Numerical simulation of a stratified two-dimensional turbulent flow around an obstacle

Large Eddy Simulation (L.E.S.) approach of Navier-Stokes equations within the Boussinesq approximation are carried out in order to study in two dimensional and time dependent, the turbulent flow around an obstacle. The relative dynamic study to the formation of coherent structures provides informations about the pace of the flow and specify its principal features (shear layer, recirculation and reattachment). In presence of temperature discontinuity (stable stratification), the flow in downstream of the obstacle is unstable. Moreover, we note the presence of pairing process thin of Kelvin-Helmholtz instabilities. In fact. we have shown that forcing injected by recirculation fluctuations governs this instabilities development. We point out that with increasing stratification the vortex pairing is inhibited, as well as the shear layer growth, and the development of gravity waves. A particular interest has been reserved to the development of baroclinic instability. This important mechanism, coming from the horizontal density gradients, insures a homogenization of the temperature field and constitutes an additional part of the turbulent mixing process. Eddy structures of the flow present striking analogies with the stratified mixing process. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.