| Summary: | 碩士 === 大仁科技大學 === 環境管理研究所 === 101 === This study way conducted to investigate the effectiveness of air-dhrouding nozzles on reducing noise levels at audible frequencies. Shrouding air will decrease the velocity gradient formed between the high-speed jet and the ambient air. Thus, this approach is effective for reducing the levels of noise generated near the nozzle exit, which falls mostly in the ultrasound frequency range for a small-exit nozzle because the peak frequency of jet noise is inversely proportional to the exit diameter. It's effect on the levels of lower-frequency noise, which are more closely associated with hearing damage, is still unclear. We used a nozzle with an exit diameter of 2.8 mm and measured the noise levels with or without shrouding air.The exit velocity for the main exit is supersonic. The frequency-band analysis of the noise showed that the shrouding air might reduce noise levels near the peak frequency. However, the noise levels at audible frequencies are affected very little if the additional momentum due the shrouding air is considered.
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