Non-Adiabatic Surface Effects on Step-Induced Boundary-Layer Transition

The effect on step-induced boundary-layer transition of surface temperatures different from the adiabatic-wall temperature was investigated for a (quasi-) two-dimensional flow at large Reynolds numbers and at both low and high subsonic Mach numbers. Sharp forward-facing steps were mounted on a flat plate and transition was studied non-intrusively by means of the temperature-sensitive paint technique. The experiments were conducted in the Cryogenic Ludwieg-Tube Göttingen with various streamwise pressure gradients and temperature differences between flow and model surface. A reduction of the ratio between surface and adiabatic-wall temperatures had a favorable influence on step-induced transition up to moderate values of the step Reynolds number and of the step height relative to the boundary-layer displacement thickness, leading to larger transition Reynolds numbers. However, at larger values of the non-dimensional step parameters, the increase in transition Reynolds number for a given reduction in the wall temperature ratio became smaller. Transition was found to be insensitive to changes in the wall temperature ratio for step Reynolds numbers above a certain value. Up to this limiting value, the relation between the relative change in transition location (with respect to its value for a smooth surface) and the non-dimensional step parameter was essentially unaffected by variations in the wall temperature ratio. The present choice of non-dimensional parameters allows the effect of the steps on transition to be isolated from the influence of variations in the other factors, provided that both transition locations on the step and smooth configurations are measured at the same conditions.