Roughness-induced instabilities in the boundary layer of a generic hypersonic forebody at Mach 6

In hypersonic flows, it is often necessary to be able to trip the transition to turbulence, upstream of air intakes, for example. Direct numerical simulations have been performed to identify the roughness-induced transition mechanisms on a wedgelike forebody at Mach 6 and unit Reynolds number Re=11 million (/m). Good agreement with the experiments performed in the Boeing/Air Force Office of Scientific Research Mach-6 Quiet Tunnel at Purdue University was obtained in terms of wall heat-flux and wall-pressure fluctuations. First, an isolated roughness was considered. The presence of the roughness in the span-inhomogeneous base flow leads to the formation of a crossflowlike vortex. High-frequency secondary instabilities of the stationary crossflow vortex are observed in the wake and are found to be responsible for the breakdown to turbulence. Spatial linear modal instability analysis of this flow has been performed at selected streamwise locations. The linear stability approach is found to give accurate predictions in terms of mode shapes, most-amplified disturbance frequencies, and growth rates, as it only underpredicts the N-factor of the most unstable mode by 10% compared to the direct numerical simulations. Unsteady simulations were then carried out for a trip array configuration and showed that it does not change the transition mechanisms, but the frequencies of the most unstable secondary instabilities were found to be higher