Experimental investigation of surface imperfection effects on boundary-layer transition using temperature-sensitive paints

Abstract

The effect of sharp forward-facing steps and sinusoidal bumps 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 and Mach number. 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 imperfection location with increaslng non-dimensional imperfection parameters. Stronger flow acceleration led to an increase in the transltlon Reynolds number even in the presence of forward-facing steps and bumps; this favorable influence became, however, less pronounced at larger values of the non-dimensional lmperfection parameters.