Turbulent subsonic particle-laden flow over an open backward-facing step with and without countercurrent shear at {R}e=3000

We present a numerical study of the subsonic particle-laden flow over an open backward-facing step at a Reynolds number of $Re$=3000. The compressible Navier-Stokes equations are simulated without turbulence models using a multidomain spectral method. Point particles are tracked assuming a Stokes flow model. We focus on the compressibility effects and the effect of countercurrent shear on the zero and first order turbulence statistics and flow topology for low to moderate Mach numbers. At $Mach$=0.1 our results compare well with published incompressible numerical simulations and experiments. At $Mach$=0.4 we show that the shear layer reattaches 10\% closer the step than at the nearly incompressible $Mach$=0.1 as a result of transient compressibility effects related to pressure-dilatational dissipation. The application of a moderate level of countercurrent shear destabilizes the shear layer immediately increasing turbulence intensity and particle deposition behind the step, and decreasing the reattachment length with 12\%. Countercurrent shear also creates large structures that shed with a distinct period frequency.