To develop improved methods of transition prediction for isolated roughness, based on the growth of disturbances in the roughness wake, the underlying insta- bility mechanisms must first be characterized. A cylindrical roughness element was used to introduce instabilities into the laminar nozzle-wall boundary layer in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. Instabilities were detected in the roughness wake using flush-mounted pressure sensors, at both near-effective and near-critical conditions. These are the first such instabilities measured at hypersonic speeds.;Experimentally-observed instabilities were compared to computations performed by others for a large roughness with a height of 1.2 times the boundary-layer thickness. Direct numerical simulations allowed a detailed analysis of the entire flow field, while experimental measurements discovered the real flow physics and confirmed the findings of the computations. For a large roughness height of 10.2 mm, the dominant mechanism for transition was identified. An instability with a frequency near 21 kHz was detected upstream of the roughness, as predicted by the computations, suggesting that the instability originates within the separation region. Unstable shear layers and horseshoe vortices appeared to cause transition downstream of the roughness for this case. As the roughness height was reduced, there appeared to be a change in the dominant instability mechanism. Several possible instabilities were identified for smaller, near-critical roughness heights that caused incipient transition on the nozzle wall.
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