Roughness-induced instability in a laminar boundary layer at Mach 6

Abstract

Roughness can cause a boundary layer to become turbulent, increasing aeroheating from the laminar rate. Empirical correlations are currently used to predict the onset of roughness-induced transition but do not take into account the flow physics that cause it. More accurate physics-based prediction methods must be developed, based on the growth of instabilities within the wake of the roughness. An isolated roughness element was used to introduce instabilities into a laminar nozzle-wall boundary layer in the Boeing/AFOSR Mach-6 Quiet Tunnel. Qualitative temperature-sensitive paint measurements show several hot streaks within the wake of the roughness. Pitot and hot-wire probes were used to measure an instability in the wake of the roughness. These are believed to be the first such measurements at hypersonic speeds. The instability was observed to grow downstream of the roughness and was strongest off the wake centerline, at a height near the roughness height. Mean-flow pitot pressures in the wake of the roughness were recorded for comparison to future computations of the wake. Further characterization of this instability can assist development and validation of a physics-based transition prediction method for roughness-induced transition