Numerical study of the supersonic impinging jet oscillation and tone generation mechanism

Supersonic impinging tones have been attracting significant interest because high-intensity discrete-frequency tones pose substantial risks to structural safety in applications such as rocket launch and recovery, and space vehicle attitude adjustment. However, various issues remain to be addressed regarding the jet oscillation and tone generation mechanism. In this study, a numerical simulation of the supersonic impinging jet with a nozzle pressure ratio of 4.03 and an impingement distance of 2.08 times the nozzle exit diameter is conducted. The results show good consistency with the reference data by other researchers. A phase-locked averaging analysis of 2960 flow field snapshots is employed to investigate jet structure oscillation dynamics and the tone generation mechanism. The phase-locked averaged images reveal that the pressure variation induced by Kelvin-Helmholtz vortices as they pass through the reflected shock results in the periodic motions of the reflected shock and Mach disk. The periodically oscillating Mach disk generates high-pressure fluid masses driving recirculation bubbles through a cyclic 'compression-generation-merging' oscillation. The streamline oscillation and sound-ray analyses reveal there are two distinct tone source regions: the impinging zone and the wall jet region. Consequently, it is proposed that vortex collapse in conjunction with wall jet oscillations coexist to generate the tone. According to the directivity, the tone emitted from the wall jet source region is believed to contribute to the feedback loop. These findings collectively contribute to an improved understanding of the jet plume oscillation and tone generation mechanisms of the supersonic impinging jet.