Change in separation flow regimes over obstacles in subsonic gas flow as a manifestation of viscous forces: numerical results

The unsteady subsonic viscous gas flows over forward- and backward-facing steps on a flat surface are numerically simulated at a free-stream Mach number of 0.1 by applying compact difference schemes of the 14th order of accuracy. The characteristics of the boundary layer separation zones are investigated at Reynolds numbers ranging from 10$^3$ to 10$^7$. The computations are based on the nonstationary Navier–Stokes equations supplemented, if necessary, with the formulas of the one-parameter differential SA model of turbulent viscosity. The conditions for pulsing separation zones arising in the boundary layer are determined, and the subsequent transition to a turbulent flow regime is physically justified.