Roughness-Induced Instability in a Hypersonic Laminar Boundary Layer

A cylindrical roughness element was used to introduce instabilities into a laminar nozzle-wall boundary, layer in the Purdue Mach-6 Quiet Tunnel. A pitot probe, hot-wire probes, and wall-mounted pressure sensors were used to detect an instability in the wake of the roughness. This is the first such instability measured 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. Computations have confirmed the presence of the instability, which originates upstream of the roughness in the separation region. Further characterization of this instability can assist development and validation of physics-based prediction methods for roughness-induced transition.