Effect of wall-seeping gas film on hypersonic laminar and transitional boundary layers

The wall-seeping gas film (WSGF) is a promising technology for controlling hypersonic boundary layer transition. Experiments were conducted in a Mach 6 hypersonic quiet wind tunnel with freestream unit Reynolds numbers Re∞ = 7.3 × 106 m−1 and 14.6 × 106 m−1, corresponding to the WSGF being present in the laminar and transitional boundary layers, respectively. The effect of seeping ratios on the hypersonic boundary layer instability and transition was investigated using nano-tracer-based planar laser scattering (NPLS) technique and high-frequency wall fluctuating pressure sensors. The results indicate that WSGF exhibits opposite effects in the laminar and transitional boundary layers. Under all examined seeping ratios, WSGF significantly promotes transition at Re∞ = 7.3 × 106 m−1, accompanied by the premature appearance of second-mode instability waves. The maximum peak amplitude occurs at x = 420 mm, where the nonlinear interactions of instability waves are strongest. However, at Re∞ = 14.6 × 106 m−1, WSGF with moderate seeping ratios (0.072%–0.280%) extends the development distance of the second-mode waves characterized by a frequency range of 80–140 kHz. The boundary layer relaminarizes downstream of the seeping wall. The turbulence burst point was delayed by up to 14% at a seeping ratio of 0.141%. These opposite effects of WSGF on the laminar and transitional boundary layers further confirm the notion that the relative position of the WSGF to the synchronization point is a more critical factor.