Two level simulation of Schmidt number effect on passive scalar transport in wall-bounded turbulent flows

In this study, the two-level simulation (TLS) modeling strategy developed earlier for momentum mixing is assessed in turbulent flows with passive scalar mixing and transport. Compared to the conventional algebraic and transport equation based subgrid-scale closures for largeeddy simulation (LES), where the effects of the unresolved small scales (SS) of motion on the large scales (LS) are modeled, in the TLS model, both large and small scales are explicitly evolved in a coupled two-scale decomposition method called TLS. Earlier studies demonstrated the ability of the SS model in the TLS to simulate high Reynolds number wall-bounded flows with a reasonable LS grid resolution. In the present study, this two-scale strategy is extended to model passive scalar mixing in turbulent channel flows. A comprehensive a priori assessment of the scalar SS model is performed using direct numerical simulation datasets for passive scalar transport in a fully developed turbulent channel flow at frictional Reynolds number Re-tau = 395 and at Schmidt numbers Sc = 1 and 4. This assessment is carried out to study the evolution of the large and the small scales of the scalar field in physical and spectral space and also to assess the SS prediction of quantities relevant for modeling of scalars such as scalar dissipation rate, counter-gradient transport, and backscatter. A TLS model is, then, used in a posteriori near-wall study using a hybrid TLS-LES approach to assess the two-scale scalar model.