Effect of Forward-Facing Steps on Boundary Layer Transition at a Subsonic Mach Number

Experiments were conducted at a subsonic Mach number in the Cryogenic Ludwieg-Tube Göttingen 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 and various streamwise pressure gradients. Transition, detected non-intrusively by means of the Temperature-Sensitive Paint technique, was found to gradually move upstream towards the step location with increasing step Reynolds numbers and relative step height. Larger flow acceleration had a favorable influence on the transition Reynolds number even in the presence of forward-facing steps, but also increased the sensitivity of boundary layer transition to the surface excrescences. The plots of the relative change in transition location as a function of step Reynolds number and relative step height gave good correlations of the results obtained with both wind tunnel models. The correlations were practically independent of the streamwise pressure gradient. Criteria for allowable tolerances on low-sweep Natural Laminar Flow surfaces can now be established from the functional relations determined in this work