Turbulent stratified interfaces

In a buoyancy reversing system, a stratified interface between two fluids is known to be unstable to strong perturbations only for relatively large values of a buoyancy reversal parameter. This instability leads to accelerating entrainment, i.e., an entrainment process in which both the size of the entraining eddy and the entrainment rate increase rapidly with time. The relevant parameter is defined as the maximum density increase due to mixing of the two fluids normalized by their density difference before mixing. Stirring grid turbulence with zero mean shear in a water tank has been used to further investigate the interaction phenomena between the buoyancy reversal, the stratification, and the type of disturbance in a two-layer buoyancy reversing system. It is found that, even for relatively large buoyancy reversal parameter, the interface remains stable at large Richardson number. In order to explore independently the role of Reynolds number in stratified flows, a second experiment was carried out in which an aqueous turbulent jet impinged on a stably stratified interface (without buoyancy reversal). Laser-induced fluorescence of a pH indicator revealed the region of mixed fluid. A mixing transition was detected for the stratified interface, analogous to that of nonstratified free-shear flows. This seems to explain why the disturbance (with buoyancy reversal) at the stratified interface must be fully turbulent before it triggers the mixing-induced buoyancy reversal instability. A simple model of the interface has three requirements for buoyancy reversal instability: the mixed fluid must be sufficiently dense, the vortices must complete a rotation, and they must be above the mixing transition condition if the fluids are liquids. This implies certain limits on the buoyancy reversal parameter and the Richardson, Reynolds, and Schmidt numbers.