Turbulence evolution and mixing in a two-layer stably stratified fluid

The results of an experimental study of shear-free decaying grid-generated turbulence on both sides of a sharp interface between two homogeneous layers of different densities are presented. The evolution of turbulence and mixing were examined by simultaneously mapping the velocity (u, w) and density fields (ρ) and the vertical mass flux $F(=\overline{\rho w}/\rho\prime w\prime)$ together with flow visualization in a low-noise water tunnel. Buoyancy was induced by salinity differences so the value of the Schmidt number S_c = 700. Density stratification altered the inertial-buoyancy force balance (most simply expressed by Nt, the product of the buoyancy frequency N and turbulent timescale t) so as to attenuate turbulent velocity fluctuations, vertical motions and interfacial convolutions, normalized density fluctuations, vertical flux mass, and mean interfacial thickness. Vertical velocity fluctuations w′ were found to increase with distance from the interface, whereas the u′-distribution can be non-monotonic. The maximum value of the mass flux, F, was found to be about 0.5 which was less than the typical value of 0.7 for thermally stratified wind tunnel experiments for which S_c = 0.7. The vertical mass flux can be a combination of down-gradient and counter-gradient transport with the ratio varying with Nt (e.g. at Nt ≈ 5, the flux is counter-gradient). The flux Richardson number R_f was found to increase monotonically to values of approximately 0.05.