Novel Subgrid Modeling of the LES Equations Under Supercritical Pressure

Transitional states obtained from Direct Numerical Simulation (DNS) of a supercritical mixing layer are analyzed for studying small-scale behavior and assessing the ability of Subgrid Scale (SGS) models to duplicate that behavior. Initially, the mixing layer contains a single chemical species in each of the two streams, and a perturbation promotes rollup and a double pairing of the four spanwise vortices initially present. The database encompasses three combinations of chemical species, several perturbation wavelengths and amplitudes, and several initial Reynolds numbers specifically chosen for the sole purpose of achieving transition. The Large Eddy Simulation (LES) equations are derived from the DNS ones through filtering. This filtering leads to two types of additional terms in the LES compared to the DNS equations : SGS fluxes and other terms for which either assumptions or models are necessary. The magnitude of all terms in the LES conservation equations is analyzed on the DNS database, with special attention to terms that could possibly be neglected. It is shown that in contrast to atmospheric-pressure gaseous flows, there are two new terms that must be modeled: one in each of the momentum and the energy equations. Discussed is a model for the momentum-equation additional term. This model performs well at small filter size but deteriorates as the filter size increases, highlighting the necessity of ensuring appropriate grid resolution in LES.