Generation of acoustic tones in round jets at a Mach number of 0.9 impinging on a plate with and without a hole

Abstract

The generation of acoustic tones in four round jets at a Mach number of 0.9 impinging on a plate at a distance $L=6r_0$ from the nozzle exit, where $r_0$ is the nozzle radius, has been investigated by large-eddy simulation. Three plates are perforated by holes of diameters $h=2r_0$ , $3r_0$ and $4.4r_0$ , centred on the jet axis, whereas the fourth plate has no hole, in order to study the effects of the hole on the jet flow and acoustic fields. In all cases, acoustic tones emerge in the jet near field, upstream but also downstream of the plate for the perforated plates. Their frequencies are similar for all jets and close to those predicted for aeroacoustic feedback loops establishing between the nozzle and the plate, involving flow disturbances convected downstream and waves propagating upstream at the ambient speed of sound. Their levels, however, decrease with the hole diameter, by a few dB for $h\leqslant 3r_0$ but approximately by 40 dB for $h=4.4r_0$ . The features of the feedback loops are identified by computing two-dimensional space–time correlations and frequency–wavenumber spectra of the pressure fluctuations in the jet mixing layers. These loops are found to be closed by free-stream upstream-propagating guided jet waves, produced by the impingement of vortical structures on the plate for $h\leqslant 3r_0$ and by the scattering of the jet aerodynamic pressure fluctuations by the hole edges for $h=4.4r_0$ . Finally, an acoustic analogy is used to show that the contributions of the pressure fluctuations on the plate to the radiated noise are dominant.

Flow and Acoustic Fields of Rocket Jets Impinging on a Perforated Plate

“…The humps in the near-nozzle spectra for the impinging jets may thus be generated by feedback loops establishing between the nozzle and the plate closed by upstream-propagating guided jet waves. However, unlike the case of impinging jets generating intense tones [12,13], the spots of high energy in the frequency-wavenumber spectrum do not extend over all wavenumbers. Therefore, the feedback phenomenon in the present jets is probably weak, which may be due, at least in part, to the large nozzle-to-plate distance.…”

“…The numerical setup is identical to that used in recent LES of subsonic and supersonic, free [18,19], and impinging [12,13] jets. In the simulations, the unsteady compressible Navier-Stokes equations are solved in cylindrical coordinates r; θ; z using an OpenMPbased in-house solver.…”

“…In addition, the resolution of earlier numerical simulations is rather low, yielding a Strouhal cutoff number around 0.2 [2,3], which does not allow high-frequency noise components to be captured. Finally, in the studies on impinging jets at a Mach number lower than 2 [5][6][7][8][9][10][11][12][13], intense tones were also shown to be generated by feedback loops establishing between the nozzle exit and the plate. Such tones do not seem to emerge for Mach numbers around 3 [2,3], suggesting that there are no feedback loops or only weak ones in that case.…”

Mach number dependence of tone generation in impinging round jets

“…The numerical setup is the same as that of very recent LES of subsonic [14] and supersonic [29,30] impinging jets. The unsteady compressible Navier-Stokes equations are solved in cylindrical coordinates (r, θ, z) using an OpenMP based in-house solver.…”

Presence and properties of acoustic peaks near the nozzle of impinging rocket jets

“…In order to characterize them, the impingement of the gases exhausted by the engines on the launchpad can be modeled as a jet impinging on a flat plate. Such a simplified set-up has been studied experimentally and numerically for high-subsonic jets [2][3][4][5][6][7][8]. Intense tones were reported in the pressure spectra close to the jet nozzle.…”