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

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 <= 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 <= 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.