Short conical solid/perforated plug-nozzle as supersonic jet noise supressor

Abstract

Experimental studies of the acoustic far field, and the nature of the repetitive shock structure in cold jet flows from an externally expanded plug-nozzle having a short conical plug (with and without porosity) with a pointed termination, operated in a range of supercritical pressure ratios ξ = 2·0 to 4·5, are reported. The overall sound pressure levels ( OASPLs) of a short solid concial plug-nozzle (CPN) are found to be within 3 dB of those of the baseline noise levels of a minimum length contoured plug-nozzle operated at the same pressure ratio and having the same annular throat area. As compared to an equivalent convergent nozzle, the noise reductions achieved through the use of a short solid CPN are of the order of 5 dB. The porosity of the plug surface (10% distributed over almost the entire plug surface or 4% distributed over the middle third) results in further noise suppression of up to 3 dB, thus approaching the aeroacoustic performance of an equivalent contoured plug without perforations. Shadowgraphic data indicate that the presence of perforations on the plug surface results in the weakening and favorable modifications of the repetitive shock structure in the improperly expanded jet flows, leading to the observed reductions in shock-associated noise.