Hypersonic Wind-Tunnel Measurements of Boundary-Layer Transition on a Slender Cone

Boundary-layer transition was studied on a sharp 7 deg cone in two hypersonic wind tunnels at Mach numbers of 5, 6, 8, and 14 over a range of freestream Reynolds numbers between 3.3 and 15.4×106/m. High-speed schlieren measurements visualized the intermittent formation of instabilities and turbulent spots within the transitional boundary layer. Surface pressure and heat-transfer measurements revealed how the intermittent behavior of the boundary layer produces the mean character of these quantities. Transition at Mach 5 appeared to be initiated by a combination of first- and second-mode instabilities. These disturbances were isolated and surrounded by an otherwise smooth boundary layer. At higher Mach numbers, the boundary layer was dominated by second-mode instabilities, which covered most of the model before breakdown into turbulent spots. The spots remain surrounded by second-mode waves throughout the transitional region. These differences alter the pressure fluctuations and heat transfer profiles during transition. Higher frequency pressure measurements peaked upstream of the onset of transition because of the growth of second-mode instabilities. Lower frequency pressure fluctuations and the surface heat transfer did not rise significantly until further downstream where turbulent regions developed.