| An a priori analysis of subgrid-scale pressure in high pressure combustion is carried out using three different flow fields obtained by direct numerical simulation (DNS). The simulations directly solve the Navier-Stokes equations of high pressure, transitional H2/O2 mixing and reacting shear layers employing a cubic Peng-Robinson real gas state equation. Detailed chemistry, multicomponent, differential, and cross diffusion are included. The results are analyzed to provide subgrid information relevant to Large Eddy Simulation (LES) of turbulent combustion. The analysis includes a detailed comparison of the actual filtered pressure with its corresponding form evaluated with the filtered primitive variables. Although negligible for purely mixing cases, the gradient of the subgrid pressure is shown to be of the same order, or larger than, the corresponding divergence of the turbulent subgrid stresses for reacting cases. This is despite the fact that all species behave essentially as ideal gases for this flame. The analysis is conducted through both a global perspective, as well as by conditioning on specific regions of the flame; including regions of large subgrid kinetic energy, large subgrid scalar dissipation, large temperature, etc., in order to isolate its effect within certain regions of the flame. The results indicate that subgrid pressure modeling can be important for accurate LES. A dynamic similarity model for the subgrid pressure is therefore introduced and shows substantial improvement in predicting the filtered pressure for reacting flows. However, its improvement is diminished when considering the pressure gradient. |
