Wednesday 19 May 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Many flows encountered in our daily lives involve a moving boundary. The shape of a raindrop, for example, evolves as it falls through the air. Likewise, the free surface of a river deforms as it encounters obstacles. While the mathematical ingredients required to describe such flows have been known since the late 19th century and are encapsulated in the Navier-Stokes equations, solving complex flows with a moving boundary or interface still poses significant challenges and provides stimulating cross-disciplinary research opportunities. The question at the centre of the research I will present is “if information about the evolution of a moving interface is available, can we indirectly infer unknown properties of the flow?” Such a question falls in the realm of inverse problems for which one knows the effect but is looking for the cause. Specifically, I will talk about how it is possible to estimate the fluid properties of lava just by looking at how it flows or what is the best way to rotate a pan to cook the perfect crêpe.

Since 2018, Mathieu Sellier is Professor of Fluid Mechanics in the Department of Mechanical Engineering at the University of Canterbury (New Zealand). He is also Head of that Department since 2019. Prof Sellier graduated with a Master in “Modelling and Simulation in Mechanics” from Université Grenoble Alpes (France) in 2000, then got his PhD from the University of Leeds (UK) in 2003 working on the development of numerical methods to better understand the flow of thin liquid films and droplets on complex textured surfaces for coating applications. From 2003 to 2006, he was a PostDoc at the Fraunhofer Institute for Industrial Mathematics (Kaiserslautern, Germany) in the Marie-Curie Research Training Network MAGICAL (Mathematics for the Glass Industry Computations and Analysis). Prof Sellier started at the University of Canterbury as a lecturer in Theoretical Fluid Mechanics in 2006 and now leads the Interfaces and Inverse Problems lab (I&IP). His research interests are broad but typically revolve around modelling free surface or multiphase flow phenomena at small scales for which capillary and wetting phenomena dominate (droplets and thin film flows) or at large geophysical scales (river or glacier flows). Prof Sellier’s other area of research expertise is related to inverse problems for which one tries to infer the unknown causes of observed phenomena.

• ## 2021 AFMSS: From transition to turbulence control

Björn Hof
IST Austria

5.00pm* Wednesday 12 May 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

In many shear flows turbulence arises despite the linear stability of the laminar flow and this transition has remained unresolved for over a century. In this talk I will report recent progress in experiments and show that the onset of sustained turbulence in pipe and Couette flow corresponds to a directed percolation phase transition. Moreover, I will show how properties of transitional flows can be exploited to control turbulence and in particular to fully relaminarize pipe flow at much higher Reynolds numbers.

Björn Hof is a professor of Physics at the Institute of Science and Technology Austria. Previously he was a Max Planck Research Group leader at the MPI Göttingen and a lecturer and RCUK fellow at the University of Manchester. He held postdoctoral positions at the TU Delft and at the University of Manchester. He obtained his PhD in physics from the University of Manchester. His current work ranges from studies of the transition to turbulence, to polymer drag reduction, turbulence control, cardiovascular flows and epidemic modelling. He is a fellow of the American Physical Society.

• ## 2021 AFMSS: Universality in the subcritical route to turbulence

Dwight Barkley
University of Warwick

5.00pm* Wednesday 5 May 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Recent years have witnessed a profound change in our understanding of the route to turbulence in wall-bounded shear flows. In stark contrast to the classical Hopf-Landau picture where turbulence arises through an increase in the temporal complexity of fluid motion, the route to turbulence in subcritical shear flows occurs via spatio-temporal intermittency and falls in the class of non-equilibrium statistical phase transitions known as directed percolation. I will review important results in the field with focus on the spatio-temporal nature of the problem and how universality manifests itself. I will then describe recent work aimed at capturing the rare events dictating critical phenomena in wall-bounded transition.

Dwight Barkley is a professor of Mathematics at the University of Warwick. He held postdoctoral positions at Caltech, Princeton, and ENS Lyon. He obtained his PhD in Physics from the University of Texas at Austin. His research lies at the interface between high-performance computation, pattern formation and nonlinear phenomena. He has studied instabilities, waves and patterns in fluid, chemical and biological systems. His current work focuses on Euler singularities and the onset of turbulence. In 2005 he was awarded the SIAM J.D. Crawford Prize. He is a Fellow of the American Physical Society and of SIAM.

• ## 2021 AFMSS: Hidden physics models

Maziar Raissi

4.00pm Wednesday 28 April 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

A grand challenge with great opportunities is to develop a coherent framework that enables blending conservation laws, physical principles, and/or phenomenological behaviors expressed by differential equations with the vast data sets available in many fields of engineering, science, and technology. At the intersection of probabilistic machine learning, deep learning, and scientific computations, this work is pursuing the overall vision to establish promising new directions for harnessing the long-standing developments of classical methods in applied mathematics and mathematical physics to design learning machines with the ability to operate in complex domains without requiring large quantities of data. To materialize this vision, this work is exploring two complementary directions: (1) designing data-efficient learning machines capable of leveraging the underlying laws of physics, expressed by time dependent and non-linear differential equations, to extract patterns from high-dimensional data generated from experiments, and (2) designing novel numerical algorithms that can seamlessly blend equations and noisy multi-fidelity data, infer latent quantities of interest (e.g., the solution to a differential equation), and naturally quantify uncertainty in computations.

I am currently an Assistant Professor of Applied Mathematics at the University of Colorado Boulder. I received my Ph.D. in Applied Mathematics & Statistics, and Scientific Computations from University of Maryland College Park. I then moved to Brown University to carry out my postdoctoral research in the Division of Applied Mathematics. I then worked at NVIDIA in Silicon Valley for a little more than one year as a Senior Software Engineer before moving to Boulder. My expertise lies at the intersection of Probabilistic Machine Learning, Deep Learning, and Data Driven Scientific Computing. In particular, I have been actively involved in the design of learning machines that leverage the underlying physical laws and/or governing equations to extract patterns from high-dimensional data generated from experiments.

• ## 2021 AFMSS: Gravity currents in heterogeneous porous media

Edward Hinton
University of Melbourne

4.00pm Wednesday 21 April 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Many analytical results have been obtained for gravity-driven flow in uniform porous media, which provide checks on numerical methods and furnish insight into the underlying physical processes. However, subsurface porous rocks are typically highly heterogeneous, which strongly influences the flow. In this talk, I examine the effect of some simple heterogeneities on the classical solutions for flow in porous media and relate the results to applications in CO2 sequestration. First, the interaction of free-surface flow with a cylindrical inclusion of different permeability is explored. The flow thickness and streamlines are obtained through simple flux-balance arguments in various regimes for the relative inclusion width and permeability. Second, I will discuss displacement flow in a confined layer with a vertical gradient of permeability and analyse how gravity and permeability variations compete to drive the flow and control the interface evolution.

Edward Hinton is a Harcourt-Doig research fellow at the University of Melbourne (https://blogs.unimelb.edu.au/edward-hinton/). He previously held a London Mathematical Society research fellowship at The University of Bristol, UK and prior to that completed his PhD at the University of Cambridge, UK. His PhD thesis investigated various low Reynolds number environmental flows. Edward’s research aims to deploy low order models to capture the dominant aspects of a particular problem and then use numerical methods, laboratory experiments and mathematical techniques to provide insights into the underlying physical behaviour.

• ## 2021 AFMSS: Surface wave control of bacterial biofilms

Hua Xia
Australian National University

4.00pm Wednesday 14 April 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Faraday waves have been shown to produce a rich array of phenomena, from pattern formation, order to disorder transition, to the generation of two-dimensional turbulent flows [Shats, Phys. Rev. Lett., 2012; Xia, Phys. Rev. Lett., 2012; Francois, Phys. Rev. Letters, 2013, Xia, Nature comm. 2013]. The Faraday wave driven surface flows influence turbulent mixing, transport of inertial particles [Xia, J. Fluid. Mech., 2018; Yang, Phys. Fluids, 2019] and can be utilized to design turbulence-driven rotors [Francois, Phys. Rev. Fluids, 2018; Francois, Phys. Rev. Lett., 2020] and self-propelled surface vehicles [Yang, Phys. Rev. Fluids, 2019]. Wave-assisted technology has great potential in engineering of new biomaterials based on bacterial biofilms. Bacterial biofilm is the most abundant form of life on Earth. The understanding and control of the biofilm formation has been a research focus for the past 20 years mainly due to its roles in biofouling of industrial pipelines, antibiotic resistance, and in chronic infections. On the other hand, industrial wastewater treatment plants and agricultural technologies utilise the benefits of biofilms. The search for new sustainable technologies has led to the fast development of non-conventional materials produced by certain bacteria, such as bacterial cellulose. Understanding environmental factors affecting the attachment and the early stages of the biofilm development is the key to promoting and discouraging the biofilm growth. We show that surface wave driven flow can be used to control the formation of the biofilm formation at the liquid solid interface [Hong, Science Advances, 2020]. By controlling the wavelength, amplitude, and horizontal mobility of the waves, one can shape the biofilm, and either enhance the growth, or discourage the formation of the biofilm. Wave driven flows are the strongest at the liquid-air interface, which makes a good candidate for the control of biofilm formed at the liquid-air interface. We apply this approach to engineer biofilms whose morphology is controlled by the wave-driven flows. This allows to create different biofilm forms, such as for example spherical beads, or planar pellicles which are used in many industrial, scientific and agricultural applications.

Dr Hua Xia has been working at the ANU since 2006 on physics of fluids and their applications in atmospheric, oceanic and biological systems. Her research has attracted funding from the ARC including several Discovery projects, a DECRA award (2012-2014) and a Future Fellowship (2015-2019). Dr Xia's current research focus is on the physics approaches to microbiological systems for novel biophysical applications. The work presented here is part of her ARC DP19 project 'Transport control in multi-species fluid suspensions for biomedical applications' (2019-2022).

• ## 2021 AFMSS: Controlling and Modelling Transport Phenomena using Surface Engineering

Shervin Bagheri
Royal Institute of Technology (KTH)

4.00pm Wednesday 7 April 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Engineered surfaces with texture, chemical contrasts, pores, and compliance may significantly modify transport pro-cesses. In this talk, I give a few examples of our work focusing on how complex surfaces and flowing fluids interact at different length scales. We demonstrate; (i) how very small changes in either surface texture or chemistry modify lubrication forces, resulting in non-trivial trajectories of particles traveling parallel to surfaces; ii) how liquid-infused surfaces modify turbulent friction drag, in particular, when capillary waves develop on trapped lubricant; iii) how to accurately model transport processes between free flows and porous/rough materials using effective boundary conditions, that is, without resolving every microscopic feature of the surface. These problems illustrate the potential of surface engineering for control of transport phenomena and our ability to efficiently model flows over complex surfaces.

Shervin Bagheri is a Professor at the Royal Institute of Technology, KTH in Stockholm. He is a Wallenberg Academy Fellow and one of the twenty recipients of the Future Research Leaders grant awarded by the Swedish Foundation for Strategic Research. His research group (www.bagherigroup.com) focuses on understanding how flowing fluids and surfaces behave and interact across length scales, including modelling moving triple-phase moving contact lines, lubrication forces, transport in porous media and wall-bounded turbulent flows. His group uses mainly numerical simulations in combination with multiscale, data-driven and/or modal techniques for analyzing and controlling fluid-surface interaction.

• ## 2021 AFMSS: Aerodynamically driven rupture of a liquid film by turbulent shear flow

Melissa Kozul
University of Melbourne

4.00pm Wednesday 31 March 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

The rupture of a liquid film due to co-flowing turbulent shear flows in the gas phase is studied using a volume-of-fluid method. This was done by ‘sandwiching’ the liquid film between two fully developed gas-phase boundary layers from a turbulent channel simulation. The film deforms and eventually ruptures within the shear zone created by the co-flows. Previous theoretical work has indicated that aerodynamic forces drive the film’s evolution. We employ a classical aerodynamics approach to quantify the role of the inviscid lift and drag forces over the deforming film, which while suggested by previous authors, has not been systematically studied before. A cumulative lift force is introduced to capture the effect of the alternating pressure minima and maxima forming over the film, which amplify and eventually rupture the film. Our novel, efficient setup allows systematic variation of physical parameters to gauge their role in the aerodynamically driven deformation and rupture of a liquid film under fully developed sheared turbulence.

Melissa completed her PhD at the University of Melbourne in 2018 under the supervision of A/Prof. Daniel Chung and Prof. Jason Monty, having completed undergraduate degrees at the same institution. In early 2018 she moved to Norway and was a Postdoctoral Fellow within the Thermo Fluids Research Group at NTNU in Trondheim for three years. She has recently been engaged as a Research Fellow at the University of Melbourne upon returning to Australia. Melissa’s research expertise is in the high-fidelity simulations of fundamental turbulent flows that feature critically in energy and transport technologies, for which she often employs strategically-designed numerical ‘thought experiments’ to decouple effects that are conflated, or add controlled physics. To date her research interests have included wall-bounded flows, homogeneous isotropic turbulence, turbulent multiphase flows as well as particle tracking and sizing algorithms for use with laboratory turbulent flows.

• ## 2021 AFMSS: Swept-wing boundary layer transition

Yury S. Kachanov
Khristianovich Institute of Theoretical and Applied Mechanics

7.00pm* Wednesday 24 March 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Yury S. Kachanov is a professor in Fluid and Plasma Mechanics, Main Research Scientist and Head of Research Group in Khristianovich Institute of Theoretical and Applied Mechanics of Siberian Branch of the Russian Academy of Sciences Novosibirsk, Russian Federation. He has received the Alexander von Humboldt Research Award in 2002, the highest prize of the German Aerospace Society: “Ludwig-Prandtl-Ring” in 2008, and the Prof. G.I. Petrov’s Prize of the Russian National Committee on Theoretical and Applied Mechanics in 2010. His fields of interest are receptivity and stability characteristics of boundary layers to various external perturbations, traveling and steady instability modes, nonlinear mechanisms of breakdown of laminar boundary layers, nonlinear interactions of instability modes in boundary layers, mechanisms of formation, development, and breakdown of coherent structures in transitional and turbulent flows, mechanisms of turbulence production at late stages of transition.

• ## 2021 AFMSS: Numerical analysis of heat transfer characteristics of wall impinging spray flames under CI engine-like conditions

Abhishek L. Pillai
Kyoto University

4.00pm Wednesday 17 March 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Design of Compression Ignition (CI) engines with improved thermal efficiencies requires further investigation of the heat transfer mechanism from spray flame to the combustion chamber wall. Heat transfer during the interaction between impinging spray flame and wall under CI engine-like conditions, is investigated in this study using 3-D numerical simulations. The main objective is to examine the influence of fuel spray injection velocity on the heat transfer between impinging spray flame and wall. Details of the numerical methods and models employed in these simulations will be discussed. Results obtained from the simulations indicate that the total heat flux (sum of convective and radiative heat fluxes) at the wall surface increases with the fuel injection velocity. It is observed that the total wall heat flux is largest in the stagnation zone where the spray flame impinges directly upon the wall surface, while the radiative heat flux at the wall surface becomes greater as the distance form this stagnation zone increases. Additionally, it is found that the influence of fuel injection velocity on the radiative heat flow rate at the wall surface is rather insignificant. This radiative heat flow rate when expressed as a percentage of the total wall heat flow rate, ranges from ≈ 18% to 30% (depending on the 3 cases investigated), indicating that its contribution cannot be neglected for the CI engine-like conditions under which the present simulations are performed. Furthermore, correlations between the Nusselt number Nu (corresponding to the wall heat loss) and Reynolds number Re (of the flow-field) of the form $Nu \propto Re^n$ , are analysed and compared with those of existing experimental studies to assess their applicability for accurately estimating the heat transfer coefficient.

Abhishek is an Assistant Professor in the Department of Mechanical Engineering and Science at Kyoto University. He completed his PhD (Engineering) at Kyoto University under the supervision of Prof Ryoichi Kurose and is a recipient of the prestigious Japanese Government (Monbukagakusho) PhD Scholarship. He holds a Master’s degree in Aerospace Engineering from the Indian Institute of Technology (IIT) Bombay. He is also a member of the Japan Society of Mechanical Engineers (JSME). His main research interests lie in numerical investigations of problems pertaining to turbulent combustion, occurring under conditions relevant to practical energy conversion devices (viz., gas-turbines, IC engines). Therefore, he performs large-scale parallel simulations on HPC infrastructures, such as Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES), for studying turbulent gas-phase and spray combustion problems. His recent research focuses on thermo-acoustic instabilities, combustion noise using hybrid CFD/CAA (Computational Fluid Dynamics/Computational Aero-Acoustics) simulations, and wall-bounded turbulent reacting flows.

• ## 2021 AFMSS: High-order finite-difference schemes on a cartesian mesh: application to active flow control of turbulent flows

Sylvain Laizet
Imperial College London

6.00pm* Wednesday 10 March 2021 (*Please note change in start time)
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

With recent impressive developments in computer technology, High Performance Computing (HPC) is currently transitioning to the exascale era with far-reaching consequences for scientific research. HPC is expected to open the doors to solving highly complex turbulence problems that were until very recently beyond our imagination. Computational Fluid Dynamics (CFD) is now a critical complement to experiments and theories in order to understand turbulent flows and discover strategies to manipulate them. One of the biggest challenges for the CFD community is to be able to efficiently exploit the current and next generation of HPC systems. In this talk, I will introduce the open-source framework Xcompact3d, dedicated to the study of turbulent flows on HPC systems based on CPUs. Based on high-order finite-difference schemes on a Cartesian mesh, it combines accuracy, efficiency, versatility and scalability. Active flow control solutions for wall-bounded flows, free-shear flows and wind turbines will be presented to highlight the potential of the Xcompact3d framework.

Sylvain Laizet is a reader in the Department of Aeronautics at Imperial College London (ICL). He holds a PhD and an Habilitation à Diriger des Recherches from the University of Poitiers in France in the field of Computational Fluid Dynamics (CFD) applied to turbulence. Understanding turbulent flows and how to manipulate them in various engineering applications is the motivation behind his research. With his collaborators at Imperial College, in France and in Brazil, he has developed over the years high-order finite-difference highly-scalable flow solvers dedicated to the study turbulent flows. Within the turbulence simulation group at ICL, he is currently investigating wake-to-wake interactions in wind farms, Bayesian optimisation techniques for drag reduction and energy saving, active control solutions for free-shear flows, immersed boundary methods for moving objects, neural networks applied to CFD and particle-laden gravity currents.

• ## 2021 AFMSS: Eddy structures and very-large-scale motions in turbulent round jets

Queen's University

4.00pm Wednesday 3 March 2021
Online Event (2021 Australasian Fluid Mechanics Seminar Series)

Coherent structures in the fully turbulent region of round jets are evaluated for up to Re_d=50,000 with the aid of two-point measurements and an existing direct numerical simulation (DNS) dataset. The experimental data comprise simultaneous velocity time-series acquired with both radial and azimuthal separations between the sensors. A spectral correlation analysis is applied to these data that reveal that the coherent structures in the jet flow consist of two rincipal configurations, which correspond to two main spectral domains. One spectral domain, which is signified by small to medium wavelengths, is associated with hierarchical eddy structures (ESs) for which a physical aspect ratio of 1.2:1:1 in the axial, radial, and azimuthal directions is observed. The other spectral domain indicated by large wavelengths is associated with very-large-scale-motions (VLSMs). The wavelength marking the boundary between these spectral domains is used to decompose the velocity fluctuations into ES and VLSM components, and the corresponding ES and VLSM components of two-point correlations are obtained from the experimental data. The VLSM component of two-point correlations denotes helical structures as the dominant VLSMs in the jet turbulent region. Instantaneous axial velocity fluctuation fields from DNS support the prevalence of helical VLSMs in the jet. Moreover, the ES signatures are evident in the unwrapped axial-azimuthal planes of the DNS, indicating that the VLSMs are formed by the concatenation of ESs.

Milad is an adjunct professor and a postdoctoral research fellow at Queen’s University. Milad received his PhD degree from the University of Melbourne under the supervision of Professors Ivan Marusic and Nicholas Hutchins in 2018 before moving to Queen’s University. Milad uses experimental and theoretical methods to study various fundamental flow physics including wall bounded turbulent flows, free shear flows, entrainment, coherent structures, and active flow control.

• ## 2020 AFMSS: Host-to-Host Airborne Contagion As a Multiphase Flow Problem For Science-Based Social Distance Guidelines

Sivaramakrishnan Balachandar
University of Florida

4.00pm Wednesday 2 December 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The COVID-19 pandemic has brought sudden and broad social awareness about the fundamental role of airborne droplets and aerosols as virus carriers. Droplets are formed and emitted at high speed during a sneeze, and at lower speed while coughing, talking or breathing. But no two coughs or sneezes are alike - many aspects of these expiratory events vary from one individual to another. For example, a violent sneeze of a large person could generate a large puff containing a sizable number of potentially virus-laden droplets that extend much farther into the surrounding than that of a child. It is to be expected that the coughs and sneezes of even the same individual could vary from one to the next. It is perhaps less evident that two nearly identical coughs or sneezes may show substantial differences as a result of their turbulent nature. Infinitesimal differences in the initial exhalation process or in the ambient conditions can be dramatically amplified and send a cough or sneeze careening in different paths - this chaotic behavior is the so-called butterfly effect. Such chaotic evolution must be properly accounted in any deterministic social distancing guidelines, since such guidelines must not only safeguard under average conditions, but also take into account occasional extreme departures from the average. Despite the chaotic behavior, there are important underlying universal properties that are common across all expiratory events and the dispersive nature of the ejected droplet clouds. This work will demonstrate the ability of a simple mathematical formulation (Balachandar et al., Int J. Multiphase Flow, 2020) to accurately predict the key quantities of interest to viral contagion. Furthermore, the simulations, supported by theoretical analysis, illustrate the importance of environmental variables such as dry versus humid condition can have on the number of airborne airborne potentially virus-laden droplet nuclei.

Sivaramakrishnan Balachandar is a Distinguished Professor in Mechanical & Aerospace Engineering at the University of Florida.

• ## 2020 AFMSS: Stabilisation of exact coherent structures using time-delayed feedback in two-dimensional turbulence

Dan Lucas
Keele University

6.00pm* Wednesday 25 November 2020 (*Please note change in start time)
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Time-delayed feedback control (Pyragas 1992 Phys. Letts. 170 (6) 421-428), is a method known to stabilise periodic orbits in chaotic dynamical systems. A system dx/dt = f(x) is supplemented with G(x(t)-x(t-T) where G is a gain matrix' and T a time delay. The form of the delay term is such that it will vanish for any orbit of period T, making it an orbit of the uncontrolled system. This non-invasive feature makes the method attractive for stabilising exact coherent structures in fluid turbulence. Here we validate the method using the basic flow in Kolmogorov flow; a two-dimensional incompressible viscous flow with a sinusoidal body force. Linear predictions for the laminar basic flow are well captured by direct numerical simulation. This result demonstrates a work-around of the so-called “odd-number” limitation in flows which have a continuous symmetry. By applying an adaptive method to adjust the streamwise translation of the delay, a known nonlinear travelling wave solution is able to be stabilised up to relatively high Reynolds number. Finally an adaptive method to converge the period T is also presented to enable periodic orbits to be stabilised in a proof of concept study at low Reynolds numbers. These results demonstrate that unstable ECSs may be found by timestepping a modified set of equations, thus circumventing the usual convergence algorithms.

Dan holds an undergraduate MMath degree in Applied Mathematics from the University of St Andrews and an MSc in Atmosphere/Ocean modelling from Reading University. Following this he returned to St Andrews to study for his PhD under the supervision of Prof David Dritschel. Dan then held postdoctoral positions in Bristol (Rich Kerswell), University College Dublin (Miguel Bustamante) and Cambridge (Colm Caulfield) before starting as a lecturer in Applied Mathematics at Keele University in September 2017.

• ## 2020 AFMSS: Nonlinearity - the “frenemy” of linearised Navier-Stokes analysis

Sean Symon
University of Southampton

6.00pm* Wednesday 18 November 2020 (*Please note change in start time)
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The linearised Navier-Stokes equations are a popular tool for identifying structures in turbulent flows. There are, nevertheless, subtle differences between the structures computed directly from the flow using data-based methods (e.g. dynamic mode decomposition) and equation-based methods (e.g. resolvent analysis). In this talk, we scrutinise the nonlinear forcing term, which is often treated as white noise in space and time, for three flows of increasing complexity. The first is flow past a circular cylinder, where we deal explicitly with the nonlinear forcing. We show that linear analysis provides clues regarding the nonlinear interactions that are needed to sustain the flow. The second is an exact coherent state in plane Poiseuille flow, where we analyse the energy balance and compare it to predictions from resolvent analysis. We also investigate the quantitative agreement between nonlinear transfer modelled by an eddy viscosity profile to the true nonlinear transfer. Finally, we consider turbulent channel flow at Reτ = 2003, where we make no attempt to compute or model the nonlinear forcing. Instead, we investigate how an imperfect eddy viscosity can be corrected by limited measurements to build a useful estimator of the flow.

Sean studied Aerospace Engineering at the University of Maryland where he completed a Bachelor of Science and a Bachelor of Arts in French Language/Literature in 2012. The following year, he completed a Master of Science in Aeronautics at the California Institute of Technology. Before beginning his PhD, Sean studied at Ecole Polytechnique in Paris where he was awarded a masters in fluid mechanics in 2014. He returned to Caltech for his PhD under the supervision of Beverley McKeon. Sean defended his thesis in 2018 and became a post-doctoral research fellow in Australia for 2 years at the University of Melbourne, working with Simon Illingworth and Ivan Marusic. He began as a lecturer in the Aerodynamics and Flight Mechanics Group in 2020 at the University of Southampton.

• ## 2020 AFMSS: Turbulence structure and modeling: some adventures in the frequency domain

Tim Colonius
Caltech

6.00pm* Wednesday 11 November 2020 (*Please note change in start time)
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Amongst many available data-driven modal decompositions of utility in fluid mechanics, the frequency-domain (spectral) version of the proper orthogonal decomposition (SPOD) plays a special role in the analysis of stationary turbulence. SPOD modes are optimal in expressing structures that evolve coherently in both space and time, and they can be regarded as optimally-averaged DMD modes. Importantly, the SPOD spectrum is also related to the resolvent spectrum of the linearized dynamics and examination of the relationships between the SPOD and resolvent modes yields information about how coherent structures are forced by nonlinear interactions amongst coherent and incoherent turbulence. We discuss the application of these tools to analyze and model turbulence in high-speed jets and boundary layers. We highlight recent developments including (a) utilizing eddy-viscosity models in resolvent analysis to enable RANS-based prediction of coherent structures, and (b) nonlinear extensions of resolvent analysis to discover worst-case disturbances for laminar-turbulent transition, and (c) the development fast spatial marching methods for large-scale resolvent problems.

Tim Colonius is the Frank and Ora Lee Marble Professor of Mechanical Engineering at the California Institute of Technology. He received his B.S. from the University of Michigan in 1987 and M.S and Ph.D. in Mechanical Engineering from Stanford University in 1988 and 1994, respectively. He and his research team use numerical simulations to study a range of problems in fluid dynamics, including aeroacoustics, flow control, instabilities, shock waves, and bubble dynamics. Prof. Colonius also investigates medical applications of ultrasound, and is a member of the Medical Engineering faculty at Caltech. He is a Fellow of the American Physical Society and the Acoustical Society of America, and he is Editor-in-Chief of the journal Theoretical and Computational Fluid Dynamics. He was the recipient of the 2018 AIAA Aeroacoustics Award.

• ## 2020 AFMSS: Navigating turbulent waters

Ashleigh Hutchinson
University of the Witwatersrand

4.00pm Wednesday 4 November 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Approaches to modelling turbulence vary in complexity. From simple eddy viscosity closure models to complicated numerical simulations, all methods have their uses depending on the context in which they are to be applied. Turbulence is commonly observed in nature and in industry. Physicists and engineers alike aim to design reliable models that are of theoretical and industrial importance. However, this endeavour proves challenging and progress can only be made when theoretical, experimental, and computational approaches are integrated. Despite having these techniques at our disposal, turbulence is considered ‘one of the oldest unsolved problems’ in Physics. In this talk, provide an overview of the different approaches to modelling turbulence. We will focus on one application, turbulent wakes, which play a valuable role in the design, maintenance, and control of turbines on wind farms. Various eddy viscosity closure models will be investigated and compared.

Dr Ashleigh Jane Hutchinson is a Senior Lecturer at the University of the Witwatersrand, Johannesburg, in the School of Computer Science and Applied Mathematics, in the Faculty of Science. She is also the Assistant Director of the national DSI-NRF Centre of Excellence in Mathematical and Statistical Sciences (CoE-MaSS). The focus of Dr Hutchinson’s research is on developing mathematical models to solve real-world problems, with particular attention paid to modelling in fluid mechanics. Her research is a combination of theoretical work and practical applications.

• ## 2020 AFMSS: Patterns in 2D Turbulence: Cause, Effect and Monte-Carlo Science

Javier Jiménez

7.00pm* Wednesday 28 October 2020 (*Please note change in start time)
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The relentless growth of the speed of computers has meant that turbulence simulations that used to take long times can now be performed in minutes. Among other things, this allows us to explore ideas ‘at random’, in the hope that some of them might be interesting, but this ‘Monte-Carlo science’ is only the first step in a process in which the choice and evaluation of ideas still has to be performed by human researchers in more conventional way. The most important role of the computer is to help them overcome their prejudices. We will explore an example of such a (humanmachine) collaboration dealing with the dynamics of the reverse energy cascade in two-dimensional turbulence. It begins with the essentially random search for local flow perturbations that result in global effects, continues with the identification of the sensitive flow patterns where these perturbations are best applied, and eventually guides us to the formation process of large-scale vortex patterns. Although the (human) author accepts responsibility for most of the analysis and conclusions, he has to acknowledge that the original idea came from the computer.

Javier Jiménez is an emeritus research professor at the School of Aeronautics of the U. Politécnica de Madrid, Spain. He received his Engineering degree from the same school, and a PhD in Applied Mathematics from Caltech. After beating about several bushes, his main interest for the past few years has been to deceive himself into believing that turbulence is really simple. His coauthor in this particular talk, Mr. PLOFF, is a computer.

• ## 2020 AFMSS: High-fidelity simulations for cleanear gas turbines

Mohsen Talei
University of Melbourne

4.00pm Wednesday 21 October 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Gas turbines are the predominant prime movers in the aviation industry and account for about 20% of the installed capacity for electricity generation globally. They have fast ramp up and down capabilities, which make them complementary to intermittent renewable generation. Furthermore, with the increasing interest in producing renewable hydrogen, gas turbines can play an important role in the implementation of a successful hydrogen economy. Lean premixed combustion is the desired combustion regime in industrial gas turbines. It results in a low level of NOX emissions due to operating at lower temperatures and facilitates better combustion efficiency. However, the main issue with operating gas turbines in this regime is thermoacoustic instability, commonly initiated by combustion-generated sound. Another problem with these gas turbines is the increase in carbon monoxide (CO) emissions at part-load and low temperatures. These issues are significant drawbacks for developing cleaner gas turbines that feature stable combustion. High-fidelity simulations such as direct numerical simulation (DNS) can play a critical role here. They can provide insight into the root cause of these issues and the rich data provided by the simulations can help us develop more accurate predictive models. This presentation will provide examples from DNS studies undertaken at the University of Melbourne in this context.

Dr. Mohsen Talei is a Discovery Early Career Researcher (DECRA Fellow) of the Australian Research Council (ARC) and a Senior Lecturer in the Department of Mechanical Engineering. Mohsen joined the University of Melbourne as a staff member in 2014, having previously completed post-doctoral research at the University of New South Wales (UNSW) and a PhD at the University of Melbourne. Mohsen has 17 years of industry and academic research experience in the broad area of energy, which includes numerical and theoretical investigations of turbulent reactive flows with a focus on low-emission energy technologies. Mohsen’s research involves a significant use of high-performance computing to develop reliable models that can be used for simulating cleaner gas turbines and reciprocating engines.

• ## 2020 AFMSS: Symmetry induced turbulent scaling laws for arbitrary moments and their validation with DNS and experimental data

Martin Oberlack

4.00pm Wednesday 14 October 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Using the symmetry-based turbulence theory, we derive turbulent scaling laws in wall-bounded shear flows for arbitrarily high moments of the flow velocity $U_1$ . The key ingredients are the symmetries of classical mechanics, especially the scaling of time and space, and two statistical symmetries, which are not directly visible in Euler and Navier-Stokes equations. These symmetries are admitted by all complete theories of turbulence, i.e. the infinite hierarchy of moment and PDF equations and also by the famous Hopf functional approach. The symmetries provide a measure of intermittency and non-Gaussian behavior – properties that have been investigated for decades for turbulence and are now subject to a rigorous description. Based on this, in the near-wall log-region the symmetry theory predicts an algebraic law with the exponent $t_2 (n - 1)$ for moments $n > 1$ . Hence, the exponent $t_2$ of the 2nd moment determines the exponent of all higher moments. Moments here always refer to the instantaneous velocities $U$ and not to the fluctuations $u'$ . For the core regions of both plane and round Poiseuille flows we find a deficit law for arbitrary moments $n$ of algebraic type with a scaling exponent $n (s_2 - s_1) + 2 s_1 - s_2$ . Hence, the moments of order one and two with its scaling exponents $s_1$ and $s_2$ determine all higher exponents. All new theoretical results will be nicely validated by a new channel flow DNS at $Re_t = 10^4$ and data from the CICLoPE pipe flow experiment at $Re_t = 3.5 \times 10^4$ .

Martin Oberlack is Professor of Mechanical Engineering at Darmstadt University and holder of the chair for fluid dynamics. He received both his Diploma (1988) and his Ph.D. (1994) from RWTH Aachen. Prof. Oberlack pioneered the use of Lie symmetry methods for the study of turbulence and modelling concepts and has written widely on this with a special focus on turbulent shear flows. His current interests include: Lie symmetries of the Lundgren-PDF and Hopf equation of turbulence, construction of conservation laws, hydrodynamic stability theory, Fokas unifying method for multi-phase problems, aerodynamic noise, combustion, high-performance and parallel computing, GPU acceleration, Discontinuous Galerkin numerical methods with special focus on singular problems such as multi-phase flows and large scale direct turbulence simulations. He was and is actively involved in various editorial boards including Fluid Dynamic Research, Theoretical and Computational Fluid Dynamics and Continuum Mechanics and Thermodynamics. He is a Fellow of the American Physical Society for his pioneering use of symmetry methods for the study of turbulence and related fields and the derivation of new conservation laws in fluid dynamics.

• ## 2020 AFMSS: Numerical investigation of shock-induced phenomena in turbulent flows

Ivan Bermejo-Moreno
University of Southern California

4.00pm Wednesday 07 October 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

This talk will address fundamental interactions between shock waves and turbulent flows relevant to high-speed flight and propulsion:
Shock-induced scalar mixing under canonical shock-turbulence interactions (STI) will be considered first, by means of Direct Numerical Simulation. The effects of relevant physical parameters (shock and turbulence Mach numbers, and Reynolds number) will be highlighted on statistical changes along the shock-normal direction of scalar variance and dissipation-rate budgets, as well as alignments of the scalar gradient with vorticity and strain-rate eigendirections. Shock-induced scalar mixing will also be addressed by tracking the downstream evolution of the geometry and physics of scalar structures initialized with a well-defined shape as they are transported and diffused by the background turbulence in STI, and compared with decaying homogeneous isotropic turbulence.
Flow-structure interactions of shock waves reflecting off turbulent boundary layers that develop along flexible walls will be addressed next, comparing results from ongoing numerical simulations with prior wind tunnel experiments. The calculations couple wall-modeled large-eddy simulation for the fluid flow, using an Arbitrary Lagrangian-Eulerian formulation, with an elastic solid structural solver that accounts for geometric nonlinearities, and a mesh deformation module based on a spring-system analogy. Strong shock/boundary-layer interactions resulting in mean flow separation and low-frequency unsteadiness that can interact with the natural frequencies of the structure will be emphasized.

Ivan Bermejo-Moreno received his Ph.D. in aeronautics (2008) from the California Institute of Technology. Afterwards, he held a postdoctoral research fellowship at the Center for Turbulence Research, Stanford University/NASA Ames Research Center (2009-2014). He joined the Aerospace and Mechanical Engineering Department at the University of Southern California in 2015. His research combines numerical methods, physical modeling and high-performance computing for the simulation and analysis of turbulent fluid flows involving multi-physics phenomena. He is a recipient of the Fulbright Fellowship, the Rolf D. Buhler Memorial Award, the William F. Ballhaus Prize and the Hans G. Hornung Prize.

• ## 2020 AFMSS: Self-excited three-dimensional laminar separation bubbles: Waypoints in a 20-year roadmap

Vassilis Theofilis
University of Liverpool

5.00pm* Wednesday 30 September 2020 (*Please note change in start time)
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Solutions of multi-dimensional initial-value and eigenvalue problems that govern linear instability of flow in the vicinity of laminar separation bubbles forming in boundary layers from incompressible to hypersonic flow have permitted placing knowledge gained from simplified local analyses on a firmer theoretical footing and revealed physical mechanisms of separated flow instability inaccessible to classic linear theory. The first part of the talk will discuss the footprints of the dominant stationary and traveling primary and secondary global eigenmodes as well as linear optimal disturbances of spanwise homogeneous incompressible laminar separated flows over wings and low pressure turbine blades. Results will be contrasted against recently obtained linear global modes on finite aspect ratio swept wings. In the second part of the talk, the laminar separation bubble formed at a compression corner in supersonic flow will be shown to sustain linear instabilities in which the recompression shock forms an integral part of the amplitude functions of the dominant global modes, a result which renders questionable instability analyses in which shocks (or their effects) are neglected. An analogous conclusion is reached in shock-induced laminar separation bubble in hypersonic flow over double cones and double wedges, where both two- and three-dimensional global instability analysis has shown the intimate connection (through the global mode amplitude functions) of instability in the laminar separation zone, the triple point, the slip line and the shock system downstream of the laminar separation bubble.

Prof. Theofilis obtained his MSc in Applied Mathematics and PhD in Aerospace Engineering at the University of Manchester. After a post-doc at the Department of Applied Mathematics of University of Twente, he has been Alexander von Humboldt research fellow at DLR Goettingen, Germany and Ramon y Cajal Research Professor at the School of Aeronautics in Madrid. He held visiting appointments at Caltech, Arizona, Maryland and the Universidad Federal Fluminense (Rio de Janeiro). In 2016 he was appointed at the Chair of Aerospace Engineering at the University of Liverpool and since 2019 he is also Full Professor at the Escola Politecnica of Universidade São Paulo. His research interests lie in development and application of accurate numerical methods for the solution of large-scale eigenvalue and singular value problems, as applied to fluid flow nstability from the incompressible to the hypersonic regime.

• ## 2020 AFMSS: Spectral properties of turbulence kinetic energy in turbulent boundary layers

Woutijn Baars
TU Delft

4.00pm Wednesday 23 September 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Even though wall-bounded turbulence is highly broadband in nature, it does comprise a strong coherence in for instance the velocity fields. This seminar will primarily focus on a spectral characterization of organized motions in wall-bounded turbulence. An organization is evidenced by the longstanding classification of different flow structures, such as hairpin packets and very large-scale motions. Since the turbulence statistics of different categories of structures comprise dissimilar scaling behaviors, their coexistence complicates the development of scaling laws or models for velocity spectra and even the integrated turbulence kinetic energy. Via multi-point data analyses, we present data-driven spectral filters for stochastically decomposing velocity spectra into sub-components, which are interpreted as representations of different types of ‘building blocks’ of the wall-bounded turbulent flow. In the process we reveal a Reynolds-number invariant wall-scaling for a portion of the outer-region turbulence that is coherent with the near-wall region; this supports the existence of wall-attached self-similar turbulent structures. Throughout this seminar we will also indicate how the results assist in developing frameworks for predicting statistics at high-Reynolds-number conditions, how they assist in designing active control methods and how they assist in creating models for coherent turbulence in atmospheric surface layers.

Woutijn Baars is an Assistant Professor in the Faculty of Aerospace Engineering at Delft University of Technology, within the sections of aerodynamics and aeroacoustics. His research interests span various topics related to turbulence, including wall-bounded turbulence and passive/active methods for flow-control, and the aeroacoustics of turbulent jet flows rotor systems (propulsion of drones and urban air mobility vehicles). Woutijn received his MSc degree (2009) from Delft University of Technology and his PhD degree (2013) from the University of Texas at Austin, where he primarily focused on experimental jet aeroacoustics. From 2013 to 2018 he was a Research Fellow at the University of Melbourne, focusing on the flow physics of high-Reynolds-number wall-bounded turbulence.

• ## 2020 AFMSS: Mixing enhancement in binary fluids by adjoint-based optimization

Peter Schmid
Imperial College London

4.00pm Wednesday 16 September 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The mixing of binary fluids by stirring is a ubiquitous process in a wide range of industrial applications. The food processing, pharmaceutical and consumer product industries are dominated by mixing processes, and enhancing mixing efficiency for these applications, even by a very modest amount, would translate into considerable savings and benefits. We develop and use an adjoint-based nonlinear optimization scheme to minimize the mix-norm of a passive scalar. The mixing is accomplished by two cylindrical stirrers, moving on concentric circular paths inside a circular container. In a first attempt, the velocities of these stirrers is optimized to reach improved mixedness over a finite time horizon. The enhanced stirring protocol is shown to consist of a complicated interplay of vortical structures which have been created and exploited by the stirrers’ action. In a second attempt, and within the same computational framework, we seek to optimize the cross-sectional shape of the stirrers. In both scenarios, substantial mixing enhancement could be achieved. Further extensions of the mathematical formalism and the physical setup will be discussed, and remaining challenges of this research effort will be addressed.

Prof. Peter Schmid received his Engineer's Degree from the Technical University Munich and his PhD in mathematics from the Massachusetts Institute of Technology. He then joined the Department of Applied Mathematics of the University of Washington, Seattle before taking up a research director position with the French Scientific Research Agency (CNRS) at the Laboratoire d'Hydrodynamique (LadHyX) of the Ecole Polytechnique near Paris. He is currently Chair of Applied Mathematics and Mathematical Physics in the Department of Mathematics of Imperial College London. His research focuses on stability theory, flow control, model reduction and computational fluid dynamics. He is also interested in data decomposition techniques and algorithms for the extraction of pertinent flow processes from numerical and experimental data.

• ## 2020 AFMSS: Canopy Flows - An Outsider's Look

Ricardo Garcia-Mayoral
University of Cambridge

4.00pm Wednesday 09 September 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The turbulent flow over and within canopies is a subject of interest to both engineering applications and to hydrology and atmospheric fluid dynamics. By 'canopy' we refer to a collection of elements that obstruct the flow, which are "tall" -in a rather vague sense- to differentiate them from conventional roughness. This talk will discuss our recent work on canopy flows, approaching the problem from a background in turbulence over textured surfaces and flow control. We focus mainly on the character of the flow for homogeneous canopies of different densities, and use evidence from direct simulations. We observe that the flow dynamics arise from the interplay between the overlying turbulence, the element-induced flow, and the mixing-layer instability that can be induced at the canopy-tip plane. For sparse canopies, only the first two mechanisms are present. The turbulence in-between elements is essentially unobstructed, and is thus reminiscent of that over a smooth wall, but scaling with the fluid shear at each height. As the canopy density increases, mixing-layer rollers develop and the flow within the canopy becomes more quiescent, showing only a damped footprint of the overlying rollers. The element height has a secondary influence on the flow, as a lack of sufficient canopy depth inhibits the mixing-layer instability. From the evidence in our simulations, we argue that the ratio of the element spacing to the turbulent scales is the essential parameter determining whether the canopy behaves as dense or sparse.

Dr. Ricardo Garcia-Mayoral received his PhD from Universidad Politécnica de Madrid in 2011 under Prof. Javier Jiménez, and subsequently was a Postdoctoral Scholar at the Center for Turbulence Research in Stanford University in 2012-2013. Since 2013 he has been a Lecturer in Fluid Mechanics at the Department of Engineering in the University of Cambridge, UK. His group's research focuses on understanding the dynamics of turbulent flows over non-conventional surfaces, and on how these surfaces can modify and control the flow. The group has worked on roughness, riblets, superhydrophobic surfaces, canopies, and permeable substrates.

• ## 2020 AFMSS: DMD-based methods to identify flow patterns

4.00pm Wednesday 02 September 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Complex flows model a wide range of industrial and biological applications. Although these flows have been studied for decades, due to their high complexity, to elucidate their spatio-temporal structure is still considered as an open topic in a large quantity of relevant cases. Hence, the deep study and understanding of complex flow behavior, such as transitional or turbulent flows, is a research topic of high interest. In this talk we will introduce a method suitable to detect spatio-temporal patterns in complex non-linear dynamics. This is an extension of the well-known technique dynamic mode decomposition (DMD), called as higher order dynamic mode decomposition (HODMD). The method combines singular value decomposition (SVD) with DMD and Takens' delay embedding theorem to approximate the main dynamics describing a signal. HODMD has been successfully applied to study several problems covering from non-linear dynamical systems (i.e. complex Ginzburg-Landau equation, Lorenz system), to complex fluid dynamic problems (i.e. analysis of noisy experiments and transitional flows in synthetic jets, turbulent channel flows, etcetera) and has also been applied as a reduced order model for data forecasting in compressible flows and wind turbines. In the second part of the talk, a method known as spatio temporal Koopman decomposition (STKD) will be introduced, presenting a good alternative to perform spatio-temporal DMD analyses in a very efficient way. This method identifies coherent structures as a group of traveling waves. The method will be tested in some examples including the detection of spatio-temporal flow structures describing the wake of a wind turbine or global instabilities in elastoviscoplastic fluids and turbulent channel flows with anisotropic porous wall. The main flow patterns will be identified, and this information will be used to create a reduced order model for data forecasting.

Dr. Soledad Le Clainche holds a Lectureship in Applied Mathematics at the School of Aerospace Engineering of UPM. She received a Master in Mechanical Engineering by UPCT, in Aerospace Engineering by UPM, and in Fluid Mechanics by the Von Karman Institute. In 2013 she completed her PhD in Aerospace Engineering at UPM. Her research focusses on computational fluid dynamics and in the development of novel tools for data analysis enabling the detection of spatio-temporal patterns. Recently, she has developed reduced order models taking advantage of the insights provided by data analysis tools. In addition, she has contributed to the fields of flow control, global stability analysis, synthetic jets, analysis of flow structures in complex flows (transitional and turbulent) using data-driven methods, and prediction of temporal patterns using machine learning and soft computing methods.

• ## 2020 AFMSS: Prediction of Structural Wind Loading and Aerodynamic Scaling Effects due to Atmospheric Boundary Layer Turbulence

Matthew Emes

4.00pm Wednesday 26 August 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Wind load predictions for small-scale structures, such as heliostats, are not provided in design codes for buildings because of their thin-walled structural components and the changes in wind velocity and turbulence in the lowest 10 m of the atmospheric boundary layer (ABL). This seminar will provide an insight into the generation and analysis of turbulence characteristics within simulated ABLs in the large-scale University of Adelaide wind tunnel. Through comparison of semi-empirical models in experimental fluid mechanics, wind engineering and flat plate aerodynamics, the effect of turbulence intensity and integral length scales in different roughness terrains on the design wind loads on flat plates will be discussed. This is important to provide accurate estimation of the maximum wind loads on heliostats to avoid structural failure, but also reduce the cost of over-engineered structural components. The Australian Solar Thermal Research Institute (ASTRI), led by the University of Adelaide in collaboration with CSIRO, ANU and DLR, have investigated the static and dynamic wind loads for the maximum operational and stow survival cases that represent the worst case scenario loading orientations of a heliostat with respect to the wind. The findings highlight the importance of accurately reproducing and resolving the critical range of high-frequency turbulence characteristics through an approach considering the effects of aerodynamic model scaling of the lower surface layer within the ABL.

Dr Matthew Emes is a University of Adelaide postdoctoral researcher in the School of Mechanical Engineering, with a background in experimental fluid mechanics, atmospheric boundary layer turbulence, and a focus on the effects of turbulence on the aerodynamics and wind loads of heliostats in concentrating solar thermal technology. His research involves developing innovative techniques for measuring and characterising the temporal and spatial fluctuations of atmospheric turbulence in wind tunnel and field experiments. Dr Emes’ research is funded by the Australian Renewable Energy Agency (ARENA), as part of the Australian Solar Thermal Research Institute (ASTRI) heliostat project.

• ## 2020 AFMSS: The National Flying Laboratory Centre – Cranfield University: In-Flight Measurement and Research

Nicholas Lawson
Cranfield University

4.00pm Wednesday 19 August 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The National Flying Laboratory at Cranfield University operates three aircraft for research and teaching. The aircraft range from a 7000kg Jetstream 31 turboprop with 19 seats, to two tandem-seat 1000kg aerobatic piston engine aircraft. A Saab340B will replace the Jetstream in 2021. This presentation will describe the aircraft and their application in a research environment, with work ranging from the development of fibre optic pressure and strain sensors in-flight, to the study and validation of an unsteady DES CFD model, of a stalling Slingsby light aircraft. In the latter case, there is a focus on using in-flight data to validate the DES model and to study the wake interaction with the tail plane. In all cases, the challenges of in-flight measurement will be discussed and the benefit of combining in-flight testing with CFD models.

Professor Nicholas Lawson holds a Chair in Aerodynamics and Airborne Measurement and heads the National Flying Laboratory Centre at Cranfield University. He is also a Chartered Engineer, a Fellow of the Royal Aeronautical Society and a member of the Society of Flight Test Engineers. Professor Lawson has extensive experience in the development and application of particle image velocimetry (PIV) and laser Doppler anemometry (LDA) to high speed, large scale, non-Newtonian and multi-phase flows. Recent work has involved Rolls Royce plc, Airbus UK, Jaguar Racing and Meggitt, as well as EU partners including DLR and TU Delft. His current research focus is on the application and development of advanced instrumentation for airborne and aerodynamic measurement, where fibre optic sensors are a key focus of his work, in airborne and wind tunnel environments. He holds a commercial pilots licence with multi-engine piston, instrument and instructor ratings, a Jetstream 31/32 type rating and is one of the pilots flying for the National Flying Laboratory Centre.

• ## 2020 AFMSS: The fluid mechanics of a large pendulum chain

Geoffrey Vasil
University of Sydney

4.00pm Wednesday 12 August 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

I’ll discuss a particular high-dimensional system that displays subtle behaviour found in the continuum limit. The only catch is that it formally shouldn’t, which raises a few questions. When is a discrete system large enough to be called continuous? When are approximate (broken) symmetries good enough to be treated like the real thing? When and why does a fluid approximation work as well as we like to assume? What does all this say about observables and the approach to equilibria? The particular system I have in mind is a large ideal pendulum chain, and it’s cousin the continuous flexible string. I propose that this is a perfect model system to study notoriously difficult phenomena such as vortical turbulence and cascades, but with many fewer degrees of freedom than a three-dimensional fluid.

Geoff Vasil is a Senior Lecturer in Applied Mathematics at the University of Sydney. He did his PhD in Atmospheric, Oceanic and Planetary Sciences at the University of Colorado. After that, he did postdoctoral work in Astronomy Astrophysics at the University of Toronto and University of California Berkeley. His work focuses on numerical and high-performance computational methods applied to geophysics and astrophysical fluid dynamics. He is one of the core developers of the Dedalus project, which is an open-source community toolkit for solving general partial differential equations; http://dedalus-project.org.

• ## 2020 AFMSS: Predictions in wall-bounded turbulence through recurrent and convolutional neural networks

Ricardo Vinuesa
KTH

4.00pm Wednesday 05 August 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

The advent of new powerful deep neural networks (DNNs) has fostered their application in a wide range of research areas, including more recently in fluid mechanics. In this work we explore the capabilities of DNNs to perform two types of predictions in turbulent flows: first, we employ recurrent neural networks (RNNs) to perform temporal predictions of a low-order model of the near-wall cycle of turbulence. Our results indicate that the RNNs are indeed able to successfully reproduce the dynamics of the reference database. Second, we will use convolutional neural networks (CNNs) for non intrusive sensing, i.e. to predict the flow in a turbulent open channel based on quantities measured at the wall. We show that it is possible to obtain very good flow predictions, outperforming traditional linear models, and we showcase the potential of transfer learning between friction Reynolds numbers of 180 and 550. These non intrusive sensing models will play an important role in applications related to closed-loop control of wall-bounded turbulence.

Dr. Ricardo Vinuesa is an Associate Professor at the Department of Engineering Mechanics, at KTH Royal Institute of Technology in Stockholm. He received his PhD in Mechanical and Aerospace Engineering from the Illinois Institute of Technology in Chicago. His research combines numerical simulations and data-driven methods to understand and model complex wall-bounded turbulent flows, such as the boundary layers developing around wings, obstacles, or the flow through ducted geometries. Dr. Vinuesa’s research is funded by the Swedish Research Council (VR) and the Swedish e Science Research Centre (SeRC). He has also received the Göran Gustafsson Award for Young Researchers.

• ## 2020 AFMSS: The challenge of validation in large-scale highly-turbulent computational fluid dynamics models

Xinqian (Sophia) Leng
University of Bordeaux

4.00pm Wednesday 29 July 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Validation of numerical and Computational Fluid Dynamics (CFD) models are sometimes not straight forward, and often just absent. Even when validation dataset exists, whether it is prototype data or experimental measurements, the validation process can still be complicated, time consuming, and misleading at times, especially for complicated flow behaviours such as large scale turbulence flows, aerated flows, unsteady or rapidly-varying flows. In this presentation, I will show a few studies where numerical (CFD) modelling is used, and validation is needed. The presentation will show the importance, challenges and compromises of the validation processes. The goal is to spark thoughts and generate discussions for possible improvements for future numerical modellers.

Dr. Xinqian (Sophia) Leng’s research interests include physical and numerical (CFD) modelling of unsteady turbulent flows e.g. breaking waves, bores and positive surges, field investigations of tidal bores, and hydraulic design of fish-friendly culverts. Graduated in 2018 with a PhD in Civil Engineering (University of Queensland), she has authored/co-authored over 30 peer-reviewed publications, including 21 international journal papers and 16 conference proceedings. Dr. Leng is the recipient of the 2018 Institution of Civil Engineers (UK) Baker Medal, 2019 IdEx Post-doctoral Fellowship awarded by Université de Bordeaux (France) and is currently working at lab I2M, TREFLE (SITE ENSCBP), Université de Bordeaux. She was an invited speaker at Sichuan University International Young Scholars Forum, Water Resource and Hydropower session (May 2019), Chengdu, China, and an invited speaker for the International Symposium of Hydraulic Structures, Chile, 2020 (event cancelled due to Covid-19).

• ## 2020 AFMSS: What’s in a mean? Towards nonlinear models of wall turbulence

Beverley McKeon
Caltech

4.00pm Wednesday 22 July 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

In this talk I will discuss several approaches to understanding the nonlinear interactions sustaining turbulence near a wall. Exploiting information embedded in the turbulent mean field (and how it gets there) using resolvent analysis, emphasis will be placed on discovering strict rules governing the important scale-to-scale interactions. First, exact coherent states will be used as a model problem, before the techniques are extended to turbulent channel flow. A dramatic reduction in complexity of these problems can be obtained, associated with sparsity, self-similarity and low rank behavior in the resolvent (attributable to the information encoded in the turbulent mean field). In this talk, we will synthesize analysis and observations to distill clues guiding reconstruction of the full nonlinear flow field from the most amplified spatio-temporal modes, drawing connections with other approaches to related problems.

The support of the U.S. Air Force Office of Scientific Research under grant FA 9550-16-1-0361 and the U.S. Office of Naval Research under grants N00014-17-1-2307 & N00014-17-1-3022 is gratefully acknowledged.

Beverley McKeon is Theodore von Karman Professor of Aeronautics at the Graduate Aerospace Laboratories at Caltech (GALCIT). Her research interests include interdisciplinary approaches to manipulation of boundary layer flows using morphing surfaces, fundamental investigations of wall turbulence at high Reynolds number, the development of resolvent analysis for modeling turbulent flows, and assimilation of experimental data for efficient low-order flow modeling. She was the recipient of a Vannevar Bush Faculty Fellowship from the Department of Defense in 2017, the Presidential Early Career Award (PECASE) in 2009 and an NSF CAREER Award in 2008, and is a Fellow of the American Physical Society and the American Institute of Aeronautics and Astronautics. She is the past editor-in-chief of Experimental Thermal and Fluid Science and currently serves as an associate editor of Physical Review Fluids, as well as on the editorial boards of the AIAA J., Annual Review of Fluid Mechanics and Experiments in Fluids. She is the APS representative and Vice Chair of the US National Committee on Theoretical and Applied Mechanics.

• ## 2020 AFMSS: Prediction of inertial particle focusing in curved microfluidic ducts

Yvonne Stokes

4.00pm Wednesday 15 July 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Inertial lift and drag forces on particles suspended in flow through a curved microfluidic duct cause their migration in the cross section of the duct and focusing by size to different regions of the cross section. This has been used in microfluidic devices for particle sorting as required for “liquid biopsy”, the isolation of cancer cells in a routine blood sample. I will discuss a recently developed model to predict the migration of a spherical particle under the assumption that the particle Reynolds number is small. This extends an asymptotic model of inertial lift force previously developed to study inertial migration in straight ducts. Of particular interest is the existence and location of stable equilibria within the cross-sectional plane. Depending on its initial position, a particle migrates towards one of these. The Navier-Stokes equations determine the hydrodynamic forces acting on a particle. A leading order model of the forces within the cross-sectional plane is obtained through the use of a rotating coordinate system and a perturbation expansion in the particle Reynolds number of the disturbance flow. The model is used to predict the behaviour of neutrally buoyant particles at low flow rates and examine the variation in focusing position with respect to particle size and bend radius, independent of the flow rate. In this regime, the lateral focusing position of particles approximately collapses with respect to a dimensionless parameter dependent on three length scales, specifically the particle radius, duct height, and duct bend radius. We will consider ducts with rectangular and trapezoidal shaped cross-sections in order to demonstrate how changes in the cross-section design influence the dynamics of particles. This is joint work with Dr Brendan Harding and Prof Andrea Bertozzi.

Yvonne Stokes is an ARC Future Fellow and a Professor in the School of Mathematical Sciences at The University of Adelaide. She completed her PhD in Applied Mathematics at The University of Adelaide where she subsequently held an ARC Postdoctoral Fellowship (2000-2002) before obtaining a tenurable Lecturer position. She is a member of the Australian Academy of Science National Committee for Mechanical and Engineering Sciences, and an ANZIAM representative to ICIAM. She has also served on the Executive Committee of the Women in Mathematics Special Interest Group of the Australian Mathematical Society.

Yvonne enjoys mathematical modelling, particularly where differential equations are employed. Her primary research interests are in the field of fluid dynamics, in particular free-surface and viscous flow problems. One application area of interest is the drawing of micro-structured optical fibres; another is particle focusing in flows along curved microfluidic ducts. She is also interested in problems in mathematical biology, such as nutrient transport and uptake and chemical signalling, with applications in assisted reproduction technologies.

• ## 2020 AFMSS: Wavy drag reduction works!

Wolfgang Schroeder
RWTH Aachen University

4.00pm Wednesday 08 July 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Drag reduction in turbulent boundary layers is key for substantial energy savings in aerodynamics. Large parts of the flow over the wing of modern aircraft are turbulent such that even net energy savings of a few percent lead to high cost savings. Active drag reduction methods have shown to be capable of significantly reducing the drag in generic external turbulent wall-bounded flows. First, some fundamental knowledge of the technique of spanwise traveling transversal surface waves will be developed for the flat plate turbulent boundary flow. These results will also be used to derive via machine learning a model that allows drag reduction prediction for an extended parameter range. Based on this input of these generic studies the technique of spanwise traveling transversal surface waves will be applied to two aerodynamically completely different wing sections - DRA2303 and NACA4412 - at a chord-based Reynolds number of Rec = 400,000. Several parameter combinations are tested for maximum drag reduction and maximum net power saving. The results show a reduction of the total drag of up to 8.5 percent and a decrease of the viscous drag by up to 12.9 percent. Note that this includes all actuated and non-actuated parts of the surface, i.e., locally a much higher decrease of the wall-shear stress is achieved. Additionally the lift is slightly increased and positive net power saving is obtained.

Wolfgang Schröder received his doctorate degree at RWTH Aachen University 1987. After his postdoc time at the California Institute of Technology in Applied Math, he was in charge of the Aerothermodynamics Department, Space Infrastructure at the German Aerospace AG. Then, he joined the University of Applied Sciences in Braunschweig, before he took over the Chair of Fluid Mechanics and became Head of the Institute of Aerodynamics of the RWTH Aachen University in 1998. His main research interests are in the fields of turbulence, aeroacoustics, vortex dynamics, biological and medical flows, and particle-laden flows. He generally emphasizes a coupled numerical and experimental approach, i.e., simulations and measurements are used to doublecheck the findings and to develop innovative theoretical models. Wolfgang Schröder is a Member of the Academy of Sciences North-Rhine Westphalia and a Membre Titulaire de l'Académie de l'Air et de l'Espace. He has been Treasurer of EUROMECH from 2004 through 2015. From 2010 through 2012 he was Dean of the Department of Mechanical Engineering, RWTH Aachen University and since 2013 he is Member of the Strategy Board of RWTH Aachen University. In 2013 he received the Carl Friedrich Gauß Preis of the Braunschweig Society of Sciences.

• ## 2020 AFMSS: Cause-and-effect analysis of wall-turbulence theory and applications

Stanford University

4.00pm Wednesday 01 July 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Even after more than a century, turbulence research is deemed to be in its infancy by the most critical researchers in the field. And they might be right. Although we possess a crude practical understanding of turbulence, we still lack a theory capable of providing the accurate predictions demanded by the industry at an affordable computational cost. In the present talk, we discuss advancements to quantify cause-and-effect links in fluid dynamics based on Information Theory. The approach is used to tackle three important problems in fluid mechanics. In the first one, we unveil, in a simple manner, that the dynamics of energy-containing eddies at a given scale is essentially universal and independent of the eddy-size. In the second problem, we leverage the previous knowledge to forecast turbulent velocity signals via machine learning. Finally, we show improvements in the aerodynamic efficiency of a full aircraft in stall at realistic Reynolds number via reduced-order modelling of active flow control.

Dr. Adrián Lozano-Durán is currently a Postdoctoral Research Fellow at the Center for Turbulence Research at Stanford University. He received his PhD in Aerospace Engineering from the Technical University of Madrid at the Fluid Mechanics Lab advised by Professor Javier Jiménez. The overarching theme of his research is physics and modelling of wall-bounded turbulence via simulation, with creativity as a core value. His work covers a wide range of topics such as turbulence theory and modelling by machine learning, large-eddy simulation for external aerodynamics, high-speed flows, and multiphase flows, among others. In January 2021, Adrián will begin his appointment as an Assistant Professor at the Aerospace Department at MIT.

• ## 2020 AFMSS: Optimising geometry and hemodynamics within an arterio-venous fistula

Tracie Barber
University of New South Wales

4.00pm Wednesday 24 June 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Surgically created arteriovenous fistulae (AVF) are used to enable vascular access in kidney failure patients. The access is used to allow hemodialysis which provides extracorporeal blood filtration when the kidneys are no longer able to complete this function. However, complications such as stenosis are very common in AVFs, and access failure is a costly and life-threatening problem for dialysis patients. These complications are attributed to hemodynamics perturbations including turbulent flow, pathophysiological wall shear stress (WSS) and flow recirculation zones. We have used a novel methodology for generation of patient-specific CFD vasculature models. Models were generated using an in-house designed 3D freehand ultrasound setup for the vessel geometry scanning, complemented with spectral Doppler measured ECG-synchronised flowrates, forming boundary conditions for the CFD solution. Our team works closely with a local hospital which has allowed the collection of over 100 patient scans, including a longitudinal study of one patient for 15 weeks.

In the longitudinal study we were able to determine the first results for fistula maturation, demonstrating that while flow and cross-sectional area increased over time in the proximal artery and vein, wall shear stress remained fairly constant, while at a higher than baseline level. Our data has also allowed the effects of different surgical configurations to be modelled. Earlier work in our group showed that the traditional AVF geometry, with an acute angle of vein to artery at the juxta anastomotic region, produced detrimental flow conditions. A smooth loop’’ geometry, when modelled computationally, was seen to result in a more beneficial flow circuit. This has now been proven surgically, with patients who have the smooth loop’’ geometry – often created via the use of a stent during an intervention – performing better longer term.

In order to create a predictive tool, we have considered a number of flow related parameters to determine if there is any correlation between these and AVF success rate. Hemodynamic impedance through the vascular access was calculated by combining the proximal artery and vein resistances. In one patient case, the first scan featured low flowrates with stenosis noted in two locations of the outflow vein. The second scan was taken after the AVF underwent straight stent implantation in the stenotic region. While greater flowrates results, a new stenosis region formed near the new stent. A further stenting procedure in the juxta-anastomotic region provided a smooth loop geometry and removed all stenoses. Oscillatory shear behaviours were seen at different locations in all scans, localised in regions of low WSS, however impedance was significantly reduced in the third scan, indicating successful restoration of the vascular access.

A comparative study of all scans to date shows excellent agreement between low impedance and AVF success.

Prof. Barber is a UNSW fluid dynamics researcher with a background in ground effect aerodynamics, and a focus in the last ten years on biomedical applications. Her research has impact in the fields of fluid dynamics via fundamental understanding gained, and also in a more applied sense, to direct clinical situations. She works with both CFD (computational fluid dynamics) and experimental methods, mostly laser based methods including PIV and LDA. Her group’s interdisciplinary approach takes engineering technologies and applies them to real world medical problems; this translational work builds on fundamental findings in flow physics (for example how turbulence varies spatially and temporally in a pulsatile, stenosed arterial flow) to apply this insight to real problems (for example, how an over-sized stent causes turbulence which can then cause re-stenosis). Prof. Barber has over 200 refereed publications across both biomedical journals (including IEEE Transactions on Biomedical Engineering, Journal of Controlled Release and Annals of Biomedical Engineering) and fluid mechanics (including Journal of Fluid Mechanics, Physics of Fluids and Experiments in Fluids). She is a frequent invited or Keynote speaker at medical conferences, including 87th Annual Scientific Congress of the Royal Australasian College of Surgeons, the Symposium of Australia & NZ Society of Interventional Nephrologists, Verve Symposium, and the Annual Scientific Meeting of the Renal Society of Australasia.

• ## 2020 AFMSS: Linear modelling of turbulent shear flow

Peter Jordan
Institut Pprime
CNRS-Université de Poitiers-ENSMA

4.00pm Wednesday 17 June 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Mean-flow-based (MFB) modelling is increasingly used to study turbulent shear flow, with applications ranging from low-Reynolds-number, incompressible channel flow to high-Reynolds-number, compressible jets. The framework, most often cast in resolvent, or input-output form, holds promise for the improvement of our understanding turbulent flows, for the elaboration of simplified models and for the conception of experimentally viable control strategies. The talk will overview four recent developments. The first concerns an ambiguity associated with the fact that the mean flow is not a fixed point of the Navier-Stokes equations. This observation illustrates how results of MFB modelling should be interpreted with caution; it highlights the importance of carefully considering the non-linear forcing terms implicit in such models; and it opens up new possibilities for model optimisation. In the second part of the talk we use the jet-noise example to illustrate how the MFB resolvent framework can be tailored to explore and model the mechanisms by which turbulent flows drive an observable of interest. The two final parts of the talk address sensor-based estimation and control. We propose a resolvent-based linear estimator that can be constructed for flow systems of large dimension, and which provides optimal linear estimation for a given set of sensors. We close the presentation with an experimental demonstration of linear, closed-loop control of stochastic disturbances in a turbulent jet.

Peter Jordan, born in Cork, Ireland in 1974, is a CNRS Research Director at the Pprime Institute, Poitiers, France, where he has been since 2001. He holds B.A., B.A.I. and Ph.D. degrees in Mechanical Engineering from Trinity College Dublin, Ireland. His main research interests are in fluid mechanics, with a focus on aeroacoustics, flow instability and turbulence.

• ## 2020 AFMSS: Challenging hydraulic engineering of the 21st century

Hubert Chanson
University of Queensland

4.00pm Wednesday 10 June 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Hydraulic engineering is the science of water in motion and its interactions with the atmosphere, channel bed and fauna and flora. Current design approaches tend to be conservative not differing much from ancient designs. Modern designs are often based upon simplistic steady flow concepts to optimise their performances. Today hydraulic engineers must embrace new challenges, as water plays a key role in human perception and is indispensable to all forms of life. The talk will raise questions about current hydraulic design practices, too often based upon the simplistic assumptions of steady monophase Newtonian flow situations and optimised for a design discharge. During the seminar, the speaker discusses these challenges in terms of self-aeration processes at hydraulic structures and upstream fish passage in culverts. It is argued that a number of technical solutions are not satisfactory, e.g. in terms of aquatic fauna, fluid structure interactions and operational restrictions.

Altogether, the technical challenges in hydraulic engineering and design are gigantic for the 21st century hydraulic engineers. The speaker aims to share his passion for hydraulic engineering, as well as share some advice for early-career academics and researchers.

Hubert Chanson is Professor of Civil Engineering at the University of Queensland, where he has been since 1990, having previously enjoyed an industrial career for six years. His main field of expertise is environmental fluid mechanics and hydraulic engineering, both in terms of theoretical fundamentals, physical and numerical modelling. He leads a group of 5-10 researchers, largely targeting flows around hydraulic structures, two-phase (gas-liquid and solid-liquid) free-surface flows, turbulence in steady and unsteady open channel flows, using computation, lab-scale experiments, field work and analysis. He has published over 1,000 peer reviewed publications. He serves on the editorial boards of International Journal of Multiphase Flow, Flow Measurement and Instrumentation, and Environmental Fluid Mechanics, the latter of which he is currently a senior Editor. He co-chairs the Organisation of the 22nd Australasian Fluid Mechanics Conference to be held in Brisbane, Australia

• ## 2020 AFMSS: Ensemble Kalman filter estimation of heat transfer parameters in a coupled climate model

Vassili Kitsios
Monash University

4.00pm Wednesday 3 June 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

General circulation models (GCM) are unable to explicitly capture all of the scales of motion present in the climate system, ranging from millimetres to hundreds of thousands of kilometres. One, therefore, resorts to parameterising the influence of the unresolved small scale processes. Many of these parameters are known with little precision, which is one of the main contributors to model bias and limited predictability. The adopted GCM is a coupled atmospheric, oceanic, and sea-ice climate model with approximately 100 million degrees of freedom. Using an ensemble Kalman filter (EnKF) we develop a systematic and objective means of determining the model initial states and spatio-temporally varying parameters that minimise the difference between a 96 member ensemble of short term model forecasts and a network of real world observations of the Earth system. The parameters of interest are those governing the heat transfer between the atmosphere and ocean. The estimated parameter maps resemble known model biases, and are shown to improve model skill in the in-sample DA experiments, and in out-of-sample multi-year climate forecasts.

Dr Kitsios completed a PhD with the University of Melbourne and the Université de Poitiers on fluid dynamical stability and model reduction (2006-2010). He then undertook post-doctoral research with the CSIRO Oceans and Atmosphere (O&A) division (2010-2013) and the Monash University Laboratory for Turbulence Research in Aerospace and Combustion (2013-2016), on the numerical simulation and parameterisation of atmospheric, oceanic and boundary layer flows. He then held an industrial research position at a hedge fund (2016-2017) developing trading algorithms on the basis of macroeconomic themes and market conditions. Since rejoining CSIRO O&A in 2017, he has been undertaking research on the: parameterisation, data assimilation and ensemble initialisation methods for decadal climate prediction; and quantification of the influence of climate variability and change on socio-economic indicators.

• ## 2020 AFMSS: Fluid-Structure Interaction Problems in Aerospace Applications

Gareth Vio
University of Sydney

4.00pm Thursday 28 May 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Fluids Structure Interaction problems arise in aeronautical application in different forms. This talk will introduce a number of areas of interest, namely shockwave boundary layer interaction, cavity flow as well as its application topology optimisation problems. The talk will touch on problem of parallelisation of CFD codes for FSI problems as well as converge issue in Topology optimisation problems. I will conclude with a summary of some challenges and future opportunities.

Gareth A. Vio received his bachelor's degree in aerospace engineering, and PhD degree in Aeronautical Engineering from The University of Manchester in 1999 and 2005 respectively. He was a postdoctoral researcher at The University of Manchester and Liverpool in the UK, working on non-linear prediction problems, aeroelastic tailoring and system identification. He is currently a Senior Lecturer with the School of Aerospace, Mechanical and Mechatronic Engineering at The University of Sydney. His research interests include multidisciplinary design and topology optimisation, shock wave/boundary layer interaction, nonlinear structural dynamics and energy harvesting.

• ## 2020 AFMSS: Recent progress in experimental two-phase flows

Agisilaos Kourmatzis
University of Sydney

4.00pm Wednesday 20 May 2020
Online Event (2020 Australasian Fluid Mechanics Seminar Series)

Two-phase flows surround us. Understanding their behaviour and being able to control them, is critical for a multitude of applications, ranging from fuel injector design, to agricultural technologies and pharmaceutical aerosols. The aim of today's talk is to initially provide a general overview of some of the recent two-phase flows research in the group, and then delve into a bit of detail into 2 key areas of ongoing research. The first area being turbulent atomization and the role of instabilities, and the second area being pharmaceutical powder based flows. I will conclude with a summary of key findings and where some challenges and future opportunities are.

• ## Advanced acoustic microfluidic manipulation for active micro-control

David Collins
University of Melbourne

3.30pm Friday 28 June 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

How can we move something too small to touch? Very high frequency sound fields (>100 MHz) provide a promising avenue for microscale manipulation, yielding acoustic wavelengths on the order of individual cells. Such micromanipulation has a wide range of biomedical applications. Using surface acoustic waves (SAW), a dynamic actuation method uniquely suited to generating microscale forces, I've performed deterministic sorting, nanoparticle concentration, droplet generation and the first acoustic 2D patterning of individual cells. In recent work, I'm creating microscale acoustic waveguides for even more refined activities. Combined with other approaches, acoustic forces show substantial promise for structuring individual cells and human tissues at the microscale.

I recently joined the Department of Biomedical Engineering at the University of Melbourne, having come here from a joint Postdoctoral Research Fellow at the Massachusetts Institute of Technology (MIT) and the Singapore University of Technology and Design (SUTD). I have 26 publications on novel acoustic actuation methods and the physics of advanced microscale manipulation, including work appearing in Physical Review Letters, Nature Communications and Science Advances, and have given multiple invited talks on my work. Interests include novel acoustofluidic waveguides and arbitrary acoustic field generation for non-uniform micropatterning in tissue engineering applications. I completed my undergraduate degree at in Biomedical Engineering from the University of Melbourne, and was awarded the Bill Melbourne Medal for best engineering PhD thesis at Monash University.

• ## Transition to turbulence in the ocean boundary layer beneath ice shelves

Catherine Vreugdenhill
DAMTP, University of Cambridge

3.30pm Friday 21 June 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The melting of ice shelves around Antarctica has implications for ocean circulation and sea-level rise. However, the picture of ocean-driven melting is incomplete. Here, we use large-eddy numerical simulations to resolve all but the smallest scales of turbulence in an idealised ocean domain of a current underneath an ice shelf. The domain is bounded from above by the ice shelf base under melting conditions that lead to a vertical stratification in both temperature and salinity. The mixing of warm, salty water towards the ice base drives melting, but the resulting meltwater freshens the water column and stabilizes the stratification, suppressing turbulence and mixing. The transition from stratified to fully turbulent flow is examined and found to strongly influence the melt rate. Conditions are chosen similar to those at Antarctic sites to compare with observations and allow exploration of future scenarios.

Cat is a Postdoctoral Research Associate with the Atmosphere–Oceans Group at the University of Cambridge. Her research centres on small-scale processes in the ocean that can have important impacts on ocean dynamics and climate. In 2017, Cat completed a PhD in Geophysical Fluid Dynamics at the Australian National University where she used numerical simulations, laboratory experiments and theory to examine transport by deep convection in the ocean. Cat's postdoc research investigates the ocean-driven melting of ice shelves in collaboration with researchers at the British Antarctic Survey. By improving our knowledge of these small-scale processes she hopes to help improve their representation in larger scale ocean and climate models.

• ## Microfluidics for biomedical applications

Reza Nosrati
Monash University

3.30pm Friday 14 June 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Microfluidic methods offer several advantages over their traditional macro-scale counterparts by extending the possibility of biomedical research based on the idea of miniaturization. These miniaturized platforms provide opportunities to manipulate cells and biological processes at the single-cell level and develop nature-inspired technologies for diagnostic and therapeutic applications. For example, in the context of fertility, microfluidics can match the geometry of micro-confined regions within the female reproductive tract, thus, presenting opportunities for biomimicry-based selection of sperm that reflect the in vivo process. In this talk, I will provide an overview of our work in developing microfluidic technologies for (a) sperm analysis and selection, and (b) understanding bacterial motility. These microfluidic platforms present several promising avenues to address our large-scale health and environmental challenges.

Dr. Reza Nosrati is a Lecturer in the Department of Mechanical & Aerospace Engineering at Monash University. Prior to joining Monash University, he was an NSERC postdoctoral fellow in the Department of Chemical Engineering at Queen's University (2016–2018). Dr. Nosrati received his Ph.D. in Mechanical Engineering from the University of Toronto (2016), his M.Sc. in Mechanical Engineering from the University of Tehran (2010) and his B.Sc. in Mechanical Engineering from Amirkabir University of Technology (2007).

Dr. Nosrati's research focuses on small-scale fluid mechanics (microfluidics and nanofluidics), with applications in cell biology, chemistry and medicine. He is a pioneer in microfluidics for male fertility and assisted reproduction. He has authored papers in top-tier scientific journals like Nature Communications, Nature Reviews Urology and Clinical Chemistry. Dr. Nosrati has received numerous prestigious awards and recognitions, including the 2016 Colton Medal for research excellence in Microsystems and Nanotechnology, and the 2018 ROYAN International Research Award for outstanding contributions to Embryology and Andrology.

• ## Effect of deformations on the flow over a ringwing

Christian Thomas
Monash University

3.30pm Friday 31 May 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Deformations on a wing surface can bring about significant changes in flight performance characteristics. The ability to accurately predict laminar–turbulent transition processes is fundamental to improving the design of future aircraft. Several numerical methods are introduced and applied to a model airfoil with small indentations that are sufficient to establish separated flow and enhance boundary layer instability.

Dr. Christian Thomas received his PhD in 2007 from Cardiff University in fluid dynamics. Following his PhD, Christian undertook postdoctoral positions at the University of Western Australia and Imperial College London. Christian spent 2013 working with Airbus Group Innovations and has been a Lecturer at Monash University since June 2017.

• ## Microfluidics of left-right symmetry breaking in mammal embryos

Andrey Kuznetsov
North Carolina State University

3.30pm Friday 24 May 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

We developed an approximate method for modelling the flow of embryonic fluid in a ventral node. We simplified the problem as flow in a 2D cavity; the effect of rotating cilia was modeled by specifying a constant vorticity at the edge of the ciliated layer. We also developed an approximate solution for morphogen transport in the nodal pit. The solutions were obtained utilizing the proper generalized decomposition (PGD) method. We compared our approximate solutions with the results of numerical simulation of flow caused by the rotation of 81 cilia, and obtained reasonable agreement in most of the flow domain. We discuss locations where agreement is less accurate. The obtained semi-analytical solutions simplify the analysis of flow and morphogen distribution in a nodal pit.

Dr. Kuznetsov joined Department of Mechanical and Aerospace Engineering at NC State University in 1998 after his postdoctoral appointments at Ruhr-University of Bochum (Germany), Ohio State University, and Vienna University of Technology. He received PhD in Mechanical Engineering from Russian Academy of Sciences in 1992. Dr. Kuznetsov's research interests are in the general area of numerical modeling, including fluid mechanics, transport in porous media, transport in living tissues, bioheat transport, bioconvective sedimentation, Newtonian and non-Newtonian flows, flows in microgravity, and turbulence. His most recent research addresses axonal transport, left-right symmetry breaking in mammal embryos, modeling of electroporation, and thermal dose optimization in cancer treatment using hyperthermia. He attracted funding from many national and international agencies, including DARPA, NSF, NASA, EPA, NATO, USDA, DTRA, NTC, and Eastman Chemical. He is also an affiliate faculty member of the UNC/NCSU Biomedical Engineering Department, Fellow of American Society of Mechanical Engineering, Associate Editor of the ASME Journal of Heat Transfer and the Journal of Porous Media, and a winner of a prestigious Humboldt Research Award. In 2014, Dr. Kuznetsov was elected as a Member of the Scientific Council of the International Center of Heat and Mass Transfer (ICHMT).

• ## Air quality, winds, and atmospheric physics in the western U.S.

Heather A. Holmes
Atmospheric Turbulence and Air Quality Laboratory | Website
University of Nevada, Reno

3pm Thursday 23 May 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Left: wildfire smoke plume near Reno, NV; right: temperature inversion in the Salt Lake Valley.

Nearly 70 million people live in the western U.S. and growth continues as this region experiences rapid population increase. According to the 2010 U.S. Census regional population growth, from 2000–2010, in the western U.S. was 13.8%, the second largest growth region in the nation. While the population density in this region is low and correspondingly there should be fewer sources of anthropogenic air pollution, many areas in the western U.S. are currently designated as maintenance areas or are in non-attainment for at least one of the criteria air pollutants as defined by the National Ambient Air Quality Standards (NAAQS). This is exacerbated by unique air pollution sources (e.g. wind blown dust and wildfire smoke) and the local meteorological and orographical affects in the Intermountain Region. The mountainous terrain and elevation changes create synoptic weather patterns that lead to complex winds and atmospheric mixing that impact air pollution transport, dispersion, and accumulation.

This talk will focus on two distinct conditions with complex atmospheric physics that impact air quality throughout the western U.S. The first, is when local stagnation events, initiated by slow moving high pressure systems, cause pollutants to become trapped in cold dense air on valley floors, often referred to as temperature inversions. Research advancements in quantifying the turbulent fluxes and surface energy balance closure during temperature inversions will be shown using observations from the wintertime Persistent Cold-Air Pool Study (PCAPS) in the Salt Lake Valley, Utah. The second, is wildfire events that are common in the western U.S. when complex flows lead to terrain induced wildfires from dry, downslope winds, especially during drought conditions. Recent advancements in satellite remote sensing algorithms to quantify aerosols will be discussed and results of ongoing air quality modeling efforts to estimate wildfire smoke exposures in the western U.S. will be presented.

Heather Holmes is an Assistant Professor in the Department of Physics and Atmospheric Sciences Program at the University of Nevada, Reno. She received her PhD in Mechanical Engineering from the University of Utah in 2010, where she focused on experimental investigations to study air pollution, turbulence, meteorology, and chemistry in the atmospheric boundary layer. She has three years of postdoctoral training, including two years at Georgia Tech where her focus was to analyze air pollution data to better understand how emission sources impact air quality and to develop exposure metrics for health effects studies. Her current research group uses ground-based monitors, atmospheric models, and satellite remote sensing to investigate atmospheric physics, air pollution sources, transport and dispersion, and provide data for human health and public policy assessments.

• ## Nonlinear exact coherent structures in pipe flow and their instabilities

Ozge Ozcakir
Monash University

3.30pm Friday 17 May 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Research on coherent structures in cylindrical pipes is an area of much recent activity. Of particular interest are traveling wave (TW) states at large Reynolds number R. The existence and stability of these states, and their connection in phase space play important roles in understanding both transition and large R behavior of pipe flows. In this talk, we consider possible asymptotic structures at large Reynolds number R and identify numerically calculated solutions as finite R realisation of a nonlinear viscous core (NVC) state that collapse towards the pipe center with increasing R at a rate R−1/4. We also identify previous numerically calculated states as finite R realisation of a vortex wave interacting (VWI) state with asymptotic structure similar to the ones in channel flows studied earlier by Hall & Sherwin (J. Fluid Mech., vol. 661, 2010, pp. 178–205). In addition, stability features of both VWI and NVC states are investigated. Numerical calculations confirm lower-branch travelling waves have slow and low dimensional unstable manifolds suggesting their relevance to transition in turbulence.

Ozge Ozcakir obtained her PhD in Mathematics from the Ohio State University. After completion of her degree, she worked as a full-time lecturer briefly at OSU. She joined School of Mathematics at Monash University in 2015 as a Research Fellow under supervision of Prof. Philip Hall. She has recently been appointed as a Lecturer at Monash University. Her primary research interest is numerical investigation of nonlinear coherent structures to understand the mechanism that leads to turbulence in different geometries. Using numerical and asymptotic techniques, she has studied coherent structures at large Reynolds numbers. Her current work focuses on investigating the effects of changes in viscosity parameter and the effect of periodic suction/blowing at the pipe wall on coherent structures numerical simulations.

• ## An update on K41 versus K62

Robert A. Antonia
University of Newcastle

3.30pm Friday 3 May 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Although Kolmogorov's (1962) correction (K62) to his 1941 theory (K41) has been embraced by many turbulence researchers, our recent work suggests that there are no valid reasons for abandoning K41. In particular, analytical considerations, based on the NS equations, which take into account the finite Reynolds number effect, together with the available experimental and numerical data, seem to confirm a tendency towards the simple and elegant predictions of K41 as the Reynolds number increases. This is especially true when the focus is on scales which lie within the dissipative range. Since K62 is predicated on the idea that the effect of large scales on small scales continues to increase as the Reynolds number increases, it cannot be reconciled with the 4/5 law, a result that is exact only when the Reynolds number approaches infinity and the effect of the large scales is negligible.

Robert Antonia studied Mechanical Engineering at the University of Sydney, and received his PhD in 1970. Following a post-doctoral year at Imperial College on a CSIRO fellowship, he joined the University of Sydney as a lecturer in Mechanical Engineering in 1972. He was appointed to the Chair of Mechanical Engineering at the University of Newcastle in 1976. Over the period 2001–2005, he was ARC Professorial Fellow at the University of Newcastle. In 2004, he was awarded a Citation Laureate for Engineering by Thomson ISI and was elected to the Australian Academy of Science. Since 2005, he has been an Emeritus Professor at the University of Newcastle.

• ## Molecular fluid dynamics

Edward Smith
Brunel University

3.30pm Friday 5 April 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Left: turbulent minimal channel flow simulated using molecular dynamics (MD) and compared to CFD. Top right: posts on a wall at the molecular scale and Bottom right: two phase flow with the moving contact line and liquid-vapour interface

Short of quantum mechanics, molecular dynamics (MD) is the most accurate model of fluid motion we have. By simulating the motion of individual molecules, we get a complete picture down to the nanoscale, capturing the complete energy cascade, visco-elastic behaviour, slip boundaries, liquid-vapour interfaces and phase change.

Far from the kinetic picture of occasional molecular collisions, complex fluid behaviour emerges from the evolving structure of a particle lattice. From the solution of Newton's law between particles, emerges the entire spectrum of fluid phenomena. In this talk I will introduce the MD methodology, highlighting the link between traditional continuum fluid dynamics and demonstrating similarities with Couette flow. I will outline the unique insights provided, including the microscopic origins of stress and viscosity, what happens near boundaries, the liquid-vapour interfaces, molecular heat flux and bubble nucleation. Finally, I will talk about the simulation of the Couette minimal channel flow (Re=400) using molecular dynamics, and discuss the insight this can provide into our understanding of fundamental fluid phenomena and turbulence.

Edward Smith (www.edwardsmith.co.uk) works on multi-scale simulation which aims to combine particle and continuum methods. He earned his PhD at Imperial College London developing theoretical and computational techniques for the coupled simulation of molecular dynamics (MD) and computational fluid dynamics (CFD). He was awarded the post-doctoral excellence fellowship and simulated the first example of near-wall turbulence using MD. He spent some time working in Swinburne Australia, before moving to Chemical Engineering at Imperial to work on multi-phase flow and the moving contact line. His next move was to Civil Engineering at Imperial to develop software (www.cpl-library.org) and techniques to link particles and continuum for granular flows, before taking up a permanent position at Brunel University London.

• ## An alternate length scale for Reynolds number and Rossby number in the context of insect-like wings decouples the aspect-ratio effects

Shantanu Bhat
Monash University

3.30pm Friday 29 March 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

A stable leading-edge vortex formed over a rotating or flapping insect-like wing is known to be a primary reason behind an extra lift acting on the wing. Inspired from earlier studies at high Reynolds-number on the high aspect-ratio aircraft wings, the wing chord has been used as the reference length-scale, even for the insect-scaled wings. However, the flow structure on a typical low aspect-ratio insect wing is highly three-dimensional. As per the conventional scaling, this flow structure has been observed to be influenced by aspect ratio, Reynolds number, and Rossby number. Our work shows that the flow structure scales better with the wingspan, which we propose as the new reference scale in the context of insect wings. Use of this modified scaling for the Reynolds number and Rossby number decouples the effects of the aspect ratio. Interestingly, this also helps reconcile the apparently conflicting trends in the previous aspect-ratio studies on insect wings.

Shantanu Bhat is working as a research officer in Fluids Laboratory for Aeronautical and Industrial Research (FLAIR) at the Department of Mechanical and Aerospace Engineering, Monash University. He received his PhD from Monash University in August 2018. He has been working on the aerodynamics of the flapping wings of insects and rotating winged seeds. In the past, he has worked on the stall flutter of small turbomachine blades at the Indian Institute of Science, where he obtained his masters by research. He has also served as a lead engineer for GE Aviation in India, where he worked on the aero-design of GE's LEAP engine combustor. His research interests broadly involve vortex dynamics, biolocomotion, and bluff body flows.

• ## What is data assimilation and how can we use it to improve models of physical systems?

Craig Bishop
School of Earth Sciences and ARC Centre of Excellence for Climate Extremes, University of Melbourne

3.30pm Friday 22 March 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

On the one hand, you have an imperfect computer model of some idealized or filtered version of a physical system such as the atmosphere; on the other hand, you have been taking imperfect observations of corresponding parts of this system. How can you combine these two entities to obtain state estimates to initialize computer model forecasts and/or expose computer model error? These are the central questions addressed by data assimilation. In this talk, I'll give a brief introduction to data assimilation and highlight how my research has been approaching some of the major challenges currently facing the weather and climate prediction communities.

Prof Bishop completed his PhD at Monash University and performed his postdoctoral research at the University of Reading where he was awarded the Royal Meteorological Society's L. F. Richardson prize. He was then Professor at Pennsylvania State University before turning to operational weather prediction at the Naval Research Laboratory in Monterey, California. He returned to Melbourne in June 2018. His current research mainly focusses on the data assimilation science of using models, observations and advanced estimation theory to initialize ensemble forecasts and to identify and account for systematic and stochastic aspects of model error in ensemble forecasting. Prof Bishop's ensemble-based data assimilation and ensemble-forecasting techniques are now used by leading environmental forecasting agencies such as the European Center for Medium Range Weather Forecasting, the UK Met Office, the German weather service, the Swiss weather service, the US National Weather Service, the US Navy and the Japanese, Korean and Brazilian Meteorological agencies.

• ## Tornado-like vortices in coriolis–centrifugal convection

Susanne Horn
UCLA

3.30pm Friday 15 March 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Rotating thermal convection is one of the most important mechanisms for generating turbulence in geophysical settings, such as planetary interiors, atmospheres, and oceans. Rotating Rayleigh–Bénard convection, in which a fluid heated from below, cooled from above and rotated about its vertical axis, is the canonical model used in laboratory experiments and numerical simulations that captures the essential flow physics. However, in most theoretical and numerical studies, rotation has only been considered in terms of the Coriolis force, whereas the centrifugal force has been neglected. Hence, it remains largely unknown how flows are altered by centrifugal buoyancy, in particular, in the turbulent regime. We have recently begun to address this deficit by numerically characterising rotating convection including the full inertial term, i.e., by including both Coriolis and centrifugal forces. This work has revealed that in Coriolis–centrifugal convection storm-like structures can develop, ranging from eyes and secondary eyewalls found in hurricanes and typhoons, to concentrated helical upflows characteristic of tornadoes. Here, I will mainly focus on the tornado-like vortices. These vortices are not only self-consistently generated, but also exhibit the physical and visual features of type I tornadoes, i.e. tornadoes that form within mesocyclones contained in supercell thunderstorms. I will show that centrifugal buoyancy is, in fact, highly relevant for the understanding of these geophysical vortices, and likely a key component in next-generation models of tornado physics.

Susanne Horn is a DFG (German Research Foundation) Research Fellow at the Department of Earth, Planetary, and Space Sciences at the University of California, Los Angeles where she investigates fluid problems that are relevant in a geophysical context, including tornado dynamics and planetary core liquid metal convection. Prior to this, she was a Postdoc at the Department of Mathematics at Imperial College London, working on data-driven decomposition methods with application to aeroacoustic noise. She has received her PhD in physics from the University of Göttingen.

• ## Some recent developments in the low-order modelling and estimation of fluid flows

Ati Sharma
University of Southampton

3.30pm Friday 8 March 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Modelling and estimating fluid flows is a difficult problem. Fluid flows are well described by the Navier–Stokes equations, but these are nonlinear PDEs, which are difficult to solve in a general way. Much recent work has focused on finding low-dimensional approximations to fluid flow systems, either by abstracting them from data generated from experiment and simulation or by finding suitable approximations to the equations. This talk will discuss recent approaches; Dynamic Mode Decomposition (DMD), Koopman mode analysis and resolvent analysis. The approaches will be explained, and recent applications to flow analysis and estimation will be presented.

Ati Sharma is Associate Professor at the University of Southampton. Ati's undergraduate degree is in Physics (University College London) and PhD and postdoc in Control Engineering (Imperial College). Over the last fifteen years, Ati has turned to modelling, estimation and control fluid flows and has published extensively in this area. His most cited paper is from 2010 introducing resolvent analysis for turbulent flows. Somewhere in there he was also an options trader for JP Morgan.

• ## Advanced low noise aircraft configurations and their assessment–past, present and future

Zoltán Spakovszky
MIT Gas Turbine Laboratory

3pm Monday 4 March 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Aircraft noise remains the key inhibitor of the growth of air transportation but the focus of the noise mitigation strategies has changed. As the propulsor fan pressure ratio (FPR) is decreased and bypass ratio (BPR) is increased for improved fuel burn and reduced environmental impact, the propulsion system noise is reduced near or even below the noise level of the airframe. Jet noise has become less of a concern and, during approach and landing, the acoustic signature is predominantly set by the airframe. Novel aircraft concepts and architectures, enabled by distributed, more integrated, and boundary layer ingesting propulsion systems, pose new aero-acoustic problems which require innovative approaches and call for teaming and collaboration as the technological challenges cut across disciplines. One past example of such a collaborative research effort was the Silent Aircraft Initiative (SAI), aimed at the conceptual design of an aircraft imperceptible to the human ear outside the airport perimeter. The initiative brought together researchers from academia, industry and government agencies. This talk gives a brief summary of the Silent Aircraft Initiative, the SAX-40 aircraft design, and the noise reduction technologies which were pursued. A decade past SAI, novel aircraft architectures such as the D8 double bubble aircraft, the outcome of a joint effort between MIT, Aurora Flight Sciences and Pratt & Whitney, are being pursued in the quest of reducing the climate impact of aviation. With regulations continuing to reduce the allowable aviation noise emission levels, both new challenges and new opportunities are emerging. Electric, hybrid, and turbo-electric aircraft concepts are currently being investigated as potential game-changers. Independent of the level of electrification, noise will remain a major issue as air transportation is growing and mobility might become a key driver. The talk will discuss a selection of enabling technologies and their implications on acoustics and noise and will give a perspective on future trends and new directions in aero-acoustics required to address the challenges.

Dr. Spakovszky is Professor of Aeronautics and Astronautics at the Massachusetts Institute of Technology and the director of the Gas Turbine Laboratory. He obtained his Dipl. Ing. degree in Mechanical Engineering from the Swiss Federal Institute of Technology (ETH) Zürich and his MS and Ph.D. degrees in Aeronautics and Astronautics from MIT. Dr. Spakovszky's principal fields of interest include internal flows in turbomachinery, compressor aerodynamics and stability, dynamic system modeling of aircraft gas turbine engines, micro-scale gas bearing dynamics, and aero-acoustics. He currently directs analytical and experimental research in these areas and teaches graduate and undergraduate courses in thermodynamics, propulsion and fluid mechanics, and aero-acoustics. He has authored a large number of technical papers in refereed journals and has been awarded several ASME International Gas Turbine Institute best paper awards, the ASME Melville Medal, the ASME Gas Turbine Award, the ASME John P. Davis Award, a NASA Honor Award, several Aero-Astro Undergraduate Advising/Teaching Awards, and the Ruth and Joel Spira Award for Excellence in Teaching. Dr. Spakovszky is a technical consultant to industry and government agencies, a Fellow of the ASME, an Associate Fellow of the AIAA, and served as the chair of the turbomachinery committee and review chair of the ASME International Gas Turbine Institute, and as an associate editor for the ASME Journal of Turbomachinery.

• ## Dense and dilute particulate flows: sediment erosion and transport phenomena

Aman Kidanemariam
University of Melbourne

3.30pm Friday 22 February 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In this talk I will present my previous work on the problems of sediment transport and subaqueous pattern formation by means of high-fidelity direct numerical simulations. The peculiarity of the simulations lies on the fact that all relevant scales of the turbulent flow are taken into account, even the near-field around each individual sediment grain. The numerical method employed features an immersed boundary technique for the treatment of the moving fluid–solid interfaces and a soft-sphere model to realistically treat the inter-particle contacts. Our study has provided, first and foremost, a unique set of spatially and temporally resolved information on the flow field and the motion of individual particles which make up the sediment bed. Furthermore, based on the rigorous analysis of the generated data, the fluid flow and particle motion over the evolving sediment bed are studied in great detail, providing novel insight into the different mechanisms involved in the processes of sediment pattern formation.

Dr. Kidanemariam is a DFG (German Research Foundation) Research Fellow who recently joined the Fluid Mechanics Research Group here at Melbourne. He earned his PhD degree in 2015, in Civil Engineering, Geo and Environmental sciences at Karlsruhe Institute of Technology (KIT), Germany. His research background and interests lie in the field of computational fluid dynamics and high-performance computing applied to environmental flow problems. His previous research has particularly emphasized on dilute and dense particulate flows, turbulence-particle interaction, and shear flow driven sediment erosion and transport phenomena. He is a recipient of the prestigious Ercoftac Da Vinci Award 2015 for outstanding quality of PhD research as well as several other scholarships and awards. Part of his work has been featured in 'Focus on Fluids' article in Journal of Fluid Mechanics, highlighting its novelty and impact in the field of sediment transport modeling.

• ## On the origin of wall turbulence: where is the wall?

Ricardo García-Mayoral
University of Cambridge

3.30pm Friday 25 January 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Surfaces that exhibit small textured features can interact with near-wall turbulence and increase or reduce drag, which is of great interest to the aerospace, naval, transport and energy industries. This talk will discuss some of the dynamic mechanisms at play in that interaction, with particular emphasis on flows over transitional roughness and over superhydrophobic and anisotropically permeable substrates. In flows over smooth surfaces, an origin for the wall-normal coordinate can be defined unambiguously at the surface itself, where all three velocity components vanish. Over complex surfaces, in contrast, different components of the flow can experience different virtual origins, where they perceive the apparent presence of a smooth wall. In the limit of vanishingly small texture, in what constitutes the 'viscous' or 'linear' regime, the main effect of the surface is an offset between the origins perceived by the mean flow and by the background turbulence, which remains otherwise smooth-like. For larger texture sizes, richer dynamic mechanisms produce a deviation from this viscous regime. These mechanisms vary from texture to texture, but we will discuss two frequent ones. The first is the direct influence of the texture granularity, which induces a texture-coherent flow, and which we will discuss in the framework of rough and superhydrophobic surfaces. The second is the appearance of spanwise-coherent structures, arising from a Kelvin–Helmholtz-like instability connected with surface transpiration, which we will discuss in the framework of permeable substrates.

Dr. García-Mayoral is a lecturer in the Department of Engineering at the University of Cambridge. He obtained his PhD from Universidad Politécnica de Madrid, and in 2011, conducted postdoctoral research at the Center for Turbulence Research (CTR) at Stanford University before joining Cambridge. His research interest is in wall-bounded turbulence with a particular emphasis on complex surfaces.

• ## Properties of the mean momentum balance in polymer drag-reduced channel flow

Chris White
University of New Hampshire

3.30pm Friday 18 January 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In the first part of the talk, Dr. White will briefly outline his ongoing research projects. In particular, recent work on the development of a simple dynamical model of the turbulent boundary layer will be presented. The formulation of the model is based on recent findings that reveal that at large Reynolds numbers the inertially dominated region of the turbulent boundary layer is composed of large-scale zones of nearly uniform momentum segregated by narrow fissures of concentrated vorticity. It will be shown that a simple model that exploits these essential elements of the turbulent boundary layer structure can reproduce statistical profiles of the streamwise velocity that agree remarkably well with those acquired from direct numerical simulation at high Reynolds number.

The main part of the talk will discuss research related to the phenomenon of polymer drag reduction in wall-bounded turbulent flows. Here a mean momentum equation based analysis of polymer drag reduced channel flow is performed to evaluate the redistribution of mean momentum and the mechanisms underlying the redistribution processes. Similar to channel flow of Newtonian fluids, polymer drag reduced channel flow is shown to exhibit a four layer structure in the mean balance of forces that also connects, via the mean momentum equation, to an underlying scaling layer hierarchy. The self-similar properties of the flow related to the layer hierarchy appear to persist, but in an altered form (different from the Newtonian fluid flow), and dependent on the level of drag reduction. With increasing drag reduction, polymer stress usurps the role of the inertial mechanism, and because of this the wall-normal position where inertially dominated mean dynamics occurs moves outward, and viscous effects become increasingly important farther from the wall. For the high drag reduction flows of the present study, viscous effects become non-negligible across the entire hierarchy and an inertially dominated logarithmic scaling region ceases to exist. It follows that the state of maximum drag reduction is attained only after the inertial sublayer is eradicated. According to the present mean equation theory, this coincides with the loss of a region of logarithmic dependence in the mean profile.

Dr. White received his Ph.D. in Mechanical Engineering from Yale University in 2001. From 2001–2004 he was Postdoctoral Research Fellow at Stanford University. Following his post-doctoral work, he joined Sandia National Laboratories as a Senior Member of the Technical Staff in the Combustion Research Facility. In 2006, he joined the Mechanical Engineering Faculty at the University of New Hampshire. Dr. White's research expertise is in the thermal-fluid sciences with a focus on the turbulent transport of mass, momentum, and energy. His research to date is of both fundamental and applied nature in the areas of polymer drag reduction, turbulent boundary layers, piston engines, flow induced erosion, biomass, and thermal management. Dr. White received an NSF CAREER award in 2009. In 2016, Dr. White's and co-author Godfrey Mungal's 2008 Annual Review of Fluid Mechanics paper "Mechanics and prediction of turbulent drag reduction with polymer additives" was designated as a Highly Cited Paper (top 1% in the field of Physics) by the Thompson Reuters Essential Science Indicators. Dr. White currently has funding from NSF, ONR, NAVAIR, and NAVSEA.

• ## A tale of two quasi-linear dynamical systems: modulated waves and shear-driven instabilities

Greg Chini
University of New Hampshire

3.30pm Friday 11 January 2019
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The quasi-linear (QL) approximation has facilitated the prediction and understanding of a broad variety of fluid dynamical phenomena, ranging from the quasi-biennial oscillation of the zonal winds in the equatorial stratosphere to the emergence of exact coherent states (ECS) in wall-bounded turbulent shear flows. The QL reduction involves a decomposition into mean and fluctuation components and retention of fluctuation/fluctuation nonlinearities only where they feed back on the mean dynamics. Although sometimes invoked as an ad hoc simplification, the QL approximation can be justified asymptotically for certain flows exhibiting temporal scale separation, as will be demonstrated here through two complementary examples. In the first example, a new type of acoustically driven mean flow is identified and analyzed. Specifically, it is shown that when a high-frequency acoustic wave of small amplitude ε interacts with a stratified fluid, an unusually strong form of acoustic streaming can occur, with the time-mean flow arising at O(ε) rather than the more commonly realized O(ε2) value. The resulting two-way coupling between the wave and streaming flow is self-consistently captured in a QL dynamical system. In the second illustration, a QL model of strongly stratified turbulent shear flows is derived. Spectrally non-local energy transfers, associated with small-scale non-hydrostatic instabilities induced by the relative horizontal motion of large-scale hydrostatic eddies, are economically represented. The model is used to compute ECS in strongly stratified Kolmogorov flow and to evaluate the mixing efficiency achieved by these nonlinear states. For both the wave- and shear-driven systems, new asymptotic analyses are developed that enable integration of the dynamics strictly on the slow time scale associated with the mean flow, yielding significant computational efficiencies while simultaneously promoting physical insight.

Greg Chini is Professor of Mechanical Engineering and Co-Director of the Integrated Applied Mathematics Ph.D. Program at the University of New Hampshire. He earned his doctorate in Aerospace Engineering at Cornell University, with a focus on a fluid mechanics and applied mathematics, and has held visiting positions at Nottingham University, Caltech, the Institute for Pure and Applied Mathematics (IPAM) at UCLA, and the Kavli Institute for Theoretical Physics (KITP) at UCSB. In 2016, he was elected to the faculty of the Woods Hole Summer Program in Geophysical Fluid Dynamics. Prof Chini's research centers on the application of nonlinear mathematics and high-fidelity numerical simulations to important environmental, energy, and resource challenges facing society. Specific research themes include self-organization and extreme transport in fluid turbulence, multiscale phenomena in geophysical fluid dynamics and geophysics, and mathematical modeling of energy and resource systems.

• ## Some implications of self-similarity in canonical wall turbulence

Beverley McKeon
Caltech

3.30pm Monday 17 December 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

There has been much recent progress with regards to characterizing self-similar behavior in wall turbulence in experiments, in simulation, and in the mean and instantaneous forms of the Navier–Stokes equations. We identify commonalities and differences between these observations, and draw some conclusions concerning the requirements for self-similarity and self-sustaining processes in wall turbulence. Recent developments with respect to resolvent analysis are exploited to identify low-rank representations of these processes, their signatures and their limitations in physical and spectral space. We close with a discussion of some outstanding challenges related to the existence, self-sustenance and modeling of self-similar solutions and structures in the canonical flows.

The support of the U.S. Air Force Office of Scientific Research under grant FA 9550-16-1-0361 and the U.S. Office of Naval Research under grant N00014-17-1-2307 is gratefully acknowledged.

Beverley McKeon is Theodore von Karman Professor of Aeronautics at the Graduate Aerospace Laboratories at Caltech (GALCIT). Her research interests include interdisciplinary approaches to manipulation of boundary layer flows using morphing surfaces, fundamental investigations of wall turbulence at high Reynolds number, the development of resolvent analysis for modeling turbulent flows, and assimilation of experimental data for efficient low-order flow modeling. She was the recipient of a Vannevar Bush Faculty Fellowship from the DoD in 2017, the Presidential Early Career Award (PECASE) in 2009 and an NSF CAREER Award in 2008, and is an APS Fellow and AIAA Associate Fellow. She is the past editor-in-chief of Experimental Thermal and Fluid Science and currently serves as an associate editor of Physical Review Fluids, and on the editorial boards of the AIAA J., Annual Review of Fluid Mechanics and Experiments in Fluids. She is the APS representative and Vice Chair Elect of the US National Committee on Theoretical and Applied Mechanics.

• ## Harnessing the energy of wave driven turbulence at a fluid surface

Nicolas Francois
Australian National University

3.30pm Friday 7 December 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

When a laminar flow becomes turbulent, its energy is spread over a range of scales in a process named energy cascade. It has recently been discovered that turbulent flows can be forced by steep Faraday waves at a fluid surface. Those flows possess features of two-dimensional turbulence. In particular, an inverse energy cascade has been identified and a substantial amount of energy is stored into the turbulent fluctuations. An interesting question is whether it is possible to efficiently use the energy of this strongly out-of-equilibrium state.

In the wave driven turbulence, we show how to create floating devices able to extract energy from the turbulent motion fluctuations by coupling with underlying features of the energy cascade. The operational principle of these devices relies on the rectification of the chaotic motion of correlated bundles of fluid trajectories. By changing the shape of the device, we can turn it into a vehicle or a rotor powered by turbulence.

Nicolas Francois was awarded his PhD in physics of fluids and polymers from the Université de Bordeaux. Since 2012, Nicolas has worked as an experimentalist in the Physics of Fluids Laboratory at the Australian National University. Nicolas studies Lagrangian aspects of Turbulence and surface hydrodynamics notably in the recently discovered Faraday wave driven turbulence. He is also interested in the physics of complex fluids and granular matter.

• ## Multi-scale interactions between the lower atmosphere and the urban canopy

Karin Blackman
CNRS

12.30pm Tuesday 20 November 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The urban boundary layer consists of complex coherent structures, such as large-scale low momentum regions and intermittent turbulent sweeps and ejections, which are responsible for the transport of heat, momentum and pollution. Although these structures have been well identified qualitatively, their quantitative relationship is still unknown. Wind tunnel modelling of flow over simplified rough terrain consisting of either three-dimensional or two-dimensional roughness elements are able to reproduce these structures and are used to investigate the nonlinear relationship between large-scale momentum regions and small-scales induced by the presence of the roughness. As the temporally resolved small-scale signal is not available Linear Stochastic Estimation is used to decompose the flow into large and small-scales and confirm that the large-scale structures within the overlying boundary layer influence the small-scales close to the roughness through a nonlinear mechanism similar to amplitude modulation. Changing terrain configuration from 3D to 2D roughness results in a modification of the nonlinear relationship closer to the shear layer that develops near the top of the obstacles. Triple decomposition of the kinetic energy budget confirms that the nonlinear relationship that exists between large-scale momentum regions and small-scales close to the roughness is related to energy transfer between these structures. Application of an existing predictive model shows that the canopy flow regimes influences both the superposition and amplitude modulation close to the roughness. Finally, a combination of roughness geometries and Reynolds numbers are used to validate the predictive model in the urban boundary layer.

Dr Blackman obtained her PhD from Ecole Centrale de Nantes and is now a researcher in atmospheric mechanics and turbulence at CNRS.

• ## Vortex formation on surging airfoils and high advance ratio rotors

Anya Jones
University of Maryland

3.30pm Thursday 8 November 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Experiments were performed on wing surging in a water tank, a swept wing pitching in a wind tunnel, and a Mach-scaled slowed rotor at high advance ratios. Time-resolved particle image velocimetry was used to characterize the flow field around a blade element in the reverse flow region of the rotor, and near the center of the wing in the non-rotating experiments. On the rotor, four dominant flow structures were observed: the reverse flow starting vortex, the blunt trailing edge wake sheet, the reverse flow dynamic stall vortex, and the tip vortex. As advance ratio increases, the duration of reduced time that the blade element spends in the reverse flow region also increases. This affects the strength, trajectory, and predicted vortex-induced pitching moment of the reverse flow dynamic stall vortex. The results of this characterization and sensitivity study are compared to the more canonical models of flow reversal and separation. The three-dimensional rotor flows are found to have many similarities to canonical two-dimensional models.

Anya R. Jones is an Associate Professor in the Department of Aerospace Engineering at the University of Maryland, College Park. She received her PhD in Aerodynamics from the University of Cambridge, United Kingdom, her S.M. in Aeronautics and Astronautics from MIT, and her B.S. in Aeronautical and Mechanical Engineering from Rensselaer Polytechnic Institute. Her research is focused on the experimental fluid dynamics of unsteady and separated flows. Her current projects focus on the flow physics of large-amplitude gust encounters, separated and reverse flow rotor aerodynamics, and flight through airwakes and other unsteady environments. Prof. Jones has been awarded the AFOSR Young Investigator Award, NSF CAREER Award, and the Presidential Early Career Award for Scientists and Engineers (PECASE). Recently, she was awarded a Fulbright Scholar Award to the Technion in Haifa, Israel and an Alexander von Humboldt Research Fellowship to TU Braunschweig in Germany. She is currently chair of a NATO Research Technology Organization task group on gust response and unsteady aerodynamics, an associate fellow of AIAA, and a member of the Alfred Gessow Rotorcraft Center.

• ## Spatially localized structures in driven dissipative systems: theory and applications

Edgar Knobloch
University of California Berkeley

3.30pm Friday 2 November 2018
KevinMechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)
Spatially localized structures arise frequently in driven dissipative systems. In this talk I will describe a number of examples from different physical systems, followed by a discussion of the basic ideas behind the phenomenon of nonlinear self-localization that is responsible for their existence. I will illustrate these ideas using a simple phenomenological model and explain why the qualitative predictions of this model help us understand the properties of much more complicated systems exhibiting spatial localization, and specifically those arising in fluid mechanics.

Edgar Knobloch studied Mathematics at the University of Cambridge. In 1974 he received a J.F. Kennedy Scholarship to study theoretical astrophysics at Harvard University. After receiving his PhD in 1978 he was a Junior Fellow of the Harvard Society of Fellows and a Research Fellow at St John's College, Cambridge. He has been a Professor of Physics at the University of California, Berkeley, ever since. His interests range from dynamical systems theory, chaotic dynamics, pattern formation, all the way to materials science and geophysical and astrophysical fluid dynamics. He is a Fellow of the American Physical Society and the Society for Industrial and Applied Mathematics. He holds honorary doctorates from Université Paul Sabatier in Toulouse, France, and Universidad Politécnica de Madrid, Spain.

• ## Experimental measurement of three-dimensional density fields and the coupling of direct numerical simulations to experiments

Callum Atkinson
Monash University

3.30pm Friday 26 October 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

While the last 20 years have seen significant advances in our ability to experimentally measure micro to metre scale instantaneous velocity fields (including time-resolved volumetric measurements), the measurement of fluid density and temperature fields is far less evolved. For many quantities in compressible and heated flows, this requires us to rely on computationally expensive simulations, generally at Reynolds numbers far below those we desire. Even in incompressible flows, our best velocity measurements, while able to readily capture large scale information, are able to match the fidelity of direct numerical simulations. This presentation will focus on recent developments towards providing volumetric instantaneous density measurement in compressible flows, along with attempts to use large scale experimental data as an input to drive the convergence of direct numerical simulations, similar to how field observations are used to enhance weather predictions.

Dr. Atkinson received his PhD in Mechanical Engineering in 2012 as a Cotutelle PhD between Monash University and Ecole Centrale de Lille in France on the development and application of three-dimensional measurement techniques for investigating the 3D structure of wall-bounded turbulence. This included the development of the MLOS approach that is now widely used in commercial software for tomographic particle image velocimetry. His post-doc work focused on the experimental establishment of a self-similar adverse pressure gradient turbulent boundary layer and complementary direct numerical simulations. He was awarded an ARC Discovery Early Career Researcher Award (DECRA) fellowship in 2016 and is currently developing experimental techniques and facilities for the simultaneous measurement of fluid temperature and velocity fluctuations in heated boundary layers over natural and engineered surface roughness, while looking at means to directly couple the relative strengths of the optical measurements with higher fidelity direct numerical simulations.

• ## Multi-phase fluid behaviour and thermal characteristics in flow through curved ducts

Tilak Chandratilleke
Curtin University

3.30pm Friday 19 October 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Fluid flow behaviour in curved passages is fundamentally different to that in straight channels due to the secondary flow induced by centrifugal effects from passage curvature. Such flows have characteristic vortex structures producing spiralling fluid motion through curved passages, hence promoting fluid mixing even under laminar flow conditions and enhancing wall heat transfer process. Secondary flow also leads to hydrodynamic instability under certain flow conditions, introducing additional vortices to the flow for more pronounced fluid mixing. The secondary flow structures in multi-phase fluids are much more complex than in single phase fluid since the individual fluid phases are affected differently by the centrifugal forces. This presentation outlines the current research knowledge in this field, focussing on specific contributions made by the author towards the scientific understanding of the single and two-phase flow through curved ducts and their thermal characteristics.

Professor Tilak Chandratilleke is the Head of Mechanical Engineering Discipline at Curtin University, Perth, Western Australia. He obtained his PhD in two-phase flow and boiling heat transfer from the Cambridge University in the United Kingdom, supported by Trinity College, Cambridge and the UK Atomic Energy Authority. He has worked as a consulting engineer in the UK and the USA for several years prior to joining the academia. His experimental and numerical research spans from the discovery of microlayer theory for bubble growth in boiling to pulsed-jet electronic cooling, microfluidics, thermal radiation, and single and two-phase heat transfer in curved flow passages (topic of the presentation). He has contributed to thermal industry applications with unique and patented concepts, including the radiation shield for preventing excessive heating of water pipe network in Western Australia and the development of Thermoelectric-based fridge/freezer. He serves as an editor for the Institution of Mechanical Engineers (UK) Journals, as national chair of the Australasian Thermal Engineering Society (AFTES), and an executive member of the Engineers Australia WA Mechanical Panel. He was a member of the ARC College of Experts until early this year.

• ## Subcritical transition in wall-bounded shear flows

Jacob Cohen
Technion

3.30pm Friday 12 October 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The current study focuses on a sub-critical transition scenario of wall-bounded flows which are stable with respect to infinitesimal small disturbances (Couette flow and Plane Poiseuille flow for subcritical Reynolds numbers). Accordingly, a linear transient growth mechanism is initiated by four decaying normal modes, the initial structure of which corresponds to counter-rotating vortex pairs. It is shown that the four modes are enough to capture the transient growth mechanism. More importantly, it is demonstrated that the kinetic energy growth of the initial disturbance is not the key parameter in this transition mechanism. Rather, it is the ability of the transient growth process to generate an inflection point in the wall-normal or spanwise directions and consequently to make the flow susceptible to a three-dimensional disturbance leading to transition to turbulence. The model utilizes separation of scales between the slowly evolving base-flow and the rapidly evolving secondary disturbance. Because of the minimal number of modes participating in the transition process, it is possible to follow most of the key stages analytically, using the multiple time scales method. It is only due to nonlinear effects that the base flow becomes unstable with respect to an infinitesimal disturbance. The theoretical predictions are compared with direct numerical simulations and very good agreement with respect to the growth of the disturbance energy and associated vortical structures is observed, up to the final stage just before the breakdown to turbulence. Finally, the mechanism governing these transition stages (in the odd transition scenario) is very similar to the one described by the vortex dynamics model, previously proposed by the authors to explain the experimentally observed generation of a train of hairpins.

Jacob Cohen is a Professor and Sydney Goldstein Chair in Aeronautical Engineering at the Faculty of Aerospace Engineering, Technion – Israel Institute of Technology. He is the former Dean of the faculty (2015-2018) and currently serves as the head of the Technion Wind Tunnel Complex. Jacob received his Bachelor (1980) and Master (1982) degrees in Mechanical Engineering at the University of Tel-Aviv, Israel, and PhD (1986) at the AME department in University of Arizona. He then completed two and a half years as a Postdoc fellow at MIT before returning to the Israel (Technion). His main research interests are in experimental, theoretical and numerical study of laminar-turbulent transition, study of the evolution and control of coherent structures in wall bounded and free shear flows, hydrodynamic and thermal instabilities and unsteady phenomena.

• ## Reconstruction and estimation of flows using resolvent analysis and data-assimilation

Sean Symon
University of Melbourne

3.30pm Friday 5 October 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)
A flow reconstruction methodology is presented for incompressible, statistically stationary flows using resolvent analysis and data-assimilation. The only inputs necessary for the procedure are a rough approximation of the mean profile and a single time-resolved measurement. The objective is to estimate both the mean and fluctuating states of experimental flows with limited measurements which do not include pressure. The input data may be incomplete, in the sense that measurements near a body are difficult to obtain with techniques such as particle image velocimetry (PIV), or contaminated by noise. The tools developed in this talk are capable of filling in missing data and reducing the amount of measurement noise by leveraging the governing equations. The reconstructed flow is capable of estimating fluctuations where time-resolved data are not available and solving the flow on larger domains where the mean profile is not known.

The first part of the talk is centered on developing the tools necessary for this procedure. The second part of the talk discusses the reconstruction of flow around a NACA 0018 airfoil at zero angle of attack and a chord-based Reynolds number of 10250. The mean profile, obtained from PIV, is data-assimilated and used as an input to resolvent analysis to educe coherent structures in the flow. The resolvent operator for non-amplified temporal frequencies is forced by an approximated nonlinear forcing. The amplitude and phase of the modes are obtained from the discrete Fourier-transform of a time-resolved probe point measurement. The final reconstruction contains less measurement noise compared to the PIV snapshots and obeys the incompressible Navier–Stokes equations.

Sean Symon completed his undergraduate degree in Aerospace Engineering at the University of Maryland. He then completed his masters in Aeronautics at the California Institute of Technology and participated in the masters exchange program between GALCIT and Ecole Polytechnique. In France, he received a masters in fluid mechanics and worked with David Quéré studying drop dynamics on macro-textured superhydrophobic surfaces. Upon returning to Caltech, Sean joined Beverley McKeon's group working on data-assimilation and resolvent analysis. He is now working as a post-doc with Simon Illingworth and Ivan Marusic studying the attached eddy model in the context of the linearized Navier–Stokes equations.

• ## Enhancing heat transfer by inducing resonance in natural convection boundary layers

Chengwang Lei
University of Sydney

3.30pm Thursday 27 September 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In natural convection systems, the rate of heat transfer depends on the state of the thermal boundary layers forming on heat transfer surfaces, which may be laminar, transitional or turbulent. The heat transfer rate is much higher in the turbulent state than that in the laminar state. The transition of the thermal boundary layers from laminar to turbulent state may be advanced by either active or passive techniques. In this presentation, I will introduce a unique strategy to enhance heat transfer by triggering resonance in thermal boundary layers. Resonance occurs when a system is excited at frequencies close to its natural frequency. As a result, strong oscillations of the system are induced. Resonance may take place in a thermal boundary layer if it is perturbed at its characteristic (natural) frequency. I will describe the process for determining the characteristic frequency of thermal boundary layers and the application of resonance for enhancing heat transfer. Both active and passive strategies for triggering resonance and enhancing heat transfer will be demonstrated.

Chengwang Lei is currently a professor and Deputy Head of School of Civil Engineering at The University of Sydney, Australia. He is also the Deputy Director of the Centre for Wind, Waves and Water. Chengwang received his Bachelor (1988) and Master (1992) degrees in Mechanical Engineering at Huazhong University of Science & Technology in China and PhD (2000) in Civil and Resource Engineering at The University of Western Australia. His main research interest is in fundamental fluid mechanics related to buoyancy driven flows with environmental, industrial and domestic applications. His research involves experimental modelling, numerical simulation and analytical investigation of diverse thermal flow problems. In recent years Chengwang has been conducting concentrated research on solar thermal based passive strategies for building ventilation and thermal comfort. He has published widely in the international literature and at leading national and international conferences. More information about Prof Chengwang Lei may be found on his profile page here.

• ## On the scaling of the statistics of adverse-pressure-gradient turbulent boundary layers

Atsushi Sekimoto
Osaka University

12.30pm Tuesday 25 September 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

A characteristic boundary layer thickness is introduced to scale turbulence statistics in zero-/adverse-pressure gradient turbulent boundary layers (ZPG-/APG-TBLs). The characteristic length scale, which is termed the shear thickness', corresponds to the location which corresponds to the end of an actively sheared region in a turbulent shear flow, where the nondimensional Corrsin shear parameter is approximately constant. Using the friction and pressure velocity, the Reynolds stresses in TBLs over a wide range of APGs collapse with those in a ZPG-TBL. The present scaling is used to analyse the mean velocity and the kinetic energy balance in TBLs, and compare them to other shear flows. Furthermore, a scaling for small-scale properties will also be presented. The present scaling for TBLs over a wide range of pressure gradients is considered to be key to the development and application of turbulent models.

In 2011, the speaker completed his Ph.D. in thermo-fluid mechanics group at Osaka University, and he joined the Fluid Mechanics group in the Technical University of Madrid (UPM) as a post-doctoral fellow. In 2016, he worked as a research fellow in the Laboratory for Turbulence Research in Aerospace & Combustion (LTRAC) at Monash University. He is now Assistant Professor in the Dept. of Chemical Engineering in the School of Engineering Science at Osaka University. His main research interests are on the turbulence structures and the control of the temporal-space dynamics.

• ## Accurate and efficient methods for reduced-complexity modelling in fluid mechanics

Scott Dawson
Caltech

3.30pm Friday 21 September 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

This talk will explore several recent developments that improve upon the efficiency, accuracy, and theoretical understanding of methods for modal decomposition and reduced-order modelling in fluid mechanics. First, I will propose a method for the analytic approximation of the shape of resolvent modes in shear-driven turbulence, based on approximations to pseudospectral modes of scalar operators. This approach provides a theoretical framework for understanding the origin of observed structures, and gives a method for mode estimation without the need for large numerical computations. Next, I will focus on the dynamic mode decomposition (DMD), which provides a means of extracting dynamical information from fluids datasets. I will show that DMD is biased to sensor noise, and will subsequently present a number of modifications to the DMD algorithm that eliminate this bias, even when the noise characteristics are unknown. Lastly, I will discuss a number of approaches by which linear data-driven modelling techniques may be utilised and extended for accurate modelling of nonlinear systems.

Scott Dawson is currently a postdoctoral scholar within the Graduate Aerospace Laboratories at the California Institute of Technology. Prior to this, he completed his Ph.D. in Mechanical and Aerospace Engineering at Princeton University, and honours degrees in Mechanical Engineering and Mathematics at Monash University. In January, he will commence a position as an assistant professor at the Illinois Institute of Technology. His research interests include modelling, optimisation and control in fluid mechanics, with a particular focus on turbulent shear flows and unsteady aerodynamic systems.

• ## Incomplete mixing in porous media

Daniel Lester
RMIT University

3.30pm Friday 14 September 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Experimental image of evolving dye plumes resulting from Stokes flow over packed glass spheres.

Mixing, transport, reaction and dispersion of fluids and solutes in heterogeneous porous media is a fundamental problem of widespread importance in nature and engineered systems, ranging from geophysical and biological systems to microfluidics and chemical processing. Despite over a century of research, quantification of fluid mixing in heterogeneous porous media is largely based on a macro-dispersion (well-mixed) paradigm that does not resolve incomplete mixing at either the pore- or Darcy-scales. In recent years, significant advances have been made to better understand, predict and control mixing, transport and dispersion across scales using a combination of Lagrangian methods (dynamical systems theory, Hamiltonian chaos) and stochastic modeling (continuous time random walks, Markov models) to develop ab initio models of such phenomena. In this talk I shall present a brief overview of these advances and models for fluid mixing at the pore- and Darcy-scales, and discuss future research directions at the interface of porous and open flows.

Daniel completed his PhD in Chemical Engineering at University of Melbourne in 2003 on the deformation, flow and separation of concentrated colloidal suspensions prior to undertaking a Postdoc in the Advanced Thermofluids Lab at CSIRO on the topic of Lagrangian chaos and mixing in 2005. He joined the (then) Mathematics, Statistics and Informatics division of CSIRO as a Research Scientist in 2007 prior to joining RMIT University as a Senior Lecturer in 2014, where his current research interests involve the rheology and flow of complex fluids, fluid chaos and mixing, and transport and reactions in porous media.

• ## Broadband noise predictions of an axial compressor operating at low Reynolds number

Oscar Wilsby
University of Melbourne

3.30pm Friday 7 September 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Government regulations on noise are moving the importance of acoustic requirements to earlier stages of consumer product design. For turbomachinery applications such as axial fans and compressors, it is crucial for the design engineer to be able to accurately capture noise trends and understand the minimum noise level of a particular design. To be able to make noise predictions in an industrial setting, low order CFD must be used to produce accurate statistics of turbulence. However the modeling assumptions inherent in these methods make this a challenging task. This talk investigates the use of wall-resolved LES to study noise sources of an axial compressor operating at low Reynolds number and highlights the deficiency of current RANS based CFD for acoustic predictions.

Oscar Wilsby completed his undergraduate, masters and PhD at the University of Cambridge, working on aeroacoustic predictions for industry under sponsorship of Dyson Technologies Ltd. He recently started working in Professor Richard Sandberg's group on developing new data driven models for airfoil noise prediction.

• ## The role of convection on the basal melting of Antarctic ice shelves

Mainak Mondal
Australian National University

3.30pm Friday 31 August September 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Schematic of a sloping Antarctic ice shelf along with a enlarged snapshot of the boundary layer flow field (inset). The along-slope velocity (ζη-plane) and cross-slope convective velocity (yη-plane) and spatial distribution of meltrate (ζy-plane) at the ice interface is shown here at ambient temperature, Tb = 1° C and ambient current, Ub = 0.05 m/s.

Melting of Antarctic ice shelves has a large impact on ocean circulation, future sea level rise and the global climate. Most of the ice shelves in Antarctica are sloped forward into the ocean, forming an ice cavity underneath. The turbulent transport of heat and salt into the ice interface melts the ice and drives convective wall plumes that play a crucial role in the basal melting. Ice bathymetry and various ambient flows like tides, waves and sub-mesoscale eddies further modify the plumes. The regional and global ocean models work at scales over 100 meters and rely on crude sub-grid scale parameterization of convection and turbulent processes at the ice-ocean boundary layer, causing uncertainties in the estimation of the melt rate.

I have examined the role of micro scale turbulent processes at the ice ocean boundary using Direct Numerical Simulation (fully resolving convection and turbulence, see figure). I carry out simulations by varying the slope of the ice shelves, changing the strength of ambient flow and including sub-glacial discharge. The results show that the melt rate is controlled by the slope of the ice face with decreasing melt rate at shallower slopes. Over the geophysical flow regime, convection is the key parameter that controls the heat and salt transfer into the ice face and hence the melt rate. The results from this study significantly widen our present understanding of basal melting and can improve the ice-ocean parameterizations for large-scale models.

Mainak did his bachelors with major in physics and later a postgraduate diploma in astrophysics from BIFR India. His earlier research interest was on stellar dynamics. As a summer intern, he did an observational project on 'Morphological Evolution of Planetary Nebulae' from Indian Institute of Astrophysics. He later moved into geophysical fluid dynamics and did a master in atmospheric science from IIT Kharagpur. His master thesis was on 'Effects of aerosol on BOB tropical cyclone Phailin'. He became interested in boundary layer turbulence and found the very interesting project at ANU on 'Turbulent melting of the Antarctic ice shelves'. Over his PhD he explored the turbulence properties under the ice ocean boundary and how melting occurs right at the ice ocean boundary.

• ## Bluff body wake control

Jonathan Morrison
Imperial College

3.30pm Friday 24 August 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The coherent structures of a turbulent wake generated behind a bluff axisymmetric body are investigated experimentally at a diameter based Reynolds number ~ 2 × 105. Spectral and proper orthogonal decomposition of base pressure measurements indicates that the most energetic coherent structures retain the structure of the symmetry-breaking laminar instabilities appearing as unsteady vortex shedding with azimuthal wavenumber, m = ±1. In a rotating reference frame, the shedding preserves the reflectional symmetry and is linked with a reflectionally symmetric mean pressure distribution on the base. Due to a slow rotation of symmetry plane of the turbulent wake around the axis of the body, statistical axisymmetry is recovered in the long time average.

We investigate the effects of pulsed jet blowing on the turbulent wake. The jet is formed from an annular orifice situated immediately below the trailing edge and oriented in the direction of the freestream. By varying the frequency and amplitude of the perturbation, we achieve a mean pressure increase on the base of the body of up to 33%. Modal decomposition of the base-pressure fluctuations reveals a nonlinear coupling between the symmetric (m = 0) perturbation and higher order azimuthal modes (m = ±1, ±2) that results in an asymmetric mean pressure distribution. The pressure recovery is shown to be a broadband suppression of energy across all modes with no preferential selection.

Lastly, we apply the modelling approach for the axisymmetric body described above to the bistable mode of a rectilinear bluff body wake. We demonstrate the validity of the model and its Reynolds number independence through time-resolved base pressure measurements of the natural wake. Further, oscillating flaps are used to investigate the dynamics and timescales of the instability associated with the "flipping" process, demonstrating that they are largely independent of Reynolds number. The modelling approach is then used to design a feedback controller that uses the actuators to suppress the bistable mode. The controller is successful, leading to concomitant reductions in both lateral and stream-wise forces. Most importantly the controller is found to be efficient, the actuator requiring only 24% of the aerodynamic power saving.

Jonathan Morrison holds the chair of Experimental Fluid Mechanics in the Department of Aeronautics at Imperial College. Recent work has focused on fundamentals of turbulent flow and control including novel approaches to flow control. He is currently exploring instabilities in transitional flows as part of the LFC-UK Programme Grant and the correspondence between the scalar and momentum fields as part of the Transpiration Cooling Systems Programme Grant. Other recent work has developed novel drag-reduction techniques for bluff-body flows both in open loop and with feedback control. He is also exploring travelling surface waves for turbulent drag reduction with support from Airbus. He is currently a member of the "Aircraft of the Future" Advisory Group of the ATI. He is Director of, and chairs the Management Board of the National Wind Tunnel Facility (NWTF), funded by EPSRC and the ATI. He led an exhibit at the Royal Society Summer Exhibition, 2014, "Smart Wing Design: Science Imitating Nature".

• ## High-fidelity computational fluid dynamics (CFD) in industry: A retrospective and next steps

Vittorio Michelassi
Baker Hughes

3.30pm Friday 17 August 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The experience with non-reactive high-fidelity computational fluid dynamics (CFD) for aero-thermal applications in General Electric spans across low-pressure turbines, high-pressure turbines, radial and axial compressors and turbine center-frames. Direct numerical simulation (DNS) and large-eddy simulation (LES) were used for the investigation of design parameters impact on performance across a wide design space leading the way for additional testing and design changes that unlock further performance improvement. The scale-resolved simulations data-set was also used for detailed post-processing that shed light on some of the fundamental physical processes that ultimately drive aerodynamic and thermal efficiency. The large CFD data base was also used to improve unsteady Reynolds-averaged Navier–Stokes (URANS) calculations, that is and will be the workhorse in daily design iterations. This talk will first summarize the results and impact obtained so far, and indicate the possible next steps necessary for the full exploitation of scale-resolving CFD in industry.

Prof Dr Vittorio Michelassi is Chief Consulting Engineer for Aerodynamics at Baker Hughes, a GE Company, and GE Aviation. In his role he overlooks the aero-thermal design of gas turbine components as well as the improvement of design tools for all energy conversion related activities. Prior to joining GE Aviation he worked in General Electric Global Research as aero-thermal technologies Chief Engineer and from 2003 till 2011, he was principal engineer and Manager Aero Design of General Electric Oil and Gas. Before 2003 he was Professor of Gas Dynamics and Turbomachinery Aerodynamics at the Universities of Firenze and Roma Tre, Italy. He received a Master in Engineering from University of Florence, Italy, and a Master in Fluid Dynamics from the Von Karman Institute for Fluid Dynamics, Rhode Saint Genese, Belgium. He received his PhD from the University of Florence after having conducted research at NASA Lewis Research Centre and Karlsruhe Institute of Technology. He was visiting scientist at NASA Lewis, at the Center for Turbulence Research in Stanford, at Karlsruhe Institute of Technology. He participated several EU-granted research consortiums with focus on turbomachinery and combustions where he gained a wide experience in internal aerodynamics, with focus on turbine unsteady aerodynamics. He worked in turbulence modelling applied to steady and unsteady Reynolds-averaged Navier–Stokes simulations, as well as large-eddy and direct numerical simulations, where he pioneered the industrial application of high-fidelity CFD as a design investigation tool in cooperation with academia. He is a member of ASME Turbomachinery Committee and serves as a reviewer for international journals. He authored and co-authored several journal and conference papers in the area of turbomachinery design.

• ## Optimal growth and two-dimensionalisation in liquid-metal duct flows under a uniform transverse magnetic field

Greg Sheard
Monash University

3.30pm Friday 10 August 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

A transient growth analysis of linearised three-dimensional disturbances is conducted on a liquid metal flow in a duct with square cross section under a uniform vertical magnetic field. This analysis reproduces published energy amplifications at low Hartmann number, and for the first time reveals a two-dimensionalisation of optimal disturbance structures in the magnetic field direction at higher Hartmann number. Thereafter the predicted energy amplifications are in excellent agreement with a quasi-two-dimensional model for MHD flow developed by Sommeria & Moreau (1982).

Greg Sheard obtained his PhD from Monash University in 2004. He subsequently held an Australian Postdoctoral Fellowship, and since 2006 has been a faculty member of the Department of Mechanical and Aerospace Engineering at Monash University. His research is concerned with the application of high-order numerical methods for computational fluid dynamics to the study of fluid flows and their stability. This has seen him explore problems in bluff-body wakes, rotating flows, natural convection, and magnetohydrodynamics.

• ## The character and mechanics of flow-induced noise production from a finite span airfoil

Danielle Moreau
University of New South Wales

3.30pm Friday 27 July 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Many real-world technologies employ an airfoil that is wall-mounted and finite in length with boundary layer impingement at the airfoil-wall junction and flow over the tip. Examples include submarine hydrofoils mounted to a hull, wind turbine blades mounted to a hub or the stators in an aeroengine that are connected to an outer wall. An important aspect of airfoil noise production that has received little attention in the past is the influence of airfoil three-dimensionality, boundary layer impingement and flow at the tip on noise generation, which is the focus of this seminar. Recent results will be presented from a series of airfoil flow and noise measurement campaigns conducted in anechoic wind tunnels at Virginia Tech (USA), the Brandenburg University of Technology (Germany) and UNSW. A combination of acoustic array measurements, flow visualizations, surface pressure and unsteady wake data are used to gain insight into the turbulent noise sources and the role of three-dimensional vortex flow near the airfoil tip and wall junction in noise production.

Danielle Moreau obtained her PhD from the University of Adelaide in 2010. Following PhD completion, she worked as a research associate at the University of Adelaide for five years. During this time, Danielle investigated the mechanics of bio-inspired quiet airfoils and submarine hydrofoil noise generation. In 2015, Danielle moved to UNSW in the position of lecturer. Her research focuses on the understanding and control of flow-induced noise with the aim of quietening modern technologies.

• ## Buoyancy and convection drive the ocean circulation

Taimoor Sohail
Australian National University

3.30pm Friday 13 July 2018
Mechanical Enginneering Seminar Room Level 3 (Room 311, Bldg 170)

Convection is a small-scale flow feature which exists in most fluid flows. In the global ocean, convection is present wherever a buoyancy gradient is present. Given the ever-present nature of convection in the ocean, it is unfortunate that large-scale ocean models cannot resolve this process. As a result, there is a significant gap in the understanding of the impact of convection on ocean circulation. Whilst research in the area is still scant, it has become increasingly clear that convection may have profound impacts on ocean circulation, particularly in the North Atlantic and Southern Ocean.

To explore this, we employ a high-resolution Direct Numerical Simulation to model the Southern Ocean, whilst explicitly resolving all scales of fluid flow, from millimetre-scale turbulent convection to large-scale jets and overturning circulation. The idealised channel model consists of a surface temperature gradient and constant surface wind stress, mimicking the climatological conditions in the Southern Ocean. We find that convection significantly enhances mixing and plays a major role in modifying the fluxes of potential and kinetic energy the system. The energy fluxes are tightly controlled by the buoyancy forcing (and resulting convection), with little sensitivity to changing surface wind stress. In addition, the overall mass- and heat-transport appears to be largely buoyancy-dominated, with wind stress having a minimal impact on the system.

This research indicates that when fully resolved, buoyancy-forcing and the small-scale flows associated with it may have a major role in driving ocean circulation compared to surface wind stress, overturning conventional wisdom on the topic.

Taimoor Sohail is a PhD Candidate in the Climate and Fluid Physics group at the Research School of Earth Sciences, Australian National University, Canberra. Completing a Bachelor's degree in Mechanical Engineering from Lafayette College, USA, in 2014, Taimoor moved on to climate change policy research in Pakistan, working for a local think tank for two years. It was there he was inspired to further explore climate science, and found it to be a good fit for the engineering skills he had picked up over the years. He began his PhD in 2016 under the supervision of Dr. Bishakhdatta Gayen and Dr. Andy Hogg. Taimoor's main research focus is small-scale flow processes in the ocean, including submesoscale fronts, internal waves and convection. His eventual aim is to work towards tying large-scale ocean modelling work to more recent small-scale computational research conducted with DNS and LES.

• ## Does aeolian sedimentation contribute to coral cay accretion and island development in the Maldives?

Mike Hilton
University of Otago

3.30pm Friday 6 July 2018
Mechanical Enginneering Seminar Room Level 3 (Room 311, Bldg 170)

The atolls of the Maldives contain over 1200 reef islands; including atoll lagoon islands and numerous sand cays on reef platforms. Sand cays are comprised entirely of biogenic sediments, primarily coral sand. Cay formation at nodal zones has been attributed to wave refraction and diffraction around and over reef platforms. They are unstable deposits that may meander within these zones. Unvegetated sand cays are low and commonly over-washed by local waves during high astronomical tides. They may also be inundated by swell waves generated in the southern Indian Ocean and tsunami. These processes are known to deposit sediment on vegetated islands, but over-wash is unlikely to contribute to cay accretion in the absence of vegetation. There remains, therefore, the conundrum of how cays transition to stable and forested islands? I hypothesize that aeolian sedimentation, in conjunction with stranded organic debris and the growth of early successional plant species, may contribute sufficient depth of sediment to allow the development of non-saline groundwater and the colonisation of islands by forest tree species.

Wind speed and direction was observed at 1Hz over and 8-day period in February 2018 using Windsonic 2D anemometers mounted on a mast at 5.8m, 0.53m and 0.05m. Additional anemometers were positioned across the surface of this terrace. Aeolian sedimentation was observed using Wenglor laser particle counters over a period of 8 days, with swinging sand traps and erosion/accretion stakes. Surface topography, nabkha and island morphology was surveyed using a laser level to a datum established by RTK-GPS and local mean sea level (derived from RBR deployments).

Incident wind direction was mainly from the north east during the period of fieldwork, consistent with the Northeast Monsoon (October to April). Incident wind speed was typically 4–6m/s at 5.8m, however, periods of wind speed in excess of 12m/s were recorded on two occasions. These events, which were characterised by periods of relatively high onshore wind speed lasting several minutes, followed by a reverse in wind direction, appear to be driven by low-level outbursts under cumulonimbus clouds. Wind speed at ground level (0.05m) during these events reached 8m/s. Saltation and ripple development occurred during these events, albeit the rates of sand flux were not high (with Wenglor counts up to 200/s). Our observations occurred during the relatively weak Northeast Monsoon. We conclude that aeolian sedimentation probably occurs frequently on Maaodagalla, particularly during the more energetic Southwest Monsoon.

The development of nabkha appears to make a significant contribution to island elevation. The elevation of Maaodagalla does not exceed 1.0m above the level of spring high tides and 0.6m above observed maximum wave run-up. Therefore, nabkha development has contributed most of the elevation (0.5m) of the island above the level of wave run-up. Of course, aeolian sedimentation may be occurring concomitant with overwash processes. The analysis of sediment texture and the GPR data is ongoing to determine the nature of the aeolian facies across the island and on Mahutigalla. A freshwater lens was found under Maaodagalla in almost all of the excavated pits, which indicates the cays do not need to accrete much to generate conditions for freshwater accumulation and the establishment of forest tree species.

Associate Professor Hilton is a coastal geomorphologist at the University of Otago, with interests in dune geomorphology, the biogeomorphology of foredunes, flow over dunes, beach-dune sedimentation, foredune response to environmental stress, and the impact of invasive species on dune form and function. He is the Secretary of the International Society for Aeolian Research, and past executive member of the New Zealand Geographical Society, New Zealand Coastal Society, IGBP-LOICZ and received the New Zealand Order of Merit in 2018 for services to coastal science and conservation.

• ## Completion: Computed coronary arterial flow dynamics: from stented to rough surfaces

Winson Chen
University of Melbourne

3.30pm Friday 22 June 2018
Mechanical Enginneering Seminar Room Level 3 (Room 311, Bldg 170)

Coronary stenting as a standard treatment for coronary arterial diseases has achieved a high success rate in recent years. However, post-stenting complications still affect a small but significant subset of patients. Better understanding of the in-stent haemodynamic environment represents the key to reveal the physics associated with these complications. Computational fluid dynamic toolkits are employed to simulate pulsatile blood flow inside stented curved arteries. Arterial curvature and stent malapposition introduce pronounced adverse haemodynamic behaviours such as flow recirculation. It consequentially enhances localised in-stent restenosis and aggregation of activated platelets.

To understand the effect of stent-induced roughness on laminar pulsatile blood flow, simplifications of stented artery geometries were made such that key roughness parameters can be systematically investigated. The effect of roughness parameters on laminar pulsatile pipe flow is quantified in a 2D parametric space which could be relevant in the stent design to minimise adverse haemodynamic behaviours.

• ## Mass, energy and vorticity conservation for the rotating shallow water equations on a non-affine cubed sphere geometry using mixed mimetic spectral elements

Dave Lee
Monash University

3.30pm Friday 15 June 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The preservation of conservation laws and leading order balance relations in the discrete form helps to mitigate against biases and improves the representation of dynamical processes for geophysical flows over long time integrations. In this talk we will discuss the use of the mixed mimetic spectral element method to preserve mass, vorticity and energy conservation, as well as geostrophic balance, for the rotating shallow water equations with optimal error convergence on the cubed sphere. The method is based on the recently developed spectral element edge functions, which exactly satisfy the fundamental theorem of calculus with respect to the standard nodal basis functions. Compatible high order finite element spaces are constructed via tensor product combinations of nodal and edge functions. These allow for the annihilation of the curl by the divergence and the preservation of the divergence theorem in the strong form, and the annihilation of the gradient by the curl and the preservation of the circulation theorem in the weak form, via Galerkin projections onto the appropriate function spaces.

These mimetic properties are preserved independent of geometry, and the use of the generalised Piola transformation between canonical and physical space allows for both the preservation of conservation laws and the spectral convergence of errors on the non-affine geometry of the cubed sphere. These properties are confirmed via results for standard test cases. Preliminary results for 2D turbulence on the sphere and the dispersion relation for high frequency waves will also be presented.

Dave completed his PhD in Applied Mathematics at Monash at 2014, where he worked on the development of spectral element methods to model weakly nonlinear internal ocean waves. He then undertook a post doctoral position at Los Alamos National Laboratory, where he worked on the development of hybrid semi-Lagrangian / discontinuous Galerkin methods for transport in ocean models, and with the generous support of his supervisors initiated an independent research project into the use of mixed mimetic spectral elements for geophysical flows. Prior to his PhD he worked as a computational scientist at VPAC. He is currently a research associate in the Department of Mechanical and Aerospace Engineering at Monash.

• ## The cascade of energy in an anisotropic homogeneous turbulence experiment

Douglas Carter
University of Minnesota

3.30pm Friday 8 June 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Douglas Carter is a PhD candidate from the University of Minnesota and graduate student researcher at Saint Anthony Falls Laboratory in Minneapolis, Minnesota. Prior to graduate school he obtained a BS in Mechanical Engineering from the University of New Hampshire in 2014. His interests include fundamental turbulence, innovation in designing and performing measurements, as well as environmental flows such as dust storms (particle–turbulence interaction) and rain clouds (droplet–turbulence interaction).

• ## Electro-mechanical energy harvesting from flutter of cantilevers in axial- and cross-flows

Richard Howell
Curtin University | Website

3.30pm Friday 1 June 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The presentation will consist of two talks given at FSSIC 2017.

1. We model the fluid-structure interaction of non-linear flutter of a cantilever mounted upon a non-linear spring at the clamp in a uniform axial flow. This permits us to compare results with those from a hybrid non-linear system (a linear system mounted on a non-linear spring) and so to assess the change in fundamental physical phenomena owing to the introduction of full non-linear structural- and fluid-mechanics. We use numerical simulation for the non-linear system while our state-space solution of the corresponding linear system is used to guide the choice of parameters in the investigation. We show that above the flow speed of flutter-onset for small disturbances, amplitude growth leads to non-linear saturation so that the system settles into finite-amplitude oscillations. The frequencies of these oscillations evidence the dual-frequency characteristics of mount oscillation observed in physical experiments. When the natural frequency of the mount is low, we show that for a range of increases above the linear critical speed the linear hybrid and non-linear systems evidence the same frequency phenomena. However, the linear hybrid system evidences larger oscillation amplitudes than the non-linear system. Therefore, the stabilising effect of the non-linear structural terms outweighs the destabilising effect of the non-linear fluid terms.

2. A twin cantilever system is spring-mounted on a bearing and investigated experimentally in a cross flow in a wind tunnel; two types of bearing are used, cylindrical with one degree of freedom and spherical with three. By comparing the two systems, it is found that the spherical bearing system is slightly more unstable and operates at a higher oscillation frequency, hence having more favourable conditions for energy harvesting. The physical manifestation of the destabilising effect of the spherical bearing is seen in the yawing figure-of-eight forward and backward motion of the cantilevers about the mount, which can be observed by looking directly down on the set-up. For the range of spring stiffnesses used in this investigation, the power producing capabilities of both systems continue to increase as system natural frequency reaches higher values.

• ## Fluid dynamic properties of irregular, multi-scale rough surfaces

Thomas Jelly
University of Melbourne

3.30pm Friday 25 May 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Surface roughness increases fluid dynamic drag and induces a rise in the mean momentum deficit called the roughness function. This has implications for engineering systems which involve surfaces that, hydrodynamically speaking, are rough as opposed to smooth. Some examples include: the aero-thermal characteristics of ablated turbine vanes; the frictional resistance of bio-fouled marine vessels and the levels of wall shear stress in industrial-grade steel pipes. As a result, the fluid dynamic properties of practical roughness topographies are of significant interest.

The roughness function is itself a function of the roughness height and the roughness topography. Engineering rough surfaces with an equal roughness height but different topographies can give rise to roughness functions that vary by a factor of four. While the relationship between drag and the roughness height is well-understood, the relationship between roughness topography and fluid dynamic properties remains less clear. Accurately predicting the fluid dynamic properties of a given roughness topography is therefore non-trivial.

In this work, surface simulation methods from tribology have been implemented in order to generate "realistic" rough surfaces with specified topographical parameters (e.g. skewness and correlation lengths). Direct numerical simulations (DNS) of fully-developed turbulent channel flow over the generated surfaces have been performed in order to obtain their fluid dynamic properties with the aim of establishing relationships between topographical parameters and quantities such as the roughness function, "form-induced" dispersive stresses and turbulence-induced Reynolds stresses.

Dr. Thomas Jelly received his PhD in Mechanical Engineering from Imperial College London. He has held previous post-doctoral positions at the University of Cambridge and at the University of Glasgow. As of March 2018, Dr. Jelly joined Prof. Andrew Ooi's research group in order to investigate the fluid dynamic properties of rough-wall pulsatile pipe flows.

• ## Large-eddy simulation of environmental turbulence: Langmuir cells in the ocean and aeolian morphodynamics on Mars

William Anderson
Mechanical Engineering Department, The University of Texas at Dallas, USA

3.30pm Friday 18 May 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Results from large-eddy simulation of microscale turbulence in Earth's ocean, and in the atmosphere of Mars, are presented. The ocean mixed layer - the first ∼ 100 m of the ocean, and the zone most closely modulated by atmospheric forcing - regulates sequestration of anthropogenic carbon and heat from the atmosphere, accelerates the dispersion of surface-laden oil, and mixes agricultural nitrogen released at coastal inlets. Large-eddy simulation has been used to model Langmuir turbulence in coastal zones, which is accomplished by integration of the grid-filtered Craik-Leibovich equations. The forcing required to sustain the counter-rotating Langmuir cells is prescribed via the curl of vorticity and Stokes drift - an idealized velocity profile representing the aggregate motion due to waves. In contrast to the open ocean, where the mechanism sustaining Langmuir cells eventually vanishes, it is found that the presence of bottom-boundary layer shear results in distinctly different turbulence morphology. The scale of Langmuir cells does not remain constant, and instead cells associated with downwelling occupy the entire column; it is shown that this ostensible "cell thickening" is a product of mechanical shear imposed by the seafloor. This result has implications for benthic zone sequestration of surface quantities, and for sediment erosion and suspension in coastal zones. Results from simulation of atmospheric flow over crater-like geometries resembling those found on Mars are shown. Mars is a dry planet with a thin atmosphere. Aeolian processes - wind-driven mobilization of sediment and dust - are the dominant mode of landscape variability on Mars. Craters are common topographic features on the surface of Mars, and many craters on Mars contain a prominent central mound (NASA's Curiosity rover was landed in Gale crater). Using density-normalized large-eddy simulations, we have modeled turbulent flows over crater-like topographies that feature a central mound. Resultant datasets suggest a deflationary mechanism wherein vortices shed from the upwind crater rim are realigned to conform to the crater profile via stretching and tilting. This was accomplished using three-dimensional datasets (momentum and vorticity) retrieved from LES. As a result, helical vortices occupy the inner region of the crater and, therefore, are primarily responsible for aeolian morphodynamics in the crater. These results suggest that secondary flows - originating from flow separation at the crater - have played an important role in shaping landscape features observed in craters (including the dune fields observed on Mars, many of which are actively evolving).

Anderson received his PhD in Mechanical Engineering from The Johns Hopkins University in July 2011. He began as a tenure-track faculty in the Mechanical Engineering Department at Baylor University in Fall 2011, and moved to the University of Texas at Dallas in Fall 2014. His research interests focus on inertial-dominated turbulent flows in the environment. His research is supported by the Army Research Office (ARO), the Air Force Office of Scientific Research (AFOSR), the National Science Foundation (NSF), and the Texas General Land Office (TGLO). He is a 2014 recipient of the AFOSR Young Investigator Program award.

• ## Applications of synchrotron light to multi-phase fluid mechanics

Daniel Duke
Monash University

3.30pm Friday 11 May 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Non-invasive fluid mechanics experiments are often limited to flows that are optically accessible. This quickly becomes problematic when multiple phases are present, such as in sprays, cavitating flows and particle-laden flows, or under extreme temperatures and pressures. Refraction at gas/liquid interfaces and multiple-scattering from large numbers of particles, droplets or bubbles can make quantitative optical measurements challenging. X-rays offer a number of advantages over optical diagnostics due to the unusual ways they interact with matter. X-rays can penetrate opaque media, but also refract very weakly, allowing us to see through dense droplet and bubble clouds without refractive effects. Synchrotron radiation provides a collimated source of x-rays with orders of magnitude more flux than benchtop or CT scanner x-ray sources, and with tunable wavelength. Synchrotron beams can approach micron spatial resolution and sub-microsecond time resolution. This presentation will introduce a range of synchrotron diagnostics for fluid mechanics and example use cases. These includes radiography, phase-contrast imaging, hard and mid-range fluorescence spectroscopy, and small-angle and ultra-small angle x-ray scattering. These techniques are expanding the range of possible environments in which fluid mechanics experiments can be conducted. They provide an opportunity to gain new insight into the underlying physics of multi-phase flows.

Dr. Daniel Duke received his PhD in Mechanical Engineering from Monash University in 2013, studying the aerodynamic breakup of liquid sheets. During his PhD, Daniel spent two years in the United States on a Fulbright Scholarship, working at the Advanced Photon Source at Argonne National Laboratory. He joined a research group that was developing methods of using synchrotron radiation to study the structure of high-pressure diesel fuel injection sprays. After graduation, Daniel returned to the US as a postdoc and spent three years at the synchrotron developing novel x-ray measurement techniques for cavitating flows, multi-jet sprays and flash-evaporating medical sprays. Daniel returned to Australia in 2017 as an ARC DECRA Fellow in the Department of Mechanical & Aerospace Engineering at Monash. His current work concerns the development of optical and x-ray diagnostics for pharmaceutical sprays.

• ## Stability and transition of natural convection flow in differentially heated cavities

Steve Armfield
University of Sydney

3.30pm Friday 4 May 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Natural convection flow in a differentially heated square cavity has been widely studied, providing a canonical representation of a large range of buoyancy driven flows. In the standard configuration one sidewall is heated, the opposing sidewall cooled, with all other walls, ceiling and floor adiabatic. The flow consists of natural convection boundary layers forming on the heated/cooled walls, entraining fluid from, and discharging to, the stratified interior. The overall flow acts to transport heat from the heated to the cooled wall, with the details of the flow depending on the temperature difference between the sidewalls, typically characterised by a Rayleigh (or Grashof) number, and on the Prandtl number of the fluid. Start-up, transition and full development have been extensively investigated showing increasingly unsteady and complex behaviour with increasing Rayleigh number. Early results on the basic flow structure and stability properties, together with the nature and Prandtl number dependence of the transition to chaotic flow at full development, will be reviewed. Recent work will be presented on the absolute stability of the flow, together with a new three-dimensional steady turning flow instability, in the context of the tilted cavity.

Steve Armfield is Professor of Computational Fluid Dynamics in the School of Aerospace, Mechanical and Mechatronic Engineering at the University of Sydney. He completed his PhD, at the University of Sydney, in 1987, and has held appointments at the University of Western Australia and the University of New South Wales, as well as visiting positions at Stanford University, Cambridge University, Saitama University, Tohoku University and Auckland University. His research is focused on stability, transition and mixing in buoyant and stratified flows.

• ## Nonlinear Tollmien–Schlichting waves in a high-speed channel flow

Kengo Deguchi
Monash University

3.30pm Friday 20 April 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Plane Poiseuille flow has long served as the simplest testing ground for Tollmien–Schlichting wave instability. The aim of this talk is to give a comprehensive comparison of equilibrium Tollmien–Schlichting wave solutions arising from new high-resolution Navier–Stokes calculations and the corresponding predictions of various large-Reynolds-number asymptotic theories developed in the last century, such as double-deck theory, viscous nonlinear critical layer theory and strongly nonlinear critical layer theory. In the relatively small to moderate amplitude regime, the theories excellently predict the behaviour of the numerical solutions at Reynolds numbers of order 106 and above, whilst for larger amplitudes the computations suggest the need for further asymptotic theories to be developed.

Kengo Deguchi is currently a lecturer in School of Mathematics at Monash University. He obtained his PhD in 2013 at Kyoto University and then held a research associate position at Imperial College London before moving to Melbourne in 2016. He is interested in mathematical fluid dynamics of high speed flows, in particular those using the method of matched asymptotic expansions and dynamical systems theory.

• ## Turbulent channel flow in the low polymer drag reduction regime

John Elsnab
University of Melbourne

3.30pm Friday 13 April 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Streamwise velocity profiles and their wall-normal derivatives were used to investigate the properties of turbulent channel flow in the low polymer drag reduction (DR) regime, DR = 6.5% to 26%. Streamwise velocity data were obtained over a friction Reynolds number ranging from 650 to 1800 using the single velocity component version of molecular tagging velocimetry (1c-MTV). This adaptation of the MTV technique has the ability to accurately capture instantaneous profiles at very high spatial resolution (> 850 data points per wall-normal profile), and thus generate well-resolved derivative information as well. Owing to this ability, the present study is able to build upon and extend the recent numerical simulation analysis of White et al. (J. Fluid Mech., vol. 834, 2018, pp. 409–433) that examined the mean dynamical structure of polymer drag reduced channel flow at friction Reynolds numbers up to 1000. Consistently, the present mean velocity profiles indicate that the extent of the logarithmic region diminishes with increasing polymer concentration, while statistically significant increases in the logarithmic profile slope begin to occur for drag reductions less than 10%. Profiles of the streamwise velocity r.m.s. indicate that reductions in drag correlate with the location of the maximum moving farther from the wall and increasing in magnitude. Similarly, with increasing drag reduction, the profile of the combined Reynolds and polymer shear stress exhibits a decrease in its maximum value that also moves farther from the wall. Correlations are presented that estimate the location and value of the maximum r.m.s. streamwise velocity and combined Reynolds and polymer shear stress. Over the range of DR investigated, these effects consistently exhibit approximately linear trends with increasing DR. The present measurements allow reconstruction of the mean momentum balance (MMB) for channel flow, which provides further insights regarding the physics described in the study by White et al. (2018). In particular, the present findings support a physical scenario in which the self-similar properties on the inertial domain identified from the leading order structure of the MMB begin to detectably and continuously vary for drag reductions less than 10%.

John Elsnab received his PhD from the University of Utah in 2008 and has been a Research Fellow at the University of Melbourne since 2011.

• ## Illuminating the mechanics of relaminarization and roughness induced transition–a vorticity perspective

Garry Brown
Princeton University

3.30pm Friday 6 April 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

There is a long history over more than a century of experiments on turbulent shear flows. It is now more than 100 years since Taylor (1915) first drew attention to the connection between the transport of vorticity and the Reynolds stress. The relationship is considered in detail but in a plane, wall-bounded shear flow, such as channel flow, it is simply

(∂/∂y) <−u'v'> = <v'ω'z> − <w'ω'y>.

The long history of the 'turbulence problem' can be partly explained by the central role of structure, or of the mechanics of the unsteady vorticity. A simple example describes the two very different perspectives provided by a vorticity transport and a Reynolds stress perspective. Results for free turbulent shear flows and wall-bounded turbulent shear flows, from a vorticity transport perspective, are presented.

The flux of vorticity across a channel is constant. Near one wall the transfer from viscous stress to Reynolds stress is inextricably linked to the transport of vorticity and the dominant turbulent vorticity transport is <w'ωy'>. The origin of this correlation, its role as a counter gradient vorticity flux, the statistical behavior and the physical explanation for its strongly asymmetrical PDF are discussed. The sources, transport and dissipation of both ωx2 and ωy2 are described and quantified and their central role in the correlation, <w'ωy'>, is shown. By contrast for the outer flow the dominant vorticity transport is <v'ωz'> and its necessary connection, at high Reynolds number, with <w'ωy'> raises important 'vorticity structure' questions.

These results are used to explain recent numerical results on re-laminarization and roughness induced transition. The first has been in collaboration with ON Ramesh and his group at IISc (Bangalore) and the second with David Goldstein and Saikishan Suryanarayanan at UT Austin.

Relaminarization in a strongly favourable pressure gradient followed by re-transition: skin friction coefficient and wall shear stress versus distance downstream.

In important respects they seem opposite sides of the same coin. A vorticity perspective which has been enabled by numerical simulation and the calculation of vorticity transport in both cases sheds light on puzzling aspects of the mechanics. It offers a perspective on the mechanics of turbulent shear flows in general and provides a basis for a renewed emphasis on vorticity transport, as first proposed by GI Taylor in 1915.

Garry Brown is the Emeritus Robert Porter Patterson Professor of Engineering at Princeton University. He received a first class Honors Degree in Engineering from the University of Adelaide in 1964, was awarded a Rhodes scholarship, completed his D.Phil at Oxford and was then a research fellow/senior research fellow at GALCIT, Caltech. In 1971 he returned to the University of Adelaide and in 1978 returned to Caltech as full professor. He was asked to serve as Director of the Australian Aeronautical Research Laboratory and held this position from 1981–1990 after which he joined the faculty at Princeton, serving as Chair of the Department of Mechanical and Aerospace Engineering from 1990 to 1998.

His best known work is in the study of turbulence. Fifty years after the inception of the Journal of Fluid Mechanics, his 1974 paper with Professor Roshko "On density effects and large structure in turbulent mixing layers," was the most frequently cited paper in the history of the journal. Since joining Princeton he has explored new research horizons while continuing his abiding interest in turbulence.

He has also made significant contributions, as a consultant to the American aerospace industry, that include the root cause of failure and redesign of the solid rocket motor for the Titan IV, the cause of early failure and development of the thrust-vectoring system for AIM-9X and the resolution of critical issues for Tactical Tomahawk and for the Standard Missile-3 Programs. He played a leading role in the failure investigation and redesign of early air-cooled test cells for the after-burning F100 engine.

He is a Fellow of the Institution of Engineers of Australia, Fellow of the American Physical Society, and a Fellow of the AIAA.

• ## Making a LIST and checking it twice: Length scales of instabilities and stratified turbulence

Colm-cille P. Caulfield
BP Institute and DAMTP, University of Cambridge

3.30pm Friday 23 March 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Stratified shear flows, where the background' velocity and density distribution vary over some characteristic length scales, are ubiquitous in the atmosphere and the ocean. At sufficiently high Reynolds number, such flows are commonly believed to play a key role in the transition to turbulence, and hence to be central to irreversible mixing of the density field. Parameterizations of such irreversible mixing within larger scale models of the ocean in particular is a major area of uncertainty, not least because there is a wide range of highly scattered and apparently inconsistent experimental and observational data. It is becoming increasingly appreciated that appropriately defined characteristic length scales of the flow are critically important to all stages of the flow's evolution, and that such data scatter is associated with differing length scales being important in different experiments and observations. Here, I review some of the recent progress using modern mathematical techniques in developing understanding of instability, transition, turbulence and mixing in stratified shear flows, focussing in particular on the crucial role of various length scales. I highlight certain non-intuitive aspects of the subtle interplay between the ostensibly stabilizing effect of stratification and destabilizing effect of velocity shear, especially when the density distribution has layers, i.e. relatively deep and well-mixed regions separated by relatively thin `interfaces' of substantially enhanced density gradient.

Colm-cille P. Caulfield is Professor of Environmental and Industrial Fluid Dynamics at the University of Cambridge, where he is a member of both the BP Institute and the Department of Applied Mathematics and Theoretical Physics. Before taking up his position in Cambridge in 2005, he also held faculty positions in Environmental Engineering at the University of California, San Diego, and in Mathematics at the University of Bristol. His research focuses on stability, transition, turbulence and mixing in environmental and industrial flows, particularly where density differences play a dynamically significant role. He is an associate editor of the Journal of Fluid Mechanics (Rapids).

• ## Turbulent–laminar patterns

Laurette Tuckerman
CNRS/ESPCI Paris | Website

3.30pm Friday 16 March 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The transition to turbulence is characterized by coexistence of laminar and turbulent regions. In plane Couette and Poiseuille flow, this coexistence takes the form of statistically stationary alternating oblique bands of turbulent and laminar flow whose wavelength and orientation with respect to the streamwise direction are fixed. Since the wavelength of these astonishing patterns is much larger than the gap, they were first discovered in very large aspect ratio experiments. We study these patterns via full direct numerical simulation and reduced models. This is joint work with Mat Chantry and Dwight Barkley.

Laurette Tuckerman is a senior researcher at the CNRS (Centre National de la Recherche Scientifique) at the ESPCI (Ecole Superieure de Physique et de Chimie Industrielles) in France. Prior to moving to France, she was on the mathematics faculty of the University of Texas at Austin and she obtained her bachelors and PhD from Princeton and MIT, respectively. She studies hydrodynamic instabilities using the methods of computational fluid dynamics and of bifurcation theory. She has studied spherical, plane, and Taylor–Couette flow; Rayleigh–Bénard, Marangoni, and binary fluid convection; and the Eckhaus and Faraday instabilities. She is a Fellow of the American Physical Society and of Euromech.

• ## Feedback mechanisms in high-speed jet resonance

Daniel Edgington-Mitchell
Monash University

3.30pm Friday 9 March 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

High-speed jets show a proclivity for self-excitation across a range of operating conditions. This resonant behaviour is the result of a number of complex fluid mechanic and aeroacoustic phenomena. The feedback cycle can typically be broken into four discrete components:
1) A receptivity process in the near-nozzle shear layer, where an upstream propagating wave produces a perturbation.
2) The unstable growth of this perturbation into a downstream-propagating hydrodynamic wave.
3) Some downstream interaction of this hydrodynamic wave with shocks or solid objects that produces an upstream-propagating wave.
4) The upstream propagation of this wave to the near-nozzle region.
This talk will consider current developments in understanding of these four components of resonance mechanisms in shock-containing supersonic jets: free jets, multi-jets and impinging jets. Recent developments in understanding of the downstream wavemaker, and the upstream-propagating wave will be discussed, understanding that has been achieved through a combination of experimental measurement and the application of stability theory.

Dr Daniel Edgington-Mitchell is a Senior Lecturer in Monash University's Department of Mechanical and Aerospace Engineering, where he works in the Laboratory for Turbulence Research in Aerospace and Combustion. His research focus is the study of compressible and multiphase fluid mechanics using a primarily experimental approach. He maintains a website with some pretty pictures of fluid flow at daniel.edgington-mitchell.com.

• ## Advanced numerical methods for fluid-structure interactions and their applications

Fangbao Tian
University of New South Wales Canberra

3.30pm Friday 2 March 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

This talk will introduce three versions of immersed boundary methods for a variety of fluid-structure interaction (FSI) problems in biological flows, energy harvester and hypersonic flows. The fundamental idea of the immersed boundary method is to apply a source term into governing equations, which are discretised on Cartesian mesh, to approximately achieve the boundary conditions. Recently, immersed boundary method has been incorporated into the second-order finite difference method (for incompressible FSI), high-order WENO scheme (for compressible FSI) and lattice Boltzmann method (for incompressible moderate and high Reynolds number FSI). New elements including adaptive mesh, multiphase flows and non-Newtonian rheology have been considered. Validations are presented to show the accuracy and efficiency of these methods. Applications of the methods are briefly introduced.

Dr Fangbao Tian is a Senior Lecturer in the School of Engineering and Information Technology of UNSW, Canberra. His research focuses on numerical methods for fluid-structure interaction and complex flows, as well as their applications. He was awarded a PhD in Engineering Mechanics in June 2011 by the University of Science and Technology of China. In September 2011, he joined the Computational Flow Physics Laboratory at Vanderbilt University, USA, as a Postdoctoral Researcher. In August 2013, he moved to SEIT as a Research Associate and got a Lecturer position here in 2014. He was promoted to Senior Lecturer in 2017.

• ## What does a JFM Editor look for when assessing a paper?

Ivan Marusic
University of Melbourne

3.30pm Friday 23 February 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## Completion: Experimental factors influencing the quality of PIV results

Kristian Grayson
University of Melbourne

4pm Wednesday 14 February 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Experimental error sources have the potential to generate the bulk of errors encountered in a PIV experiment. Therefore, thorough consideration and analysis of these error causes, and informed modifications to the experimental setup can yield significant improvements to the quality of a measurement. This talk will cover a series of important, practical considerations to the setup of PIV experiments (as well as other laser-based flow visualisation techniques), which can greatly enhance the performance and accuracy of results. The matching and alignment of laser light sheets is one (often underappreciated) factor which can be crucial to achieving high quality PIV measurements. Additionally, an awareness of a laser's stability and transient warm up characteristics is also beneficial when performing laser experiments. The emphasis of this presentation will be on the practical actions an experimentalist can take to avoid these issues.

• ## On the fundamentals of Rayleigh–Taylor instability and interfacial Rayleigh–Taylor mixing

Snezhana I. Abarzhi
University of Western Australia

3.30pm Friday 9 February 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Rayleigh–Taylor instability (RTI) develops when fluids of different densities are accelerated against their density gradient. Extensive interfacial mixing of the fluids ensues with time. The Rayleigh–Taylor (RT) mixing controls a broad range of processes in nature and technology. Examples include inertial confinement fusion, core-collapse supernova, stellar and planetary convection, reactive and super-critical fluids, fossil fuel recovery, and nano-electronics. In this work, we focus on the classical problem RT mixing induced by variable acceleration with power law time dependence. By applying group theory, we find symmetries, invariants, scaling, correlations and spectra of the RT mixing, and quantify its sensitivity to the initial conditions. In a broad range of the acceleration parameters, critical points are identified at which the RT dynamics is ballistics, quasi-Kolmogorov, steady flex, diffusive, and dissipative. For super-ballistics and super-Kolmogorov dynamics, RT mixing has greater degree of order when compared to canonical turbulence. For up-steady-flex and super-diffusion—larger velocity fluctuations occur at larger scales, whereas for sub-diffusion and dissipation—larger velocities correspond to smaller scales. The properties of RT mixing depart substantially from those of canonical turbulence thus opening new perspectives for better understanding and control of RT dynamics in nature and technology.

Snezhana Abarzhi works at the University of Western Australia as Professor and Chair of Applied Mathematics. Her research interests are in Applied Mathematics (applied analysis, partial differential equations, dynamical systems) and in Theoretical Applied and Physics (plasmas, fluids, materials). The focus of Dr. Abarzhi's research is on Rayleigh–Taylor instabilities and interfacial mixing. Her contribution to this field is in rigorous physics-based theory for fundamentals of unstable interfacial dynamics.

• ## Direct numerical simulation of internal compressible flows at high Reynolds number

Davide Modesti
University of Melbourne

3.30pm Friday 2 February 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Instantaneous flow field for square duct flow at friction Reynolds number Reτ = 1000. Streamwise velocity contours in the cross-stream (yz) and wall-parallel (at y+ = z+ = 15) planes are shown. The mean cross-flow structure is also visualized using streamfunction isolines.

The talk will focus on the results of a recently developed DNS database which includes plane channel, pipe and duct flow, across a wide range of Mach (Mb = 0.2–3), and Reynolds numbers (Reτ = 180–1000) (Modesti & Pirozzoli, J. Sci. Comput., 2017; Modesti & Pirozzoli, Int. J. Heat Fluid Flow, 2016). The effect of compressibility on the mean flow statistics will be discussed for plane channel and pipe flow, focusing on transformations which allow to map the compressible flow statistics onto the incompressible ones. The talk will also address some recent findings on the role of secondary motions in square duct flow and their effect on the mean velocity field (Pirozzoli, Modesti, Orlandi & Grasso, J. Fluid Mech., 2018). Despite their effect of redistributing the wall shear stress along the duct perimeter, we find that secondary motions do not have large influence on the mean velocity field, which can be characterized with good accuracy as that resulting from the concurrent effect of four independent flat walls, each controlling a quarter of the flow domain. Further insight on the role of these motions is gained by performing numerical experiments in which secondary flows are artificially suppressed, revealing that they act as a sort of self-regulating mechanism of turbulence which acts to mitigate the effect of the corners.

Davide Modesti is postdoc at the University of Melbourne. He earned a PhD in Theoretical and Applied Mechanics from La Sapienza Università di Roma in 2017, where he carried out direct numerical simulations (DNS) of compressible wall-bounded flows at high Reynolds number. After completing his PhD he has been postdoc at DynFluid laboratory (ENSAM-CNAM) in Paris, where he studied secondary motions in rectangular duct flow using DNS. Since January 2018 he joined the Fluid Mechanics Research Group at the University of Melbourne.

• ## Thermoacoustic oscillations in annular combustors: symmetry, eigenstructure, perturbation methods, and nonlinear phenomena

Jonas Moeck
Technical University of Berlin and Norwegian University of Science and Technology

3.30pm Tuesday 19 December 2017
Old Metallurgy Masters Seminar Room 1 (Room 103, Bldg 166)

Thermoacoustic instabilities frequently appear in various combustion systems in the form of high-amplitude pressure oscillations. This undesirable dynamic phenomenon originates from the interaction of flame oscillations and the acoustic modes of the combustion chamber. It has been particularly plaguing for the development of high-efficiency, low-emission gas turbine technology. After a brief introduction to the topic, I will discuss some aspects that are specific to annular combustion chambers, as they are found in aeroengines and in most gas turbines for power generation. These combustors exhibit special dynamical features associated with their discrete rotational symmetry. After introducing the key aspects of the linear eigenstructure of this type of system, I will present some recent tools for stability assessment, uncertainty quantification, optimization, and nonlinear analysis of annular thermoacoustic systems.

Jonas Moeck is an Associate Professor at the Technical University Berlin and the Norwegian University of Science and Technology. He received engineering degrees from the University of Michigan and the Technical University Berlin, a PhD from the latter institution and was a postdoctoral scholar at Laboratoire EM2C, Ecole Centrale Paris. His research interests include flame dynamics, combustion control, low-order modeling and stability analysis, pulsed detonation, and plasma-assisted combustion.

• ## Color of turbulence

Mihailo Jovanovic
University of Southern California

3.30pm Monday 18 December 2017
Old Metallurgy Masters Seminar Room 1 (Room 103, Bldg 166)

This talk describes how to account for second-order statistics of turbulent flows using low-complexity stochastic dynamical models based on the linearized Navier-Stokes (NS) equations. The complexity is quantified by the number of degrees of freedom in the linearized evolution model that are directly influenced by stochastic excitation sources. For the case where only a subset of correlations are known, we develop a framework to complete unavailable second-order statistics in a way that is consistent with linearization around turbulent mean velocity. In general, white-in-time stochastic forcing is not sufficient to explain turbulent flow statistics. We develop models for colored-in-time forcing using a maximum entropy formulation together with a regularization that serves as a proxy for rank minimization. We show that colored-in-time excitation of the NS equations can also be interpreted as a low-rank modification to the generator of the linearized dynamics. Our method provides a data-driven refinement of models that originate from first principles and it captures complex dynamics of turbulent flows in a way that is tractable for analysis, optimization, and control design.

Mihailo Jovanovic (http://ee.usc.edu/mihailo/) is a professor in the Ming Hsieh Department of Electrical Engineering and the founding director of the Center for Systems and Control at the University of Southern California. He was a faculty in the Department of Electrical and Computer Engineering at the University of Minnesota, Minneapolis, from December 2004 until January 2017, and has held visiting positions with Stanford University and the Institute for Mathematics and its Applications. His current research focuses on the design of controller architectures, dynamics and control of fluid flows, and fundamental limitations in the control of large networks of dynamical systems. He serves as the Chair of the APS External Affairs Committee, an Associate Editor of the SIAM Journal on Control and Optimization and of the IEEE Transactions on Control of Network Systems, and had served as an Associate Editor of the IEEE Control Systems Society Conference Editorial Board from July 2006 until December 2010. Prof. Jovanovic is a fellow of APS and a senior member of IEEE. He received a CAREER Award from the National Science Foundation in 2007, the George S. Axelby Outstanding Paper Award from the IEEE Control Systems Society in 2013, and the Distinguished Alumni Award from UC Santa Barbara in 2014.

• ## Identification, decomposition and analysis of dynamic large-scale structures in turbulent Rayleigh–Bénard Convection

Yulia Peet
Arizona State University

11.30am Friday 15 December 2017
Old Metallurgy Masters Seminar Room 1 (Room 103, Bldg 166)

At high Rayleigh numbers in moderate aspect-ratio cylindrical domains turbulent Rayleigh–Bénard convection (RBC) exhibits coherent large-scale motions that organize themselves into a collection of three-dimensional "roll cells". In the current talk, we present the results and analysis of Direct Numerical Simulation of the RBC in a cylindrical domain with a 6.3 aspect ratio and the Rayleigh number of 9.6×107. The analysis of the long-time data series that are collected for the duration of over 100 eddy turnovers shows that the spatial organization of the roll cells in the investigated domain can be well described by the azimuthal Fourier modes. A hub-and-spoke mode-3 pattern first emerges and dominates the flow for the first 20 eddy turnovers, which then transitions into a mode-2 pattern that persists for the remainder of the simulations. A spatial inhomogeneity of the observed mode-3 and mode-2 structures is investigated. A conclusion follows that the cylindrical geometry constraint applies a "squeezing" effect to the large-scale structures, which forces them to align with a strong azimuthal periodicity.

Yulia Peet is an Assistant Professor of Mechanical and Aerospace Engineering at the School for Engineering of Matter, Transport and Energy at Arizona State University since Fall 2012. Her Ph.D. degree is in Aeronautics and Astronautics from Stanford (2006), M.S. and B.S. degrees are from Moscow Institute of Physics and Technology (1999 and 1997). Her previous appointments include a Postdoctoral position at the University of Pierre and Marie Curie in Paris in 2006–2008, and a dual appointment as an NSF Fellow at Northwestern and Assistant Computational Scientist at Argonne National Laboratory in 2009–2012. Her research interests include computational methods and high-performance computing, fluid mechanics and turbulence, with applications in wind energy, aerospace, and biological fluid mechanics.

• ## Wind farm modeling and control for power grid support

Dennice Gayme
Johns Hopkins University

3.30pm Monday 11 December 2017
Old Metallurgy Masters Seminar Room 1 (Room 103, Bldg 166)

Traditional wind farm modeling and control strategies have focused on layout design and maximizing wind power output. However, transitioning into the role of a major power system supplier necessitates new models and control designs that enable wind farms to provide the grid services that are often required of conventional generators. This talk introduces a model-based wind farm control approach for tracking a time-varying power signal such as a frequency regulation command. The underlying time-varying wake model extends commonly used static models to account for wake advection and lateral wake interactions. We perform numerical studies of the controlled wind farm using a large eddy simulation (LES) with actuator disks as a wind farm model. Our results show that embedding this type of dynamic wake model within a model-based receding horizon control framework leads to a controlled wind farm that qualifies to participate in markets for correcting short-term imbalances in active power generation and load on the power grid (frequency regulation). We also demonstrate that explicitly accounting for the aerodynamic interactions between turbines within the proposed control strategy yields large increases in efficiency over prevailing approaches by achieving commensurate up-regulation with smaller derates (reductions in wind farm power set points). This potential for derate reduction has important economic implications because smaller derates directly correspond to reductions in the loss of bulk power revenue associated with participating in regulation markets.

Dennice F. Gayme is an Assistant Professor and the Carol Croft Linde Faculty Scholar in Mechanical Engineering at the Johns Hopkins University. She earned her B. Eng. & Society from McMaster University in 1997 and an M.S. from the University of California at Berkeley in 1998, both in Mechanical Engineering. She received her Ph.D. in Control and Dynamical Systems in 2010 from the California Institute of Technology, where she was a recipient of the P.E.O. scholar award in 2007 and the James Irvine Foundation Graduate Fellowship in 2003. Her research interests are in modeling, analysis and control for spatially distributed and large-scale networked systems in applications such as wall-bounded turbulent flows, wind farms, power grids and vehicular networks. She was a recipient of the JHU Catalyst Award in 2015, a 2017 ONR Young Investigator award, and an NSF CAREER award in 2017.

• ## Modelling the infra-red signatures of aircraft and missiles: current and future practice

Nigel Smith
Defence Science Technology Group

3.30pm Friday 8 December 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The appropriate application of fluid mechanics modelling is essential to successfully predicting the infra-red signatures of aircraft and missiles. It is, however, but one element of a larger framework of necessary models. This talk will provide a qualitative understanding of the key phenomena associated with the IR signatures of airborne systems, the physical processes to be modelled and practical issues arising. The roadmap for future development will be outlined.

Nigel Smith completed a PhD in Mechanical Engineering in 1994 at the University of Sydney. The topic of this PhD and subsequent postdoctoral fellowship at Stanford University and the NASA Ames Research Center was the numerical modelling of turbulent combustion. He joined DST Group in 1996, and has since worked on combustion, rocket and air-breathing propulsion, and infra-red signatures. From 2009 he has led the Infrared Signatures and Aerothermodynamics Group (IRSA) in Aerospace Division. IRSA is an integrated team responsible for measuring, modelling and managing the signatures of aircraft and other systems for Defence.

• ## Advanced understanding of turbulent combustion processes

University of Sydney

12pm Friday 17 November 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

This seminar is jointly organised with the Thermodynamics Research Group. For more information, see: Thermodynamics seminars.

• ## Local transport of passive scalar released from a point source in a turbulent boundary layer

Kapil Chauhan
University of Sydney

3.30pm Friday 10 November 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Simultaneous measurements of velocity and concentration fluctuations for a horizontal plume released at eight different locations within a turbulent boundary layer (TBL) will be discussed. Statistics based on cross-correlation of velocity and concentration reveal the organisation of flow structures and the meandering plume. The time-averaged streamwise concentration flux is found to be positive and negative respectively, below and above the plume centreline. This behaviour is a result of wall-normal velocity fluctuations and Reynolds shear stress that play a dominant role in the vertical spread of scalar plume. Results of cross-correlation coefficient show that high- and low-momentum regions have a distinctive role in the transport of passive scalar. Above the plume centreline, low-speed structures have a lead over the meandering plume, while high-momentum regions are seen to lag behind the plume below its centreline. Based on these observations, a physical model is proposed for the relative arrangement of a passive scalar plume with respect to large-scale structures in the flow.

Dr. Kapil Chauhan is a lecturer in the School of Civil Engineering at The University of Sydney where he undertakes research within the Centre forWind, Waves, and Water and industry-linked projects in the Boundary Layer Wind Tunnel. Dr. Chauhan's research examines how fluids such as water and air flow over various surfaces under different conditions. In the BLWT, he has utilized his expertise in turbulent boundary layers and experimental methods to offer unique measurement capabilities for wind engineering applications. His research interests include wind energy, entrainment in boundary layers, scalar transport and natural convection. Dr. Chauhan graduated with a PhD from Illinois Institute of Technology, Chicago in 2007 and was a post-doctoral research fellow at University of Melbourne from 2009 to 2014.

• ## The fluid mechanics of microbial communities

Douglas Brumley
School of Mathematics and Statistics, University of Melbourne | Website

3.30pm Friday 3 November 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Fluid mechanics at the microscale governs a myriad of physical, chemical and biological processes. In this talk I will discuss several examples in which the ability to deconstruct complex biological systems and confine, control and visualise them, has enabled the development of new mathematical frameworks, from the active search strategies of marine bacteria to the dynamics of invading pathogens in the intestine. In each case, I will highlight how access to modern experimental results can uniquely inform the development of models with far-reaching ecological insights.

Douglas Brumley is a Lecturer in Applied Mathematics at The University of Melbourne. Before his current position, he was a Postdoctoral Researcher and Fellow of the Human Frontier Science Program, working with Professor Roman Stocker at MIT and ETH Zurich. He completed his PhD in the Department of Applied Mathematics and Theoretical Physics (DAMTP) at The University of Cambridge, under the supervision of Professor Timothy Pedley, FRS. During this time, Douglas was a Gates Cambridge Scholar and External Research Scholar at Trinity College. Douglas' research integrates experimental approaches and mathematical models to investigate microscale fluid flows in biology.

• ## Completion: Drag reduction via manipulation of large-scale coherent structures in a high Reynolds-number turbulent boundary layer

Reza Abbassi
University of Melbourne

3.30pm Friday 27 October 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The large-scale coherent structures in the outer region of a high-Reynolds-number turbulent boundary layer have been shown to be highly energetic with an influence that extends down to the wall and thus affecting skin friction. These structures possess bilateral characteristics; their instantaneous streamwise velocity are either higher or lower than the mean streamwise velocity at each wall-normal height, with these high- and low-speed regions accompanied by respective down- and up-ward wall-normal velocity components. In a conditional sense, counter-rotating roll modes are manifested in the spanwise-wall-normal planes.

In this talk, results of a real-time drag reduction strategy will be presented where actuation involves using wall-normal jets. For an actuation strategy where the jets are synchronized with the high-speed regions results in a reduction of the energy associated with the large-scale structures in the outer region (∼30% reduction of large-scale spectral energy was observed in the log-region). Additionally, a maximum mean wall-shear stress reduction of 3.2% was measured at 1.6 boundary layer thicknesses downstream of the actuators. Additional measurements involving random firing of the jets, and synchronisation with low-speed events show that the above-mentioned results are not a by-product of the injection of the wall-normal jet airflow into the turbulent boundary layer, rather it is due to the implicit synchronization with the down-wash sections of the counter-rotating roll modes which accompany the high-speed regions and led to an opposition mechanism of the control scheme.

• ## Flow external to a rotating torus (or a sphere)

Sophie Calabretto
Macquarie University

3.30pm Thursday 26 October 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The unsteady flow generated due to the impulsive motion of a torus or sphere is a paradigm for the study of many temporally developing boundary layers. The boundary layer is known to exhibit a finite-time singularity at the equator. We present results of a study that focuses upon the behaviour of the flow after the onset of this singularity. Our computational results demonstrate that the singularity in the boundary layer manifests as the ejection of a radial jet. This radial jet is preceded by a toroidal starting vortex pair, which detaches and propagates away from the sphere. The radial jet subsequently develops an absolute instability, which propagates upstream towards the sphere surface.

Sophie Calabretto is a Lecturer in Applied Mathematics at Macquarie University. Her research in fluid dynamics seeks to understand and predict the behaviour of rotating fluids, exploring fundamental questions in fluid physics, with the potential to impact fields as diverse as aerodynamics and climate science.

• ## Nonlinear internal tide dynamics and diapycnal mixing on continental shelves

Nicole Jones
University of Western Australia

3.30pm Friday 20 October 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The coastal shelf in Northwest Australia is a region of strong nonlinear internal wave generation and dissipation that results from strong tidal flows. Here we focus on a combination of moored and ship-based observations collected in 100 m of water 4–22 April 2012. The moored instrumentation observed both mean vertical temperature and velocity structure as well as turbulence quantities. The nonlinear internal wave (NLIW) packets were characterised by isotherm displacements of up to 60 m, occurred once per semi-diurnal tidal cycle and generally persisted for 4 h, with periods ranging from 10–30 minutes. The NLIWs were not consistently phase-locked to either the surface tide or internal tide. The largest turbulent overturns observed coincided with the downward isotherm propagation phase of each of the NLIWs within a packet. The observed diffusivity spanned six orders of magnitude and the median value was over four orders of magnitude greater than molecular diffusion. The highest values of diapycnal diffusivity were event driven, for example, during the occurrence of nonlinear internal wave packets when the stratification weakened due to straining of the background density gradient and there were comparatively large turbulent overturns. The diffusivity was often larger at the mid-water column compared with the near-bed, due to isopycnal straining and shear. Our direct estimates of diffusivity demonstrate that using the Osborn model with a constant mixing efficiency can both vastly over-predict and under-predict the diffusivity. Our diffusivity estimates can be used to improve predictions of processes such as the transport of nutrients, sediment and pollutants.

Dr Jones graduated from Stanford University with a PhD in Civil and Environmental Engineering in 2007. She is currently a Senior Lecturer at the University of Western Australia. Dr Jones' research focus is environmental and geophysical fluid dynamics, in particular the interaction between the physics and biogeochemistry in natural aquatic environments. She uses a combination of field observations and numerical modelling to study topics such as: The elucidation of internal wave generation, propagation and dissipation; The role of complex topography in circulation and mixing in a macro-tidal environment; Ocean mixing; The role of non-linear internal waves in stimulating primary productivity; The ocean response to tropical cyclone forcing; The influence of whitecapping waves on the vertical structure of turbulence in shallow water environments; The hydrodynamic control of phytoplankton loss to the benthos in estuarine environments; Plume dispersion on fringing coral reefs; The development of field techniques for studying environmental fluid mechanics.

• ## Some applications of the resolvent analysis

Paco Gómez
RMIT University

3.30pm Friday 13 October 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In the first part of the talk, I will present a reduced-order model for unsteady flows arising from the resolvent analysis of McKeon & Sharma (J. Fluid Mech., 2010). The inputs required for the model are the mean flow field and a small set of velocity time-series data obtained at isolated measurement points, which are used to fix relevant frequencies, amplitudes and phases of a limited number of resolvent modes that, together with the mean flow, constitute the reduced-order model. Some applications of the model include the identification of relevant flow structures, the reconstruction of dynamics and the estimation of unsteady aerodynamic forces. The technique will be applied to derive a model for three different flows: (i) an unsteady three-dimensional flow in a lid-driven cavity, (ii) an oscillatory flow past a square cylinder and (iii) a turbulent pipe flow.

The second part will explore how to control self-sustained oscillations using a data-driven decomposition based on the resolvent analysis. The decomposition consists of a temporal correlation between POD modes of fluctuating velocity and nonlinear forcing, and its physical interpretation is the simultaneous identification of the most energetic self-sustained motions in the flow and the corresponding forcing that excite them. Applications to flow control strategies in which a suppression of the forcing inhibits the vortex shedding past 2D and 3D bluff bodies will be shown.

Francisco (Paco) Gómez is a Lecturer in Computational Aerodynamics at RMIT University. He obtained a PhD in Aerospace Engineering from the Technical University of Madrid and he was a Postdoctoral Research Fellow at Monash University. His research interests include flow instability, flow control and reduced-order modelling.

• ## Secondary eyewall formation in tropical cyclones

Jeff Kepert
Head, High Impact Weather Research, Bureau of Meteorology

3.30pm Friday 6 October 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Roughly half of all intense tropical cyclones experience an eyewall replacement cycle. In these events, a new eyewall forms concentrically around the original one. This secondary eyewall develops its own wind maximum, and both the secondary eye and its wind maximum typically intensify and contract, whilst the original eyewall and wind maximum weaken and eventually dissipate. Although the evolution of a storm with concentric eyewalls is reasonably well understood, the mechanism or mechanisms by which the outer eyewall forms remain elusive. Understanding secondary eyewall formation is an important problem, for the subsequent eyewall replacement cycle can significantly affect the intensity of the storm, and the formation process and replacement cycle are usually associated with a major expansion of the outer wind field. Both these factors significantly affect the cyclone's impact.

We investigate a high resolution numerical simulation of an eyewall replacement cycle. We show that horizontal convergence within the boundary layer due to friction substantially influences the evolution of the convection. We show how this convergence is sensitive to the details of the vortex structure, best understood in terms of vorticity, and how it is quite different to classical Ekman pumping. Using this theory, we present evidence for a positive feedback involving convection, vorticity and frictional convergence that governs the subsequent evolution of the system. In this feedback, frictional convergence strengthens the eyewall clouds, stretching of vortex tubes in the buoyant updrafts in those clouds increases the vorticity, and the vorticity structure of the storm determines the strength and location of the frictional updraft.

Dr Jeff Kepert leads the High Impact Weather research team at the Bureau of Meteorology, which is responsible for improving understanding and prediction severe weather in Australia, including tropical cyclones, fire weather, severe thunderstorms and heatwaves. He is widely known for his tropical cyclone research, particularly into the dynamics and characteristics of the near-surface flow. More recently, he has branched out into fire weather research, including case studies of some severe fire events, contributing to the development of a prototype ensemble fire prediction system, and work on coupled fire-atmosphere modelling. He has a strong interest in analysing and quantifying uncertainties in weather forecasts, with the multiple aims of making forecasts more useful by including error bars, improving our analysis of the current atmospheric state, and better understanding atmospheric dynamics.

Jeff completed his undergraduate studies at the University of Western Australia and his postgraduate degrees at Monash University. Most of his career at the Bureau of Meteorology has been spent in the research centre, but he has also worked as a forecaster and in the training branch.

• ## Transition modelling for RANS/URANS approaches and applications to turbomachinery flows

Roberto Pacciani
University of Florence, Italy

3.30pm Friday 15 September 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Current models for transition in turbomachinery boundary layers are described in the framework of Reynolds-averaged Navier–Stokes (RANS or URANS) description of the flow. These include the γ–Reθ,t and laminar-kinetic-energy (LKE) based families of models. Emphasis is put on the discussion on the way transition mechanisms are expressed by model ingredients. Model equations are analyzed on a consistent physical basis. The predictive quality of the models is assessed through applications to the prediction of steady/unsteady flows in cascades operating in low pressure turbine conditions. The strong and weak features of each formulation are also evidenced and discussed.

After completing his graduate degree in mechanical engineering at the University of Florence, Roberto Pacciani received his Ph.D. from the Technical University of Bari in 1997. Since 1999, he has been a member of the faculty of mechanical engineering at the University of Florence. He is presently Associate Professor in the same institution. His research interests involve the development of Computational Fluid Dynamics methodologies with a special focus on turbomachinery design and analysis, and particular emphasis on turbulence/transition modelling and unsteady component interactions. Applications mainly concern the advanced aerodynamic design of aeronautical and industrial turbomachinery.

• ## Monte Carlo methods for oscillatory nanoscale gas flows

University of Melbourne

3.30pm Friday 1 September 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Gas flows generated by resonating nanoscale devices inherently occur in the non-continuum, low Mach number regime. As the continuum assumption underpinning the Navier-Stokes equations is violated, these flows must be modelled using an approach that accounts for the non-continuum nature of the gas, i.e. the Boltzmann equation. Solution of the Boltzmann equation is difficult using standard numerical approaches–this has motivated the development of a range of Monte Carlo methods for simulating gas flows, the most well known of which is direct simulation Monte Carlo (DSMC).

We begin this talk with an introduction to the Boltzmann equation applied to nanoscale flows, followed by a discussion of methods which solve this equation using a Monte Carlo approach. We then present a frequency-domain Monte Carlo method which may be applied to oscillatory low Mach number gas flows. This circumvents the requirement of current Monte Carlo methods to operate in the time-domain, providing direct access to amplitude and phase information using a pseudo-steady algorithm. This frequency-domain method is shown to provide a significant improvement in computational speed compared to existing time-domain Monte Carlo methods.

Daniel received his PhD in applied mathematics from the University of Melbourne in 2016, where he is currently a postdoctoral researcher. His areas of research include analytic and numerical methods for solving the Boltzmann equation, particularly in the context of nanoscale gas flows.

• ## Coherent structures and particles in turbulent boundary layers

Ellen Longmire
University of Minnesota, USA

3.30pm Friday 18 August 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

This talk will focus on our experience employing planar and volumetric velocimetry techniques to investigate the nature of turbulent boundary layers and the motions of finite particles within them. Fluid velocity fields are analyzed to quantify and understand the organization of coherent eddies as well as their relation to regions of long streamwise coherence. In the logarithmic region, extended packets of eddies can be tracked over significant distances. Our efforts to manipulate the packet organization and to understand packet recovery from perturbations will be discussed. In addition, our first results characterizing translation and rotation of spherical particles within the boundary layer will be presented.

Ellen Longmire received an A.B. in physics (1982) from Princeton University and M.S. (1985) and Ph.D. (1991) degrees in mechanical engineering from Stanford University. Prior to receiving her Ph.D., she worked as an engineer at Hauni-Werke Koerber & Co in Germany and at Honeywell and SAIC in the U.S. Since 1990, she has taught and directed research in the Department of Aerospace Engineering and Mechanics at the University of Minnesota where she holds the rank of Professor. She uses experimentation and analysis to answer fundamental questions in fluid dynamics that affect industrial, environmental, and biological applications. She is a Fellow of the American Physical Society and received the UM Distinguished Women Scholars Award (2007), the McKnight Land-Grant Professorship (1994), and the NSF National Young Investigator Award (1994). She serves as Editor-in-Chief of Experiments in Fluids. She previously served as Chair of the American Physical Society Division of Fluid Dynamics (2016), member of the U.S. National Committee on Theoretical and Applied Mechanics (2011-2015) and Associate Editor of Physics of Fluids (2008-2013).

• ## Completion: Sub-miniature hot-wire anemometry for high Reynolds number turbulent flows

University of Melbourne

4pm Friday 11 August 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Despite several decades of research in wall-bounded turbulence there is still controversy over the behaviour of streamwise turbulence intensities near the wall, especially at high Reynolds numbers. Much of it stems from the uncertainty in measurement due to finite spatial resolution. Conventional hot-wire anemometry is limited for high Reynolds number measurements due to limited spatial and temporal resolution issues that cause attenuation in the streamwise turbulence intensity profile near the wall. To address this issue we use the NSTAP (nano-scale thermal anemometry probe) developed at Princeton University to conduct velocity measurements in the high Reynolds number boundary layer facility at the University of Melbourne. NSTAP is almost one order of magnitude shorter than conventional hot-wires. This enables us to acquire fully-resolved velocity measurements of turbulent boundary layers up to a friction Reynolds number of 20,000.

• ## Wall-resolved large-eddy simulation of flow about smooth-walled and span-wise grooved cylinders

Dale Pullin
Graduate Aerospace Laboratories California Institute of Technology, USA

3.30pm Friday 4 August 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Wall-resolved large-eddy simulation (LES) of flow over both smooth- and grooved-walled cylinders will be discussed. The stretched-vortex sub-grid scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear, body-fitting mesh. The grooved cylinder consists of 32 equal, sinusoidal, spanwise grooves around the cylinder circumference, each of equal height ε. The LES is first used to investigate the effect of groove height in 0 ≤ ε/D ≤ 1/32 at ReD = 3.9 x 103. A second set of LES uses fixed ε/D = 1/32 with 3.9 x 103 ≤ ReD ≤ 6 x 104, the latter value reaching the transcritical range. The presence of grooves substantially reduces the ReD range of the drag crisis. Both smooth-walled and grooved-wall cylinder flows exhibit mean-flow secondary separation bubbles embedded within the large-scale, separated flow region at subcritical ReD. These transform to prior separation bubbles as ReD is increased through and beyond the respective drag crises. For the smooth-wall flow this is associated with local transition to turbulence and a strong drag crisis. For the grooved-cylinder case the flow remains laminar but unsteady through its drag crisis and into the early transcritical flow range.

This research is performed in collaboration with W. Cheng and R. Samtaney (King Abdullah University of Science and Technology).

Dale Pullin is the Robert H. Goddard Professor at the Graduate Aerospace Laboratories California Institute of Technology. His research interests include computational and theoretical fluid mechanics, vortex dynamics, compressible flow and shock dynamics, turbulence, and large-eddy simulation of turbulent flows.

• ## Confirmation: Development of algebraic Reynolds stress models for low pressure turbines using machine learning techniques

Harshal Akolekar
University of Melbourne

2pm Wednesday 2 August 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Enhancing the efficiency of the low pressure turbine (LPT) of an aircraft engine leads to the reduction in specific fuel consumption and carbon emissions. This enhancement can be achieved by accurately capturing two important phenomena in the designing and modelling process, namely: separation-induced transition and turbulent wake mixing. Direct numerical simulations (DNS) offer the most accurate solution to this problem but are still computationally expensive. On the other hand, some RANS-based turbulence and transition models, six of which are used in this work to analyse the T106A LPT profile, estimate the separation to some degree of accuracy, but predict inaccurate wake-loss profiles. This is partly due to the use of the Boussinesq approximation to derive stress-strain relationships. An approach is therefore sought to improve the Boussinesq relationship by the development of algebraic Reynolds stress models. Instead of using a conventional analytical approach, this work uses an evolutionary Gene Expression Programming algorithm (a machine learning approach) to develop algebraic models based on reference DNS data sets. Preliminary analysis has shown that the developed algebraic Reynolds stress models improve the turbulent wake-mixing and consequently wake-loss profiles.

• ## Confirmation: Turbulent flow over surfaces with spanwise heterogeneity

Dea Wangsawijaya
University of Melbourne

3.30pm Friday 28 July 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Examples of wall-bounded turbulent flows over surfaces with heterogeneous roughness are abundant in both engineering applications and nature. A specific case of heterogeneity is a surface with roughness variation in the spanwise direction. Such surface generates secondary flow in the form of roll modes along the streamwise direction. Studies regarding the behaviour of wall-turbulent flows over spanwise heterogeneous roughness found contradictory results in terms of the sign of the roll modes and the mean streamwise velocity component U. The latter, as shown by a recent study by Chung, Monty & Hutchins (2017), might be affected by the ratio of spanwise roughness wavelength to δ. In this initial study, hot-wire measurements were performed over surfaces with spanwise heterogeneous roughness with a certain half-wavelength Λ, yielding a range of Λ/δ from 0.32 to 6.50. It is found that as Λ/δ becomes very large, secondary flows are restricted to the interface between high and low shear stress region, leaving the high and low stress region away from the interface to behave as if the surface becomes homogeneous based on the local stress. Similarly, measurement results of the very small Λ/δ test case show that secondary flows are confined within the near-wall region. Away from the wall, flow over the small Λ/δ case becomes spanwise homogeneous. As Λ/δ ≈ 1, U above the high and low shear stress region switches sign: U is higher above high shear stress region and lower above low stress region. Further analysis of 1-D energy spectra profile shows the occurrence of a second peak in the wake region, which might be related to an instability of secondary flows (Kevin et al. 2017).

• ## Completion: Boundary layer and bulk behaviour in vertical natural convection

Chong Shen Ng
University of Melbourne

3.30pm Friday 21 July 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Thermal convection is ubiquitous in nature and many engineering applications, for example, from the scale of convection in stars and in Earth's atmosphere, down to the scale of convection-driven heating or cooling of the built environment and in the computer that you are using to read this text. In this research, we consider the buoyancy driven flow that is sustained between two vertical walls, where the left wall is heated and the right wall cooled. We refer to this setup as vertical natural convection. Results from direct numerical simulations for Rayleigh numbers 105–109 and Prandtl number 0.709 are found to support a generalised applicability of the Grossmann–Lohse theory for thermal convection, originally developed for Rayleigh–Bénard convection. Similar to Rayleigh-Bénard convection, a pure power-law relationship between the Nusselt, Rayleigh and Prandtl numbers is not the best description for vertical natural convection and existing empirical relationships should be recalibrated to better reflect the underlying physics.

In this talk, we show that for the present Rayleigh number range, the mean laminar-like boundary layers coexist with local near-wall patches that exhibit turbulent behaviour. Thus, the Nusselt number versus Rayleigh number scaling relation is contaminated by dynamics of the two regions. When the walls are removed and boundary layers omitted, the new setup mimics turbulent bulk-dominated thermal convection and we show that the Nusselt number versus Rayleigh number scaling relation instead follows the 1/2 power-law asymptotic scaling relation, in agreement with the theoretical predictions of Kraichnan (Phys. Fluids, 1962, vol. 5, pp. 1374–1389) and Grossmann and Lohse (J. Fluid Mech., 2000, vol. 407, pp. 27–56). An extension of this result to the wall-bounded setup is discussed.

• ## Completion: Characterisation of a microfluidic hydrotrap to study the effect of straining flow on waterborne microorganisms

Farzan Akbaridoust
University of Melbourne

3.30pm Friday 30 June 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

We present a systematic study on the effect of straining flow on the harmful filamentous cyanobacterium Anabaena circinalis using a cross-slot type microfluidic device equipped with an advanced image-based real-time control system and simultaneously quantified by micron-resolution particle image velocimetry (micro-PIV). This enables us to monitor potential morphological damage to, and simultaneously compute the forces on, filaments in real-time while the filaments of the cyanobacterium are exposed to high strain rates.

• ## Interaction of synthetic jets with turbulent boundary layers

Tim Berk
University of Southampton

3.30pm Friday 23 June 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Synthetic jets are zero-net-mass-flux actuators that can be used in a range of flow control applications. For several pulsed/synthetic jet in cross-flow applications, two physical phenomena are important: 1) The variation of the jet trajectory in the mean flow with jet and boundary layer parameters, which will provide an indication of the penetration depth of the pulsed/synthetic jet in to a boundary layer and 2) the losses associated with this penetration and the various physical mechanisms that are responsible for it. In this talk, I will present results that discuss both the variation of trajectories with different jet and boundary layer parameters and the mechanisms that are responsible for the losses.

The first part will focus on the mechanisms that are responsible for momentum losses created by the interaction of a synthetic jet with a turbulent boundary layer. Previous studies have ascribed the momentum deficit caused by the jet to a combination of viscous blockage of the crossflow and the upwash of low-momentum fluid. However, these contributions have not been quantified systematically. In order to resolve these contributions, detailed Particle Image Velocimetry measurements were carried out to examine the interaction between a synthetic jet and a low-Reynolds-number turbulent boundary layer at the University of Southampton. Based on an analysis of the vortical structures identified in the PIV data, the momentum deficit is attributed to the streamwise velocity induced by these vortical structures. Using Biot-Savart calculations, this induced velocity can be determined from the vorticity field obtained from PIV. The resulting momentum deficit is shown to account for 90% of the total momentum deficit, indicating that the advection of vortices is the main source of momentum deficit in these flows.

In the second part of the talk I will present results that compare the mean flow trajectories of the jet at low Reynolds number to the data obtained in the High Reynolds Number Boundary Layer Wind Tunnel at Melbourne. Trajectories of the synthetic jet in the turbulent boundary layer are measured for a range of actuation parameters in both low and high Reynolds numbers. The important parameters influencing the trajectory are determined from these comparisons. The Reynolds number of the boundary layer is shown to only have a small effect on the trajectory. In fact, the critical parameters are found to be the Strouhal number of the jet based on jet dimensions as well as the velocity ratio of the jet (defined as a ratio between mean jet blowing velocity and the freestream velocity). An expression for the trajectory of the synthetic (or pulsed) jet is derived from the data, which (in the limit) is consistent with known expressions for the trajectory of a steady jet in a cross-flow.

Tim received his BSc in Advanced Technology (2011), MSc in Mechanical Engineering (2014) and MSc in Sustainable Energy Technology (2014) from the University of Twente in the Netherlands. He is currently a PhD student at the University of Southampton under supervision of Bharath Ganapathisubramani, hoping to finish his PhD at the end of this year. For the past two months Tim has visited the University of Melbourne, performing experiments in the High Reynolds Number Boundary Layer Wind Tunnel.

• ## Turbulent mixing and entrainment in density stratified riverine flows

Michael Kirkpatrick
University of Sydney

3.30pm Friday 16 June 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Maintaining the health of our river systems requires management of a limited resource, and involves balancing the water needs of agriculture, fisheries, urban water supplies, power generators, industry, tourism, recreational users and the environment. As a result, many rivers are now highly regulated and have a large proportion of their water withdrawn for human uses, leading to flow rates significantly below their natural levels. Lowering of flow rates can lead to serious water quality issues due to the combined effects of longer residence times, decreased turbulent mixing and increased levels of stabilizing density stratification due to solar heating and/or salinity. The resulting stagnant conditions are characterized by low levels of dissolved oxygen and the accumulation of contaminants and nutrients, which can lead to long-term, systemic damage to riverine ecosystems. Thermal stratification combined with low turbulence, and high radiation and nutrient loads are also conducive to the proliferation and accumulation of toxic cyanobacteria (blue-green algae) and other micro-organisms. Persistence of these conditions for more than a few days has been identified as a key risk factor for cyanobacterial outbreaks.

In this talk I will give an overview of our research on turbulent mixing and entrainment in density stratified riverine flows. The first half of the talk will focus on experimental and numerical studies of entrainment and mixing processes in saline pools that form commonly in river bed depressions. The second half of the talk will focus on recent Direct Numerical Simulation studies of thermal stratification in turbulent open channel flow due to radiative solar heating. I will present our recently published findings (J. Fluid Mech., 2015) regarding the equilibrium states reached by turbulent open channel flow with solar heating. I will then discuss our current work in which we are investigating the primary physical processes that break down these stratified equilibrium states, namely nocturnal surface cooling, wind shear and river bends.

The aim of our research is to develop scaling relations that can be used in combination with other data, such as weather forecasts, to predict the likely occurrence of persistent stratification events. These improved modelling capabilities will enable river managers to initiate short time-scale tactical responses such as an environmental flow release from upstream reservoirs in response to drought or heat-wave conditions in order to maintain water quality and river health. It will also enable them to better optimize long-term water management strategies such as the distribution of water allocations amongst stakeholders, taking into account predicted changes in land-use patterns and climate.

Michael Kirkpatrick graduated with a degree in Mechanical Engineering from the University of Sydney in 1996. He then worked with the international building engineering consultancy Ove Arup & Partners in Sydney and their London office. Michael returned to Australia in 1998 to study for a PhD at the University of Sydney. On completion of his PhD in 2002, he was awarded a two year Postdoctoral Fellowship with the Center for Turbulence Research at Stanford University and NASA Ames. In 2004 he accepted a lecturing position in the School of Engineering at the University of Tasmania. In 2006 Michael returned to the University of Sydney where he is currently an Associate Professor in Fluids, Energy and Environment in the School of Aerospace, Mechanical and Mechatronic Engineering.

• ## Generation of a classical vortex ring and its axial interaction with a cylinder

Debopam Das
Department of Aerospace Engineering, Indian Institute of Technology Kanpur

3.30pm Wednesday 14 June 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In this talk I will discuss a novel method for generating vortex rings that circumvents some of the drawbacks associated with existing methods. The predominant effects that occur in previously used methods are due to the presence of some of the other vortices such as, stopping vortex, piston vortex, image vortex and orifice lip generated vortices in the early stage of development. These disturbances influence the geometric, kinematic and dynamic characteristics of a vortex ring and lead to mismatches with classical theoretical predictions. It is shown in the present study that the disturbance free vortex rings produced follow the classical theory. Flow visualization and PIV experiments are carried out in the Reynolds number (defined as ratio of circulation Γ and kinematic viscosity ν) range, 2270 < ReΓ < 6790, to find the translational velocity, total and core circulation, core diameter, ring diameter and bubble diameter. In reference to the earlier studies, significant differences are noted in the variations of the vortex ring diameter and core diameter. A model for core diameter during formation stage is proposed. The translational velocity variation with time shows that the second-order accurate formula derived using Hamilton's equation by Fraenkel (J. Fluid Mech., 1972, vol. 51, pp. 119–135) predicts it best.

Subsequently the axial interaction of this classical vortex ring with a thin circular cylinder has been studied. It is observed that due to the presence of the cylinder, there is an increase in the velocity of the vortex ring. Also, noticeable changes in the characteristic properties of vortex ring such as core circulation, core diameter and ring diameter have been observed. To justify these experimental observations quantitatively, an analytical study of the interaction under inviscid assumption is performed. The inviscid analysis does predict the increase in velocity during the interaction, but fails to predict the values observed in experiments. However, when the theory is used to correct the velocity change through incorporation of the effects of axisymmetric induced boundary layer region over the cylinder, modelled as an annular vortex sheet of varying strength, the changes in the translational velocities of vortex rings match closely with the experimental values.

Dr. Debopam Das is a professor of the Department of Aerospace Engineering, IIT Kanpur, India. He obtained PhD degree from Indian Institute of Science Bangalore. He has worked as Senior Research Scientist, Temasek Laboratory at National University of Singapore, Postdoctoral Research fellow in Florida State University and as an Assistant Professor, in the Department of Mechanical Engineering, IIT, Guwahati, India. His research interest is in flapping flight, incompressible and compressible vortex rings, instabilityo and transition of unsteady internal flows and instability of buoyant and non-buoyant free shear flows. He received Excellence in Aerospace Education award for 2015 by Aeronautical Society of India.

• ## Flow generated by surface waves

Horst Punzmann
Research School of Physics and Engineering, Australian National University

3.30pm Friday 9 June 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Trajectories of fluid parcels on the surface have been described analytically for progressing irrotational waves, where particles move in the direction of wave propagation. Waves in the laboratory and in nature are more complex due to the development of instabilities that render ideal planar 2D propagating waves into complex 3D waves. In this talk I will present experiments in the short wavelength gravity-capillary range that demonstrate different surface flow phenomena. Propagating waves, driven by a vertically oscillating plunger, can lead to a flow reversal when wave instabilities render planer wave fronts into wave packets at high amplitude. At small-amplitude, stable flow pattern can be achieved in stationary (rotating) two-dimensional wave fields that are produced by phase-synchronised wave paddles. The role of surface vorticity generation by waves will be discussed.

Dr Horst Punzmann is research engineer at the Research School of Physics and Engineering at the Australian National University (ANU), Canberra. He obtained his undergraduate degree in Electrical Engineering at the University of Applied Science in Regensburg, Germany and worked in analog integrated circuit design in the automotive industry for 5 years. After this, he did his PhD in high-temperature plasma physics at the ANU and studied plasma turbulence and particle transport while being facilities manager of the Australian Plasma Fusion Research Facility for 10 years. He switched to hydrodynamic turbulence a while ago and his current research interest in the Physics of Fluids group is related to the interaction of surface waves and flows.

• ## Confirmation: Air-sea interaction: laboratory study on the air-side turbulence of wind-wave

Tunggul Bhirawa
University of Melbourne

4pm Friday 2 June 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Wind is the main driving force in ocean wave generation, however the process of energy transfer from wind to wave is poorly understood. In order to explain the process comprehensively, we need to study how the atmosphere (air) and sea interacts. Air-sea interaction is relevant to understanding the coupled atmosphere-ocean system. The interactions involve small- and large-scale transfers of momentum, heat and mass, which affect our weather and climate. The knowledge of air-sea interaction is essential for industries related to the ocean system (ports and harbors, shipping, oil and gas offshore platforms) and for the ocean research community as well.

The common problem in studying this subject is to measure important quantities (sea drag, roughness length) close to the ocean surface which brings engineering and logistical challenges. In extreme conditions, such as in tropical cyclones, the uncertainty is higher since predictions are often extrapolated from lower wind speeds. This is where laboratory study is needed to make measurements possible.

PIV measurements were conducted to investigate the air-side turbulence above wind generated waves at the Extreme Air-Sea Interaction (EASI) facility in the Michell Hydrodynamics Laboratory. The objective is to study the behaviour of sea drag coefficient and roughness length under different wind speed and sea state conditions, which have been the most sought-after measurements in the study of air-sea interaction (Donelan et al. 1993).

• ## Dynamic behaviours of composite panel subjected to underwater blast: fluid-structure interaction (FSI) experiment and numerical simulation

Jonathan Tran
Department of Infrastructure Engineering, University of Melbourne

4pm Friday 26 May 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Designing lightweight high-performance materials that can sustain underwater impulsive loadings is of great interest for marine applications. In this study, underwater experiments are conducted and validated with a novel finite element fluid-structure interaction to understand the deformation and failure mechanisms of both monolithic and E-glass/Vinylester/H250 PVC sandwich composite panels. The experimental setup simulates fluid-structure interactions (FSI) encountered in various applications of interest. To generate impulsive loading similar to blast, a specially designed flyer plate impact experiment was designed and implemented. The design is based on scaling analysis to achieve a laboratory scale apparatus that can capture essential features in the deformation and failure of large scale naval structures. In the FSI setup, a water chamber made of a steel tube is incorporated into a gas gun apparatus. A scaled structure is fixed at one end of the steel tube and a water piston seals the other end. A flyer plate impacts the water piston and produces an exponentially decaying pressure history in lieu of explosive detonation. The pressure induced by the flyer plate propagates and imposes an impulse to the structure (panel specimen), which response elicits bubble formation and water cavitation. Failure modes, damage mechanisms and their distributions are identified and quantified for composite monolithic and sandwich panels subjected to typical blast loadings. The temporal evolutions of panel deflection and centre deflection histories were obtained from shadow Moiré fringes acquired in real time by means of high speed photography.

Dr Jonathan (Phuong) Tran received his PhD degree in Theoretical and Applied Mechanics at the University of Illinois, Urbana Champaign (UIUC, USA, 2010) on the development of Laser-induced compressive shockwave technique to investigate the dynamic delamination of multilayer thin film structures. Following his PhD, Dr Tran has been working as a postdoctoral researcher in Mechanical Engineering department at the Northwestern University (Evanston, IL, USA) on an ONR-funded project to investigate the performance of composite material subjected to underwater blast loading. His current position is a lecturer in the department of Infrastructure Engineering, the University of Melbourne. He has been involved and contributed to number of successful ARC Linkage and Discovery projects, CRC-Project and other defence-related grants from Defence Materials Technology Centre (DMTC) and CSIRO Manufacturing Flagship. His research interests include dynamic behaviours of composite structure under extreme loadings (air/underwater blast, ballistics impacts, wind and fire) and design optimisation of bio-mimicry and lightweight composite materials. He has published 50 journal papers and a book chapter on related topics.

• ## Natural convection in the near shore regions of lakes and reservoirs induced by the absorption of solar radiation

John Patterson
School of Civil Engineering, University of Sydney

4pm Friday 19 May 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In the near shore region of lakes and reservoirs where the depth decreases towards the shore, the incidence of solar radiation results in a complex motion which provides a mechanism for natural convection heat and mass transfer between the shore and the main water body. This has potentially significant consequences for water quality issues and therefore management of the resource. The flow also has wider implications for any contained fluid heated by radiation. The mechanism operates as follows. Incident radiation is absorbed by the water column in an exponentially decaying manner. The absorption of the radiation in this fashion gives rise to a stable temperature stratification in the upper regions of the water column. However, in the shallower parts, some radiation reaches the bed, and is re-emitted as a heat flux. There are two consequences of this: first, the volumetric rate of heating is greater in the shallow part than in the deep, which gives rise to a horizontal temperature gradient and therefore to a circulation up the slope and outward at the surface from the shore line; and second, the emission of heat from the bed gives rise to a potentially unstable temperature gradient at the bed which may cause intermittent rising plumes. In this presentation I discuss the development of the understanding of the key characteristics of this complex flow, including recent numerical, scaling analysis and experimental investigations. In particular I report some recent laboratory experiments using concurrent PIV and shadowgraph for measurement and visualisation which demonstrate both the underlying circulation and the presence of the rising plumes. The characteristics of these flows are compared with earlier scaling analysis results, and the underlying mechanism descriptions verified. Finally, linear stability analysis provides some of the characteristics of the rising plumes.

John received his PhD in Applied Mechanics from the University of Queensland. He moved to University of Western Australia as postdoctoral fellow, took up an academic post in the Department Civil Engineering, and eventually became Head of the School of Environmental Engineering. He moved to James Cook University in Townsville in 1997, and became the founding head of the merged School of Engineering. In 2009 he joined the School of Civil Engineering at the University of Sydney as Professor of Fluid Mechanics and Director of the Wind Waves and Water Centre. From 2011–2015 he was Associate Dean Research for the Faculty of Engineering and IT at the University of Sydney. John has served on a number of journal editorial boards, and has been a member of ARC assessment panels, including what is now called the College of Experts, Centre of Excellence selection panels, and the ERA Evaluation Committee. He has held visiting research appointments in Canada, Germany, UK, and the USA. He has published over 200 articles in the international literature. He has generated in excess of \$7M in ARC Discovery or equivalent research grants. John's research interests are historically in mixing and transport in lakes and reservoirs and the implications for water quality, and more recently, in natural convection flows, with environmental and industrial applications. His research encompasses theoretical, experimental and numerical investigations.

• ## Adjoint-based optimal flow control and invariant solutions in compressible 2D cavity flows

Javier Otero
University of Melbourne

4pm Friday 12 May 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The advances in the understanding of compressible flows challenge researchers to tackle more ambitious problems that surpass human intuition. Hence, it becomes necessary to rely on methods that give an insight into the underlying physical mechanisms that govern these complex flows to perform non-trivial tasks such as flow control. To this end, an adjoint-based optimal flow control framework for compressible flows has been appended to an existing in-house DNS code (HiPSTAR). In particular, we focus our efforts on a 2D cavity flow at Re=5000, where we aim to reduce noise levels at the sensor location, by either reducing the overall sound radiation or altering the sound directivity. In addition, with minor coding effort, this framework is extended to permit the computation of both (stable and unstable) exact steady and periodic flow solutions in compressible flows over complex geometries. Thus, we present the families of exact periodic and steady solutions across Mach number, using the same 2D cavity flow setup with a lower Reynolds number (Re=2000). These two families of flow solutions are found to meet at the quasi-incompressible flow regime, where a stability analysis on both periodic and equilibrium solutions shows that they form a subcritical Hopf bifurcation

Javier did his undergraduate in Aerospace Engineering at the Technical University of Madrid (UPM), Spain (2008–2012); an MSc in Computational Fluid Dynamics at Cranfield University, UK (2012–2013); and PhD in development and application of an adjoint-based optimal flow control framework for compressible DNS at Southampton University, UK (2013–2017). He is now a postdoc working with Prof Richard Sandberg.

• ## Shock driven instabilities in two-fluid plasmas

Vincent Wheatley
University of Queensland

4pm Friday 28 April 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In inertial confinement fusion (ICF) experiments, a capsule filled with fuel is imploded by shock waves generated by laser-driven ablation. The goal is to ignite a fusion burn at the centre of the implosion, and have that burn propagate through and consume the inertially confined fuel. Hydrodynamic instabilities cause mixing between the capsule material and the fuel, which is highly detrimental to the propagation of a fusion burn. One of the key hydrodynamic instabilities in ICF is the Richtmyer-Meshkov instability (RMI). This occurs when a shock interacts with a perturbed density interface, such as the interface between the capsule and the fuel. In ICF and astrophysical applications, the RMI typically occurs in plasmas. Here, we study the RMI in the context of the ideal two-fluid, ion-electron, continuum equations. These couple a separate set of conservation equations for each species to the full Maxwell equations for the evolution of the electromagnetic fields. We focus on cases with and without an imposed magnetic fields and with Debye lengths ranging from a thousandth to a tenth of the interface perturbation wavelength. For all cases investigated, the behaviour of the flow is substantially different from that predicted by the Euler or ideal magnetohydrodynamics equations. Electric fields generated by charge separation cause interface oscillations, particularly in the electrons, that drive a secondary high-wavenumber instability. Consequently, the density interface is substantially more unstable than predicted by the Euler equations for all cases investigated. Self-generated magnetic fields are predicted within our simulations, but their orientation is such that they do not dampen the RMI. In ideal magnetohydrodynamics (MHD), it has been shown that in the presence of such a seed magnetic field, the growth of the RMI is suppressed by the transport of vorticity from the interface by MHD shocks. Our two-fluid plasma simulations reveal that while the RMI is suppressed in the presence of the seed field, the suppression mechanism varies depending on the plasma length-scales. Two-fluid plasma RMI simulations also reveal that the secondary, high-wavenumber, electron-driven interface instability is not suppressed by the presence of the seed field.

Dr Vincent Wheatley is a senior lecturer in the Centre for Hypersonics within the School of Mechanical and Mining Engineering at the University of Queensland. He obtained his PhD in Aeronautics from the California Institute of Technology in 2005. He also earned an MEngSc (Mechanical) and a BE (Mechanical and Space) from the University of Queensland (UQ). After completing his PhD in the US, Dr Wheatley spent two years as post-doctoral fellow at ETH Zurich. He was then a Lecturer in Aerospace Engineering at the University of Adelaide before taking up his position at UQ in 2009.

• ## Non-laminar solutions for grooved Couette flow

University of Melbourne

4pm Friday 21 April 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The dynamical systems approach to turbulence has gained a lot of attention since the turn of the century. A large set of exact, symmetrical solutions of the Navier-Stokes equations have been found for canonical wall-bounded flows such as Couette, channel, and pipe flows at low Reynolds numbers (~ 400). The defining characteristic of these solutions is the vortex-streak structure that is known to play a crucial role in sustaining near-wall turbulence. In state-space, some of these solutions lie in the region that is densely visited by turbulence, while some others resemble the laminar solution and regulate laminar-turbulent transition. These solutions, along with their connections in state-space, form a skeleton for trajectories of turbulent flow. In addition to providing a description of turbulence dynamics, this approach also produces an equation-based, low-rank basis for use in flow control.

This vision of turbulence has not been extended to rough-walled flows despite their practical significance. We have taken the first steps in this direction by computing exact invariant solutions for plane Couette flow with (longitudinal) grooved walls, through continuation of known solutions for smooth-walled flows using a simple domain transformation method. We find that the principal role of such grooves is to localize the vortex-streak structure near the wall, as well as to reduce the bulk energy density while increasing the dissipation rate. Similar localization of the structure is considered to produce drag reduction (~ 10%) in riblet-mounted boundary layers. While these results are fairly primitive, especially due to the low Reynolds numbers involved, we believe that further efforts in this direction can help in optimizing riblet geometries for drag reduction, and provide insight on the influence of surface-roughness on the near-wall flow.

Sabarish Vadarevu is a research fellow in the fluids group at the University of Melbourne. He worked on his PhD (awaiting completion) in Aerospace Engineering at the University of Southampton, on computing exact invariant solutions in grooved plane shear flows. Prior to that, he obtained his Bachelors and Masters degrees from the Indian Institute of Technology-Madras, with his Masters thesis involving a stability analysis of partially premixed flames in laminar mixing layers. His current research is focused on linear modelling and control of wall-bounded turbulence.

• ## LES for loss prediction in an axial compressor cascade at off-design incidences

Jake Leggett
University of Southampton

4pm Tuesday 11 April 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The work focuses on the prediction of loss in axial compressors. The majority of modern design methods use cheaper more accessible models, such as RANS, to predict flow performance. While these methods are reliable and easier to use they do not always provide the accuracy needed. The aim of the work presented is to improve our understanding of how models like RANS behave under more challenging conditions. Using higher fidelity LES simulations it is possible to assess the performance of RANS under such conditions and aid designers by highlighting short comings and improving the application of such tools.

Jake Leggett is a PhD candidate at the University of Southampton, studying loss prediction in axial compressors. Having previously done a bachelors in mechanical engineering followed by an MSc in CFD at Imperial College London. His interests are in compressible fluid dynamics and numerical methods, focussing on modelling behavior and the performance of numerical models in capturing physically realistic flows.

• ## Confirmation: Numerical investigations of hemodynamic changes in stented coronary arteries

Bo Jiang
University of Melbourne

9am Wednesday 5 April 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Incomplete stent apposition (ISA) is sometimes found in stent deployment at complex lesions, and it is considered to be one of the causes of post-stenting complications, such as late stent thrombosis and restenosis. The presence of ISA leads to large recirculation bubbles behind the stent struts, which can reduce shear stress at the arterial wall that retards neointimal formation process and thus lead to complications. Computational fluid dynamics (CFD) simulations are performed on simplified two-dimensional axisymmetric arterial models with stents struts of square and circular cross-sectional shapes at a malapposition distance of 120 μm from the arterial wall. Under the condition of the same flow rate, both square and circular strut cases show that shorter period provides greater flow deceleration, leading to the formation of a larger recirculation bubble. With the same thickness, circular strut has a significant improvement over the square strut in terms of the size of the recirculation bubble, and therefore less likely to lead to complications. We also carried out three-dimensional computational fluid dynamics studies on a partially embedded coronary stent. Stent struts are partially embedded in portion of the circular and elliptical artery's circumference and malaposed elsewhere. Maximum malapposition distances (MD) used in this study are 0.255 mm (moderate) and 0.555 mm (severe). Time-averaged wall shear stress (TAWSS) is used as the haemodynamic metric to evaluate the clinical significance of ISA. TAWSS decreases along the circumferential direction as MD decreases from the maximum ISA side to the embedded side. The case with severe ISA has a larger area of high TAWSS on the arterial wall at the proximal end and low TAWSS at the distal end of the stent.

• ## Theoretical framework for the upscaling of physical interactions in aquatic mobile-boundary flows

University of Melbourne, University of Aberdeen

4pm Tuesday 28 March 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Most environmental flows often exhibit high levels of spatial and temporal heterogeneity in hydrodynamic fields due to the effects of multi-scale roughness elements and their mobility. These effects are especially profound in the near-bed region. The development of a unifying framework for the integration and upscaling of the fluid mechanical, ecological and biomechanical processes occurring in such conditions is needed. Particular focus in this study is on the interactions of the fluid motion with the motions of aquatic plants and sediments in aquatic systems.

To cope with the temporal and spatial heterogeneity of the flow field near the flow interface with a second non-fluid phase, the governing equations of motion are averaged over time and space. To deal with the possible discontinuity of the averaged fields within the averaging domains, appropriate definitions and theorems for time and space averaging are used.

Appropriately formulated coupled double-averaged conservation equations are developed for fluid, sediment, and plant motions. Complementary equations for the second-order velocity moments introduced by the averaging process are also proposed. Due to the double-averaging methodology (i) the governing equations are up-scaled to the scales relevant to applications, (ii) the fluid motion is rigorously coupled with the non-fluid (plants or sediments) motions, and (iii) the effect of the moving interfacial boundary is introduced explicitly in the governing averaged equations.

This presentation introduces to the main concepts and key steps involved in the derivation of the double-averaged equations outlined above, discusses their application to the analysis of high-resolution data sets and identifies potential applications in mobile-boundary flow studies.

Konstantinos performed his PhD research on flow-biota and flow-sediment interactions in rivers and open-channel flows at the University of Aberdeen, where he was on a Marie-Curie Fellowship. Prior to that, he obtained an MSc in coastal engineering at the National Technical University of Athens (2013) and MEng in environmental engineering at the Technical University of Crete (2009).

• ## High resolution simulation of dissolving ice-shelves in sea water

Bishakhdatta Gayen
Research School of Earth Sciences, Australian National University

4pm Friday 24 March 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Precise knowledge of ocean dynamics and interactions with the grounded ice at high latitudes is very crucial for predicting the sea-level rise and further development of adaptation strategies in a warming global climate. The physics of these ocean–ice interactions particularly related to small scale processes, is poorly understood which, along with limited observation constraints, leads to uncertainties in the predictions of future melt rate. We perform high resolution numerical simulation to investigate dissolving of ice into cold and salty sea water. The three coupled interface equations are used, along with the Boussinesq and non-hydrostatic governing equations of motion and equation of state for seawater, to solve for interface temperature, salinity, and melt rate. The main focus is on the rate of dissolving of ice at ambient water temperatures between −1°C and 2°C and salinity around 35 psu and the dependence on stratification (as characterizes many sites around Antarctica). Our simulation also shows boundary layer next to the ice face is dominated by turbulent motions. It is also important to quantify the difference between the melting of a vertical ice wall and the melting of a sloping ice shelf. The basal slope is observed to vary significantly, due to the formation of crevasses, channels and terraces. Our high-resolution simulations are designed for direct comparison with laboratory measurements and theory. The temperature and density structures found under Pine Island Glacier show several layers having a vertical scale that can also be explained by this study.

Bishakhdatta Gayen received a degree in Bachelor of Mechanical Engineering from Jadavpur University in Kolkata, India in 2006, and a M.S. degree in Engineering Science, majoring in Fluid Mechanics, from the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, in 2007. He was then awarded a University of California Graduate Fellowship and received an M.S. (2010) and Ph.D. (2012) from University of California, San Diego. He pursued research work on "Turbulence and Internal Waves in Tidal Flow over Topography" under the guidance of Prof. Sutanu Sarkar and was awarded the Andreas Acrivos Award for Outstanding Dissertation in Fluid Dynamics from the American Physical Society. He moved to Australia to pursue his postdoctoral research with Prof. Ross W. Griffiths at the Australian National University in Canberra. Bishakh is currently an Australian Research Council Discovery Early Career Fellow. His current research interests are nonlinear internal waves in the ocean, turbulent convection, modeling of Antarctic ice melting and Southern ocean dynamics.

• ## The vortices of V. Strouhal

David Lo Jacono
Institut de Mécanique des Fluides de Toulouse

4pm Friday 17 March 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

During this seminar, I will talk about the link between Strouhal's findings and vortices. We learn that the Strouhal number characterises the shedding frequency of vortices behind a bluff-body, yet Strouhal never encountered a vortex. I will try to show the role played by various well-known researchers (Rayleigh, Bénard, Kármán, etc.), and others that are less well known yet have a key role towards understanding wake dynamics. Starting from the motivation of Strouhal and ending with the modern analysis of wakes in 1930, I will try to build an incomplete and brief history of 19th-20th century (wake) fluid mechanics focussing on motivation and experimental insights. This work was originally part of my ScD dissertation and further completed for the recently held colloquium "A century of Fluid Mechanics 1870-1970" celebrating the IMFT century anniversary.

• ## Skin-friction and vorticity fields in wall-bounded flows and the attached eddy hypothesis

Min Chong
University of Melbourne

4pm Friday 10 March 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The invariants of the velocity gradient tensor have been used to study turbulent flow structures in order to extract information regarding the scales, kinematics and dynamics of these structures. These invariants cannot be used to study structures at a no-slip wall since they are all zero at the wall. However, the flow structures at the wall can be studied in terms of the invariants of the "no-slip tensor". Employing surface flow patterns generated using local solutions of the Navier-Stokes equations, the relationship between the surface skin-friction field and vorticity field will be explored. These local solutions, together with data from the Direct Numerical Simulations of channel flows, pipe flows and boundary layer flows may perhaps lead to a better model for the structure of attached eddies in wall bounded flows.

• ## Linear estimation of large-scale structures in channel flow at Reτ = 1000

Simon Illingworth
University of Melbourne

4pm Friday 3 March 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Given the time-resolved velocity field in a plane at a single wall-normal height, how well can one estimate, using a linear model alone, the time-resolved velocity field at other wall-normal heights? This question will be explored for channel flow at Reτ = 1000 using data from the John Hopkins Turbulence database. Two different linear models will be explored. Each linear model has its origins in the Navier–Stokes equations.

• ## Subfilter-scale stress modelling for large-eddy simulations

Amirreza Rouhi
University of Melbourne

4pm Friday 24 February 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

A subfilter-scale (SFS) stress model is developed for large-eddy simulations (LES) and is tested on various benchmark problems in both wall-resolved and wall-modelled LES. The basic ingredients of the proposed model are the model length-scale, and the model parameter. The model length-scale is defined as a fraction of the integral scale of the flow, decoupled from the grid. The portion of the resolved scales (LES resolution) appears as a user-defined model parameter, an advantage that the user decides the LES resolution.

The model parameter is determined based on a measure of LES resolution, the SFS activity. The user decides a value for the SFS activity (based on the affordable computational budget and expected accuracy), and the model parameter is calculated dynamically. Depending on how the SFS activity is enforced, two SFS models are proposed. In one approach the user assigns the global (volume averaged) contribution of SFS to the transport (global model), while in the second model (local model), SFS activity is decided locally (locally averaged). The models are tested on isotropic turbulence, channel flow, backward-facing step and separating boundary layer.

In wall-resolved LES, both global and local models perform quite accurately. Due to their near-wall behaviour, they result in accurate prediction of the flow on coarse grids. The backward-facing step also highlights the advantage of decoupling the model length-scale from the mesh. Despite the sharply refined grid near the step, the proposed SFS models yield a smooth, while physically consistent filter-width distribution, which minimizes errors when grid discontinuity is present.

Finally the model application is extended to wall-modelled LES and is tested on channel flow and separating boundary layer. Given the coarse resolution used in wall-modelled LES, near the wall most of the eddies become SFS and SFS activity is required to be locally increased. The results are in very good agreement with the data for the channel. Errors in the prediction of separation and reattachment are observed in the separated flow, that are somewhat improved with some modifications to the wall-layer model.

Amirreza Rouhi is a postdoctoral fellow in the fluids group at the university of Melbourne. Amirreza received his PhD in Mechanical Engineering from Queen's University of Canada under supervision of Prof. Piomelli, doing subfilter-scale (SFS) stress modelling as his PhD thesis. His other research interests include wall-modelled LES, spectral methods and rotating turbulence.

• ## Completion: The minimal-span channel for rough-wall turbulent flows

Michael MacDonald
University of Melbourne

4pm Friday 10 February 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Turbulent flows over roughness are ubiquitous in engineering and geophysical applications, however their effects are primarily given through semi-empirical models and approximations. The accuracy of these methods is sensitive to the model and roughness topology in question, so that laboratory experiments and conventional direct numerical simulations (DNS) remain the desired standard in rough-wall studies. However, these techniques are expensive for both industry and researchers, making design predictions and the examination of rough-wall flows challenging. In this talk, we outline a framework termed the minimal-span channel in which fully resolved numerical simulations of rough-wall flows can be conducted at a reduced cost compared to conventional DNS. The minimal-span channel is used to simulate turbulent flow over a variety of roughness geometries that would otherwise be prohibitively expensive to study. Special attention is given to recent simulations of rectangular bars aligned in the spanwise direction, commonly called d-type roughness.

• ## Completion: Reorganising turbulence using directional surface patterns

Kevin
University of Melbourne

4pm Wednesday 1 February 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

We attempt to passively reorganise wall turbulence using transitionally-rough surface patterns in the form of herring-bone riblets. The flow fields are investigated experimentally using large field-of-view particle image velocimetry in all orthogonal planes. The pronounced modification of the boundary layer suggests that a preferential arrangement of the naturally-occurring turbulence events may have been introduced. The spatial information captured in these multiple orientations enable us to clearly observe distinct turbulence events, such as the unstable outer layer, non-symmetrical vortical motions and strong streamwise-periodic events. Interestingly, our recent analysis indicates that the aforementioned events we thought was induced by the surface pattern, though weaker, are actually present in the smooth-wall (canonical) flows. In the average picture however, these structural attributes are masked by their random occurrences in space. This further suggests that we can passively reposition and perhaps manipulate large turbulence structures.

• ## DNS study on relation between vorticity and vortex

Chaoqun Liu
University of Texas at Arlington, Texas, USA

2pm Monday 23 January 2017
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Vorticity, vorticity line, vorticity tube have rigorous mathematical definition, but the rigorous definition of vortex is still an open question. However, for long time until now, in many research papers and text books, vortex is defined as vorticity tube confined by vorticity lines without vorticity line leakage. Vortex is also considered as congregation of vorticity lines with larger vorticity. Our DNS study shows that vortex is not a vortex tube but open for vorticity line penetration, is not a congregation of vorticity lines but a dispersion of vorticity lines, is not a concentration of vorticity with larger vorticity but with smaller vorticity in most 3-D cases. In general, vortex is an intuitive concept of rotation core with weak dissipation, e.g. zero dissipation when it becomes rigid rotation. In addition, vortex is not a vorticity tube, being different from what suggested by many research papers and textbooks. At the laminar boundary layer, vorticity is large but we have no vortex. A core with 2 nearly pure rotations per second, where vorticity is small, is a vortex, but a core with 10000 strongly sheared rotation, where vorticity is very large but deformation is very large as well, may not be a vortex. Therefore, vorticity magnitude and vortex are irrelevant. Unlike solid body, vortex is always a mixture of vorticity and deformation and vortex is really defined as an open area (not tube) where deformation is weak and vorticity is dominant. A function so-called "Omega" is defined to identify the vortex and Omega=0.52 well represents the vortex area boundary. Vortex is mathematically defined as Omega >0.5 which means a place where vorticity overtakes deformation. The minimum gradient of "Omega" is well representing the vortex axis. Several examples including DNS for late flow transition and LES for shock vortex interaction have been tested and the outcome is promising.

Chaoqun Liu is a Distinguished Professor and CNSM Center Director in the Department of Mathematics, University of Texas at Arlington, Arlington, Texas, USA.

• ## Completion: The structure and scaling of rough-wall turbulent boundary layers

Dougal Squire
University of Melbourne

2pm Wednesday 21 December 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Turbulent wall layers are a pervasive and influential feature in nature and engineering; common examples include the atmospheric and benthic layer (relevant to weather prediction and pollutant dispersion, for example), boundary layers developing on aerial, marine and terrestrial vehicles (such as aeroplanes, naval vessels, cars and trains), and flows in piping networks. These flows are characterised by high Reynolds numbers and, more often than not, surface roughness that exerts a dynamical effect on the flow. The latter may result from manufacturing defects, erosion and/or deposition, including that of living organisms. In this talk, we will present results from recent measurements of rough-wall turbulent boundary layers spanning a very wide range of friction and roughness Reynolds numbers. The results comprise 38 datasets and four experimental techniques, including hot-wire anemometry and particle image velocimetry. Our analysis will focus broadly on the relationship between the outer region flow and near-wall structures which are directly influenced by the roughness scale(s). Features of this relationship will be discussed using single-point statistics, measures of spatial structure, and the inner-outer interaction model of Marusic et al. (Science, 2010, vol. 329, pp. 193–196).

• ## Confirmation: Entrainment and interface dynamics of turbulent plumes

Himanshu Mishra
University of Melbourne

10am Wednesday 9 November 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Turbulent plumes form when a fluid of one density is injected into another quiescent fluid with a different density. From violent volcanic eruptions to the smoke rising from a cigarette, turbulent plumes are omnipresent in nature at wide range of scales. One of the fundamental aspects in the understanding of turbulent plumes is the process of 'entrainment', the mixing of surrounding fluid into the plume. Unlike non-buoyant flows, plumes pose a challenge in using common optical measurement techniques like particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF), because of the local changes in refractive index, when two fluids mix. This has led to most of the previous research being focused on global measurements of entrainment, whereas the local measurements, which are required for clearer understanding the entrainment phenomenon are practically non-existent. One of the ways to circumvent this problem is to match the refractive index of two solutions while maintaining the density difference, by adding certain chemicals to them. Alternatively, a measurement technique named Background Oriented Schlieren (BOS), which uses the local refractive index changes to quantify the local density variations, can be used.

With the final aim of understanding the process of entrainment in turbulent plumes, we present preliminary results for two experimental studies. (i) Velocity measurements in a vertical round axisymmetric turbulent jet in a newly constructed experimental facility, and (ii) free settling sphere in a sharp density interface using BOS.

• ## Hydrodynamic forces on geometrically porous structures

Scott Draper
University of Western Australia

4pm Friday 28 October 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Prediction of hydrodynamic forces on geometrically porous structures is important for many applied problems in offshore engineering. However, the accurate prediction of these forces is complicated by the fact that the bulk velocity of the flow passing through a porous structure is a function of its porosity. In this talk I will present a theoretical model built on control volume arguments which may be used to estimate the bulk velocity of the flow passing through an isolated porous structure and, in turn, the resulting hydrodynamic force on the structure. Extensions of the model will be introduced to consider multiple porous structures in close proximity, inviscid shear flow and non-uniform porosity. It will be shown how these extensions have been used in recent work to optimise the layout of wind and tidal turbine arrays, estimate forces on offshore space frame structures and predict the flow through patches of aquatic vegetation.

Dr Scott Draper is a senior lecturer at the University of Western Australia. His research focuses on offshore fluid mechanics applied mostly to the oil and gas and renewable energy industries. This has included research on the stability of subsea infrastructure, the optimum arrangement of marine renewable energy devices and the hydrodynamics of floating bodies. He is fortunate to work with numerous industry partners including Shell, Woodside and Carnegie Wave Energy.

• ## Pulse-burst PIV in high-speed flows

Steven Beresh
Aerosciences Department Sandia National Laboratories

4pm Wednesday 26 October 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Time-resolved particle image velocimetry (TR-PIV) has been achieved in a high-speed wind tunnel and a shock tube, providing velocity field movies of compressible turbulence events. The requirements of high-speed flows demand greater energy at faster pulse rates than possible with the TR-PIV systems developed for low-speed flows. This has been realized using a pulse-burst laser to obtain movies at up to 50 kHz with higher speeds possible at the cost of spatial resolution. The constraints imposed by use of a pulse-burst laser are a limited burst duration of 10.2 ms and a low duty cycle for data acquisition. Pulse-burst PIV has been demonstrated in a supersonic jet exhausting into a transonic crossflow and in transonic flow over a rectangular cavity. The velocity field sequences reveal the passage of turbulent structures and can be used to find velocity power spectra at every point in the field, providing spatial distributions of acoustic modes and revealing turbulence scaling laws. Additional applications in a shock tube show the transient onset of a von Kármán vortex street shed from a cylinder and particle drag in a shocked dense gas-solid flow. The present work represents the first use of TR-PIV in a high-speed ground test facility.

Steven J. Beresh is a Distinguished Member of the Technical Staff at Sandia National Laboratories in Albuquerque, New Mexico, U.S.A., where he has worked since 1999 and currently leads the Experimental Aerosciences Facility. He received his B.S. in Mechanical Engineering from Michigan State University in 1994 and his Ph.D. in Aerospace Engineering from The University of Texas at Austin in 1999. His research interests emphasize the use of optical diagnostics for compressible aerodynamics, particularly particle image velocimetry, but utilize a variety of laser-based instrumentation techniques and high-frequency surface sensors. He also is responsible for a wide range of wind tunnel testing and facility operation. He is an Associate Fellow of the American Institute for Aeronautics and Astronautics, the Chair for the AIAA Aerodynamic Measurement Technology Technical Committee, and is a past President of the Supersonic Tunnel Association International.

• ## Development effects in turbulent boundary layers: zero and adverse-pressure-gradients

Ramis Örlü
KTH Royal Institute of Technology Sweden | Website

4pm Friday 14 October 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In this talk, we discuss a number of different aspects pertinent to zero pressure gradient (ZPG) turbulent boundary layers (TBLs), and extend these to moderate and strong adverse pressure gradients (APGs). We start out with inflow and tripping effects in ZPG TBLs and show their persistence in the outer layer beyond Reynolds numbers (Re) that could be reached just few years ago by means of direct numerical simulations (DNS). When it comes to APG TBLs, the situation has usually been more blurry when considering literature data, due to their dependency on the pressure-gradient strength and streamwise history, besides Reynolds number effects. Based on different APG TBLs developing on flat plates and the suction side of a wing, we aim at distinguishing the importance of these parameters based on well-resolved in-house large-eddy and direct numerical simulations as well as wind tunnel experiments.

A recurring tool in the investigation of these topics is the diagnostic-plot concept, which indicates a linear dependence between the turbulence intensity and its mean velocity and turns out to be useful when establishing a well-behaved state in the outer layer of wall-bounded turbulent flows and determining the boundary layer edge in strong PG cases or TBLs on curved surfaces. A possible analogy between high-Re ZPG TBL flows and strong APG TBLs is also considered in light of negative wall-shear stress events.

Dr. Ramis Örlü received his M.Sc. (Dipl-.Ing.) in 2003 from the Ruhr University of Bochum, Germany in Mechanical Engineering and holds a Ph.D. in Fluid Mechanics (2009) KTH Royal Institute of Technology, Stockholm, Sweden. His research is focused on experimental methods and wall-bounded turbulent flows. Since 2009 and 2015 he works as a researcher and docent (in Experimental Fluid Physics), respectively, at the Linné FLOW Centre located at KTH. His research portfolio covers the experimental investigation of wall-bounded turbulent flows with strong numerical collaborations. Other active areas cover measurement technique development/correction for high-Reynolds number wall-bounded flows as well as applications in internal combustion engine related flows and various flow control strategies for separation delay and skin-friction drag reduction. Dr. Örlü has about 50 journal publications, and a wide network with other researchers in wall-bounded turbulence research.

• ## Experimental investigation of two-phase flows in naval hydrodynamics

Paul Brandner
Australian Maritime College University of Tasmania

4pm Thursday 29 September 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Flow about ships and submarines vary from pure liquid-gas to pure liquid-vapour two-phase flows. They involve a large range of spatial and temporal scales and physical phenomena such that gaining insight remains a challenge via either experiment or computation. Naturally occurring microbubble populations in the ocean are of particular interest due to their potential to both affect and be affected by various phenomena and flow processes. They provide nuclei for cavitation inception on lifting surfaces of ships and submarines. Surface ships are prolific sources of polydisperse bubble populations that add to bubbly disperse flows about lifting surfaces. Cavitation and turbulence ultimately create complex long-lived large-scale microbubble laden wakes with properties significantly altered compared with those of the outer fluid. Techniques and results on various two-phase flow experiments in naval hydrodynamics are presented including microbubble generation for artificial seeding of nuclei and for PIV, microbubble disperse flows, cavitation inception, macroscopic cavitation phenomena, supercavitation, millimetre-bubble break and coalescence in turbulent shear flows and fluid-structure interaction.

Paul Brandner is research leader of the Cavitation Research Laboratory at the Australian Maritime College. His research interests include cavitation inception and dynamics, bubbly flows, supercavitation, fluid-structure interaction and hydroacoustics.

• ## Completion: Numerical simulations of two-phase flow: Eulerian and Lagrangian predictions

Shuang Zhu
University of Melbourne

4.15pm Friday 16 September 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The term two-phase flow refers to any fluids flow of two coexisting media in motion. The difference between the media can be its thermodynamic state, called phase (e.g. gas, liquid or solid) and/or its multiple chemical components. Two-phase flow is common in many environmental systems including rain, snow, sandstorms, avalanches, sediment transport debris flow and countless other natural phenomena. It is also of extreme importance in many industrial and engineering applications, such as fluidised bed, droplet spray process, drug aerosol delivery, dense gas dispersion, particle deposition and pollution control.

A persistent theme throughout the study of two-phase flow is the need to model the detailed behaviour of these flow phenomena as the ability to predict the behaviour of these flow systems is central to safety, efficiency and effectiveness of those events and processes. Similar to single-phase flow research, the development of predictive models for the two-phase flow follows along three parallel paths, namely theoretical models, laboratory experiments and numerical simulations. There are some cases where full-scale experimental models and/or accurate theoretical models are possible. However, in many other cases, the use of those models can be impossible for many of reasons. Consequently, the predictive capability and physical understanding of two-phase flow problems heavily rely on the employment of numerical simulations. The study of dynamics of two-phase flow using high resolution numerical simulations has gained considerable momentum with the recent development in computational fluid dynamics (CFD) methodologies. In particular, the use of direct numerical simulations (DNS) resolves the entire range of spatial and temporal scales of the fluid motion, which provides new insights into the structure and dynamics of the two-phase flow.

Current two-phase flow simulations modelling can be broadly classified based on the treatment of the dispersed phase. Two types of approaches are prevalent, Eulerian approach and Lagrangian approach, which are based on the concept of the continuous phase described in the Eulerian reference frame in a flow domain with the dispersed phase described either in the Eulerian reference frame as the continuous phase, leading to the Eulerian approach (commonly known as the "two-fluid" model), or in the Lagrangain frame, leading to the Lagrangain approach (also known as the "trajectory" model). The primary motivation behind the present work is to perform high fidelity numerical simulations to study two-phase flow using both Eulerian and Lagrangian approaches on three different two-phase flow problems, namely gravity currents, particle-laden currents and enhanced targeted drug delivery in a vascular tree.This PhD work provides a much needed knowledge on different two-phase flow numerical modelling approaches by solving the aforementioned real life scenarios. Specifically, the success of this PhD work would lead to better two-phase computational fluid dynamics strategy.

• ## Statistical characteristics of fully nonlinear potential deep water wave fields

Elena Sanina
Department of Infrastructure Engineering, University of Melbourne

4pm Friday 12 August 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

We present an analysis of long-term wave simulations performed using a fully nonlinear potential deep water wave model. The results of the simulations are compared with the spectra obtained using a variety of directional methods and are discussed in the context of their applications. The short-crestedness of a wave field is investigated in terms of the three-dimensional steepness defined as the vector whose magnitude is equal to the average steepness calculated along the vector direction in a horizontal plane. Several statistical characteristics of the surface elevation field and the wave spectrum development such as a non-uniformity of the wave spectrum and a migration of its peaks are discussed. The appearance of coherent structures on the ocean surface closely related to the tendency of high waves to occur in groups is analysed. Various features of the identified groups such as velocity of the groups, their lengths, lifetime and steepness are studied. A general analysis of the number of detected groups is also performed for the computed wave fields.

Dr. Elena Sanina is a Research Fellow in Ocean Engineering at the Department of Infrastructure Engineering, University of Melbourne. With Bachelor and Master degrees in Pure Mathematics from the Voronezh State University (Russia), she continued her studies performing research in the area of applied mathematics. In 2015 she completed her PhD in the Centre for Ocean Engineering, Science and Technology (COEST) at Swinburne University of Technology, then continued her work at the University of Melbourne in May 2016. Her research interests include non-linear waves, extreme waves and wave statistics.

• ## Completion: Evolution of zero pressure gradient turbulent boundary layers

Will Lee
University of Melbourne

4pm Monday 8 August 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

An experimental investigation of evolving turbulent boundary layers in a tow tank facility is presented. The main aim is to study the dynamics and the development of coherent features in a turbulent boundary layer from the trip to a high-Reynolds number state. With this goal in mind, time-resolved particle image velocimetry measurements are performed with a towed plate to provide a unique view of the streamwise evolution of turbulent boundary layers. The merit of this temporally-resolved dataset is it enables us to investigate the evolving dynamic properties of coherent features specifically in the outer region of turbulent boundary layers. The results reveal the formation mechanism and temporal evolution of shear layers and their associated large-scale coherent motions. Based on these findings, a conceptual model which describes dynamic interactions of coherent features is proposed and discussed.

• ## Self-sustaining motions and periodic orbits in statistically stationary homogeneous shear turbulence

Atsushi Sekimoto
Monash University

4pm Friday 5 August 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Homogeneous shear turbulence (HST) is the most canonical flow to investigate the shear-induced turbulence, where it is known that there are coherent structures, like velocity streaks and streamwise elongated vortices, similar to wall-bounded flows. The coherent structures and their dynamics are considered as incomplete realisations of nonlinear invariant solutions in the incompressible Navier–Stokes equation, i.e. equilibrium solutions or periodic orbits, which have been reported in the plane Couette, Poiseuille, pipe flow, isotropic turbulence, and so on. In this study, unstable periodic orbits (UPOs) in HST are investigated. The ideal HST grows indefinitely, and in simulations, the growing integral scale reaches the size of the computational domain. The largest-scale motion is restricted by the computational domain, and it grows, break-downs, and regenerates quasi-periodically, reminiscent to the self-sustaining motion and bursts in wall-bounded flow. The long term simulation of HST reaches the statistically stationary state. Direct numerical simulations (DNS) of statistically stationary homogeneous shear turbulence (SS-HST) are performed by a newly developed code. The box dependency is investigated to establish 'healthy' turbulence in the sense that the turbulence statistics are comparable to wall-bounded flow. SS-HST is essentially a minimal flow, and constrained by the spanwise box dimension as in minimal channel flow (Sekimoto, Dong & Jiménez, 2016 Phys. Fluids 28:035101; Flores & Jiménez 2010 Phys. Fluids 22:071704). In these good boxes, UPOs are numerically obtained. It represents a regeneration cycle of streamwise vortices and a streak, similar to the self-sustaining process (SSP) in wall-bounded flow.

The speaker received a PhD in thermo-fluid mechanics from Osaka University in 2011. He joined the Fluid Mechanics group in the Technical University of Madrid (UPM) as postdoctoral fellow. He is now a research fellow in the Laboratory for Turbulence Research in Aerospace & Combustion (LTRAC) in Monash University.

• ## Fluid mechanic challenges at DST Group: from submarines to hypersonic vehicles

Malcolm Jones
Defence Science and Technology Group

4pm Friday 29 July 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The physics of fluid flow plays a dominant role in modelling vehicles traveling through the atmosphere or oceans. Whether it is predicting the drag experienced on a submarine, the lift generated by a flapping wing micro air vehicle or the heating loads acting a hypersonic vehicle. The Navier–Stokes equations governing these flows are known, but except for a few trivial cases cannot be solved directly. For this reason a range of alternative methods must be used. These include: approximations to the equations, numerical solutions, laboratory scale experiments and field trials. This talk will discuss how these methodologies are applied Defence Science and Technology Group to three example research areas: submarine hydrodynamics, flapping wing aerodynamics and hypersonic aerodynamics. While these vehicles appear diverse there is a common theme in the technical challenges and hence in the scientific approach taken.

Malcolm Jones received a PhD in fluid mechanics from The University of Melbourne in 1998. He received a degree in mechanical engineering from the same university in 1994. He joined the Defence Science and Technology Group in 2007 where he is currently employed as a Research Scientist in the Aerospace Division. Malcolm currently works on a range of diverse research projects which include: aerodynamics of flapping wings, aerodynamics of hypersonic vehicles (HIFiRE program), cavity aeroacoustics, boundary layer transition, and submarine hydrodynamics. Prior to joining DSTO he was employed as Research Fellow and lecturer in the Department of Mechanical Engineering at The University of Melbourne and also at the School of Mathematical Sciences, Queensland University of Technology.

• ## Confirmation: DNS of a turbulent line plume in a confined region

Nitheesh George
University of Melbourne

4pm Thursday 28 July 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

We present results from a direct numerical simulation (DNS) of a turbulent line plume in a confined region with adiabatic side, top and bottom walls. The plume originates from a local line heat source of length, L, located at the centre of the bottom wall (z/H = 0) and it rises until it hits the top wall (z/H = 1) and spreads laterally to produce a buoyant fluid layer. Since the region is confined, the continuous supply of buoyant fluid forces the layer downwards, until it reaches the bottom wall, where the flow is said to be the asymptotic state (Baines and Turner 1969). In the present case, two Reynolds numbers, 3840 and 7680, are selected for plume lengths, L/H = 1, 2 and 4, where the Reynolds number of the plume is based on H and the buoyant velocity scale, F01/3, where F0 is buoyancy flux per unit length. The current simulations are validated against the analytical model presented by Baines and Turner (1969). The simulations exhibit a slow flapping motion of the confined line plume in the asymptotic state, which precludes a straightforward comparison with Baines and Turner's analytical model. For the purpose of comparing the analytical model, we have adapted a shifting method introduced by Hubner (2004), which improves agreement with the analytical model.

• ## Confirmation: Optimal actuator and sensor placement for feedback flow control using the complex Ginzburg–Landau equation

Stephan Oehler
University of Melbourne

4pm Friday 8 July 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The process of selecting the optimal actuator and sensor positions for a single-input single-output flow control problem is investigated and its limitations are discussed. Multiple unstable modes and time delays between actuating and sensing often result in a challenging flow control problem. Previous approaches have focused on the use of modern control schemes, but these do not always provide the full picture and difficult control problems are often predicated on fundamental limitations, which are not made clear in a modern control framework. In this paper, a previously developed control scheme for the complex Ginzburg–Landau equation is presented, the challenges for different set-ups are analysed and it is shown how fundamental limitations can lead to an intractable control problem. The research explores the control schemes developed for the complex Ginzburg–Landau equation from the fundamental perspective, while further connecting the two fields of fluid dynamics and control theory.

• ## The mitigation of pulsation in ventilated supercavities

Grant Skidmore
University of Melbourne

4pm Friday 1 July 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

It is possible for an underwater body to greatly reduce the drag caused by skin friction, using ventilated supercavitation. While the idea of ventilated supercavitation works well in theory, the process of generating a ventilated supercavity in practice is often plagued by pulsation. When a supercavity pulsates, the walls of the supercavity begin to periodically expand and contract. This can lead to the supercavity walls clipping the body, which can become problematic for stability of the supercavitating body. This seminar will explore the internal cavity pressure and near-field noise generated by experimental and computational supercavities. The results of the acoustic study revealed that the radiated acoustic pressure of pulsating supercavities is at least 40 dB greater than comparable twin vortex and re-entrant jet supercavity closure regimes. For pulsating supercavities it was also found that, at the pulsation frequency, the cavity interior pressure spectrum level was related to the near-field and far-field noise spectrum level through spherical spreading of the sound waves from the supercavity interface. As a result, the cavity interior pressure can be used as a measure of the radiated noise. The oscillatory nature of the internal cavity pressure time history was used to develop a method to mitigate supercavity pulsation. The method was explored with a numerical model, experiments, and CFD. The method is based on modulating the ventilation rate injected into a ventilated supercavity with the addition of a sinusoidal component. The effect of this modulation is the ventilated supercavity being effectively driven away from the resonance frequency. A wide range of ventilation rate modulation frequencies can cause the pulsating supercavity to transition into twin vortex closure. A reduction in the radiated noise accompanies the transition from pulsation to twin vortex closure, oftentimes 35 dB or more. Other modulation frequencies do not suppress pulsation but change the supercavity pulsation frequency.

Grant Skidmore is a postdoctoral researcher in Mechanical Engineering at the University of Melbourne. Grant obtained his PhD in aerospace engineering at the Pennsylvania State University. His research interests include supercavitation, drag reduction, hydrodynamic stability, free surface flows, multiphase flows, and turbulent wakes.

• ## Confirmation: Geometric aspects of wall-flow dynamics

Rina Perven
University of Melbourne

4pm Tuesday 28 June 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Complex but coherent motions form and rapidly evolve within wall-bounded turbulent flows. Research over the past two decades broadly indicates that the momentum transported across the flow derives from the dynamics underlying these coherent motions. This spatial organization, and its inherent connection to the dynamics, motivates the present dissertation research. The local field line geometry pertaining to curvature (κ) and torsion (τ) has apparent connection to the dynamics of the flow, and preliminary results indicate that these geometrical properties change significantly with wall-normal position. One part of this research is thus to clarify the observed changes in the field line geometry with the known structure and scaling behaviours of the mean momentum equation. Towards this aim, the planar curvature of the streamlines at each point in streamwise–spanwise slices of existing boundary layer DNS has been computed for different wall-locations. The computation of κ and τ arise from the local construction of the Frenet–Serret coordinate frame. The present methods for estimating κ and τ are briefly described, as is the need to improve the efficiency of this computation. The present results are briefly described, as are the broader aims to better understand the relationships between the geometry and dynamics of wall-flows. A schedule of the future work to be conducted is outlined.

• ## Laser light sheet profile and alignment effects on PIV performance

Kristian Grayson
University of Melbourne

11am Monday 27 June 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The sensitivity and impact of laser profile misalignment and shape mismatch on Particle Image Velocimetry (PIV) measurements are investigated in this study. While the effects of laser profile misalignment can be equivalent to an out-of-plane velocity component, light sheet mismatch can be identified and corrected prior to an experiment, decreasing PIV uncertainties. Synthetic particle image simulations are used to isolate and systematically vary laser profile mismatch parameters between successive PIV laser pulses. Two simulation cases are discussed, analysing the effects of a misalignment between two otherwise identical laser pulses, as well as a mismatch in the width of two laser profiles. Our results reveal a steady degradation in mean correlation coefficient as the laser profiles are increasingly mismatched in shape and alignment, coupled with a rapid rise in the detection of spurious vectors. These findings reinforce the need to consider laser sheet alignment and intensity distribution when seeking to capture high quality PIV measurements. The design of a modular and inexpensive laser profiling camera is outlined to enable robust and repeatable quantification of laser sheet overlap and beam characteristics. The profiling system is also found to be a valuable tool for laser diagnostics and aiding the setup of experiments. Various potential applications of this device are presented for PIV and other laser-based measurement techniques. Finally, preliminary results from PIV experiments which involve the deliberate misalignment of laser profiles are discussed. These data reiterate the trends observed in simulations, but also emphasise the coupled complexity of laser profile mismatch behaviour in experimental scenarios, placing the idealised simulation results in some context. Collectively, our findings highlight the importance of well-matched laser profiles. A more rigorous experimental quantification of these behaviours has the potential to enhance the quality of PIV results.

• ## Towards a Moody-like diagram for turbulent boundary layers

Dale Pullin
Graduate Aerospace Laboratories, California Institute of Technology

4pm Friday 24 June 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

An empirical model is presented that describes a fully developed turbulent boundary layer in the presence of surface roughness with a nominal roughness length-scale that varies parametrically with stream-wise distance. For Reynolds numbers based on the outer velocity and stream-wise distance that are large, use is made of a simple model of the local turbulent mean-velocity profile that contains the Hama roughness correction for the asymptotic, fully rough regime. It is then shown that the skin friction coefficient is constant in the streamwise direction only for a linear streamwise roughness variation. This then gives a two parameter family of solutions for which the principal mean-flow parameters can be readily calculated. Results from this model are discussed for the zero-pressure-gradient turbulent boundary layer, and some comparisons with the experimental measurements of Kameda et al. (2008) are made. Trends obtained from the model are supported by wall-modeled, large-eddy simulation (LES) of the zero-pressure-gradient turbulent boundary layer at very large Reynolds numbers. Both model and LES results are consistent with the self-preservation arguments of Talluru et al. (2016). It is argued that the present model/LES can be interpreted as providing the asymptotically rough-wall equivalent of a Moody-like diagram for turbulent boundary layers in the presence of small-scale wall roughness.

The speaker is the von Kármán Professor of Aeronautics at the California Institute of Technology. His research interests include computational and theoretical fluid mechanics, vortex dynamics, compressible flow and shock dynamics, turbulence, and large-eddy simulation of turbulent flows.

• ## The universal nature of freestream coherent structures

Philip Hall
Monash University

4pm Friday 17 June 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Exact coherent structures are nonlinear solutions of the Navier Stokes equations not necessarily arising as bifurcations from a base state. The solutions are thought to be relevant to bypass transition and fully turbulent flows. There are two fundamental types of solutions: vortex-wave interaction states and freestream coherent structures. A brief survey of the field is given and then freestream coherent structures are discussed in detail. These are shown to be driven in a nonlinear layer located where the base flow adjusts to its freestream value through exponentially small terms. It is shown that they are implicated in the production of passive near wall streaks in a layer distant log R from the wall where R is the Reynolds number. They are shown to be canonical states relevant to any 2D boundary layer, vortex sheet, jet or wake and relevant to weakly 3D forms of the latter flows. Remarkably it turns out that they also exist in any fully developed flow having a local maximum of the unperturbed velocity field. The relevance to fully turbulent flows is discussed.

Professor Philip Hall is the Head of School of Mathematics, Monash University. Professor Hall is presently on leave from Imperial College, where he became the first Director of the Institute of Mathematical Sciences and where he has been the Director of LFC (Laminar Flow Control)-UK. His research interests are in applied mathematics, particularly in nonlinear hydrodynamic stability theory, computational fluid dynamics, boundary layer control, convection, lubrication theory, chaotic fluid motion, geomorphology of rivers and coherent structures in high Reynolds number flows.

• ## A voyage from the shifting grounds of existing data on zero-pressure-gradient turbulent boundary layers to infinite Reynolds number

Hassan M. Nagib
Illinois Institute of Technology, Chicago, USA

4pm Friday 10 June 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Aided by the requirement of consistency with the Reynolds-averaged momentum equation, the 'shifting grounds' are sufficiently consolidated to allow some firm conclusions on the asymptotic expansion of the streamwise normal stress normalized with the friction velocity; i.e. <u+u+>. A detailed analysis of direct numerical simulation data very close to the wall reveals that its inner near-wall asymptotic expansion must be of the form f0(y+) − f1(y+) / U+ + O(U+)−2, where f0 and f1 are O(1) functions fitted to data. This means, in particular, that the inner peak of the normal stress does not increase indefinitely as the logarithm of the Reynolds number but reaches a finite limit. The outer expansion of the normal stress <u+u+>, on the other hand, is constructed by fitting a large number of data from various sources. This exercise, aided by estimates of turbulence production and dissipation, reveals that the overlap region between inner and outer expansions of <u+u+> is its plateau or second maximum, extending to ybreak+ = O(U+), where the outher logarithmic decrease towards the boundary layer edge starts. The common part of the two expansions of <u+u+>, i.e. the height of the plateau or second maximum, is of the form A − B / U+ + ... with A and B constant. As a consequence, the logarithmic slope of the outer <u+u+> cannot be independent of the Reynolds number as suggested by 'attached eddy' models but must slowly decrease as 1 / U+. A speculative explanation is proposed for the puzzling finding that the overlap region of <u+u+> is centered near the lower edge of the mean velocity overlap, itself centered at y+ = O(1 / Reδ*0.5) with Reδ* the Reynolds number based on free stream velocity and displacement thickness. Similarities and differences between <u+u+> in ZPG TBLs and in pipe flow will be briefly discussed. Finally, the composite profile of <u+u+> is used with our mean velocity composite profile to demonstrate the detailed behavior of the so-called "diagnostic plot" with Reynolds number.

Professor Nagib is the John T. Rettaliata Distinguished Professor of Mechanical and Aerospace Engineering at the Illinois Institute of Technology, Chicago, Illinois, and the Founding Director of the Institute's Fluid Dynamics Research Center. His field of specialty is in fluid mechanics, turbulent flow and flow management and control. Professor Nagib is the recipient of a number of prestigious honors including being a Fellow of the American Physical Society, the American Association of Advancement of Science, the American Institute of Aeronautics and Astronautics, and the American Society of Mechanical Engineers.

• ## Large-amplitude flapping of an inverted-flag in a uniform steady flow

School of Mathematics and Statistics, University of Melbourne | Website

4pm Friday 3 June 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The dynamics of a cantilevered elastic sheet, with a uniform steady flow impinging on its clamped-end, have been studied widely and provide insight into the stability of flags and biological phenomena. Recent measurements show that reversing the sheet's orientation, with the flow impinging on its free-edge, dramatically alters its dynamics. In contrast to the conventional flag, which exhibits (small-amplitude) flutter above a critical flow speed, the inverted-flag displays large-amplitude flapping over a finite band of flow speeds. In this talk, a combination of mathematical theory, scaling analysis and measurement is used to investigate the origin of this large-amplitude flapping motion. Flapping is found to be periodic predominantly, with a transition to chaos as flow speed increases. These findings have implications to leaf motion and other biological processes, such as the dynamics of hair follicles, because they also can present an inverted-flag configuration. This collaborative work is with the Gharib group at the California Institute of Technology.

John E. Sader is a Professor in the School of Mathematics and Statistics, The University of Melbourne. He leads a theoretical group studying a range of topics including the dynamic response of nanoparticles under femtosecond laser excitation, mechanics of nanoelectromechanical devices, high Reynolds number flow of thin films and rarefied gas dynamics in nanoscale systems.

• ## Turbulent horizontal convection and heat transfer

Bishakhdatta Gayen
Research School of Earth Sciences, Australian National University | Website

4pm Friday 27 May 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Horizontal convection (HC) is driven by a horizontal difference in temperature or heat flux at a single horizontal boundary of a fluid. In a thermally equilibrated state, net heat flux over the boundary is zero and circulation cell involves a horizontal boundary flow, turbulent plume motion at the end wall, and interior return flow which covers the entire the flow domain. HC may be a simple model of the meridonial overturning circulation (also known as global thermohaline circulation) based on such convective flow.

I will present three-dimensional convective circulation under differential heating on a single horizontal boundary of a rectangular channel, using direct and large eddy simulations over a wide range of Rayleigh numbers, Ra ∼ 108–1015. A sequence of several stability transitions lead to a change from laminar to fully developed turbulent flow. At the smallest Ra, convection is maintained by a balance of viscous and buoyancy forces inside the thermal boundary layer, whereas at the largest Ra, inertia dominates over viscous stresses. This results in an enhancement of the overall heat transfer at Ra ≥ 1010, while both dynamical balances give Nu ∼ Ra1/5. We have recently extended our study on circulation by applying thermal forcing on a lengthscale smaller than the domain, and with variation in both horizontal directions instead of traditional unidirectional gradient over the domain scale. Simulations show turbulence throughout the domain, a regime transition to a dominant domain-scale circulation, and a region of logarithmic velocity in the boundary layer. Scaling theory shows a new regime dominated by inertia of the symmetric interior large scale circulation, coupled to thermal dissipation in the boundary layer which explains the Nu ∼ Ra1/4 behaviour.

Finally, I will show circulation in a rotating rectangular basin forced by a surface temperature difference but no wind stress. Here, our focus is on the geostrophic regime for the horizontal circulation with a strong buoyancy forcing (large Ra).

• ## Buoyancy effects on turbulent entrainment

Dominik Krug
University of Melbourne

4pm Friday 6 May 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

In this talk, we will address the question which role buoyancy plays in the entrainment process in unstable configurations such as turbulent plumes. Based on data from direct numerical simulations of a temporal plume we show that the entrainment coefficient can be determined consistently using a global entrainment analysis in an integral framework as well as via a local approach. The latter is based on a study of the local propagation of the turbulent/non-turbulent interface (TNTI) relative to the fluid. We find that locally this process is dominated by small-scale diffusion which is amplified by interface-convolutions such that the total entrained flux is independent of viscosity. Further, we identify a direct buoyancy contribution to entrainment by the baroclinic torque which accounts for 8–12% percent of the entrained flux locally, comparable to the buoyancy contribution at an integral level (15%). It is concluded that the effect of the baroclinic torque is a mechanism which might lead to higher values of the entrainment coefficient in spatial plumes compared to jets. Finally, some results will be presented from applying the local analysis to a stable configuration. Here, buoyancy is observed to reduce the entrainment rate by reducing the surface area of the TNTI and we will consider how this affects the fractal scaling.

• ## Measuring ocean turbulence and inferring ocean mixing

Greg Ivey
University of Western Australia

4pm Friday 29 April 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## Conversion: Experimental factors influencing the quality of PIV results

Kristian Grayson
University of Melbourne

2pm Wednesday 27 April 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Particle Image Velocimetry (PIV) is a useful tool for investigating the behaviour of turbulent flows, enabling the capture of undisturbed instantaneous velocity fields. The shape and alignment of the pulsed laser sheets used in PIV can have a significant impact on the quality of results, where laser profile mismatches can degrade correlation quality. This study develops two key experimental and analysis tools to aid the refinement of PIV techniques and systematically quantify the impact of unoptimised laser characteristics. Improvements to PIV simulation software enable much more experimentally realistic scenarios to be modelled and a laser profiling camera allows laser characteristics and alignment to be quantified in experimental setups for troubleshooting and analysis. The capabilities of these tools can ultimately be used to assess the practicality and possible benefits of more advanced and complex experimental PIV configurations.

• ## Symmetries and symmetry breaking in bluff-body wakes

Justin Leontini
Swinburne University of Technology

4pm Friday 22 April 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

The periodic vortex shedding in the wake of a circular cylinder is one of the most well-studied flows in fluid mechanics. It often serves as a canonical flow for understanding all bluff-body flows, as well as being important in its own right due to the common use of cylindrical cross sections in engineering structures. The periodic forcing on the structure caused by this vortex shedding can lead to large correlated forces and subsequent vibration. Often, it is desirable to control this motion to avoid failure; at other times, this motion may be deliberately amplified to be used as a source of energy harvesting. Here, I will present results looking at simple control strategies of this wake, such as periodic forcing and geometrical modifications such as streamlining. I will show that even these simple modifications to the base cylinder flow can result in significant changes in the flow, and that many of these changes can be understood by considering the interaction of the symmetries of the base flow and the imposed control.

Dr. Justin Leontini joined Swinburne in December 2013. Previous to this, he was an Australian Postdoctoral Fellow at Monash University, spent time working at the Centre for Maths and Information Science at CSIRO, and at l'Institut de la Recherche sur les Phenomenes Hors Equilibre (IRPHE) (trans. Dynamic Systems Research Institute) in Marseille, France. His research work has spanned fundamental fluid-structure interactions, planetary core flows, ship hydrodynamics, and flow stability. Recently, he has been building a project investigating the gas transport mechanisms in specialty ventilation machines in neonatal ICU wards. All of this research is focussed around understanding the fundamental physics of flow phenomena that have relevance to engineering or natural problems.

• ## Direct and large eddy simulations of flows through low-pressure turbines subject to inlet disturbances

Richard Pichler
University of Melbourne

4pm Friday 15 April 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

To reduce specific fuel consumption and cost of jet engines it is desirable to decrease the number of blades. As a result the individual blades of modern low-pressure turbines (LPT) are subjected to more severe pressure gradients that might lead to flow separation. Since laminar boundary layers are more prone to separation than turbulent ones, the actual transition location might dictate if a boundary layer remains attached or not, which in general has a significant effect on losses.

Transition is known to be influenced by the incoming flow state, in particular inlet turbulence and discrete wakes shed by the upstream blade row(s) that in essence are regions of high-turbulence and low-momentum flow. To investigate the interaction of unsteady transition and separation for varying design parameters, a combined large-eddy and direct numerical simulation study has been conducted and the data have been studied in light of loss generation.

• ## The control of near-wall turbulence by non-conventional surfaces

Ricardo García-Mayoral
University of Cambridge | Website

4pm Friday 8 April 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

Complex features on what would otherwise be a smooth wall can alter an overlying turbulent flow. This talk will focus on surfaces that exploit this capability to reduce wall friction, and will discuss three of such surfaces: riblets, permeable and superhydrophobic surfaces. Riblets are a kind of directional roughness made up of small surface grooves aligned in the direction of the flow. Permeable coatings allow the flow to penetrate into the surface to a certain extent. Superhydrophobic surfaces, when immersed in water, can entrap pockets of air, so that the water flow can effectively slip over them. In all cases, for small texture or pore size the reduction of friction increases with size, but beyond a certain size the performance begins to degrade, limiting the range of technological interest and the optimum performance achievable. The talk will discuss both the drag-reducing and the drag-degrading mechanisms for the above three types of surface.

• ## Universality aspects and coupling mechanisms of turbulence

Patrick Bechlars
University of Melbourne

4pm Friday 1 April 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

To understand, describe and model the physical processes that drive a chaotic turbulent flow a comprehensive and detailed understanding of flow features and their interconnection is needed. This can be obtained through a thorough analysis that interprets and breaks down observations across different flows and locations. This should be done to expose key features of turbulence from different points of view. The features then need to be connected to create the solution of the puzzle.

With this in mind a set of detailed time-resolved 3D flow data was sampled. The set involves data from a turbulent boundary layer, turbulent pipe flows, jet flows and a supersonic wake flow. I am happy to share these datasets and look forward to upcoming collaborations.

Besides an outline of the available datasets, some universal and non-universal aspects of turbulence across the different flows will be discussed in the presentation. Further, an analysis based on the velocity gradient invariant is applied to discuss the composition and development of turbulence across a turbulent boundary layer flow. Also, the method was extended to analyze the cascading process of kinetic energy. This analysis exposes the backscatter mechanism that transfers kinetic energy from smaller to larger scales of motion. Results for this will be shown and an underlying physical mechanism will be suggested.

• ## Completion: Structure of mean dynamics and spanwise vorticity in turbulent boundary layers

Caleb Morrill-Winter
University of Melbourne

3.30pm Thursday 24 March 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## Conditional methods for modelling turbulent combustion

Alexander Klimenko
University of Queensland

4pm Friday 18 March 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## High-fidelity simulations of noise radiation from an elastic trailing-edge

Stefan Schlanderer
University of Melbourne

4pm Friday 11 March 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## A model problem for a supersonic gas jet from a moon

Hans Hornung
California Institute of Technology

4pm Friday 4 March 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## Computing external flows with the immersed boundary method and the lattice Green's function

Tim Colonius
California Institute of Technology

3pm Friday 26 February 2016
Old Metallurgy Masters Seminar Room 2 (Room 202, Bldg 166)

• ## Coherent features in jet aero-acoustics and wall-bounded turbulence

Woutijn Baars
University of Melbourne

4pm Friday 19 February 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## Dynamics of impacting slot jets

David Lo Jacono
Institut de Mécanique des Fluides de Toulouse

4pm Friday 12 February 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

• ## Investigation of supercritical airfoil dynamic response due to transonic buffet

Robert Carrese
RMIT University

## Transition prediction for non-axisymmetric bodies of revolution

David Pook
Defence Science and Technology Group

4pm Friday 22 January 2016
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

## Postdocs

First nameLast namePrimary supervisorLocation@unimelb.edu.au

## Students

First nameLast namePrimary supervisorLocation@student.unimelb.edu.au
• ## Sea-Ice-Wind-Wave-Interaction facility (SIWI)

Funded by ARC
Contact: Prof Jason Monty | Schedule
• ## High-Reynolds-number boundary layer statistics - streamwise, spanwise and wall-normal velocity components

Reference: R. Baidya, J. Philip, N. Hutchins, J. P. Monty & I. Marusic (2021)
Spanwise velocity statistics in high-Reynolds-number turbulent boundary layers.
J. Fluid Mech. 913 A35 doi:10.1017/jfm.2020.1129

Reference: R. Baidya, J. Philip, N. Hutchins, J. P. Monty & I. Marusic (2017)
Distance-from-the-wall scaling of turbulent motions in wall-bounded flows.
Phys. Fluids 29(2) 020712 doi:10.1063/1.4974354

• ## Fully resolved measurements of turbulent boundary layer flows up to Reτ = 20,000

Database:
Reference: M. Samie, I. Marusic, N. Hutchins, M. K. Fu, Y. Fan, M. Hultmark & A. J. Smits (2018)
Fully resolved measurements of turbulent boundary layer flows up to Reτ = 20,000.
J. Fluid Mech. 851 391–415 doi:10.1017/jfm.2018.508

• ## A tool to estimate missing energy at a given spatial resolution for turbulent boundary layers

Reference: J. H. Lee, Kevin, J. P. Monty & N. Hutchins (2016)
Validating under-resolved turbulence intensities for PIV experiments in canonical wall-bounded turbulence.
Exp. Fluids 57 129 doi:10.1007/s00348-016-2209-6

• ## Predictive 'Inner-Outer Interaction Model' for turbulent boundary layers

Reference: W. J. Baars, N. Hutchins & I. Marusic (2016)
Spectral stochastic estimation of high-Reynolds-number wall-bounded turbulence for a refined inner-outer interaction model.
Phys. Rev. Fluids 1 054406 doi:10.1103/PhysRevFluids.1.054406

• ## Two-point hot-wire data in a high Reynolds number zero-pressure gradient turbulent boundary layer at Reτ ≈ 15,000

Reference: W. J. Baars, K. M. Talluru, N. Hutchins & I. Marusic (2015)
Wavelet analysis of wall turbulence to study large-scale modulation of small scales.
Exp. Fluids 56 188 doi:10.1007/s00348-015-2058-8

Reference: W. J. Baars, N. Hutchins & I. Marusic (2016)
Spectral stochastic estimation of high-Reynolds-number wall-bounded turbulence for a refined inner-outer interaction model.
Phys. Rev. Fluids 1 054406 doi:10.1103/PhysRevFluids.1.054406

• ## High Reynolds number zero-pressure gradient turbulent boundary layer flow statistics

Reference: I. Marusic, K. Chauhan, V. Kulandaivelu & N. Hutchins (2015)
Evolution of zero-pressure-gradient boundary layers from different tripping conditions.
J. Fluid Mech. 783 379–411 doi:10.1017/jfm.2015.556

• ## Multi-component high Reynolds number zero-pressure gradient turbulent boundary layer flow statistics at Reτ ≈ 15,000

Reference: K. M. Talluru, R. Baidya, N. Hutchins & I. Marusic (2014)
Amplitude modulation of all three velocity components in turbulent boundary layers.
J. Fluid Mech. 746 R1 doi:10.1017/jfm.2014.132

• ## Adverse/favourable pressure gradient turbulent boundary layer flow statistics at Reτ ≈ 3000

Reference: Z. Harun, J. P. Monty, R. Mathis & I. Marusic (2013)
Pressure gradient effects on the large-scale structure of turbulent boundary layers.
J. Fluid Mech. 715 477–498 doi:10.1017/jfm.2012.531