Effect of rotation on mixing efficiency in homogeneous stratified turbulence using unforced direct numerical simulations

Diapycnal (irreversible) mixing is analyzed using thirty direct numerical simulations (at 1024(3) resolution) of homogeneous rotating stratified turbulence (RST) in the absence of imposed shear or forcing. The influence of varied rotation and stratification rates on the energetics (in particular the dissipation rates of kinetic and potential energies) is presented. Data is also analyzed within a new parametric framework, using the turbulent Froude and Rossby numbers Fr-t = epsilon/Nk, Ro(t) = epsilon/fk, where k is the turbulent kinetic energy, epsilon its rate of dissipation, N the buoyancy frequency and f the Coriolis parameter. This framework is used to illustrate relative magnitudes of the stratification and rotation in geophysical flows and provide a useful tool for explicating the relationship between Fr-t and Ro(t) as relevant dynamic parameters in the geophysical setting. Results indicate that unforced rotation does not impact the magnitude of the irreversible mixing coefficient (Gamma = epsilon(p)/epsilon) when compared to results without rotation, where epsilon(p) is the rate of potential energy dissipation. Moreover, it is shown that the recent scaling laws for mixing efficiency in stably stratified turbulence in the absence of rotation, as exemplified in Garanaik & Venayagamoorthy (J. Fluid Mech. 867, 2019, pp. 323-333), are applicable as well for homogeneous and decaying RST. Results also highlight the ambiguity of the ratio N/f as a control parameter for the classification of small-scale RST, and thus for evaluating diapycnal mixing.