Experimental investigation on the effect of wall-seeping gas film on downstream second-mode waves in hypersonic boundary layer

As a potential method of reducing wall heat flux and friction drag in hypersonic vehicles, wall-seeping gas film (WSGF) can affect the hypersonic boundary layer stability. An experimental study investigating the WSGF effect on second-mode waves in hypersonic boundary layer downstream of the seepage zone was conducted in a Mach 6 hypersonic quiet wind tunnel under a unit Reynolds number of 5.86×106m−1. The test model was a slender cone with a 7-degree half-angle. WSGF was realized by seeping out air through permeable stainless steel. Five seeping ratios, 0, 0.041%, 0.055%, 0.066% and 0.138% were studied. The flow structure visualized by schlieren technique showed that WSGF with seeping ratio of 0.055% ∼ 0.138% induced weak shock wave. WSGF thickened density boundary layer. The cross-correlation of wall fluctuating pressure measured by PCB sensor arrays between two adjacent measuring stations showed that WSGF had no significant impact on the propagation velocity of second-mode waves. The power spectrum density of wall fluctuating pressure indicated that the peak frequency of second-mode waves decreased at each measuring station as the seeping ratio increased. The peak frequency decreased along the streamwise direction without WSGF. However, the decreasing trend slowed down as seeping ratio increased to 0.041%. At seeping ratio of 0.055%, the peak frequency first remained almost constant (around 121.2 kHz) at first three measuring stations and then gradually decreased. Surprisingly, at higher seeping ratios of 0.066% and 0.138%, the peak frequency even exhibited an initial gradual increase followed by a slight decrease downstream. Additionally, WSGF made the second-mode waves amplitude increase fast downstream of the seepage zone and decrease sharply far downstream. The maximum amplification factor is approximately 0.85 and the minimum is -1.87.