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 , the irreversible mixing efficiency in LES scales like , where and are the vertical Froude number and turbulent Prandtl number, respectively. Assuming a unit scaling coefficient and , goes to a constant value , 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 . Overall, the LES approach can reproduce mixing efficiency results similar to those from the DNS approach if , where 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.