Experimental Investigation of the Effect of Forward-facing Steps on Boundary Layer Transition

The effect of geometric forward-facing steps on boundary layer transition was experimentally investigated 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 height were installed on a spanwise invariant wind tunnel model. Streamwise pressure gradient and high chord Reynolds number were systematically varied and their effect on boundary layer transition was studied in the presence of forward-facing steps on the model surface. At all tested stability situations, the surface imperfections were shown to reduce the extent of the laminar region. Transition was observed to move gradually towards the step location with increasing step Reynolds numbers and increasing relative step height. For a given combination of step height and chord Reynolds number, more pronounced negative pressure gradients led to an increase in transition Reynolds number. The reduction in transition Reynolds number due to the effect of the surface imperfection was more marked at larger flow acceleration. The plots of the relative change in transition location as a function of the step Reynolds number and of the relative step height gave good correlation of the results. The correlations were found to be practically independent of the streamwise pressure gradient in the examined range. Criteria for the allowable tolerances on low- sweep Natural Laminar Flow surfaces can now be derived from the functional relations determined in this work.