Mixing efficiency in stably stratified growing turbulence

Abstract

Nearly steady state mixing efficiencies, as characterized by the flux Richardson number Rf, have been measured in stably stratified, turbulent, uniform gradient shear flows. The turbulent dissipation rate ϵ, the vertical mass and momentum fluxes and , and the mean and rms density and velocity fluctuations were obtained from simultaneous single point measurements of velocity and density. Rf and the gradient Richardson number Ri were calculated at a number of positions in the spatially evolving shear flow. Within the investigated range of stabilities 0.02<Ri<0.08, Rf was observed to increase with Ri. It is found that the length scale arguments successfully applied by Rohr et al. (1984) to the case of decaying, stably stratified, unsheared grid turbulence can also account for the overall behavior of the mixing efficiency (Rf) as a function of stability (Ri) in the present case of growing, stably stratified, uniform mean shear turbulence.