Effect of wall-seeping gas film under different working media on stability of conical hypersonic boundary layer

Abstract

Wall-seeping gas film (WSGF) is a promising method of controlling hypersonic boundary layer transition and reducing friction drag and heat transfer. Experiments are conducted in a Mach 6 hypersonic quiet wind tunnel by using nano-tracer planar laser scattering (NPLS) and high-frequency fluctuating pressure measuring technique. This work investigates the effects of wall-seeping helium, air, and carbon dioxide gas films under identical volume flow rate condition on conical boundary layer thickness, disturbance wave structure, wavelength, frequency, amplitude, and nonlinear interaction. The experimental results reveal that the WSGF significantly thickens the hypersonic boundary layer, with the thickest position appearing at the downstream boundary of the seeping zone. The boundary layer thickness is thinnest for helium gas film but thickest for carbon dioxide gas film. Generally, air gas film and carbon dioxide gas film induce the regular, rope-like, and interlaced second-mode waves to appear in advance in the boundary layer. However, under a higher volume flow rate for carbon dioxide gas film, the disturbance wave structure resembles interface fluctuations, with a characteristic wavelength of approximately 18 mm and a peak frequency as low as about 35 kHz, but no the rope-like interlaced characteristic. At this time, the influence of shear layer instability becomes significant. The disturbance waves do not exhibit second-mode wave characteristics for wall-seeping helium gas film, whose shape is irregular and undergoes deformation with time and space. Additionally, the power spectral density of wall fluctuating pressure exhibits insignificant variation with volume flow rate and flow direction, which is similar to the characteristic of power spectral density in the laminar boundary layer and has no peak frequency. The wavelength of second-mode waves is about 2－3 times the boundary layer thickness for air gas film, and increases to more than 3 times for carbon dioxide gas film. The application of carbon dioxide gas film results in smaller peak frequency and bandwidth of disturbance wave, larger characteristic wavelength and amplitude, longer propagation distance, and stronger nonlinear interaction than the application of air gas film. In the future, attention should be paid to understanding disturbance wave characteristics in the boundary layer for the helium gas film and shear layer instability under larger volume flow rates.