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.