Mach Number Dependence of Tone Generation by Impinging Round Jets

The Mach number dependence of tone generation by jets impinging on a plate is investigated using large-eddy simulations, for seven jets at Mach numbers M between 0.6 and 1.3 and a nozzle-to-plate distance of 8 nozzle radii. For M=0.6, the upstream sound radiation is broadband, whereas for M = 0.75, tones emerge in the near-nozzle spectra, highlighting the establishment of feedback loops. The tone frequencies are consistent with those for aeroacoustic feedback loops between the nozzle and the plate, and exhibit a staging behavior with the Mach number, staying inside or close to the ranges of the upstream-traveling free-stream guided jet waves predicted by a vortex-sheet model. The azimuthal and radial structures of the pressure fields at the tone frequencies agree with those of these waves closing the loops. The selection of the dominant tone and the staging behavior are discussed by evaluating the power gains of the shear-layer instability waves between the nozzle and the plate using linear stability. In most cases, the dominant tones emerge at the possible tone frequencies with the highest gain. For M = 0.6, the gain of the instability waves at the frequency of the guided jet waves most likely to close feedback mechanisms is lower than the gain peak values, possibly contributing to the absence of resonance.