Experimental investigation of the effect of forward-facing steps on boundary layer transition

Abstract

The effect of geometric forward-facing steps on boundary layer transition was experimentally investigated at high Reynolds numbers and at a high subsonic Mach number in a blow-down wind tunnel facility in Göttingen, Germany. Boundary layer transition was detected non-intrusively by means of the Temperature-Sensitive Paint (TSP) technique. Forward-facing steps of different heights were installed on a spanwise invariant wind tunnel model and various streamwise pressure gradients were considered. For all tested stability situations, the surface imperfections were shown to reduce the extent of the laminar region, with larger reductions being obtained with higher steps. The transition Reynolds number at a fixed combination of surface step height and streamwise pressure gradient was found to be independent of the chord Reynolds number. For a given combination of step height and chord Reynolds number, the decrease of the transition Reynolds number relative to the transition Reynolds number obtained for the smooth surface was observed to be larger at more pronounced negative pressure gradients.