Stably stratified shear-produced turbulence and large-scale waves in a lid driven cavity

We study experimentally stably stratified sheared turbulence and large-scale flows and waves in a lid driven cavity with a non-zero vertical mean temperature gradient. Geometrical properties of the large-scale vortex (e.g., its size and form) and the level of small-scale turbulence inside the vortex are controlled by the buoyancy (i.e., by the temperature stratification). The observed velocity fluctuations are produced by the shear of the large-scale vortex. At larger stratification obtained in our experiments, the strong turbulence region is located at the upper part of the cavity where the large-scale vortex exists. In this region the Brunt-Väisälä frequency is small and increases in the direction outside the large-scale vortex. This is the reason of that the large-scale internal gravity waves are observed in the regions outside the large-scale vortex. We found these waves by analyzing the non-instantaneous correlation functions of the temperature and velocity fields. The observed large-scale waves are nonlinear because the frequency of the waves determined from the temperature field measurements is two times smaller than that obtained from the velocity field measurements. The measured intensity of the waves is of the order of the level of the temperature turbulent fluctuations.