The autumn edition of the Fluid Mechanics Webinar Series will take place over ten weeks between 2nd October and 4th December 2020. Registration will remain open, and please note that if you have already registered you need not register again.
Video recordings of past webinars will be made available soon. Watch this space!
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
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.
Speaker: Petros Koumoutsakos, ETH-Zurich, Switzerland
Date/Time: Friday 5th June, 2020. 4:00pm BST/11am EDT
Title: Machine Learning for Fluid Mechanics
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.
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
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
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
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.
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
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.