Noise and noise reduction in supersonic jets

This paper describes a novel noise reduction method for supersonic jets. It involves the injection of relatively small amounts of air into the diverging section of a convergent-divergent nozzle. The air is injected through a series of injectors that are aligned axially. The injected air diverts the jet core flow and forms "fluid inserts." These inserts have the same effect as hard-walled corrugated seals, but can be modified by changes to the injector pressure ratios. The inserts change the effective area ratio of the nozzle and can enable the jet to operate closer to an ondesign condition. This has the effect of weakening the shock cell structures in the jet and reducing the broadband shock-associated noise. In addition, streamwise vortices are generated that break up the large-scale turbulent structures in the jet and result in a reduction in the supersonic mixing noise in the peak noise radiation direction. Experiments are described that examine the levels of noise reduction achieved by the fluid inserts. The effects of the number and azimuthal distribution of the inserts are examined. The effect of a change of experimental scale from small to moderate model sizes is also given. Noise reductions in the peak noise direction of up to 6 dB are obtained and broadband shock-associated noise is reduced. Ongoing plans and open questions are also discussed.