The Multi-scale Cross-spectrum Analysis Of Hypersonic Boundary Layer Transition On A Sharp Cone

Hypersonic boundary layer instability and transition is one of the most important problems to aeronautic and astronautic. The experiment had been devoted to learn more about the characters and the disturbances in the boundary layer. A model with 5-degree half-angle sharp cone was tested in a hypersonic gun tunnel at free-stream Mach number 6. The instantaneous fluctuating surface-thermal-flux signals were gained with 7 Pt-thin-film thermocouple sensors, which are located along the same generatrix on the sharp cone surface. The fluctuating surface-thermal-flux signals have been investigated using the wavelet analysis. The power spectrum, bispectrum and cross-bispectrum which are based on the wavelet transformation have been investigated. The conditional phase-averaging waveforms extracted were using for providing developing details of the instable disturbance in spectrum space.The first-mode disturbance and the second-mode disturbance were observed clearly in the wavelet analysis. Although the second-mode disturbances are present all through the flow, its power isn't the highest and the harmonic of the first-mode disturbance dominates in the initial of the flow. At the last station, the power of the second-mode disturbance are dominance. From the bispectrum analysis, the second-mode disturbance affect the hypersonic flow with together the first-mode disturbance at early stage. With the develop of the boundary layer, the effects of two modes both increase, but the effect of the second-mode is greater. The cross-bispectrum analysis show that phase-locked interactions between every frequency are apparent, and the effects of the second-mode disturbances are from slender to strong, which are consistent with the results of power spectrum and bispectrum. The power spectrum calculated for the extracted instable disturbance signals provides developing details of the instable disturbance in spectrum space. It is observed that the magnitude of disturbance waveforms increase in the initial of the flow and decrease in the middle stage. The frequence of disturbance waveforms is from 200 khz to 400 khz, which is as same as the conclusion of Wavelet coefficients magnitude contour.In the studying of this paper, it is apparent that wavelet transformation techniques provide a new method to identify the characters of the first-mode and the second-mode disturbance in a hypersonic flow. And the correlation spectrum and conditional phase-averaging waveforms extracted which are based on the wavelet transformation show that more profound understanding about the first-mode disturbance , the second-mode disturbance and instable disturbance waveforms.