Mixing efficiency in stably-stratified decaying turbulence

Abstract

The behavior of the flux Richardson number R _f, as a function of the overall Richardson number Ri ₀, was investigated for a stably stratified, grid-generated, turbulent flow evolving in a closed-loop water channel. The turbulent dissipation rate ε, the buoyancy or vertical mass flux p wbar; and the rms density fluctuation ρ′ were obtained from simultaneous single-point measurements of the horizontal and vertical velocity components and density fluctuations. From these, R _f and Ri ₀ were calculated at each point in the spatially evolving flow. The resulting curves of R _f vs. Ri ₀ exhibit the full range of behavior found in the very different case studied by Linden (1980). The length scale arguments of Gibson (1980) and Stillinger et al. (1983b) provide an underlying mechanism which successfully accounts for the shape of the R _f vs. Ri ₀ curve.