Mixing efficiency in large-eddy simulations of stratified turbulence

The irreversible mixing efficiency is studied using large-eddy simulations (LES) of stratified turbulence, where three different subgrid-scale (SGS) parameterizations are employed. For comparison, direct numerical simulations (DNS) and hyperviscosity simulations are also performed. In the regime of stratified turbulence where Fr-v similar to 1, the irreversible mixing efficiency gamma(i) in LES scales like 1/(1 + 2Pr(t)), where Fr-v and Pr-t are the vertical Froude number and turbulent Prandtl number, respectively. Assuming a unit scaling coefficient and Pr-t = 1, gamma(i )goes to a constant value 1/3, in agreement with DNS results. In addition, our results show that the irreversible mixing efficiency in LES, while consistent with this prediction, depends on SGS parameterizations and the grid spacing Delta. Overall, the LES approach can reproduce mixing efficiency results similar to those from the DNS approach if Delta less than or similar to L-0, where L(0 )is the Ozmidov scale. In this situation, the computational costs of numerical simulations are significantly reduced because LES runs require much smaller computational resources in comparison with expensive DNS runs.