Effect of forward-facing steps on boundary-layer transition in combination with stability modifiers

Experiments were conducted up to high subsonic Mach number in the Cryogenic Ludwieg-Tube Gottingen to study the influence of surface imperfections on boundary layer transition. Forward-facing steps of different height were installed on two spanwise-invariant wind tunnel models and their effect on transition was examined at high chord Reynolds numbers, various streamwise pressure gradients, and different wall temperature ratios. Transition, detected non-intrusively by means of the Temperature-Sensitive Paint technique, was found to gradually move towards the step location with increasing step Reynolds numbers and relative step height. Larger flow acceleration and reduced wall temperature ratios had a favorable influence on the transition Reynolds number even in the presence of forward-facing steps, but their effect was less pronounced as the step Reynolds number was increased. The plots of the relative change in transition location as a function of the step Reynolds number gave good correlation of the results obtained at all examined Mach numbers and with both wind tunnel models.