Assessment of subgrid-scale models in large-eddy simulations of decaying rotating stratified turbulence

We report the results of large eddy simulation (LES) using three subgrid scale models, namely: constant coefficient Smagorinsky, dynamic Smagorinsky, and a non-linear model, for rotating stratified turbulence in the absence of forcing using large-scale isotropic initial condition. The LES results are compared to in-house direct numerical simulation (DNS) for establishing grid-independence requirements. Three cases with varying ratios of Brunt-Vaisala frequency to the inertial wave frequency, $mathcal{N}/f$, have been chosen to evaluate the performance of LES models. The Reynolds number and N/f are chosen as (a) Run1: Re=3704, N/f=5, (b) Run2: Re=6667, N/f=40 and, (c) Run3: Re=6667, N/f=138. This framework is used to illustrate the relative magnitudes of the stratification and rotation which is observed in geophysical flows. Various quantities including turbulent kinetic energy (tke), turbulent potential energy (tpe), total dissipation, potential & total energy spectra, and their fluxes, are analyzed to understand the predictive capability of the various LES models. Results showed that all the SGS model predictions were very similar to each other with the classical Smagorinsky model displaying the highest deviation in comparison to DNS. The effect of an increase in value of N/f was also seen in the results of LES with an increase in the oscillation observed in the evolution of tke and tpe, and reduction in dissipation, which is exhibited by DNS. The spectral analysis shows that the non-linear and dynamic Smagorinsky models predict the large-scale physics ($kappa < 10$), while the small scales (10< κ< 64) energy is under-predicted.