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Past Webinars

Speaker: Grae Worster, University of Cambridge UK

Date/Time: Friday 7th October 2022 4pm BST/11am EDT

Title: The dynamics of super-absorbent hydrogels

Abstract: Super-absorbent polymers placed in water can form hydrogels with polymer fractions of less than 1% by volume, with the water molecules being adsorbed by the hydrophilic polymer to form an elastic material. They are used in disposable diapers, for soil remediation, for controlled drug delivery and as actuators in microfluidic devices. Hydrogels are also a component part of the xylem, through which water is transported upwards in trees. The water is not fixed in place but can flow through the porous polymer scaffold to drive swelling, drying and transpiration. We have developed a new mathematical approach to model super-absorbent hydrogels, which allows for strongly nonlinear swelling while remaining linear in deviatoric strains, in effect treating hydrogels as instantaneously incompressible, linear elastic materials with inhomogeneous elastic properties related to the differential swelling states. I will describe this model and illustrate its features by solving simple examples of swelling spheres and transpiration through cylinders, and by analysing different instabilities that can arise during swelling and drying. I will also introduce a hypothesis for a potential role of hydrogels in pumping water up very tall trees.

Enjoy free access to papers in support of Worster's webinar, courtesy of the Journal of Fluid Mechanics.

Speaker: James Moore, Imperial College London, UK

Date/Time: Friday 1st July 2022 4pm BST/11am EDT

Title: Lymphatic System Transport and Vaccine Design


Abstract: An important criterion for vaccine success is the transport of information into lymph nodes, where immune cells exist in sufficient density to facilitate adaptive immunity. This is the responsibility of the prenodal lymphatic system, which consists of porous initial lymphatics and actively pumping collecting lymphatic vessels. Antigen information can remain suspended in lymph or be captured by antigen presenting cells. Once inside the node, fluid flow and solute transport are crucial for delivering the information to specific cell types. In vivo observations of these transport processes are challenged by the lack of appropriate tools for measuring lymphatic flows. We have initiated a series of experiments and numerical simulations to investigate the roles of fluid transport in the various processes that support adaptive immunity. We have constructed porous media mathematical models for the lymph compartment, coupled to models of viscous flow in lymph node blood vessels to simulate the effects of vaccination. We have also coupled these to advection-diffusion-reaction models that simulate the migratory stimulatory effects of chemokines on immune cells. Microfluidic chamber experiments with controllable advection and ability to visualise cell migration in real time supports these models. Vaccination has strong effects on rat mesenteric collecting lymphatic pumping dynamics and fluid shifts within lymph nodes. The transport of small antigens in solution occurs primarily by diffusion in small (<1µm diameter) collagen-bundle conduit structures. These results provide insights on ways to modulate adaptive immune responses for the optimisation of vaccine design.

Speaker: Richard Bomphrey, The Royal Veterinary College, UK

Date/Time: Friday 10th June 2022 4pm BST/11am EDT

Title: Translating aerodynamic features from insects and birds to bioinspired aircraft

Note: This talk will not be recorded

Abstract: I will present two recent examples of how fundamental bioscience research can teach us about animal ecology, and also offer solutions to engineering challenges.
Flying animals must perceive and avoid obstacles, often in environments deprived of visual sensory cues. In my first example, I will show how collision-avoidance in nocturnal mosquitoes can be mediated by mechanosensory feedback, based on modulations of their own induced aerodynamic and acoustic fields as they enter ground- or wall-effect. Our computational fluid dynamics and aeroacoustic simulations are derived from detailed wing kinematics extracted from high-speed recordings of freely flying Culex quinquefasciatus mosquitoes. Results reveal areas of relative pressure changes that are associated with close proximity to the ground and wall planes and that could provide useful information to the flight controller: a mechanism we term ‘aerodynamic imaging’. Using these insights we successfully built an aerial robotic prototype carrying a bio-inspired sensor package.
In my second example, I will present our work based on measuring the changing shape of birds in flight. I will show how they minimise drag in a different way from aeronautical design, and how they remain unperturbed by strong gusts. Our detailed three-dimensional reconstructions of surface geometries show how wing elevation around the shoulder joint acts as a suspension system that rejects gusts. The mechanism works most effectively when the aerodynamic centre of pressure is aligned with the mechanical centre of percussion, and therefore can be tuned either by changing wing shape or by the distribution of mass within the wing.

Speaker: Claudia Cenedese, Woods Hole Oceanographic Institution, USA

Date/Time: Friday 6th May 2022 4pm BST/11am EDT

Title: Some like it hot, but not icebergs


Abstract: Mass discharge from the Greenland and Antarctic ice sheets has increased dramatically over the last two decades. Iceberg calving accounts for approximately half of this discharge. Icebergs impact both the regional and large-scale ocean circulation by altering the stratification. They also impact the marine ecosystem by affecting nutrient distribution and carbon cycling. Freshwater input due to iceberg melt has the potential to impact regional sea-ice distribution and the global overturning circulation. Notwithstanding their importance, our understanding of where and how icebergs melt is limited and their representation in ocean and climate models is over-simplistic, in part due to the scarcity of quantitative field data. As a result, model-based predictions of iceberg melt rates, of the fate of the melt water, and of its impact on the ocean are highly uncertain. The focus of this lecture will be on laboratory experiments investigating the influence of the ambient flow, the icebergs’ aspect ratio and the Earth rotation on iceberg melting.

Enjoy free access to a paper in support of Cenedese's webinar, courtesy of the Journal of Fluid Mechanics.

Speaker: Tim Pedley, DAMTP, University of Cambridge, UK

Date/Time: Friday 1st April 2022 4pm BST/11am EDT

Title: Fifty years of Biological Fluid Dynamics


Abstract: I joined the Physiological Flow Studies Unit at Imperial College, London, as a PDRA, in 1968; it was founded in 1965-6 by its Director, Colin Caro, and Sir James Lighthill. I have been working in Biological Fluid Dynamics (BFD) ever since. This talk will describe some the following selection of the topics on which I have worked; I hope it will give a feel for how BFM has changed over the years and how an applied mathematician has been able to contribute:

(1) Flow and pressure drop in branched tubes: prediction of inspiratory airway resistance.
(2) Unsteady boundary layer theory: calibration of hot film anemometers in reversing (arterial) flow.
(3) Flow in non-uniform and collapsible tubes and channels, including unsteady separation (experiment), vorticity waves and their possible influence on wall shear stress (WSS) in arteries (experiment and theory), self-excited oscillations in collapsible channels (theory and computation), with application to lungs, veins and arteries under a cuff (and maybe giraffes).
(4) Individual and collective behaviour in suspensions of swimming micro-organisms: bioconvection, gyrotaxis, continuum modelling for dilute suspensions (algae) ; bioconvection for chemotactic bacteria; squirmer models (Volvox) and cell-cell interactions leading to computations for non-dilute suspensions; trajectory measurements for Heterosigma akashiwo near vertical and horizontal plane boundaries – some unexplained phenomena.

Enjoy free access to papers in support of Pedley's webinar, courtesy of the Journal of Fluid Mechanics.

Read JFM Volume 705, published in 2012 in honour of Professor Pedley's 70th Birthday

Speaker: Noel Clemens, University of Texas at Austin, USA

Date/Time: Friday 4th March 2022 4pm GMT/11am EST

Title: Unsteadiness of Shock Wave / Turbulent Boundary Layer Interactions


Abstract: Shock wave / boundary layer interactions are an important feature of high-speed flow that occur in supersonic and hypersonic aircraft inlets, aircraft control surfaces, missile base flows, nozzles, and rotating machinery. These interactions are often associated with severe boundary layer separation, which is highly unsteady, and exhibits high fluctuating pressure and heat loads. The unsteady motions are characterized by a wide range of frequencies, including low-frequency motions that are about two orders of magnitude lower than the integral-scale fluctuations in the upstream boundary layer. The low-frequency motions are particularly problematic for aircraft structures as they can excite high-amplitude vibration of thin panels, which can lead to fatigue and failure. In this seminar, we will discuss experimental research on various types of shock wave / boundary layer interactions that we have conducted over the past 20 years. The discussion will focus on the use of high-speed PIV and pressure sensitive paint to investigate the physical mechanisms that drive the separated flow unsteadiness in 2D and 3D interactions.

Enjoy free access to papers in support of Clemens' webinar, courtesy of the Journal of Fluid Mechanics.

Speaker: Pierre-Thomas Brun, Princeton University, USA

Date/Time: Friday 4th February 2022 4pm GMT/11am EST

Title: Engineering interfacial flows and instabilities in solidifying liquids


Abstract: This talk is concerned with interfacial fluid mechanics in the context of solidifying liquids. Due to favorable downscaling with length, capillary effects are dominant for submillimetric objects and play a key role is a number of engineering and natural processes where they dictate the shape of drops, thin films, and coatings. As such, interfacial effects have been widely researched, albeit primarily in Newtonian fluids or in fluids whose constitutive law remains constant over time. In particular, the subtle interplay between hydrodynamics, solidification and the solid structures formed when harnessing capillary effects remain poorly understood. We will attempt to fill this gap of knowledge and study the physics of viscous jets and sheets in solidifying liquids. We will discuss experiments conducted with a model system, an elastomeric solution, so as to elucidate the competing phenomena at play in the formation of patterns and structures arising from interfacial flows and their subsequent or concomitant solidification. We will revisit classic fluid mechanics models, e.g. lubrication, and adapt them to situations where the flow is arrested in finite time. First, we will discuss droplet forming instabilities and show that the patterns they form can be engineered via the use of templates. New phenomena, e.g. self-templating, will be explored too. Second, we will revisit classic coating, drainage and imbibition problems and demonstrate that these flows can be used for materials design, e.g. fabricate soft robots. In fact, curing converts the flows we study into elastic solids, introducing the concept of “building with flows”.

Enjoy free access to papers in support of Brun's webinar, courtesy of the Journal of Fluid Mechanics.

Speaker: Stephen Monismith, Stanford University, USA

Date/Time: Friday 7th January 2022 4pm GMT/11am EST

Title: Flows through Living Roughness


Abstract: Kelp forests, seagrass beds, and coral reefs are among the most productive, bio-diverse, and beautiful of marine ecosystems. Hydrodynamics plays a central role in their function, controlling mass transfer between the organisms and the overlying flow, setting patterns and rates of transport of larvae, and affecting the extent to which the environment (temperature, pH, etc.) in these nearshore systems are different from that of the adjacent ocean. Central to this interplay of physics and ecology is the fact that the resistance to flow provided by kelp plants, seagrasses, or corals is determined by the geometry and density of these “ecosystem architects”, behavior that is described by the dynamics of complex, canopy flows. I will present field observations of these flows drawn from a coral reef in American Samoa, a seagrass bed in Palau, and a kelp forest in Baja. In the first case, drag can be related explicitly to the reef geometry at cm scales. In the second case, drag varies strongly with velocity reflecting the re-configuration of the drag elements due to bending, behavior that can be modeled with some accuracy. In the final case, I will show velocity measurements taken over the span of several years during which time the condition of the kelp forest varied between non-existent and dense. During this time rms depth-averaged velocities varied by a factor of 3, showing the importance of kelp-dependent drag to flow. However, this last case seems substantially more complicated in that drag on flow through a kelp forest is strongly influenced by surface waves, has multiple drag sources, and involves substantial inhomogeneity in the distribution of the kelp plants themselves. Nonetheless, I will present a simplified approach to inferring drag through kelp.

Speaker: Karen Flack, United States Naval Academy, USA

Date/Time: Friday 3rd December 2021 4pm GMT/11am EST

Title: Prediction of Drag for Rough Wall Boundary Layer Flows


Abstract: Significant progress has been made towards the understanding of rough-wall boundary layers and the subsequent drag penalty. Continued progress is promising since a larger range of parameter space can now be investigated experimentally and numerically. Recent advances in rapid prototyping techniques enables the generation of systematic variations of roughness scales and computationally efficient simulations with creative surface mapping techniques allows for experiments and computations to investigate similar complex roughness. While a universal drag prediction correlation is still elusive and may not be possible, predictive correlations for classes of surface roughness pertinent to engineering applications seem achievable. Three surface parameters based solely on surface statistics are showing promise in predictive correlations for a range of studies. These include a measure of surface elevation a slope parameter and the skewness of the surface elevation probability density function. Other candidate parameters that may be useful in a predictive correlation or a surface filter are the streamwise and spanwise correlation lengths. The challenges to represent this wide range of surface conditions and potential scales to characterize engineering roughness including biofouling in predictive correlations will be discussed.

Enjoy free access to a paper in support of Flack's webinar, courtesy of the Journal of Fluid Mechanics.

Anne-Virginie Salsac Photo

Speaker: Anne-Virginie Salsac, Université de Technologie de Compiègne, France

Date/Time: Friday 5th November 2021 4pm GMT/12am EDT

Title: Fluid structure interactions of a microcapsule in flow: when numerical modeling meet experiments


Abstract: Encapsulation consists in enclosing an internal medium in a solid semi-permeable membrane to protect it and control the exchanges with the environment. Being at the source of innovative applications in the fields of biotechnologies, pharmacology, or food industry, capsules offer tremendous potential in the process engineering world. But scientific challenges remain to be met, such as finding the optimal compromise between payload and membrane thickness, characterizing the membrane resistance and controlling the moment of rupture.

We will explore the challenges to use deformable liquid-core capsules of micrometric size to efficiently transport active material, with a primary focus on health-related applications. Being used suspended in a carrying fluid in flow, microcapsules constitute a formidable problem of complex fluid-structure interactions. I will present how the three-dimensional capsule-flow interactions may be modeled and how these sophisticated numerical models can dialogue with microfluidic experimentations to produce innovative techniques to characterize the mechanical properties of deformable capsules, sort them upon their rigidity or enrich suspensions. 

Enjoy free access to papers in support of Salsac's webinar, courtesy of the Journal of Fluid Mechanics.

brad marston

Speaker: Brad Marston, Brown University, USA

Date/Time: Friday 1st October 2021 4pm BST/11am EDT

Title: Topological Origin of Certain Fluid and Plasma Waves


Abstract: Symmetries and topology play central roles in our understanding of physical systems. Topology, for instance, explains the precise quantization of the Hall effect and the protection of surface states in topological insulators against scattering from disorder or bumps. However discrete symmetries and topology have so far played little role in thinking about the fluid dynamics of oceans and atmospheres. In this talk I show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorially trapped Kelvin and Yanai waves emerge as topologically protected edge modes. The non-trivial structure of the bulk Poincaré waves encoded through the first Chern number of value 2 guarantees the existence of these waves. Thus the oceans and atmosphere of Earth naturally share basic physics with topological insulators. As equatorially trapped Kelvin waves in the Pacific ocean are an important component of El Niño Southern Oscillation and other climate oscillations, these new results demonstrate that topology plays a surprising role in Earth’s climate system. We also predict that waves of topological origin will arise in magnetized plasmas. A planned experiment at UCLA’s Basic Plasma Science Facility to look for the waves is described.

Enjoy free access to papers in support of Marston's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Keith Moffatt, Cambridge University, UK

Date/Time: Friday 18th June 2021 4:30pm BST/11.30am EDT

Title: Concluding remarks and some open problems


Abstract: This series of webinars was planned in commemoration of George Batchelor's promotion of the field of fluid mechanics in all its varied aspects and applications. In rounding off the series, I shall first comment on how much George Batchelor himself would have enjoyed the 15 brilliant presentations that we have heard. It is good to know that our subject is flourishing so vigorously with the young talent that has been so evident through these presentations. I shall use the short time available to me to discuss three questions that I touched on in my JFM Perspectives paper Some topological aspects of fluid dynamics:
(i) How are corner eddies affected by increasing the Reynolds number of the remote forcing?
(ii) How does a sheet of air enter through the free-surface cusp in a tightly controlled situation?
(iii) How can knotted or linked magnetic flux tubes most efficiently jump from one minimum-energy state to another?

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Speaker: Guohua Wang, Lanzhou University, China

Date/Time: Friday 18th June 2021 4:00pm BST/11am EDT

Title: Very-large scale motions in the atmospheric surface layer


Abstract: Very-large scale motions (VLSMs) are typical structures in wall-bounded turbulence at high Reynolds number, which make important contributions to mass and energy transport. This talk will present the atmospheric surface layer observations carried out at the Qingtu Lake observation array (QLOA) site. Furthermore, some studies on the VLSMs in the atmospheric surface layer based on the observed data are introduced, including the morphological and dynamic characteristics of the VLSMs and its effect on the sand dust transportation. The influences of dust particles and heat flux on the VLSMs are discussed as well. It is found that the length scale of the VLSMs has Reynolds number invariance, and evidenced that the VLSMs in the atmospheric surface layer evolute with a top-down mechanism. In the sand-laden flow, the energy of the VLSMs increases, while their energy fraction decreases. The inclination angle of the VLSMs increases with the increase of dust concentration. It is revealed that the VLSMs dominates the streamwise transport of PM10 (tiny particles with size less than 10 μm), but suppress the vertical transport of PM10 near the surface. Finally, it is demonstrated that the temperature and PM10 in the atmospheric surface layer have large structural feature similar as the VLSMs, though the shape of temperature and PM10 structures are different with the VLSMs.

Enjoy free access to papers in support of Wang's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Axel Huerre, Laboratoire Matière et Systèmes Complexes (MSC), Université de Paris

Date/Time: Friday 11th June 2021 4:30pm BST/11:30am EDT

Title: Freezing a rivulet


Abstract: We investigate experimentally the formation of the particular ice structure obtained when a capillary trickle of water flows on a cold substrate. We show that after a few minutes the water ends up flow-ing on a tiny ice wall whose shape is permanent. We characterize and understand quantitatively the formation dynamics and the final thickness of this ice structure. In particular, we identify two growth regimes. First, a 1D solidification diffusive regime, where ice is building independently of the flowing water. And second, once the ice is thick enough, the heat flux in the water comes into play, breaking the 1D symmetry of the problem, and the ice ends up thickening linearly downward. This linear pattern is explained by considering the competition between the water cooling and its convection.

Enjoy free access to papers in support of Huerre's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Jane Bae, Harvard University, USA

Date/Time: Friday 11th June 2021 4:00pm BST/11am EDT

Title: Nonlinear interaction of the self-sustaining process in the near-wall region of wall-bounded turbulence


Abstract: We investigate the nonlinear interaction in the self-sustaining process of wall-bounded turbulence. Resolvent analysis is used to identify the principal forcing (nonlinear) mode which produces the maximum amplification in direct numerical simulations of the minimal channel for the buffer layer. The identified mode is then removed from the nonlinear term of the Navier-Stokes equations at each time step from a direct numerical simulation of a minimal channel. The results show that the removal of the principal forcing mode is able to inhibit turbulence in the buffer layer, while the removal of subsequent modes only marginally affects the flow. Analysis of the dyadic interactions in the nonlinear term shows that contributions toward the principal forcing mode come from a limited number of wavenumber interactions. Using conditional averaging, the flow structures that are responsible for generating the principal forcing mode, and thus the nonlinear interaction to self-sustain turbulence, are identified to be spanwise rolls interacting with meandering streaks.

Click here to read Bae's recent paper, published in the JFM Special Volume in celebration of the George Batchelor centenary

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Speaker: Hugues Faller, Universite Paris-Saclay, France

Date/Time: Friday 4th June 2021 4:30pm BST/11:30am EDT

Title: On the nature of intermittency in a turbulent von Kármán flow


Abstract: We have conducted an extensive study of the scaling properties of small scale turbulence using both numerical and experimental data of a flow in the same von Kármán geometry. We have computed the wavelet structure functions, and the structure functions of the vortical part of the flow and of the local energy transfers. We find that the latter obeys a generalized extended scaling, similar to that already observed for the wavelet structure functions. We compute the multi-fractal spectra of all the structure functions and show that they all coincide with each other, providing a local refined hypothesis. We find that both areas of strong vorticity and strong local energy transfer are highly intermittent and are correlated. For most cases, the location of local maximum of energy transfer is shifted with respect to the location of local maximum of vorticity. We however observe a much stronger correlation between vorticity and local energy transfer in the shear layer, that may be an indication of a self-similar quasi-singular structure that may dominate the scaling properties at large order structure functions.

Enjoy free access to papers in support of Faller's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Laura Cope, Cambridge University, UK

Date/Time: Friday 4th June 2021 4:00pm BST/11am EDT

Title: The dynamics of stratified horizontal shear flows at low Péclet number


Abstract: Stratified flows are ubiquitous; examples include atmospheres and oceans in geophysics and stellar interiors in astrophysics. The interaction of a stable stratification with a background velocity distribution can develop into stratified turbulence, key to transport processes in many systems. Geophysical flows, in which the Prandtl number Pr ∼ O(1), are often strongly stratified, nevertheless, turbulence still occurs. Density layering is key to understanding the properties of this ‘layered anisotropic stratified turbulence’ (LAST) regime that is characterised by anisotropic length scales and velocity fields. Conversely, Pr ≪ 1 for astrophysical flows, inhibiting the formation of density layers. This suggests that LAST dynamics cannot occur, raising the question of whether analogous or fundamentally different regimes exist in the limit of strong thermal diffusion. This study addresses this question for the case of a vertically stratified, horizontally-forced Kolmogorov flow using a combination of linear stability theory and direct numerical simulations. Four distinct dynamical regimes emerge, depending upon the strength of the background stratification. By considering dominant balances in the governing equations, we derive scaling laws which explain the empirical observations.

Click here to read Cope's recent paper published in the Journal of Fluid Mechanics

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Speaker: Shingo Motoki, Osaka University, Japan

Date/Time: Friday 28th May 2021 4:30pm BST/11:30am EDT

Title: Multi-scale steady solutions representing classical and ultimate scaling in thermal convection


Abstract: Rayleigh–Bénard convection is one of the most canonical flows widely observed in nature and engineering applications. The effect of buoyancy on a flow is characterised by the Rayleigh number Ra, and the flow becomes turbulent eventually as Ra increases. One of the primary interests in convective turbulence is the scaling law of the Nusselt number Nu (dimensionless vertical heat flux) with Ra. A one-third power law for Nu with Ra, referred to as the 'classical' scaling, has been reported in many experiments and numerical simulations. On the other hand, a one-half power law, referred to as the 'ultimate' scaling, has not been observed yet in conventional Rayleigh–Bénard convection (buoyancy-driven convection between horizontal impermeable walls with a constant temperature difference). In this talk, I will first discuss a multi-scale steady solution in the conventional Rayleigh–Bénard convection. It is a three-dimensional steady solution to the Boussinesq equations, found using a homotopy from the wall-to-wall optimal transport solution (Motoki et al. 2018 J. Fluid Mech., 851, R4). The exact coherent thermal convection exhibits the classical scaling and reproduces structural and statistical properties of convective turbulence. Next, I will draw attention to thermal convection between permeable walls. The permeable wall is a simple model mimicking a Darcy-type porous wall (Jiménez et al. 2001 J. Fluid Mech. 442, 89-117). The wall permeability leads to the ultimate scaling, meaning that a wall heat flux being independent of thermal conductivity, although the heat transfer on the wall is dominated by thermal conduction. Finally, I will discuss the physical mechanisms of classical and ultimate scaling.

Read Motoki's recent papers here and here, both published in the JFM Special Volume in celebration of the George Batchelor centenary.

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Speaker: Chris Howland, University of Twente, Netherlands

Date/Time: Friday 28th May 2021 4:00pm BST/11am EDT

Title: Quantifying mixing in simulations of stratified flows


Abstract: Turbulent mixing exerts a significant influence on many physical processes in the ocean. In a stably stratified Boussinesq fluid, this irreversible mixing describes the conversion of available potential energy (APE) to background potential energy (BPE). In some settings the APE framework is difficult to apply and approximate measures are used to estimate irreversible mixing. For example, numerical simulations of stratified turbulence often use triply periodic domains to increase computational efficiency. In this set-up, however, BPE is not uniquely defined and the method of Winters et al. (J. Fluid Mech., vol. 289, 1995, pp. 115–128) cannot be directly applied to calculate the APE. We propose a new technique to calculate APE in periodic domains with a mean stratification. By defining a control volume bounded by surfaces of constant buoyancy, we can construct an appropriate background buoyancy profile b∗(z,t) and accurately quantify diapycnal mixing in such systems. This technique also permits the accurate calculation of a finite-amplitude local APE density in periodic domains. The evolution of APE is analysed in various turbulent stratified flow simulations. We show that the mean dissipation rate of buoyancy variance χ provides a good approximation to the mean diapycnal mixing rate, even in flows with significant variations in local stratification. We discuss how best to interpret these results in the context of quantifying diapycnal diffusivity in real oceanographic flows.

Enjoy free access to papers in support of Howland's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Debasish Das, Strathclyde University, UK

Date/Time: Friday 21st May 2021 4:30pm BST/11:30am EDT

Title: A three-dimensional small-deformation theory for electrohydrodynamics of dielectric drops


Abstract: Electrohydrodynamics of drops is a classic fluid mechanical problem where deformations and microscale flows are generated by application of an external electric field. In weak fields, electric stresses acting on the drop surface drive quadrupolar flows inside and outside and cause the drop to adopt a steady axisymmetric shape. This phenomenon is best explained by the leaky-dielectric model under the premise that a net surface charge is present at the interface while the bulk fluids are electroneutral. In the case of dielectric drops, increasing the electric field beyond a critical value can cause the drop to start rotating spontaneously and assume a steady tilted shape. This symmetry-breaking phenomenon, called Quincke rotation, arises due to the action of the interfacial electric torque countering the viscous torque on the drop, giving rise to steady rotation in sufficiently strong fields. Here, we present a small-deformation theory for the electrohydrodynamics of dielectric drops for the complete Melcher–Taylor leaky-dielectric model in three dimensions. Our theory is valid in the limits of strong capillary forces and highly viscous drops and is able to capture the transition to Quincke rotation. A coupled set of nonlinear ordinary differential equations for the induced dipole moments and shape functions are derived whose solution matches well with experimental results in the appropriate small-deformation regime. Retention of both the straining and rotational components of the flow in the governing equation for charge transport enables us to perform a linear stability analysis and derive a criterion for the applied electric field strength that must be overcome for the onset of Quincke rotation of a viscous drop.

Enjoy free access to papers in support of Das' webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Adrian Lozano-Duran, MIT, USA

Date/Time: Friday 21st May 2021 4:00pm BST/11am EDT

Title: Cause-and-effect of linear mechanisms sustaining in wall turbulence


Abstract: Despite the nonlinear nature of turbulence, there is evidence that part of the energy-transfer mechanisms sustaining wall turbulence can be ascribed to linear processes. The different scenarios stem from linear stability theory and comprise exponential instabilities, neutral modes, transient growth from non-normal operators, and parametric instabilities from temporal mean-flow variations, among others. These mechanisms, each potentially capable of leading to the observed turbulence structure, are rooted in simplified physical models. Whether the flow follows any or a combination of them remains elusive. Here, we evaluate the linear mechanisms responsible for the energy transfer from the streamwise-averaged mean-flow U to the fluctuating velocities u’. To that end, we use cause-and-effect analysis based on interventions: manipulation of the causing variable leads to changes in the effect. This is achieved by direct numerical simulation of turbulent channel flows at low Reynolds number, in which the energy transfer from U to u’ is constrained to preclude a targeted linear mechanism. We show that transient growth is sufficient for sustaining realistic wall turbulence. Self-sustaining turbulence persists when exponential instabilities, neutral modes, and parametric instabilities of the mean flow are suppressed. We further show that a key component of transient growth is the Orr/push-over mechanism induced by spanwise variations of the base flow.

Click here to read Lozano-Duran's recent paper, published in the JFM Special Volume in celebration of the George Batchelor centenary

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Speaker: Xianyang Jiang, University of Cambridge, UK

Date/Time: Friday 14th May 2021 4:30pm BST/11:30am EDT

Title: A metamorphosis of three-dimensional wave structure in transitional and turbulent boundary layers


Abstract: Laminar-turbulent transition in boundary layers is characterized by the generation and metamorphosis of flow structures. The early transition is usually associated with a process of the evolution from a three-dimensional (3-D) wave to a Λ-vortex. To develop a deeper understanding of the spatiotemporal wave-warping process and its roles in precipitating the development of other structures (e.g. hairpin-like structure and turbulent spot), we present numerical studies of both K-regime transition and bypass transition. In this talk, I will first illustrate a qualitative comparison of flow visualizations between a K-regime zero pressure gradient case and an adverse pressure gradient case, based on the method of Lagrangian tracking of marked particles. The underlying vortex dynamics will be presented using a proposed method of Lagrangian-averaged enstrophy. Next, I will draw attention to the 3-D wave structures in bypass transition and early turbulent boundary layer and will describe similar flow behaviours between transitional and turbulent boundary layers. Finally, I will discuss a path to transition, which hypothesizes that the amplification of a 3-D wave precipitates low-speed streaks and rotational structures in wall-bounded flows.

Enjoy free access to papers in support of Jiang's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: George Fortune, University of Cambridge, UK

Date/Time: Friday 14th May 2021 4:00pm BST/11am EDT

Title: Waltzing worms: the dynamics of plant-animal collective vortex structures


Abstract: Circular milling, a stunning manifestation of collective motion, is found across the natural world, from fish shoals to army ants. It has been observed recently that the plant-animal worm Symsagittifera roscoffensis exhibits circular milling behaviour, both in shallow pools at the beach and in Petri dishes in the laboratory. Here in this talk, we investigate this phenomenon, through experiment and theory, from a fluid dynamical viewpoint, focusing on the effect that an established circular mill has on the surrounding fluid. Unlike systems such as confined bacterial suspensions and collections of molecular motors and filaments that exhibit spontaneous circulatory behaviour, and which are modelled as force dipoles, the front-back symmetry of individual worms precludes a stresslet contribution. Instead, singularities such as source dipoles and Stokes quadrupoles are expected to dominate. A series of theoretical models is presented to understand the contributions of these singularities to the azimuthal flow fields generated by a mill, in light of the particular boundary conditions that hold for flow in a Petri dish. A model that treats a circular mill as a rigid rotating disc that generates a Stokes flow is shown to capture basic experimental results well, and gives insights into the emergence and stability of multiple mill systems.

Click here to read Fortune's recent paper, published in the JFM Special Volume in celebration of the George Batchelor centenary

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Speaker: Mazi Jalaal, University of Amsterdam, Netherlands

Date/Time: Friday 7th May 2021 4:30pm BST/11:30am EDT

Title: Droplets of Yield Stress Fluids


Abstract: The impact and spreading of droplets of complex fluids over surfaces occur in a variety of industrial applications. We will study the spreading of viscoplastic droplets under surface tension and gravity. The droplet converges to a final equilibrium shape once the driving stresses inside the droplet fall below the yield stress. Scaling laws are presented for the final radius and complemented with an asymptotic analysis for shallow droplets. Moreover, numerical simulations using the volume-of-fluid method and a regularized constitutive law and experiments with an aqueous solution of Carbopol are presented. In the end, we briefly present the other applications of droplets of yield stress fluids.

Enjoy free access to papers in support of Jalaal's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Zijing Ding, Harbin Institute of Technology, China

Date/Time: Friday 7th May 2021 4:00pm BST/11am EDT

Title: Coating Liquid Films down a Vertical Fibre


Abstract: Previous experimental study by Kliakhandler et al. (JFM 429:381-390, 2001) reported three different flow patterns of a liquid film flowing down a vertical cylinder. The film flow is unstable due to the famous Plateau-Rayleigh mechanism, which evolves into organised droplets flow patterns. When the flow rate is high, steady moving droplets separated by a long-thin film were observed. When the flow rate is reduced, necklace-like flow structure was seen. When the flow rate is very small, droplets of multi-scales were observed and complex dynamical interactions between these droplets cause an unsteady flow. However, no previous theoretical model correctly predicted the droplet dynamics in the three regimes. In this talk, I will discuss the reasons accounting for poor predictions of liquid film dynamics down a vertical fibre. Then, we solve the Navier-Stokes problem and our results show excellent agreements with the experimental data by Kliakhandler et al.

Enjoy free access to papers in support of Ding's webinar, courtesy of the Journal of Fluid Mechanics.

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Speaker: Saksham Sharma, University of Cambridge, UK

Date/Time: Friday 30th April 2021 4:30pm BST/11:30am EDT

Title: On a toroidal method to solve the sessile drop oscillation problem


Abstract: The natural oscillation of a drop is a classical fluid mechanics problem. Analytical expressions for the simple case of free, spherical drops were obtained by Rayleigh, Lamb, Chandrasekhar and others using spherical coordinate system. In recent times, the focus on this problem has shifted towards a sessile drop supported on a flat substrate, as evident through some recent works. The majority of these are computational in nature. In this talk, I will present an alternative new mathematical framework, the toroidal coordinate system, to solve this long-standing problem analytically for small drops (Bond number << 1) with pinned contact lines. I start with the governing hydrodynamic equations and boundary conditions, write them in terms of the toroidal coordinate system and then obtain solutions by reducing them to an eigenmode problem. Resonant frequencies are identified for zonal, sectoral and tesseral vibration modes and compared with results presented in the literature and by other models. The impact of viscous dissipation in the bulk liquid, at the contact line, and contact line mobility is discussed qualitatively. I conclude with a discussion of the importance of conformal mapping for solving axisymmetric physical problems with complicated geometries.

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Speaker: Jesse Capecelatro, University of Michigan, USA

Date/Time: Friday 30th April 2021 4:00pm BST/11am EDT

Title: Turbulence modeling of strongly-coupled gas-particle flows


Abstract: Many natural and industrial processes involve the flow of solid particles or liquid droplets whose dynamical evolution are intimately coupled with a carrier gas. A peculiar behavior of such flows is their ability to give rise to large-scale structures (hundreds to thousands of times the size of individual particles), from dense clusters to nearly-particle-free voids. Seminal works by G.K. Batchelor has provided theoretical estimates describing the motion of collections of particles suspended in viscous flows and the notion of hindered settling under gravity. In this talk I will describe how at moderate Reynolds numbers and concentrations, momentum exchange between the phases results in enhanced settling and the generation of turbulence in the carrier phase. High-resolution simulations will be presented to reveal how multiphase interactions at the particle scale augment or restrict large-scale flow processes, and provide unique insight into the budget of turbulent kinetic energy. Finally, a new data-driven framework will be presented for model closure of the averaged two-phase flow equations.

Enjoy free access to papers in support of Capecelatro's webinar, courtesy of the Journal of Fluid Mechanics.

George Batchelor Centenary event from 29th-31st March 2021

George was a towering figure in the field of fluid mechanics, and amongst his many achievements he established the Department of Applied Mathematics and Theoretical Physics (DAMTP), founded the Journal of Fluid Mechanics and co-founded EUROMECH.

On the 29th, 30th and 31st of March 2021, the George Batchelor Centenary event was held virtually. Read more about this special celebration here.

In honour of the Centenary, a special volume of JFM has been published in tribute to George Batchelor. All papers in this volume are free to read in perpetuity and we invite you to read and share.

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Speaker: Jon Aurnou, UCLA, USA

Date/Time: Friday 19th March, 2021. 4:00 pm GMT/11am EST 

Title: Dynamo Generating Flows in Planetary Cores


Abstract: Planetary bodies typically host substantial liquid metal interior fluid layers. The fluid layers are often in turbulent motion due to planetary thermochemical evolutionary processes. This rapidly rotating turbulence can generate coherent, planetary-scale magnetic fields that can protectively shield a planet for its harsh surrounding space environment. In this talk, I will present scaling arguments for the behaviors of core flows mixed with simple table-top experiments. I will then present the most recent laboratory and theoretical models of these flows, and what aspects of dynamo observations they can and cannot yet explain.

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Speaker: Mohammad Farazmand, North Carolina State University, USA

Date/Time: Friday 12th March, 2021. 4:00 pm GMT/11am EST 

Title: Extreme Events in Fluid Dynamics: Mechanisms, Prediction and Mitigation


Abstract: A wide range of natural and engineering systems exhibit extreme events, i.e., spontaneous intermittent behavior manifested through sporadic bursts in the time series of their observables. Examples include ocean rogue waves, intermittency in turbulence, extreme weather patterns and epileptic seizure. Because of their undesirable impact on the system or the surrounding environment, the real-time prediction and mitigation of extreme events is of great interest. In this talk, I will discuss three aspects of extreme events. First, I introduce a variational method that unveils the mechanisms underpinning the formation of extreme events. Next, I show how this framework enables the data-driven, real-time prediction of extreme events. I demonstrate the application of this method with several examples, including the prediction of ocean rogue waves and the intermittent energy dissipation bursts in turbulent fluid flows. Finally, I will discuss a closed-loop adaptive control and a delay feedback control for mitigating extreme events.

Enjoy free access to this paper in support of Farazmand's webinar, courtesy of the Journal of Fluid Mechanics

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Speaker: Marina Levy, LOCEAN-IPSL, France

Date/Time: Friday 5th March, 2021. 4:00 pm GMT/11am EST 

Title: Bringing fluid dynamics to life in the ocean


Abstract: No flow, no life. Without movement in the fluid, there would barely be any life in the ocean. Fluid movements allow the continuous supply of nutrients essential to photosynthesis in the sunlit layer of the ocean. This sustains microscopic phytoplankton, an immense biodiversity, and the entire marine food web. In this talk, I will draw attention to a specific class of movements associated with sharp, surface intensified density fronts. I will show that they are disproportionally important to marine life and yet not well accounted for in current anthropogenic climate change projections based on Earth System Models.

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Speaker: Mattia Gazzola, University of Illinois at Urbana-Champaign, USA

Date/Time: Friday 26th February, 2021. 4:00 pm GMT/11am EST 

Title: Harnessing viscous streaming in complex active systems: mini-bots in fluids.


Abstract: Viscous streaming is a phenomenon where an oscillating body generates stable, predictable and robust fluid flows that can be used to manipulate the body’s local surroundings. Viscous streaming has been well explored and characterized theoretically, experimentally and computationally for simple shapes such as cylinders and spheres, and leveraged in microfluidics for transport, mixing, particle separation and assembly. However, little is known beyond simple geometries, in particular when multiple curvatures are involved. Here we explore the effects of body geometric variations on topological flow responses, and connect them with potential uses in micro-robotics.

Enjoy free access to papers in support of Gazzola's webinar, courtesy of the Journal of Fluid Mechanics

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Speaker: David Quéré, ESPCI-Paris and École Polytechnique, France

Date/Time: Friday 19th February, 2021. 4:00 pm GMT/11am EST 

Title: Acrobatics of non-stick droplets


Abstract: Making drops non-wetting increases dramatically their mobility, compared to the situations we are used to. We'll describe a few consequences of this statement, with a particular focus on jumping droplets: two non-stick drops can transfer their surface energy into kinetic energy when they merge, and thus can jump from their substrate. The materials achieving such a property might be antifogging, since condensing water is likely to be evacuated without external source of energy. We’ll discuss to what extent this optimistic view is science-fiction - wearing glasses at the time of covid, we realize how non-antifogging common materials are...

Enjoy free access to papers in support of Quéré's webinar, courtesy of the Journal of Fluid Mechanics

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Speaker: Cath Noakes, University of Leeds, UK

Date/Time: Friday 12th February, 2021. 4:00 pm GMT/11am EST 

Title: Infection transmission in the built environment – the interface of biology, fluid dynamics, design and human behaviour


Abstract: COVID-19 has presented us with the most difficult healthcare and societal challenge we have faced in living memory. To understand the mechanisms of transmission we have had to rapidly collect new evidence on the SARS-CoV-2 virus ranging from laboratory data on survival and fluid dynamics studies on droplet dispersion through to epidemiological evidence from outbreaks, contract tracing and cohort studies.

Over this time we have become acutely aware of the role that the environment plays in transmission, and how our interactions in indoor spaces determine the risk of infection. But this is not the first disease to be associated with the built environment. Many other respiratory diseases have a clear association with indoor spaces including tuberculosis and influenza. Pathogens such as pseudomonas, legionella are associated with water systems, and even hospital acquired infections such as MRSA have been linked to dispersion within the environment.

This presentation sets out what we know about transmission of pathogens in the built environment particular focus on the complex interactions between engineering systems, fluid dynamics, microbiology and the behaviour of people that determine the dispersion, transport and survival of pathogens. Engineering and modelling approaches that can be used to understand mechanisms for transmission and implement mitigation strategies are discussed. The talk discusses how research findings may be used to support practice, and where there gaps in knowledge in both understanding of fundamental processes and the real performance of engineering solutions.

Click here to read Cath Noakes' recent paper published in the Journal of Fluid Mechanics

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Speaker: Amin Doostmohammadi, University of Copenhagen, Denmark

Date/Time: Friday 5th February, 2021. 4:00 pm GMT/11am EST 

Title: Engines of life: self-pumping, bio-inspired fluids


Abstract: Sperms, bacteria, and tissues, all work as engines of life converting chemical energy into motion. These systems are known as active materials and are capable of self-pumping with prominent role in biological processes, from organ formation to tumor progression. A generic property of active fluids is the spontaneous emergence of collective flows, which often leads to chaotic flow patterns characterized by swirls, jets, and topological disclinations in their orientation field. I will first discuss examples of these collective flows helping us understand biological processes. I will then discuss various strategies to tame, otherwise chaotic, active flows, showing how hydrodynamic screening of active flows can act as a robust way of controlling and guiding active particles into dynamically ordered coherent structures. I will also explain how combining hydrodynamics with topological constraints can lead to further control of exotic morphologies of active shells.

Enjoy free access to papers in support of Doostmohammadi's webinar, courtesy of the Journal of Fluid Mechanics

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Speaker: Karen Daniels, North Carolina State University, USA

Date/Time: Friday 22nd January, 2021. 4:00 pm GMT/11am EST 

Title: Fingers, fractals, and flow in liquid metals


Abstract: A droplet of pure water placed on a clean glass surface will spread axisymmetrically, and a droplet of mercury will bead up into a spherical droplet. In both cases, the droplet is minimizing its surface energy -- creating an object with a minimized surface area -- and there is nothing to break the symmetry. Remarkably, droplets of the room-temperature liquid gallium-indium (EGaIn), which like all metals have an enormous surface tension, can nonetheless undergo fingering instabilities in the presence of an oxidizing voltage. I will describe how this oxide acts like a reversible surfactant, generating fingering instabilities, tip-splitting, and even fractals, through Marangoni instabilities. Remarkably, we find that EGaIn droplets placed in an electrolyte under an applied voltage can achieve near-zero surface tension. This effect can in turn be used to suppress the Rayleigh-Plateau instability in falling streams. Quantitative control of these effects provides a new route for the development of reconfigurable electronic, electromagnetic, and optical devices that take advantage of the metallic properties of liquid metals.

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Speaker: Isabelle Cantat, Université de Rennes, France

Date/Time: Friday 29th January, 2021. 4:00 pm GMT/11am EST 

Title: Visco-elasticity of Foam Films


Abstract: Liquid foam exhibits surprisingly high viscosity, higher than each of its phases. This dissipation enhancement has been rationalized by invoking either a geometrical confinement of the shear in the liquid phase, or the influence of the interface viscosity. However, a precise localization of the dissipation, and its mechanism at the bubble scale, is still lacking. To this aim, we simultaneously monitored the evolution of the local flow velocity, film thickness and surface tension of a five films assembly, induced by different controlled deformations. These measurements allow us to build local constitutive relations for this foam elementary brick. We first show that, for our millimetric foam films, the main part of the film has a purely elastic, reversible behavior, thus ruling out the interface viscosity to explain the observed dissipation. We then highlight a generic frustration at the menisci, controlling the interface transfer between neighbor films and resulting in the localization of a bulk shear flow close to the menisci. A model accounting for surfactant transport in these small sheared regions is developed. It is in good agreement with the experiment, and demonstrate that most of the dissipation is localized in these domains. The length of these sheared regions, determined by the physico-chemical properties of the solution, sets a transition between a large bubble regime in which the films are mainly stretched and compressed, and a small bubble regime in which they are sheared. Finally, we discuss the parameter range where a model of foam viscosity could be built on the basis of these local results.

Enjoy free access to papers in support of Cantat's webinar, courtesy of the Journal of Fluid Mechanics.