Large-Eddy Simulations of Round Jets at a Mach Number of 0.9 Impinging on a Plate with and without a Hole

Abstract

Four initially highly disturbed round jets at a Mach number of 0.9 impinging on a plate located at a distance L = 6r 0 , where r 0 is the nozzle radius, have been computed by Large-Eddy Simulation (LES). Three of the plates have a hole of diameter d = 2r 0 , 3r 0 and 4.4r 0 , centered on the jet axis, whereas the fourth one has no hole, in order to study the effects of the hole diameter on the jet flow and acoustic fields. For the cases with no hole and the holes of diameter d = 2r 0 and 3r 0 , a feedback loop establishes between the jet nozzle and the plate, which generates intense tones in the acoustic spectra. For d = 4.4r 0 , a similar feedback loop is observed, but it generates tones of weaker amplitude. For all cases, the feedback frequency is in agreement with the classical model of an aeroacoustic feedback between the nozzle and the plate. The acoustic levels are however reduced when the hole diameter increases. The same standing wave pattern between the nozzle and the plate is highlighted by a study of the aerodynamic field at the main tonal frequency. Moreover, for d = 2r 0 and 3r 0 , flow rate fluctuations are caused by turbulent structures passing through the hole at the feedback frequency, which creates a tonal radiation downstream of the plate. A model of this radiation by a monopole source of equivalent flow rate is proposed.

Large-eddy simulations of the flow and acoustic fields of a rocket jet impinging on a perforated plate

“…The overall sound pressure levels were found to decrease when the hole diameter increases, with a reduction varying from 2 dB for h = 2D up to 4 dB for h = 4D compared with the flat-plate configuration. In other studies on impinging jets at a Mach number lower than 2 [5][6][7][8], intense tones were also shown to be generated by feedback loops establishing between the nozzle exit and the plate. Such tones do not appear to emerge for Mach numbers around 3, suggesting there is no feedback loops or only weak ones in that case.…”

“…The numerical set-up is identical to that used in recent LES of subsonic and supersonic, free [13,14] and impinging [8,15] jets. In the simulations, the unsteady compressible Navier-Stokes equations are solved in cylindrical coordinates (r, θ, z) using an OpenMP based in-house solver.…”