Use of digitally filtered inflow conditions for LES of flows over backward facing steps

Large eddy simulations (LES) of subsonic and supersonic boundary layers separating at backward facing steps are performed for validating a hybrid flow solver and testing the digital filtering approach for specifying the inflow turbulence. The broadband spectra of eddies in the approaching boundary layers resulting from filtering properly trigger the shear layer instabilities leading to significant improvements in predictions of first and second order turbulence statistics when compared to those resulting from use of uncorrelated noise for generating inflow turbulence. This seems to be true even though the distance between the inflow boundary and the step is about the same or less than in most of the LES of this kind of flows reported in literature and not sufficient to establish an equilibrium boundary layer with correct phase information before the flow reaches the step. The density/entropy disturbances resulting from non-solenoidal inflow do not seem to adversely affect the predictions in the subsonic case. The digital filtering approach does, however, generate acoustic disturbances that contaminate the expansion fan generated at the corner and lead to slight overpredictions in turbulence levels downstream in the supersonic case.