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Noise reduction by interaction of flexible filaments with an underexpanded supersonic jet

Experiments were conducted at Boeing Interior Noise Test Facility (INTF) to evaluate the reduction of jet noise by a small diameter cord in a cold underexpanded jet plume. Different cord diameters, lengths and styles were used to determine the optimum configuration. The optimized configuration of the cord was multi-stranded Kevlar fibers, 3 jet diameters long and having thickness of 2% of the jet diameter. A small knot at the downstream tip of the cord stabilized it along the jet centerline. The tests were performed using a single jet and coaxial cold coflowing jets. The primary jet velocity was M=l.2, and the secondary flow was set at M=O or 0.876. Acoustic and flow visualization data showed substantial decrease of the jet noise and a significant change in the flow structure. The shock waves present in the core of the underexpanded jet were weakened and helical structures in the shear layer were suppressed. The strongest effect observed was the complete suppression of the screech tones and reduction of up to 30 dB in the shock-associated noise. This effect was stronger in the single jet than in the coaxial jet and was most noticeable in the region perpendicular to the jet at the nozzle plane. A substantial decrease of the turbulent mixing noise was obtained in the entire range of measured frequencies up to 40 kHz. This effect was most pronounced in the aft region of the jet. The filament changed both the magnitude and directivity of the sound emission. The distribution of the far field Sound Pressure Level became nearly uniform, with predominant perpendicular orientation. This paper presents an effective technique for jet noise reduction which is easy to implement and which does not carry heavy penalty in extra dead weight, loss of thrust and reduced propulsion efficiency. The technique was shown to be most effective for the underexpanded jet flow conditions described in this paper. However, tests were also 1 performed in high subsonic single and coaxial jets and in heated jet flows. In all cases certain degree of turbulent mixing noise reduction was recorded.