Propagation characteristics of acoustic wave emitted from hypersonic turbulent boundary layer

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Abstract
The background perturbations in hypersonic flows have a significant impact on boundary layer transitions. As one of the primary approaches of fluid dynamic studies, the traditional noisy wind tunnels are dominated by the acoustic disturbances radiated from hypersonic turbulent boundary layer on the nozzle. A large number of investigations have focused on the overall characteristics of acoustic radiation. However, few have looked into the variation of the characteristics with the frequency and scale of disturbances. Therefore, the propagation characteristics of acoustic wave varying with frequency is thoroughly investigated in Mach-6 hypersonic flow in this study by experimental and numerical methods. The experiment of the two hot-wire probes indicated that the convection velocity of low-frequency acoustic disturbances is ∼60%–75% of the freestream speed and tends to increase with frequency. Direct numerical simulation showed that the primary radiation angle determined by low-frequency disturbances is about 30°. The correlation analysis showed that the convection velocities normalized by the freestream velocity for high-frequency disturbances are about 83.3%, resulting in a radiation angle approaching 90°. Meanwhile, measurements by focused laser differential interferometer also revealed that the high-frequency disturbances had a preferred gradient direction of 0°, indicating a radiation angle of 90°. In general, the current study indicates that the radiation angles and convection velocities of disturbances increase from 30° to 90° and from 67% to 83.3%, respectively, as the frequency increases.

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