The term 'gravity current' is used to describe a variety of flows in which the effect of gravity on density variations in a fluid generates a horizontally spreading or propagating flow. Such currents are common in the atmosphere and oceans, where they form high-Reynolds-number turbulent flows which can be many kilometres in length but only a few metres high. This separation of scales means that depth-integrated shallow-water models provide an effective 'first-order' model of gravity currents. I will talk about some of the consequences of reducing these complex flows to shallow-water systems, and discuss one aspect of gravity currents which has received comparatively little modelling attention: the entrainment, or mixing, of ambient fluid into a current. I'll present a simple way in which entrainment can be added to a shallow-water gravity current model, and the sometimes surprising effects that this has on predictions of current behaviour.
Geological storage of carbon dioxide (CO2) in deep saline aquifers forms an integral part of many CO2 mitigation strategies. At depth CO2 is buoyant and therefore may leak into surface waters or the atmosphere. This talk will explore the rich multiphase fluid dynamics of CO2 propagation, providing constraints on the rate at which CO2 may leak from a storage site, and the dynamics of convective dissolution and capillary trapping, two leading processes by which CO2 may be stably trapped and stored within the subsurface.
We describe a novel phase transition in a non-cohesive granular gas subject to vertical vibration between two horizontal plates. We find a high-density, low temperature granular-liquid, coexisting with a low-density, high temperature granular-gas moving coherently. The mechanism responsible for the transition is traced to the existence of a negative-pressure-gradient spinodal. The dynamics of the phase separation and the existence of a non-equilibrium surface tension are also investigated.
In this talk, I will first present how confined droplets flowing in microchannels can be guided and trapped by creating external surface energy gradients, either by using a focussed laser beam or by introducing depth variations. I will also present counterintuitive experiments, conducted in presence of surfactants, where thermocapillary and solutocapillary effects enter in competition, and where three-dimensional flow reversal is observed. I will then propose a two-dimensional depth-averaged model that both allows for the simulation of the droplet transport and deformation by a carrier flow and channel geometry and for Marangoni effects to be taken into account. The model will be validated against three-dimensional solutions of the creeping flow equations and then numerically implemented by a boundary element method. This work is done in collaboration with M. Nagel, R. Dangla, S. Lee and C. Baroud.
Motivated by simple biological experiments on active gels, DNA, plant cell walls, and fungi, I will consider the sometimes puzzling and often counter-intuitive behaviour of isotropic and anisotropic bio-elastic materials under large deformations.
Mankind has been airborne for over one hundred years and advances in aeronautics during that time have been immense. Yet despite our wealth of knowledge in fundamental and applied aerodynamics we are still unable to recreate the performance exhibited by flying animals. Needless to say, they have had quite some head start over us. In the 350 million years since insects first took flight, Natural Selection has diversified a common ancestor that most resembled a modern day dragonfly into countless species and the rich assortment of shapes and sizes we see today - each one locally optimized and tuned for the tasks that define its ecological niche. Familiar trade-offs between, say, stability and manoeuvrability can be found, but in this small-scale, unsteady world, aerodynamic mechanisms we deem unconventional are, in fact, commonplace and traditional aircraft design practices are often rejected. I will highlight some of the peculiarities found in insect flight, illustrating these examples with case studies and describing the experiments that we use to reveal the mechanisms. Finally I will ask if we can learn anything that could be incorporated into the manned or unmanned air vehicles of the future.
In this work we are interested in the susceptibility of flexible structures in an axial flow to flutter instabilities. We consider the inverse point of view and examine the theoretical scope for harvesting energy from slender structures in flutter. We develop a numerical model for the non- linear system by coupling the inextensible elastic Euler-Bernoulli beam model with Lighthill???s large-amplitude elongated body theory and Taylor???s resistive force theory. Energy harvesters are modelled as strain-based damping, and are introduced as non-uniform distributions along the beam. The numerical model uses a spectral method in space and second-order implicit method in time to determine the beam response (for given flow-speed and beam geometry). In seeking insights into the optimal harvester distribution that maximises the power, we use select families of distribution functions. We show that certain non-uniform distributions are superior in har- vesting power in comparison to an equivalent constant distribution. This enhancement in power is linked to the relationship between the beam dynamic response, the localised distribution of damping and the damping coefficient itself, and maybe understood by considering the evolution of beam curvature with damping. Time permitting we consider the role of the destabilising effect of damping in structures with high fluid loading, and we explore the results in the context of some early work on fluid-structure interactions. This work was carried out with Sebastien Michelin and Emmanuel de Langre at LadHyX (Laboratoire d'Hydrodynamique, Ecole Polytechnique, Palaiseau, France).
Refreshments will be provided in the basement social area of the Schuster Lab. from 3.15pm. If you have any suggestions for future speakers, please contact Dr Draga Pihler-Puzovic.