Srinivasan K

Supersonic impinging jet noise reduction is an important problem, especially in STOL aircraft, VTOL aircraft and rockets. These types of impinging jets generate a larger amount of noise and highly unsteady flows, which lead to noisy environment, posing a hazard to humans and materials in the proximity. The present study is carried out on supersonic impinging cold jets for nozzle pressure ratio (NPR) values of 2 and 5 with nozzle to plate distance (x / D) of 5, at different swirl numbers. The swirl flows are generated using co-axial curved blades and the results were compared with non-swirl or free jets. The vane angles considered here are 20°, 40°, 60° and the swirl numbers ranged from 0 to 1.31. At NPR 2, the weak swirl number of 0.27 reduced the OASPL level by around 7 dB compared to non-swirl jets. The non-swirl jet emitted impinging tones at NPR 5 and the swirl jet eliminated the impinging tones. At a high swirl number of 1.31, at NPR 5, OASPL is lowered by around 12 dB compared to non-swirl jets. The flow visualization study shows that the swirl disintegrates the repeated shocks and reduces the length of the shock cell system.

Experimental investigations have been carried out on chevron nozzles to assess the importance of chevron parameters such as the number of chevrons (chevron count) and chevron penetration. Acoustic measurements such as overall sound pressure level, spectra, directivity, acoustic power, and broadband shock noise have been made over a range of nozzle pressure ratio from sub-critical to underexpansion levels. Shadowgraph images of the shock-cell structure of jets from various chevron nozzles have also been captured for different nozzle pressure ratios. The results indicate that a higher chevron count with a lower level of penetration yields the maximum noise suppression for low and medium nozzle pressure ratios. Of all the geometries studied, chevron nozzle with eight lobes and 0° penetration angle gives the maximum noise reduction. Chevron nozzles are found to be free from screech unlike regular nozzles. Acoustic power index has been calculated to quantitatively evaluate the performance of the various chevron nozzles. Chevron count is the pertinent parameter for noise reduction at low nozzle pressure ratios, whereas at high nozzle pressure ratios, chevron penetration is crucial. The results illustrate that by careful selection of chevron parameters substantial noise reduction can be achieved. © 2009 Elsevier Ltd. All rights reserved.

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.