Experimental analysis of step-induced transition in compressible high Reynolds number flow

Abstract

The effect of sharp forward-facing steps on boundary-layer transition was systematically investigated in this experimental work in combination with the influence of variations in the following parameters: streamwise pressure gradient, Reynolds number, Mach number, and wall temperature ratio. The experiments were conducted in a quasi-two-dimensional flow at high Reynolds numbers and at both low and high subsonic Mach numbers in the Cryogenic Ludwieg-Tube Göttingen. The adopted experimental setup enabled an independent variation of the aforementioned parameters and allowed a decoupling of their respective effects on the boundary-layer transition. Transition, measured non-intrusively by means of temperature-sensitive paint, was found to move gradually upstream towards the step location with increasing non-dimensional step parameters. Stronger flow acceleration and lower wall temperature ratios led to an increase in the transition Reynolds number even in the presence of forward-facing steps; this favorable influence became, however, less pronounced at !arger values of the non-dimensional step parameters. The representation of the results using the relative change in transition location with respect to the step location, plotted against the non-dimensional step parameters, gave good correlation. The effect of the steps on boundary-layer transition could thus be systematically investigated, being isolated from the influence of variations in the other parameters.