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.

Anya Jones
University of Maryland

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

Edgar Knobloch
University of California Berkeley

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

Callum Atkinson
Monash University

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

Tilak Chandratilleke
Curtin University

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

Jacob Cohen
Technion

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

Sean Symon
University of Melbourne

3.30pm Friday 5 October 2018
Mechanical Engineering Seminar Room Level 3 (Room 311, Bldg 170)

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.

Atsushi Sekimoto
Osaka University

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

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)

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)

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)

One of the most challenging aspects of turbulence is the dynamics of the energy cascade. This is due to its nonlinear nature, which is intimately linked to the ultimate breakdown of large eddies into small eddies. The early ideas of a physical cascade introduced by Richardson were expanded upon in the classic work of Kolmogorov such that the breakdown of eddies was thought to occur locally via adjacent scales. In more recent years evidence has mounted which shows that transfers of energy are often non-local, occur across disparate scales, and can even cascade in the inverse direction. Here we present data from a 2D particle image velocimetry (PIV) experiment performed in a jet-stirred zero-mean flow turbulence box in air. The turbulence can be tuned to have a large scale anisotropy ratio u'/v' (where u' is along the axial direction of the jets and v' along the radial direction) between 1.4 and 1.7. In this range the Reynolds number based on the Taylor microscale varies approximately between 300 and 500. Employing the second-order structure function, we show that anisotropy persists into (and through) the inertial scaling range. The third-order structure function, in the context of the Karman–Howarth–Monin equation, reveals that the cascade of energy is split such that energy moves from small to large scales along the radial direction and from large to small scales along the axial direction. We further find that the variation in large-scale turbulent kinetic energy is linked to the presence (or absence) of dissipative structures, which play a central role in the cascade. By conditioning the third-order structure function on PIV samples with "hyperactive" or "sleeping" levels of small-scale activity, we find the cascade of energy has a unique footprint for these distinct states. These results are a marked departure from the theory of Kolmogorov and imply that the central quantities which govern the overall energy transfer are non-local and reveal a complex structure in scale space.

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


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'ω_y'>. 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 ω_x² and ω_y² are described and quantified and their central role in the correlation, <w'ω_y'>, is shown. By contrast for the outer flow the dominant vorticity transport is <v'ω_z'> and its necessary connection, at high Reynolds number, with <w'ω_y'> 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.

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)

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 (M_b = 0.2–3), and Reynolds numbers (Re_τ = 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

Assaad Masri
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–Re_θ,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

Daniel Rowan Ladiges
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

Milad Samie
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 Re_D = 3.9 x 10³. A second set of LES uses fixed ε/D = 1/32 with 3.9 x 10³ ≤ Re_D ≤ 6 x 10⁴, the latter value reaching the transcritical range. The presence of grooves substantially reduces the Re_D 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 Re_D. These transform to prior separation bubbles as Re_D 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 < Re_Γ < 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

Sabarish Vadarevu
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

Konstantinos Papadopoulos
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 Re_τ = 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 Re_τ = 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, F₀^1/3, 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

John E. Sader
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 ∼ Ra^1/5. 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 ∼ Ra^1/4 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.

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.