TURBULENCE EVOLUTION AND MIXING IN A 2-LAYER STABLY STRATIFIED FLUID

The results of an experimental study of shear-free decaying grid-gener ated turbulence on both sides of a sharp interface between two homogen eous layers of different densities are presented. The evolution of tur bulence and mixing were examined by simultaneously mapping the velocit y (u, w) and density fields (p) and the vertical mass flux F(=($) over bar p ($) over bar w/p'w') together with flow visualization in a low- noise water tunnel. Buoyancy was induced by salinity differences so th e value of the Schmidt number S-c = 700. Density stratification altere d the inertial-buoyancy force balance (most simply expressed by Nt, th e product of the buoyancy frequency N and turbulent timescale t) so as to attenuate turbulent velocity fluctuations, vertical motions and in terfacial 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 th e u'-distribution can be non-monotonic. The maximum value of the mass flux, IF, was found to be about 0.5 which was less than the typical va lue 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-gradien t and counter-gradient transport with the ratio varying with Nt (e.g. at Nt approximate to 5, the flux is counter-gradient). The flux Richar dson number R(f) was found to increase monotonically to values of appr oximately 0.05.