Experimental investigation of hypersonic boundary layer transition at Mach 12

The boundary layer transition at Mach 12 over a sharp cone with 8° half-cone angle is experimentally investigated in this paper for the linear and nonlinear evolution characteristics of disturbance waves. Based on continuous wavelet transformation, the power spectral density and bicoherence of pressure fluctuations were evaluated for long-time average characteristics of multiple wave packets, as well as the evolution process of an individual wave packet. The results indicate that the transition process of the Mach 12 conical boundary layer is dominated by the second mode. The second mode first undergoes nonlinear saturation and decay, leading to a reduction in pressure fluctuation intensity. Subsequently, the spectral valleys of the disturbance frequency spectrum are gradually filled, and the pressure fluctuation intensity increases again until the transition is complete. Bicoherence analysis of an individual second mode wave packet reveals strong harmonic resonance interactions of the second mode and nonlinear interactions with low-frequency disturbances.