Effect of surface cooling on second-mode dominated hypersonic boundary layer transition

Experiments were performed in the low-enthalpy AFRL Mach 6 Ludwieg Tube to investigate the effects of surface temperature on hypersonic boundary layer transition. The test article for all experiments was a 7-degree half-angle cone with a sharp nose tip. The model was designed with a large internal cavity, capable of containing pressurized cryogenic fluids. A cooling system utilizing liquid nitrogen was designed and integrated with the test section of the Ludwieg Tube, allowing for significant surface cooling of the test article while maintaining all other conditions, such as tunnel noise spectra. The reduction of the surface temperature to values below 110 K led to a wall-to-boundary layer edge temperature ratio of 1.4, while this ratio was 4.3 for the uncooled case. Boundary layer transition behavior was observed between freestream unit Reynolds numbers ranging from 2.2 million per meter to 25.9 million per meter. Instability waves within the boundary layer were captured via time-resolved Schlieren visualization using a continuous light source. The images confirmed Mack's second mode was present for both uncooled and cooled model experiments. Turbulence intermittency statistics and frequency content of the instabilities were computed via several image processing techniques. Reducing the surface temperature of the test article decreased the boundary layer thickness and subsequently increased the frequency of the most unstable waves. Overall, the cooling delayed the boundary layer transition to turbulence for all cases observed.