Study of an Aeroacoustic Internal Feedback Loop in a High-Speed Jet Using Mode Decomposition Methods
Résumé
In a subsonic free jet, vortex sound is generated at the end of the potential core, but the mechanism of generation of that sound is still unclear. A recent study proposed that pressure waves propagating upstream in the jets interact with Kelvin-Helmholtz (K-H) instability waves, possibly creating a feedback loop that intensifies (Bogey, 2021). In this study, we applied total a least squares dynamic mode decomposition (TlsDMD) to the time series of the numerical results of an axisymmetric subsonic free jet and performed a detailed analysis of this hypothesis. We extracted the DMD mode at the K-H instability frequency, i.e., the dominant frequency peak near the nozzle exit. The findings indicate that this DMD mode contains not only K-H instability waves but also the pressure waves propagating upstream in the jet. Second, we applied a spatiotemporal Fourier analysis to the DMD mode to describe the characteristics of the pressure waves and confirmed that the pressure waves originate at the end of the jet potential core and that a feedback loop exists at the K-H instability frequency. Our results support this feedback mechanism for sound generation in subsonic free jets.
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