Shear layer flow behavior between low-speed wall-seeping helium gas film and hypersonic freestream over a cone

The wall-seeping Helium Gas Film (HGF) demonstrates high cooling efficiency in active thermal protection systems for hypersonic vehicles. Current experimental investigations focus on the shear layer flow features between the subsonic HGF and hypersonic freestream over a 7° half-angle sharp cone to elucidate the HGF cooling mechanism. The wall heat flux distributions revealed an overcooling phenomenon with cooling efficiency exceeding 100% immediately downstream of the permeable wall. It may be attributed to the internal energy dissipation, which originates from the expansion waves due to the thinning of HGF thickness. Nano-tracer planar laser scattering technique was utilized to analyze the vortex structures in shear layers. The initial propagation velocity of vortices agreed well with predictions (755 m·s -1 ) from the convective velocity formulation for shear layers. This confined shear layer exhibited a high convective Mach number (0.74) and density ratio (more than 7), which enhanced the Kelvin-Helmholtz vortex stability. Furthermore, power spectral density analysis of the wall fluctuating pressure indicated different disturbance characteristics in shear layers compared to second-mode waves. The HGF suppressed the development of high-frequency second-mode waves while amplifying the amplitude of low-frequency disturbances. This study identified that K-H vortices replaced the second-mode waves that typically dominate natural transition.