J. zyxwvuts Fluid Mech. (1995), vol. 285, pp. 41-67 Copyright zyxwvutsrq 0 1995 Cambridge University Press zyxwvut 41 Turbulence evolution and mixing in a two-layer stably stratified fluid By PABLO HUQ’ AND REX E. BRITTER’ l College of Marine Studies, University of Delaware, Newark, Delaware 19716, USA Department of Engineering, University of Cambridge, Cambridge CB2 IPZ, UK (Received 25 October 1989 and in revised form 18 May 1994) 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 zyx @) and the vertical mass flux zyxwvutsr F(= pW/p’w’) together with flow visualization in a low-noise water tunnel. Buoyancy was induced by salinity differences so the value of the Schmidt number zy S, = 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, = 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 z 5, the flux is counter-gradient). The flux Richardson number Rf was found to increase monotonically to values of approximately 0.05. 1. Introduction Increased levels of pollution globally have been accompanied by growing public concern about the quality of the environment. Therefore there is a need for quantitative assessment of the hazards associated with the release of effluents. Though the estimation of concentration levels, and hence mixing, is a significant component in the evolution of hazards, there is a lack of understanding of turbulent mixing processes, particularly those in stratified environments. Density stratification (e.g. arising from variations of temperature and salinity in lakes, estuaries and oceans, or from temperature and humidity in the atmosphere) can influence mixing profoundly; for example, strong stable stratification inhibits vertical motions and retards mixing. Stable stratification influences the turbulent field because energy is depleted by work against gravity (Turner 1973). Thus, the problem of turbulence and mixing in a density- stratified fluid is complex : it is dependent on the interaction of two dynamic scales, one due to mechanical turbulence and the other from the buoyancy of the density field. Previous efforts to understand mixing processes in density-stratified fluids have focused on the evolution of decaying grid-generated turbulence in a linearly stratified fluid column. To study this configuration, experiments were conducted in water tunnels