Turbulent gravity-stratified shear flows

Abstract

Two simple turbulent shear flows, namely a surface jet and a mixing layer, under the influence of stable gravity stratification, were investigated experimentally. The shear flows were generated in the laboratory by letting fresh water flow over saline water in a two-dimensional channel. Velocity and salinity measurements were made using a hot-film probe and a single-electrode conductivity probe. The experimental results for the two flows were correlated each using a different set of length and velocity scales. The initial development of the flows was relatively unaffected by the stable stratification. As the shear flows grew in thickness, they were observed to have a tendency to approach a ‘neutrally stable state’ in which the turbulent motion neither extracted energy from nor lost energy to the mean flow. The gradient Richardson number in this neutrally stable state was found to have the critical value predicted by linear inviscid stability theory. The decay of turbulent intensity in the longitudinal direction was observed to follow a power-law relationship similar to the one obtained by Comte-Bellot & Corrsin (1966) for the decay of grid-generated turbulence.