Experimental Study on the Hypersonic Boundary Layer Transition Over a Cone

The problem of the hypersonic boundary layer transition was experimentally investigated with a 7° half-angle cone at 0° angle of attack. Experiments were conducted in a Mach 6 wind tunnel using the nano-tracer-based planar laser scattering (NPLS) techniques, high-frequency pressure sensors, and temperature sensitive paints (TSP). The NPLS images of the complete process of the cone boundary layer developing from laminar flow to turbulence were obtained, which clearly shows the "rope-like" structure of the second mode wave. The NPLS results of the sharp/blunt cone boundary layer indicate that the second mode wave is dominant in the sharp cone boundary layer transition. However, there is a large vortex structure before the transition of the blunt cone boundary layer, and its wavelength is 5 times longer(or even longer) than the wavelength of the second mode wave, and the characteristic frequency is not higher than 31 kHz. The pressure fluctuation data was analyzed by power spectrum density(PSD), cross-correlation calculation, and N factor calculation. The amplitude of the second mode wave increases first and then decreases, the characteristic frequency gradually decreases, and the low-frequency component gradually increases for the sharp/blunt cone. It shows that the second mode develops first to saturation and then gradually attenuates while the low frequency mode develops gradually. The TSP results show that as the unit Reynolds number increases, the boundary layer transition front moves forward.