Subsonic jet noise reduction by microjets - a parametric study

The effect of injecting tiny secondary jets ('mu jets') on the radiated noise from a subsonic primary jet is studied experimentally. The Wets are injected on to the primary jet near the nozzle exit with variable port geometry, working fluid and driving pressure. A clear reduction in the overall sound pressure level in the direction of peak noise radiation is observed that improves with increasing Wet pressure. It is found that smaller diameter ports with higher driving pressure, but involving less thrust and mass fraction, can produce better noise reduction. A collection of data from the present as well as past experiments is examined in an attempt to correlate the noise reduction with the operating parameters. The results indicate that the OASPL reduction, as monitored at a shallow angle, correlates with the ratio of Wet to primary jet driving pressures normalized by the ratio of corresponding diameters (p(mu)d/p(j)D). With gaseous injection, the spectral amplitudes decrease at lower frequencies while an increase is noted at higher frequencies. The amplitude 'crossover' is thought to be at least partly due to shock-associated noise from the underexpanded Wets themselves. Such crossover is not seen with water injection apparently because the flow in that case is incompressible and there is no shock-associated noise. Centerline velocity data show that larger noise reduction is accompanied by faster jet decay as well as significant reduction in turbulence intensities. While a physical understanding of the dependence of the noise reduction on p(mu)d/p(j)D remains unclear, given this correlation, an analysis explains the observed dependence of the effect on various other parameters.