Effects of passive grids on pipe and orifice jet noise

The present study investigates the effect of passive grids on the acoustic characteristics of pipe and orifice jets for different Nozzle Pressure Ratios (NPRs). A detailed blow-down and directivity studies have been carried out for various emission angles. To understand the distribution of acoustic radiations with frequency, the spectral analysis is performed. Additionally, the acoustic power is estimated in order to signify the effects of grids. The presence of grids influences the noise level depending on the operating ranges of NPRs. At low NPRs, the grids enhance the overall sound pressure level; mainly due to the turbulent mixing noise. Whereas, at high NPRs, a considerable noise reduction is observed. However, grids assist to shift the noise level from low frequency to high frequency for all ranges of NPR. The eddy fragmentation by the grids at low NPR results in shifting the low frequency noise to a high frequency. In general, at high NPRs the shock associated noise [screech and broadband shock associated noise (BSAN)] is dominant over the turbulent mixing noise. The presence of grids successfully eliminate the dominant tones (screech) by disrupting the feedback loop and attenuate the BSAN by converting strong shock cells into weaker multiple shock-lets.