Oscillating Bow Shock Effects on Natural Boundary Layer Transition and Surface Heating on a Slender Cone in Mach 6 Flow

An experiment was conducted to investigate the effects of bow shock oscillations on natural boundary layer transition and surface heating in a Mach 6 freestream. Experiments were conducted in the Purdue University Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) under noisy and quiet flow conditions. The model tested was a 650 mm-long, 5° half-angle cone with forward-facing cavity (FFC) nosetips. The FFC nosetips produced self-sustained bow shock oscillations as has been shown in previous experiments. High-frequency pressure transducers were flush-mounted along a cone ray to track boundary layer disturbances. A high-frequency pressure transducer was embedded in the FFC nosetips to monitor bow shock oscillation amplitude and frequency. Infrared Thermography was used to visualize surface heating over the model. At 0.0° angle of attack (AoA) a boundary layer instability was observed with a spectral peak centered near 150 kHz. This is believed to be the second-mode instability. Bow shock oscillations concentrated power near the second-mode spectral region and caused earlier amplitude saturation of the second mode. At 2.5° AoA a prominent thermal striation formed at the aft end of the cone along the leeward ray. Bow shock oscillations caused the formation of secondary thermal striations and accelerated the growth of the striations in the axial direction by as much as 50 mm. Pressure measurements made in the thermal striation region showed an approximately 80 kHz disturbance. Bow shock oscillations were shown to cause this feature to transition to turbulence by an axial location of 600 mm from the model nose tip, corresponding well to the surface heating analysis.