Towards an improved subgrid-scale model for thermally driven flows

The effect of the filtering on the resolved and subgrid dynamics of turbulent Rayleigh–Bénard convection (RBC) is studied a priori using a Direct Numerical Simulation dataset. To this end, the velocity and temperature fields, split into resolved and subgrid components by a spectral cutoff filter, are analyzed with the filtered turbulent kinetic energy and temperature variance budgets. At small filter lengths, the resolved processes correspond to the exact ones except for the decreases of the dissipations which, in turn, are balanced by the sink actions of the subgrid scales. At large filters lengths, the resolved dynamics depletes close to the walls and the effect of the subgrid scales drifts from purely-dissipative to a more complex behaviour. This study highlights the possibility that eddy-viscosity and diffusivity models, commonly employed in large-eddy simulation of RBC, does not work well for large filter widths and that alternative closures should be considered.