Stratified mixing in mean shear free homogeneous isotropic turbulence: An experimental study

Stratified turbulent flow has an important role in the dynamics of aquatic and atmospheric environments. The interaction of turbulence with a buoyancy gradient is complicated and has been studied for several decades, but there remain unknowns in quantifying the primary drivers of interfacial dynamics and mass transport. We perform experiments to investigate mixing at a stable density interface with a sharp gradient in which the upper layer is turbulent and the bottom dense layer is quiescent. We generate homogeneous isotropic turbulence with negligible mean shear by randomly actuating an array of synthetic jets located at the top of a tank. We use particle image velocimetry to acquire the flow velocity, in order to compute turbulence statistics such as the integral length scale, turbulent kinetic energy, and dissipation. Simultaneously, we use laser induced fluorescence to measure the fluid density field, and to determine the location and thickness of the density interface. We also quantify the mass transport across the interface, and identify various types of instabilities and entertainment events. By changing the Richardson number, the turbulent Reynolds number, and the Prandtl number, we determine under what conditions different mixing rates and interfacial dynamics occur.