Aero-Optical and Fluidic Studies of Crossflow Transitional Waves in Hypersonic Flow

Stationary and traveling crossflow boundary-layer instabilities on a ‘slab delta’ model in M = 5.7 flow were investigated experimentally in the AFOSR–Notre Dame Large Mach-6 Quiet Tunnel under conventional noise at nominally zero angle of attack and yaw at unit Reynolds numbers from 4.1 × 10^6 m^−1 to 11.9 × 10^6 m^−1. A novel aero-optical approach — Digital Holography (DH), an interferometric wavefront measurement method — measured the path-integrated density fluctuations of the boundary layer in a circular aperture with a 40 mm field of view. Time-resolved DH measurements of wavefronts associated with wave-like transitional structures were recorded at 75 kHz. Statistical analysis of the measured wavefronts revealed the presence of both stationary and traveling crossflow waves. The stationary crossflow structures had a characteristic wavelength between 4.51 mm and 6.06 mm and a stationary wave angle between 7.07◦ and 8.91◦. Spectra in the flow-parallel direction revealed traveling waves with characteristic peaks at ≈ 13 kHz with apparent subharmonics. Aperture-averaged spectra revealed increasing peak frequencies between ≈ 14.7 kHz to 20.2 kHz, consistent with the linear stability theory. Two-point spatial correlation of the aero-optical data revealed characteristic traveling structure wavelengths of 7.26 mm and 8.99 mm for Re∞ = 8.0 × 10^6m^−1 and Re∞ = 11.9 × 10^6m^−1, respectively. Wave angles were also extracted from two-point correlation maps. Transitional structures were simultaneously measured using trios of surface- flush Kulite pressure transducers, whose results generally corroborated the DH.