Mach number dependence of tone generation in impinging round jets
Résumé
The Mach number dependence of the tones generated by round jets impinging on a flat plate, due to aeroacoustic feedback loops establishing between the jet nozzle and the plate, are investigated using large-eddy simulations. Six jets at Mach numbers varying from 0.6 to 1.1 and a nozzle-to-plate distance of 8 nozzle radii are considered. For M = 0.6, the upstream sound radiation is broadband and weak, which suggests the absence of marked feedback phenomena. For higher Mach numbers, it is tonal and intense, highlighting the establishment of feedback loops between the nozzle and the plate. Tones are present at the same frequencies in both the velocity spectra in the shear-layer and the near-nozzle acoustic spectra, highlighting a coupling between the jet flow structures and the upstream-propagating pressure waves. For M ≤ 1, the dominant tone is associated with an axisymmetric oscillation mode of the jet, whereas for M = 1.1, it is related to an helical mode, showing that the jet azimuthal structure is affected by the Mach number. For both azimuthal modes, standing-wave patterns are found inside the jets in the pressure amplitude fields at the tonal frequencies, indicating interactions between upstream and downstream-propagating waves. Moreover, the tone frequencies decrease with the Mach number. They are located in the frequency ranges of the upstream-travelling guided jet waves, indicating that these waves close the feedback loop in all cases. As for the amplitude of the tones, it increases by about 45 dB between M = 0.6 and 1, which is much higher than the rise predicted by the M 8 law. It is then reduced by 15 dB for M = 1.1, suggesting a weaker resonance for this jet.
Domaines
Sciences de l'ingénieur [physics]
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