Influence of laser-generated perturbations on hypersonic boundary-layer stability

In this paper, the boundary layer flow stability is investigated experimentally in a 7° half-angle straight cone under the condition of Mach number 6 and unit Reynolds number 3.1×106/m. Expanded shock wave generated by focusing laser in a limit space is used as the small artificial disturbance, and the influence of the laser-generated perturbation on the stability of the hypersonic boundary layer is analyzed. In the experiment, the wall fluctuation pressure is measured by the high-frequency pressure sensors whose response frequencies each reach a value on the order of megahertz. Through the short time Fourier transformation and power spectrum density analysis of the pressure data, the results show that when the laser-generated perturbation is added to the flow field, the position of the second mode wave advances and the amplitude of the disturbance wave greatly increases. Within the same flow range, the laser focusing on disturbance pushes the disturbance wave in the boundary layer from the linear development phase into the nonlinear development state. The laser-generated perturbation has a significant effect on the promotion of the development of disturbance waves in the boundary layer. At the same time, laser-generated perturbation that has different influences on the boundary layer when it focuses on different positions. When the laser focus disturbance focuses on the location X=100 mm, the amplitude of the disturbance wave with a frequency of 90 kHz in the boundary layer grows fastest, and the amplitude magnification at the position of X=500 mm is 3.81. When the laser perturbation is added to the free flow in front of the cone, the frequency of the disturbance wave with the fastest amplitude increase speed greatly decreases to 73 kHz. In the same range, the amplitude magnification is 4.51 times. It can be seen that when the laser focuses on the free stream upstream from the cone, its effect on the disturbance wave in the boundary layer is more significant.