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Harbin Institute of Technology Fluid Mechanics Seminars

Initiated by Zijing Ding and edited by Zhen Ouyang



Upcoming Seminars



Speaker: Prof. Tim Colonius, California Institute of Technology, USA

Date/Time: Tuesday, 29th June 2021, 10:00 am (GMT+8)

Title: Agreed to talk.

Abstract: Agreed to talk.

Prof. Colonius's homepage: https://eas.caltech.edu/people/colonius



Speaker: Prof. Jens G. Eggers, University of Bristol, U.K.

Date/Time: Friday, 2nd July 2021

Title: Agreed to talk.

Abstract:Agreed to talk.

 Prof. Eggers's homepage: https://people.maths.bris.ac.uk/~majge/



Speaker: Dr. Ashley P. Willis, University of Sheffield, U.K.

Date/Time: Tuesday, 6th July 2021

Title: Agreed to talk.

Abstract:Agreed to talk.

 Dr. Willis's homepage: http://apwillis.staff.shef.ac.uk/



Speaker: Prof. Matthias Heil, University of Manchester, U.K.

Date/Time: Tuesday, 13th July 2021, 4:00 pm (GMT+8)

Title: Flows past cylinders: Are the transitions between different flow regimes caused by a continuous evolution or by bifurcations?

Abstract: Solutions to the Navier-Stokes equations often go through a sequence of distinct regimes, with the flow field becoming more 'complicated' as the Reynolds number increases. These changes may occur via (i) bifurcations of the underlying solutions of the Navier-Stokes equations, or (ii) a continuous evolution of the 'complicated' flow field (with quantifiable, discrete changes to its topology).

We analyse the interplay between these two, in principle distinct, mechanisms in the context of flows past circular cylinders. If the cylinder is stationary the flow undergoes a Hopf bifurcation at a Reynolds number of approximately 46, resulting in the formation of the famous von Kármán vortex street -- a time-periodic flow in which vortices are shed downstream. While this suggests that the change to the flow topology arises via mechanism (i) we show that the transition from steady to time-periodic flow (through the Hopf bifurcation) and the formation of individual vortices are in fact distinct events that occur at slightly different Reynolds numbers.

When the cylinder performs forced oscillations transverse to the flow direction, the vortex-shedding pattern becomes significantly more complex, leading to the formation of so-called 'exotic wakes' whose character is controlled by the Reynolds number as well as the period and amplitude of the cylinder's motion. While it has generally been assumed that the transition between different wake patterns in response to changes in the amplitude occurs via mechanism (ii) we show that they are actually associated with a spatio-temporal symmetry-breaking bifurcation of the time-periodic flow.

Prof. Heil's homepage: https://personalpages.manchester.ac.uk/staff/matthias.heil/



Speaker: Prof. Vladimir S. Ajaev, Southern Methodist University, USA

Date/Time:

Title: Levitation and self-organization of microscale droplets 

Abstract: Levitating droplets of liquid condensate are known to organize themselves into highly ordered arrays over hot liquid-gas interfaces.  Our recent experimental observations show similar behavior of droplets over a dry heated solid surface at temperatures far below the Leidenfrost point.  Mathematical models are developed that predict the mechanisms of both droplet levitation and inter-droplet interaction leading to pattern formation over the dry surface; the models are shown to be in good agreement with the experimental data. Using the insights from the new experiments, we are able to resolve some long-standing controversies pertaining to the mechanism of levitation of droplets over liquid-gas interfaces and find the levitation height as a function of droplet size. Finally, by studying trajectories of levitating droplets near the contact line region we are able to obtain velocity profiles for local gas flow there. 



Speaker: Prof. Frederic Dias, University College Dublin, Ireland

Date/Time:

Title: Agreed to talk.

Abstract: Agreed to talk.

Prof. Dias's homepage: https://people.ucd.ie/frederic.dias/about



Speaker: Prof. Charles Chun Yang, Nanyang Technological University, Singapore

Date/Time:

Title: Agreed to talk.

Abstract: Agreed to talk.

Prof. Yang's homepage: https://www3.ntu.edu.sg/home/mcyang/index.html



Speaker: Prof. Michele Guala, University of Minnesota, USA

Date/Time:

Title: Agreed to talk.

Abstract: Agreed to talk.

Prof. Guala's homepage: https://cse.umn.edu/cege/michele-guala



Speaker: Prof. Victor Steinberg, Weizmann Institute of Science, Israel

Date/Time:

Title: Agreed to talk.

Abstract: Agreed to talk.

Prof. Steinberg's homepage: https://www.weizmann.ac.il/complex/steinberg/



Speaker: Prof. Neil J. Balmforth, University of British Columbia, Canada.

Date/Time:

Title: Agreed to talk.

Abstract: Agreed to talk.

Prof. Neil J. Balmforth: https://secure.math.ubc.ca/~njb/


Past Seminars



Speaker: Dr. Baole Wen, University of Michigan, Ann Arbor, USA

Date/Time: Tuesday, 15th June 2021, 10:30 am (GMT+8)

Title: Asymptotic Transport in Strongly Nonlinear Rayleigh–Bénard Convection

Abstract: Rayleigh–Bénard convection plays a significant role in important applications ranging from microfluidics engineering to climate science and astrophysics, and has been studied extensively to gain insights into the development of turbulence.  A key feature of Rayleigh–Bénard convection is heat transport, and predicting transport for large applied temperature gradients in the strongly nonlinear regime remains a major challenge for the field.  Since the 1960s two distinct scaling theories, i.e., the classical scaling theory and the ultimate scaling theory, have contended to quantitatively characterize the strongly nonlinear regime, yet no clear winner has emerged.

The computations reported here offer new evidence for one of these theories and suggest a novel way to resolve the conundrum.  Our tactic is to study relatively simple time-independent states called rolls and compare heat transport by these rolls with that of turbulent convection.  These steady rolls are not typically seen in large-Rayleigh-number simulations or experiments because they are dynamically unstable.  Nonetheless, they are part of the global attractor for the infinite-dimensional dynamical system defined by Rayleigh’s model, and recent results suggest that steady rolls may be one of the key coherent states comprising the ‘backbone’ of turbulent convection.  The new numerical technique proposed here pushes the computations of steady solutions much further up to Rayleigh number 1014, yielding clear asymptotic scalings for Rayleigh–Bénard convection between no-slip boundaries.  We observe that rolls of the horizontal periods that maximize the heat flux at each Rayleigh number display classical scaling asymptotically.  Moreover – and perhaps somewhat surprisingly – aspect ratio optimized steady convection rolls transport more heat than turbulent experiments or simulations at similar parameters.

Joint work with Charles R. Doering (University of Michigan, USA) and David Goluskin (University of Victoria, Canada).

Prof. Doering's homepage: https://lsa.umich.edu/cscs/people/core-faculty/doering.html

Dr. Wen's homepage: https://lsa.umich.edu/math/people/postdoc-faculty/baolew.html



Speaker: Prof. Xuerui Mao, Beijing Institute of Technology, Beijing, China

Date/Time: Friday, 11th June 2021

Title: Assimilation of fluid flow data.

Abstract: Data assimilation has grown to be a significant branch of fluid mechanics. In model based assimilation, considering the measured wall shear stress in a flat plate flow as the input, the incoming free-stream turbulence can be traced. Subsequently, a large part of the overall flow over the plate can be recovered and the development to the laminar-turbulence transition stage can be predicted. In data based assimilation, flow fields from various sources with various accuracy can be merged in a Gaussian Regression scheme to combine the merit of each individual set of data and mitigate their drawbacks. Here we consider the experimental data, commonly regarded as accurate but sparse, and the simulation data, which is dense but (sometimes!) inaccurate. By merging spatially and temporally discontinuous experimental data and continuous simulation one, we obtain a continuous high-fidelity set of data, shedding lights to a new relationship between experiments and simulations: rather than using one to validate the other, they can be combined! The same idea is then applied to merge DNS, LES and RANS data in an airfoil flow, aiming at using a small set of expensive and accurate data to tune the cheap and dirty one.



Speaker: Prof. Jan Kleissl, University of California, San Diego, USA

Date/Time: Thursday, 10th June 2021, 9:00 am (GMT+8)

Title: Urban Microclimatology: Implications of Realistic Urban Heating on Wind Flow and Dispersion

Abstract:As urbanization progresses, microclimate modifications are also aggravated, and more comprehensive and advanced methods are required to analyze the increasing environmental concerns. Among various factors that alter urban environments from undisturbed climates, street level air pollution due to vehicular exhausts is of major concern and is significantly affected by atmospheric motion and stability. Thermal forcing is shown to play an important role in determining flow patterns and pollutant dispersion in built environments, yet numerical studies of dispersion at microscale in urban areas are limited to simplified and uniform thermal conditions and the analyses on the effect of realistic surface heating are scarce.

To address this shortcoming, a detailed indoor-outdoor building energy model is employed to compute heat fluxes from street and building surfaces, which are then used as boundary conditiion for a Large-Eddy Simulation model. In comparison with previous studies, our model considers the transient non-uniform surface heating caused by solar insolation and inter-building shadowing, while coupling the indoor-outdoor heat transfer, flow field and passive pollution dispersion. Series of fluid flow and thermal field simulations are then performed for an idealized, compact mid-rise urban environment, and the pollution dispersion as well as turbulent exchange behavior in and above buildings are investigated. Additionally, a potentially universal characterization method of the flow field under realistic surface heating is evaluated, which aims to expand the results into a wider range of scenarios and investigate the potential correlations for various parameters of interest.

Prof. Kleissl's homepage: https://cer.ucsd.edu/_profile-pages/kleissl.html



Speaker: Prof. Demetrios Papageorgiou, Imperial College London, U.K.

Date/Time: Tuesday, 8th June 2021, 6:30 pm (GMT+8)

Title: Waves on the Microscale: Order, Chaos and its Control

Abstract:I will give an overview of the type of mathematics that needs to be invoked to study nonlinear waves in small scale geometries. Applied mathematicians are very familiar with large scale waves, such as water waves, and the activities that emanate from familiar models like the celebrated Kortweg de-Vries equation (a google search of “Kortweg de-Vries” comes up with 650,000 hits!). At large scales things are gravity driven and viscosity plays a secondary role and can be ignored. On the microscale, however, gravity’s gravity is typically diminished and viscosity rules. Interfaces between immiscible fluids (i.e. waves)  are quite happy to stay uniform and trundle along in their viscous morass. To do engineering on the microscope we need to drive them out of their equilibrium. One way to do this is by using external electric or magnetic fields, and I will begin with an overview of the mathematical models that emerge from such interventions - they involve a crucial coupling between the Navier-Stokes equations and the Maxwell equations in the right limit. The result is a host of PDEs that are derived asymptotically. Interestingly, these PDEs can produce chaotic solutions (we have rigorous proofs of this) even at zero Reynolds numbers. After deriving some of the models I will present computations of their solutions (mostly in the form of movies) and also address theoretically the problem of control and optimal control of such systems showing that this is possible and opens a gateway to useful physical exploitations.

Prof. Papageorgiou's homepage: https://www.imperial.ac.uk/people/d.papageorgiou



Speaker: Dr. Louis-Alexandre Couston, Université de Lyon, France

Date/Time: Friday, 4th June 2021, 3:30 pm (GMT+8)

Title: Subglacial hydrodynamics. Topography of ice-water interfaces.

Abstract:This talk will discuss two research topics related to ice-ocean interactions. The first topic will focus on the 400 subglacial lakes trapped beneath the Antarctic ice sheet, which are extreme, isolated, yet viable habitats for microbial life. The physical conditions within subglacial lakes are critical to evaluating how and where life may best exist. In this talk, I will demonstrate that Earth’s geothermal flux provides efficient stirring of Antarctic subglacial lake water. I will show that most lakes are in a regime of vigorous turbulent vertical convection, enabling suspension of spherical particulates with diameters up to 36 micrometers, which is essential for biome support within the water column. The second part of the talk will discuss our recent effort to model ice-ocean interactions explicitly. We have derived a phase-field model, which can be used to simulate the turbulent dynamics of water and the diffusion of heat in ice layers simultaneously, enabling to track the movement of the ice-ocean interface due to phase changes. I will show that a turbulent shear flow promotes the spontaneous generation of channels and keels in the ice aligned with the direction of the mean flow and discuss some of the implications of our work for polar ocean modelling.



Speaker: Prof. George Haller, ETH Zurich, Switzerland

Date/Time: Thursday, 3rd June 2021, 4:00 pm (GMT+8)

Title: Material Barriers to the Transport of Momentum and Vorticity in Turbulence

Abstract:I will give an overview of the type of mathematics that needs to be invoked to study nonlinear waves in small scale geometries. Applied mathematicians are very familiar with large scale waves, such as water waves, and the activities that emanate from familiar models like the celebrated Kortweg de-Vries equation (a google search of “Kortweg de-Vries” comes up with 650,000 hits!). At large scales things are gravity driven and viscosity plays a secondary role and can be ignored. On the microscale, however, gravity is typically diminished and viscosity rules. Interfaces between immiscible fluids (i.e. waves) are quite happy to stay uniform and trundle along in their viscous morass. To do engineering on the microscope we need to drive them out of their equilibrium. One way to do this is by using external electric or magnetic fields, and I will begin with an overview of the mathematical models that emerge from such interventions - they involve a crucial coupling between the Navier-Stokes equations and the Maxwell equations in the right limit. The result is a host of PDEs that are derived asymptotically. Interestingly, these PDEs can produce chaotic solutions (we have rigorous proofs of this) even at zero Reynolds numbers. After deriving some of the models I will present computations of their solutions (mostly in the form of movies) and also address theoretically the problem of control and optimal control of such systems showing that this is possible and opens a gateway to useful physical exploitations.

Prof. Haller's homepage: http://www.georgehaller.com/about/index.html



Speaker: Prof. Carlo L. Bottasso, TUM, Germany

Date/Time: Wednesday, 2nd June 2021, 4:00 pm (GMT+8)

Title: Understanding and controlling wind farm flows

Abstract:Wakes produced by upstream wind turbines have a profound influence on the performance of downstream machines. In fact, compared to clean isolated conditions, waked turbines experience a lower power output and increased loading, which in turn create cascading effects on operation & maintenance (O&M) and lifetime. Understanding and affecting wakes are extremely challenging scientific problems with very practical and concrete implications clearly felt by industry. Probably one of the most direct indications of the impact of wakes outside of the scientific literature is given by the press announcement issued in October 2019 by Ørsted (formerly DONG), the largest energy company in Denmark. In this striking announcement, Ørsted warned investors that it was not able to meet its long-term financial targets because “... of the negative impact … across our asset portfolio of … wake effects”. In addition, Ørsted stated that “… wake effects is likely to be an industry-wide issue”. This announcement had a major impact on the scientific and technical community, and sounds as a wake-up call: wake effects are one of the main priorities that need to be faced in the field of wind energy.

However, the effects of wakes go well beyond power capture and loading. In fact, with an increased penetration of wind in the energy mix, it has become necessary for wind energy systems to provide increased flexibility and services to the grid, including active power control, the provision of reserves, and the integration with storage and other power generation units. Here again, understanding and controlling wakes plays a central role in the ability to deliver such services, which build on complex behaviors such as maintaining enough reserves to hedge against wind fluctuations, or distributing fatigue damage and/or actuator duty cycle according to the status of the individual assets and their components.

In this talk we will review the recent progress in the understanding and modeling of wakes, and the latest achievements in the field of wind farm flow control. We will take the opportunity offered by some of these topics to present highlights from the work developed by the Wind Energy Institute at TUM, spanning from to digital technologies for smart operation of turbines and farms, to validation and testing.

Prof. Bottasso's homepage: https://www.professoren.tum.de/en/bottasso-carlo-l



Speaker: Prof. Mohamed Farhat, EPFL, Switzerland

Date/Time: Thursday, 27th May 2021, 3:00 pm (GMT+8)

Title: On cavitation phenomenon: From single bubble dynamics in still liquid to cavitation in turbulent flows

Abstract:The cavitation phenomenon is an issue of a long-lasting interest because of the powerful phenomena associated with the collapse of vapor bubbles. In hydraulic machinery, cavitation is always associated with severe erosion and drop in hydrodynamic performances. However, if tuned properly, cavitation can be useful in a variety of application, such as cleaning, food processing and biomedicine. In the present lecture, I will give an overview of the research performed at EPFL on the cavitation phenomenon. I will show how the case of a single bubble dynamics reveals a rich physics and let us better understand the extraordinary power of collapsing bubbles. I will then present some results related to cavitation in flowing water around a hydrofoil. I will focus on the tip vortex cavitation and the role of the gas content on the inception and development of such cavitation. I will present some promising techniques for flow control that we have developed for the mitigation of tip vortex cavitation.

 Prof. Farhat's homepage: https://people.epfl.ch/mohamed.farhat



Speaker: Prof. Genta Kawahara, Osaka University, Japan

Date/Time: Tuesday, 25th May 2021, 10:00 am (GMT+8)

Title: Ultimate heat transfer in convective and sheared turbulence

Abstract:Direct numerical simulations have been performed for turbulent heat transfer in thermal convection and shear flow between parallel permeable walls, on which the transpiration velocity is assumed to be proportional to the local pressure fluctuations (Jimenez et al. 2001 J. Fluid Mech. 442, 89-117). Turbulent heat transfer has been found to be substantially enhanced by the appearance of large-scale turbulence structures (large-scale thermal plumes in convection or large-scale spanwise rolls in shear flow) arising from the wall permeability. At high Rayleigh numbers or high Reynolds numbers we have achieved the ultimate heat transfer represented by a wall heat flux being independent of thermal conductivity, although the heat transfer on the wall is dominated by thermal conduction. The key to the achievement of the ultimate heat transfer is interpreted in terms of significant heat transfer enhancement by large-scale intense turbulence with the length scale of the order of the wall distance and with the velocity scale comparable to the buoyancy-induced terminal velocity in convection or the bulk-mean velocity in shear flow without flow separation from the permeable walls.

Papers in support of Prof. Kawahara's webinar: https://doi.org/10.1017/jfm.2020.867

Prof. Kawahara's homepage: https://rd.iai.osaka-u.ac.jp/en/187bee8bc45e082e.html



Speaker: Prof. Sjoerd W. Rienstra, Eindhoven University of Technology, Netherlands

Date/Time: Thursday, 20th May 2021, 3:00 pm (GMT+8)

Title: Sound Propagation in Shear Flow – Adiabatic Invariants of Slowly Varying Modes

Abstract:Adiabatic invariants are the holy grail in a WKB analysis of waves in a slowly varying medium. If one exists, it serves as an exact integral for the slowly varying mplitude of the wave. This is no exception for acoustic modes in a slowly varying duct with slowly varying mean flow.

Adiabatic invariants are invariants under slow variation, not any variation. Their existence ensues sometimes, but not only, from the more stringent conservation of energy. Acoustic energy in mean flow is not always conserved: it is conserved in potential flow, but not in vortical (i.e. shear) flow where the acoustic field exchanges energy with the mean flow. Adiabatic invariants are therefore common for modes in slowly varying potential flows, but so far unknown in sheared flows.

We found that: (i) in 2D shear flow the modes satisfy in general an incomplete adiabatic invariant; (ii) this reduces to a complete one for linear shear flow. This result makes the WKB approximation for a mode in a slowly varying duct almost as simple as the solution for a mode in a straight duct.

Papers in support of Prof. Rienstra's webinar: https://doi.org/10.1017/jfm.2020.687

Prof. Rienstra's homepage: https://www.win.tue.nl/~sjoerdr/