INFLUENCE OF THE NOZZLE-EXIT BOUNDARY-LAYER THICKNESS ON THE NOISE PRODUCED BY SUBSONIC IMPINGING JETS
Résumé
The influence of the nozzle-exit boundary-layer thickness on the noise produced by initially laminar subsonic impinging jets is investigated using large-eddy simulations. For that purpose, six round jets at Mach numbers of 0.6 or 0.9, with nozzle-exit boundary-layer thickness equal to 0.05r 0 , 0.1r 0 or 0.2r 0 , where r 0 is the pipe-nozzle radius, are considered. They impinge on a plate located at 6r 0 from the nozzle-exit plane. For the jets at a Mach number of 0.9, the near-nozzle pressure spectra exhibit tones, at similar frequencies in all cases. However, for a thicker boundary layer, the dominant tone is nearly 30 dB stronger. The gain in amplitude of the shear-layer instability waves developing at its frequency between the nozzle and the plate also increases for thicker boundary layers. This indicates that the rise of the peak amplitude is the result of a higher amplification of the shear-layer instability waves. For the jets at a Mach number of 0.6, a large low-frequency hump and weak narrow peaks are found in the near-nozzle pressure spectrum for the thickest boundary layers, while no peaks are observed in the other cases. This suggests that impinging jets at Mach number lower than 0.65 can become resonant as their boundary layers are thicker.
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