Influence of Surface Roughness and Free-stream Turbulence on Crossflow-instability Transition

Surface-roughness and free-stream-turbulence effects on crossflow-instability transition are investigated experimentally and theoretically. A model baseflow is established and shown to be well predicted by boundary-layer analysis. Surface roughness is systematically introduced, with a factor-of-50 variation in r.m.s. levels. Free-stream turbulence levels are varied using grids across the test section, providing roughly a factor-of-10 variation in the turbulence intensity. For low turbulence levels, the transition is dominated by stationary crossflow instabilities. As the free-stream turbulence is increased, the traveling crossflow instabilities become increasingly relevant. For higher-turbulence and lower-roughness levels the transition appears to be dominated by the traveling crossflow instabilities. The results are linked to a variable N-factor method, which is shown to provide a good basis for estimating the transition location for low turbulence levels. The variable N-factor method is used to interpret the results and infer causality between roughness, turbulence and transition.