Tutorial Talk
Speakers: Nicole Sharp (owner of @FYFD, Sharp Science Communication Consulting LLC) and Tom Crawford (owner of @TomRocksMaths, JFM Social Media Editor, Lecturer at University of Oxford)
Date/Time: Friday 4th December 2020, 4:00pm GMT/ 11am EST
Title: Promoting your Research in the Media (Social or otherwise)
This was a live event only, with no video recording available.
Abstract: Join Dr Nicole Sharp (owner of FYFD) and Dr Tom Crawford (JFM Social Media Editor and owner of Tom Rocks Maths) as they explain how to tell your science story. Drawing on their combined 15 years of experience in science communication, Tom will cover what a journalist looks for in a news story, whilst Nicole will teach you how to prepare for any media interest you receive. Together, they will also outline how to use social media to improve the reach of your work with relevant examples from Tom Rocks Maths and FYFD. In the 21st century, clever promotion of your work online can be just as important for career progression as publications, and in this talk we will show you how!
UK Fluid Networks Thesis Competition Winners
Speaker: Hannah Kreczak, University of Newcastle, UK
Date/Time: Friday 27th November, 2020. 4:00 pm GMT/11 am EST
Title: Rates of mixing in models of fluid devices with discontinuities
Video: cambridge.org/fluidwebinar/kreczak
Abstract: The dynamics of mixing by cutting and shuffling are subtle and not well understood. We present mixing dynamics arising from fundamental models capturing the essence of discontinuous stirring with diffusion. When the stirring is governed by purely cutting and shuffling, we reveal that the time to achieve a mixed condition varies polynomially with diffusivity rate with an exponent less than unity. In stirring fields which are chaotic we observe that the addition of discontinuous transformations contaminates mixing. Long-time mixing rates behave counter-intuitively when varying the diffusivity rate and a deceleration of mixing with increasing diffusion coefficient is observed, sometimes overshooting analytically derived bounds.
You can access Kreczak's thesis here.
Speaker: Peter Baddoo, Imperial College London, UK
Date/Time: Friday 27th November, 2020. 4:20 pm GMT/11:20 am EST
Title: Analytic solutions for flows through cascades
Video: cambridge.org/fluidwebinar/baddoo
Abstract: Motivated by turbomachinery aeroacoustics, we investigate the flow through a periodic array of disconnected objects, termed a “cascade”. We deploy and develop techniques from complex analysis to elucidate the mathematical structures and physical mechanisms relevant to cascade flows. Our approach considers both aerodynamic and aeroacoustic aspects but we focus efforts toward developing conformal mapping techniques amenable to cascades of complicated geometry and topology. The foundation of practical conformal mapping is the Schwarz–Christoffel (SC) formula; we extend the SC formula to periodic domains which enables analytic constructions of the periodic flow field. By utilising tools from function theory, our solutions are valid for an arbitrary number of obstacles per period window.We also present algorithmic advances that enable rapid and accurate calculations of our mappings. Our theory is applied to classical idealised problems such as steady potential flows, unsteady vortex dynamics, and free-streamline flows. These low-order techniques can be used in turbomachinery design schemes or for real-time flow control with data-driven methods.
You can access Baddoo's thesis here.
Speaker: James Steer, University College Dublin, Ireland
Date/Time: Friday 27th November, 2020. 4:40 pm GMT/11:40 am EST
Title: X-Waves and Modulation Instability
Video: cambridge.org/fluidwebinar/steer
Abstract: Modulation instability is characterised by a rapid transfer of energy from a dominant central frequency to sideband frequencies. Dispersive media such as water, optics, and Bose-Einstein condensate, all allow for modulation instability. In the space-time domain, modulation instability manifests itself as a single extreme wave crest arising from an initially periodic, constant amplitude wavetrain. Given the sudden appearance of an unexpectedly high wave crest, the modulation instability has long been proposed as a mechanism by which extreme ocean waves are formed. In this talk I will present the experimental work performed with various collaborators in both the University of Edinburgh’s circular basin (FloWave) and University College London’s combined wave-current flume. In the FloWave basin we observed the propagation of a nondispersive X-wave,a wave structure capable of transporting an extremely large wave at its centre over vast distances without dispersing. The X-wave was predicted using the three-dimensional nonlinear Schrödinger equation (NLSE) to balance nonlinear, dispersive, and diffractive wave properties. At University College London we showed experimentally how a linear vertically sheared current reduces the growth rate of modulation instability as predicted by a modified version of the NLSE, the constant vorticity NLSE (vor-NLSE).At University College London we showed experimentally how a linear vertically sheared current reduces the growth rate of modulation instability as predicted by a modified version of the NLSE, the constant vorticity NLSE (vor-NLSE).At University College London we showed experimentally how a linear vertically sheared current reduces the growth rate of modulation instability as predicted by a modified version of the NLSE, the constant vorticity NLSE (vor-NLSE).
You can access Steer's thesis here.
Speaker: Eva Kanso, University of Southern California, USA
Date/Time: Friday 20th November, 2020. 4:00 pm GMT/11 am EST
Title: Ciliary Coordination
Video: cambridge.org/fluidwebinar/kanso
Abstract: Motile cilia covering the surface of eukaryotic cells, from single-celled protozoa to epithelial surfaces in the mammalian airways, synchronize their beat to enable efficient fluid transport. The nature of the mechanisms leading to ciliary coordination remains unclear. I will review our understanding of this process based on physics-based mathematical models where either hydrodynamic or elastic forces between neighboring cilia are sufficient to lead to synchronization. Further, I will demonstrate that it is possible to reach, and transition between, multiple modes of synchrony by varying the intrinsic activity of the individual cilia and the strength of the coupling between them. I will conclude by commenting on the implications of these findings to our understanding of ciliary design and function in mammalian tissues as well as in unicellular organisms where transitions between distinct synchronization modes are crucial for the cell behavior and survival.
Speaker: Steven Balbus, University of Oxford, UK
Date/Time: Friday 13th November, 2020. 4:00 pm GMT/11 am EST
Title: Time-dependent accretion discs around Kerr black holes and tidal disruption events
Video: cambridge.org/fluidwebinar/balbus
Abstract: The theory of Newtonian astrophysical accretion discs, both in its steady and time-dependent form, dates from the mid-1970s. Relativistic thin disc theory was developed at nearly the same time, but only for steady-state conditions. I will present recent work on the fluid dynamics of time-dependent accretion discs around Kerr black holes. This work has a natural application to so-called tidal disruption events (TDEs), the tidal break-up of star when it passes too close to a galaxy's central black hole. The stellar debris collects into a disc, whose inner boundary corresponds to a radius at which the Rayleigh criterion for rotational stability is violated according to general relativity. This requires some care in its treatment, particularly with regard to the inner stress, since both the interpretation of astronomical observations and numerical simulations depend upon a detailed understanding of this innermost stable circular orbit. TDEs generally exhibit a slowly decreasing light curve that is not at present well-understood. It is, however, quite consistent with an extended evolutionary phase of time-dependent relativistic disc theory. The interplay between mathematics, fluid theory, and astrophysics is particularly striking for these objects.
Speaker: Dennice Gayme, Johns Hopkins University, USA
Date/Time: Friday 6th November, 2020. 4:00 pm GMT/11 am EST
Title: Wind farm modeling and control for power grid support
This was a live event only, with no video recording available.
Abstract: Traditional wind farm modeling and control strategies focus 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 power grid 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). 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.
Speaker: Outi Supponen, ETH Zurich, Switzerland
Date/Time: Friday 30th October, 2020. 4:00 pm GMT/11 am EST
Title: The curious dynamics of bubble oscillations and collapse
Video: cambridge.org/fluidwebinar/supponen
Abstract: Bubbles oscillate under the effect of pressure fluctuations, such as those produced by sound waves. When driven into a violent collapse, they can yield strong sound emissions, high-speed jets, and extreme heating - a behaviour known as cavitation. In this lecture, we will present ongoing research efforts to reach a fundamental understanding of the intriguing dynamics of bubbles and the resulting flows, combining ultra-high-speed experiments with theory. The broad aim of this research lies in the quest for harnessing their power for a variety of engineering applications, including ultrasound diagnostics, drug delivery, sonochemistry, surface cleaning and micro-fluidics.
Speaker: Anke Lindner, PMMH-ESPCI, Paris, France
Date/Time: Friday 23rd October, 2020. 4:00 pm BST/11 am EDT
Title: Steering microscopic particles in viscous flows via shape and deformability
Video: cambridge.org/fluidwebinar/lindner
Abstract: Understanding and controlling the transport of microscopic particles in viscous flows stems from the fundamental question of fluid-structure interactions but has also important implications for separation processes or bacterial contamination. Using recent microfabrication techniques, we produce a variety of microscopic particles and control precisely their shape and material properties. Investigating the transport dynamics of these particles in representative microfluidic flows we demonstrate how shape, mechanical properties or even activity govern particle trajectories. Combining our experimental findings with numerical and theoretical modeling performed by our collaborators we elucidate in detail the role of particle symmetry, chirality or deformability.
Speaker: Morteza Gharib, California Institute of Technology, USA
Date/Time: Friday 16th October, 2020. 4:00 pm BST/11 am EDT
Title: Toroidal plasmoid generation via extreme hydrodynamic shear
Video: cambridge.org/fluidwebinar/gharib
Abstract: Saint Elmo's fire and lightning are two known forms of naturally occurring atmospheric pressure plasmas. As a technology, non-thermal plasmas are induced from artificially created elecromagnetic or elecrostatic fields. Here we report the observation of arguably a unique case of a naturally formed such plasma created in the air at room temperature without external electromagnetic action, by impinging a high-speed microjet of deionized water on a dielectric solid surface. We demonstrate that tribo-electrification from extreme and focused hydrodynamic shear is the driving mechanism for the generation of energetic free electrons. Air ionization results in a plasma that, unlike the general family, is topologically well defined in the form of a coherent toroidal structure. Possibly confined through its self-induced electromagnetic field, this plasmoid is shown to emit strong luminescence and discrete frequency radio waves. Our experimental study suggests the discovery of a unique platform to support experimentation in low-temperature plasma science.
Speaker: François Gallaire, EPFL, Switzerland
Date/Time: Friday 9th October, 2020. 4:00 pm BST/11 am EDT
Title: Dripping down the rivulet
Video: cambridge.org/fluidwebinar/gallaire
Abstract: We revisit the canonical Rayleigh-Taylor instability and investigate the case of a thin film of liquid continuously flowing down the underside of an inclined plane. The presence of a natural flow along the plane competes with the conventional droplet forming instability and gives rise to an intricate pattern formation, which is studied experimentally, numerically and theoretically using the lubrication equation. We observe the prevalence of streamwise-invariant structures, modulated along the transverse direction perpendicular to the flow direction, called rivulets. We determine the universal thickness profile towards which they saturate and explore the origin of this pattern selection by analyzing the impulse response as well as the response to localized defect on the substrate. We eventually discuss the role of these hydrodynamic instabilities on the morphogenesis of some typical karst draperies structures encountered in limestone caves.
Speaker: Basile Gallet, CEA, Université Paris-Saclay, France
Date/Time: Friday 2nd October, 2020. 4:00 pm BST/11 am EDT
Title: A scaling theory for meridional heat transport in oceans and planetary atmospheres
Video: cambridge.org/fluidwebinar/gallet
Abstract: Developing a theory of climate requires an accurate parameterization of the transport induced by turbulent eddies. A major source of turbulence in the mid-latitude planetary atmospheres and oceans is the baroclinic instability of the large-scale flows. I will introduce idealized models of planetary atmospheres and oceanic currents, before presenting a physically based scaling theory that quantitatively predicts the turbulent diffusivity, eddy kinetic energy and mixing length of baroclinic turbulence as a function of the large-scale flow characteristics, the bottom friction and the curvature of the planet (through beta). I will then use the theory as a quantitative parameterization in the case of meridionally dependent forcing, in the fully turbulent regime. Beyond its relevance for climate theories, this work is an intriguing example of a successful closure for a fully turbulent flow.
Speaker: Lydia Bourouiba, MIT, USA
Date/Time: Friday 19th June, 2020. 4:00pm BST/11am EDT
Title: The Fluid Dynamics of Disease Transmission
This was a live event only, with no video recording available.
Abstract: The fundamental mechanisms governing infectious disease transmission and contamination by most pathogens remain poorly understood. Fluid processes and physical laws at various scales combined with biological processes are key in filling this gap. We will discuss how fluids and their dynamics are critical in shaping pathogen transport. We will present an overview of our approach, combining theory and experiments, to elucidate droplet formation and transport in the context of contamination in a range of systems.
Speaker: Dwight Barkley, University of Warwick, UK
Date/Time: Friday 12th June, 2020. 4:00pm BST/11am EDT
Title: Mechanisms and Universality in the Subcritical Route to Turbulence
Video: cambridge.org/fluidwebinar/barkley
Abstract: Recent years have witnessed a profound change in our understanding of the route to turbulence in wall-bounded shear flows. In stark contrast to the classical Hopf-Landau picture where turbulence arises through an increase in the temporal complexity of fluid motion, the route to turbulence in subcritical shear flows occurs via spatio-temporal intermittency and falls in the class of non-equilibrium statistical phase transitions known as directed percolation. In this talk I will focus on two aspects of the transition problem. The first is physical mechanisms underlying spatio-temporal intermittency in shear flows. The second is the universality in the subcritical route to turbulence.
Enjoy free access to papers in support of Dwight Barkley's webinar, courtesy of the Journal of Fluid Mechanics.
Speaker: Petros Koumoutsakos, ETH-Zurich, Switzerland
Date/Time: Friday 5th June, 2020. 4:00pm BST/11am EDT
Title: Machine Learning for Fluid Mechanics
Video: cambridge.org/fluidwebinar/koumoutsakos
Abstract: Recent advances in machine learning and an ever increasing availability of data offer new perspectives (and hope) for solving long standing fluid mechanics problems. Despite early connections dating back to Kolmogorov, the link between Fluid Mechanics and Machine Learning (ML) has not been fully explored. The situation is rapidly changing with ML algorithms entering in numerous efforts for modeling, optimising, and controlling fluid flows. In this talk I will present works from our group on the interface of Fluid Mechanics and ML ranging from low order models for turbulent flows to deep reinforcement learning algorithms and Bayesian experimental design for collective swimming. I hope to demonstrate that ML has the potential to augment, and possibly even transform, current lines of fluid mechanics research.
Enjoy free access to papers in support of Petros Koumoutsakos' webinar, courtesy of the Journal of Fluid Mechanics.
Speaker: Baylor Fox-Kemper, Brown University, USA
Date/Time: Friday 29th May, 2020. 4:00pm BST/11am EDT
Title: Affronting Ocean Models: Submesoscale Interactions between Fronts, Instabilities, and Waves
Video: cambridge.org/fluidwebinar/fox-kemper
Abstract: Ocean fronts - sharp horizontal gradients in temperature, salinity, and density - are a key feature of the upper ocean that affect the transport of pollutants and the nature of near surface flows. I will highlight some of the recent modeling and theoretical work our group and collaborators have taken on to understand how fronts, frontal instabilities and turbulence, and surface waves interact. Traditional geophysical boundary layer theory neglects horizontal variations, and so is unable to capture frontal dynamics. Some consequences of these features found in large scale modeling and observations of oil, plastics and biological tracer dispersion; boundary layers; fluid energy cycling and dissipation statistics; and finally climate sensitivity will be elucidated.
Speaker: Bérengère Dubrulle, SPEC, Université Paris-Saclay, CNRS, France
Date/Time: Friday 22nd May, 2020. 4:00pm BST/11am EDT
Title: On the nature of turbulent motions at small scale
Video: cambridge.org/fluidwebinar/dubrulle
Abstract: In 1949, Batchelor and Townsend speculated about the nature of small-scale turbulent motions on the basis of hot wire velocity measurements in the Cavendish wind tunnel. Their main conclusion was that the energy associated with small scales is intermittent in space and time and organised into strong discrete vortices. Since then, progress in computer power and image velocimetry has made it possible to investigate in more detail the nature and the properties of small scale turbulent motions, at scales of the order of or below the Kolmogorov scale.
For example, it is now well established that regions where the vorticity supersedes the strain (the so-called Q criterion) are indeed organised into small scale elongated coherent structures that may interact and reconnect iteratively, following a self-similar vortex reconnection cascade. Whether such process results in a near finite time singularity is currently an active subject of research, as such a quasi singularity may be linked with the observed constancy of the non-dimensional energy dissipation at large Reynolds number. If we take for granted that the small-scale structure of turbulent motions is very irregular, then specific tools must be built to analyse them. In this talk, I introduce and compare two scalar fields that encode the regularity properties of the small-scale motions: i) a pseudo-Holder exponent and ii) a local energy transfer. Finally, I show on numerical simulation and experimental data how these fields can be used to infer interesting information about the small-scale dynamics.
Speaker: John W. M. Bush, MIT, USA
Date/Time: Friday 15th May, 2020. 4:00pm BST/11am EDT
Title: Pilot-wave hydrodynamics, hydrodynamic quantum analogs, and hydrodynamic quantum field theory
Video: cambridge.org/fluidwebinar/bush
Abstract: In 2005, Yves Couder and Emmanuel Fort discovered that droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own wave field, and so represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results allow us to rationalize the emergence of quantum-like behavior in this hydrodynamic pilot-wave system in a number of settings, and explore its potential and limitations as a quantum analog. A new, trajectory-based description of quantumdynamics, informed by the hydrodynamic system, is proposed and explored.
This collection of papers from John Bush are free to read from Journal of Fluid Mechanics.
Speaker: Christophe Clanet, LadHyX, CNRS, École Polytechnique, Palaiseau, France
Date/Time: Friday 1st May, 2020. 4:00pm BST/11am EDT
Title: The physics of road and track cycling
Image: Lucien Jonas, Petit-Breton, étude pour The final rush, 1905, Musée de la Piscine, Roubaix
Video: cambridge.org/fluidwebinar/clanet
Abstract: Even if the first bicycle was invented in Germany in 1817 by Karl von Drais, the Physics of cycling probably started in 1869 with the work of the Scottish mechanical engineer W.J.M. Rankine entitled "On the dynamical principles of the motion of velocipedes". Among the questions which have been addressed in the subject, stability is probably the most debated. First addressed in 1890 by J. Boussinesq ("Aperçu sur la théorie de la bicyclette") and F. Klein and A. Sommerfeld ("Stabilitat des Fahrrads") research continued with tens of contributions up to 2011 (see "Historical Review of Thoughts on Bicycle Self-Stability" by Meijaard, Papadopoulos, Ruina and Schwab). The questions discussed during this webinar seminar won't however be connected to stability but related to races:
For road cycling we will wonder why three jerseys? Tour de France, Giro d’Italia and the Vuelta in Spain are the three Grand Tours of professional road cycling. Three weeks long with daily stages, these three races all use three jerseys to distinguish the leader, the best sprinter and the best climber. We will first discuss the physics of road cycling and show that these three jerseys are respectively associated with three different dynamical regimes. We will then propose a phase diagram for road cycling which enables the discussion of the different physiological characteristics observed in the peloton.
For track cycling we will wonder why Team Great Britain is so strong? Analysing the Individual pursuit of Graham Obree World Title in 1993 will be our starting point. We will then move to the qualifying 200m of Jason Kenny and finish with team pursuit. The main point will be to discuss why and how the fixed gear condition of track cycling changes the law of races.