| The future development of atmospheric and oceanic models requires improved description both of many fundamental flow processes and of the internal mixing. Recent laboratory and numerical studies have examined one aspect of this problem, the estimation of the diapycnal diffusivity in the presence of a stable background density stratification. The laboratory experiments were conducted in a closed system filled with a high Pr fluid with turbulent stirring by a horizontally oscillating vertical grid. Estimates were made of the rate of turbulent dissipation, turbulent lengthscales and diapycnal diffusivities over a very wide range of the turbulence intensities. In the energetic regime, for example, diffusivities were found to have both a different parameter dependence and to be smaller than predicted by traditional models. Some recent Direct Numerical Simulations in a density stratified shear flow at a Pr=0.7 provide an independent means of examining some of the predictions from these laboratory experiments. What emerges from the laboratory and numerical studies is a picture of how the diapycnal diffusivity smoothly makes the transition from the molecular value at very low turbulent intensities to very high diffusivities at the highest turbulent intensities attainable. |
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