An experimental study of stratified mixing in mean shear free homogeneous isotropic turbulence

Stratified turbulent flow has an important role in oceanic and atmospheric circulations. The interaction of turbulence with a density gradient has been studied for several decades, but there remain unknowns in quantifying the primary drivers of interfacial mass transport. We have designed an experimental study to investigate mixing at a stable density interface with a sharp gradient in which the upper layer is stirred continuously with turbulent forcing. A randomly actuated synthetic jet array is located at the top of the water tank to generate homogeneous isotropic turbulence with negligible mean shear in the upper layer, above a quiescent lower layer of sugar water. We use particle image velocimetry and laser induced fluorescence to simultaneously measure the spatio-temporal velocity and density fields, respectively. From velocity measurements, we quantify turbulence statistics such as integral length scale, turbulent kinetic energy, and dissipation. From LIF data, we investigate factors affecting the entrainment rate, generation of internal waves, and mechanisms that drive interfacial mass flux. By changing the Richardson number, the turbulent Reynolds number, and the Prandtl number, we can determine under what conditions different mixing rates and interfacial dynamics occur.