A STUDY OF STREAMWISE VORTEX STRUCTURE IN A STRATIFIED SHEAR-LAYER

The existence of an organized streamwise vortical structure, which is superimposed on the well known coherent spanwise vorticity in nominall y two-dimensional free shear layers, has been studied extensively. In the presence of stratification, however, buoyancy forces contribute to an additional mechanism for the generation of streamwise vorticity. A s the spanwise vorticity layer rolls up and pulls high-density fluid a bove low-density fluid, a local instability results. The purpose of th e current investigation is to force the three-dimensional instability in the stratified shear layer. In this manner, we experimentally obser ve the effect of buoyancy on the streamwise vortex tube evolution, the evolution of the buoyancy-induced instability, and the interaction be tween these two vortical structures. A simple numerical model is propo sed which captures the relevant physics of the flow evolution. It is f ound that, depending on the location, streamwise vortices resulting fr om vortex stretching may be weakened or enhanced by the stratification . Buoyancy-induced vortex structures are shown to form where the unsta ble part of the interface is tilted by the streamwise vortex tubes. Th ese vortices strengthen initially, then weaken downstream, the timesca le for this process depending upon the degree of stratification. For i nitial Richardson numbers larger than about 0.03, the baroclinically w eakened vortex tubes eventually disappear as the flow evolves downstre am and the baroclinically generated vortices dominate the three-dimens ional flow structure.