Supersonic Jet Noise Reduction Performance and Noise Reduction Mechanism of Tab Jet Noise Suppresser

Abstract Performance of tab jet noise suppresser was evaluated by ratio of jet noise reduction to jet thrust loss. To clarify tab size and shape effects on the performance, 16 kinds of rectangular shape tabs were selected. They had different height H and width B dimensions and flow blockages (0.25% to 8.54%). The far-field acoustic measurement in the polar angular direction and the direct jet thrust measurement of conical convergent nozzles fitted with the two diametrically oppositely placed tabs were carried out over ranges of jet flow Mach number 0.991 to 1.884. The jet thrust loss and the jet noise reduction in PNLT dB were estimated for each tab over the whole jet velocity cases. The results showed that the tab with the smallest flow blockage factor of 0.25% could achieve remarkable high values more than 10 PNLT dB/ % within jet Mach number range of 1.285 to 1.484. It was clarified that the tabs with flow blockage factors of 0.25% to 1.0% were very available supersonic jet noise suppressers with more than 4 PNLT dB/%. Tabs with smaller width than B/D = 0.124 (D; nozzle lip inner diameter) could efficienty eliminate screech tones as well as reduced a part of shock-associated broadband noise and jet mixing noise. However, tabs with larger width than B/D = 0.284 and larger flow blockage factors than 2% adversely regenerated discrete tones that were inferred to be screech tones. The mechanisms of elimination and regeneration of screech tones by the variation of tab geometry were investigated, using a schlieren visualization system synchronized with screech tones. From the acoustic data and the flow visualization data, it was considered that small wide tabs stabilized the large scale turbulence structure/instability wave of jet flows and resulted in screech tone elimination. While, large wide tabs regenerated screech tones to excite the instability waves, because the tabs significantly changed jet flow plume structures of conical nozzles into similar structures produced by rectangular exit nozzles and resulted in the flapping mode occurrence.