Orateur
Description
Abstract: (Vertically) stratified shear flows, where both the background horizontal velocity and buoyancy distribution vary in the vertical (i.e. the direction parallel to gravity) are ubiquitous in geophysical fluid dynamics. A key question is how such flows undergo the transition to turbulence and hence irreversibly mix vigorously. Intuitively, if the buoyancy increases upwards (anti-parallel to gravity), i.e. the fluid is `statically stable’ relative to convection, there should be a competition between the apparently stabilising effect of the buoyancy force and the destabilising effect of shear, quantified classically in terms of a Richardson number, a coupling parameter between the buoyancy and velocity fields. However, transition in stratified shear flows has proved to be significantly more subtle. Behaviour depends on the flow’s Reynolds number and Prandtl number, and indeed the turbulence near transition can be qualitatively different in stratified flows and unstratified flows.