“The problematic of energy transfer mechanisms and dissipation in stratified turbulence has still many open questions”
by Dr. Eletta Negretti
(Laboratoire d’hydrodynamique de l’Ecole Polytechnique, France)
Friday November 12, 2010, at 14:00 – Room ME B 10
Abstract :
Herein one important issue is to know if turbulent energy transfer from large vortices, typically with a “pancake” shape, may occur directly to the small dissipative scales via small scale instabilities, as for example the gravitational and the shear instability. In order to adress this question, the linear stability of an isolated pancake vortex in a stably stratified fluid is investigated. The base state is assumed to be in cyclostrophic and hydrostatic balance. We show that the vortex becomes unstable when the aspect ratio is below a critical value which scales with the Froude number, and that the dominant instability is gravitational. The numerical results agree well with the gravitational instability theory. We generalize the results to any vertical distribution of the angular velocity and almost any profile of the vortex.
Some results relative to experiments in stratified turbulence will be also presented. PIV measurements in vertical cross-sections show that the flow organises itself into thin horizontal layers. Horizontal velocity spectra obtained by PIV measurements in horizontal cross-sections exhibits a narrow inertial domain in agreement with recent DNS of forced stratified turbulence.