Experimental Analysis of Geometric, Pressure Gradient and Surface Temperature Effects on Boundary-Layer Transition in Compressible High Reynolds Number Flows

The effect of sharp forward-facing steps on boundary-layer transition was systematically investigated in this experimental work in combination with the influence of changes in the following parameters: streamwise pressure gradient, Reynolds number, Mach number, and a non-adiabatic surface. The investigations were carried out in a quasi-two-dimensional flow at high Reynolds numbers and at both low and high subsonic Mach numbers in the Cryogenic Ludwieg-Tube Gottingen. The adopted experimental setup allowed an independent variation of the aforementioned parameters and enabled 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 step Reynolds number and relative step height. 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 larger 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 and allowed the effect of the steps on boundary-layer transition to be isolated from the influence of variations in the other parameters. The present results were demonstrated to be applicable and transferable to the practical case of a natural laminar flow airfoil. Criteria for acceptable heights of forward-facing steps on unswept and moderately swept natural laminar flow surfaces can now be derived from the functional relations determined in this work.