Separation of a supersonic turbulent boundary layer by a forward facing step

Abstract

The mean flow and fluctuations were measured in the undistributed turbulent boundary layer, the separated flow region and on top of the step. The freestream Mach number was 4.0 and the boundary layer Reynolds number based on momentum thickness was 4,100. The model was large enough to allow reversed flow profile measurements and to insure two-dimensionality of the separated flow. For an aspect ratio (span between fences/step height) of 9.4, the back flow profiles show three-dimensionality effects. The mean flowfield is described in detail. In the “separation region” large normal and axial pressure gradients exist. The angle of the inviscid flow after separation is 12 1/2° In the nearly constant pressure separated flow region the forward velocity profiles up to 2H of the step are similar and nearly similar closer to the step. The entrainment rate of the forward flow λ_e = ρ_0v_0/ ρ_eu_e = .010. Close correspondence of the forward flow with the turbulent boundary layer with injection at the same λ_e was found. The reversed flow velocity reaches 0.37 u_e, close to the step (< 1H) and decays slowly to zero at separation (x/H = 4.25). The center of the recirculating vortex is closer to the step than 0.7H. On top of the step the mean turbulent boundary layer attains nearly similarity shape within one step height wheras the fluctuation distributions lag considerably. In the separated flow region most of the fluctuation energy is in random large scale low frequency fluctuations.