Step-induced transition in compressible flow: experimental results and correlation with stability analysis

Practical laminar flow wing design requires joints, panels and high-lift devices, but steps and/or gaps would arise at the junctions and can lead to premature transition to turbulence. The effect of sharp forward-facing steps on boundary-layer transition was systematically investigated in this 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. 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. The measured transition locations were also correlated with the results of linear local stability analysis for the smooth configuration, following the procedure suggested in previous work to account for the effect of steps in the eN method for transition prediction