Suppression on Shock Oscillation of Impinging Jet
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摘要: 欠膨胀冲击射流具有复杂的激波结构，并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量，对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析，射流剪切层不稳定波有对称和非对称两种模态，发现不同模态下的离散频率单音声源位于射流冲击平板后的不稳定的流动结构处.反馈声波在喷嘴唇口对剪切层扰动激励，使剪切层不稳定波向下游发展.激波结构与剪切层相互作用发生振荡，并与不稳定波模态的特征相关.通过消弱反馈声波在喷嘴唇口对剪切层的扰动，可消除离散频率的冲击单音，降低噪声，同时也抑制了激波振荡，使得激波结构趋于稳定.Abstract: Underexpanded impinging jets have complex shock structures and generate high-amplitude tones with discrete frequencies. Based on the schlieren images obtained by the high-speed camera and the results of noise measurements, the oscillations of shocks, the modes of instability waves in the jet shear layer and the generation of the discrete tones of underexpanded impinging jets were studied. In this paper, the experimental results of the impinging jet with an impact distance of 5 nozzle exit diameters were presented. It is found that there are two modes of the instability waves in the shear layer: symmetric and asymmetric modes. The sound source of the impinging tones at different modes is located at the instability flow structures after impinging on the plate. The feedback acoustic waves excite the shear layer at the nozzle lip and maintain the development of the instability waves. The oscillations of shocks are caused by the interactions between the shocks and the shear layer and are related to the characteristics of the instability modes. By weakening the disturbances of the feedback acoustic waves on the shear layer at the nozzle lip, the discrete tones are eliminated and the shock oscillations are suppressed.
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Key words:
- underexpanded jet /
- impinging jet /
- discrete tone /
- shock oscillation /
- noise reduction
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图 4 原始喷嘴欠膨胀冲击射流噪声频谱图
Figure 4. Noise spectrum of underexpanded impinging jet of original nozzle
图 5 NPR=3.0欠膨胀冲击射流激波振荡过程
Figure 5. Shock oscillation process of underexpanded impinging jet, NPR=3.0
图 6 NPR=3.5欠膨胀冲击射流激波振荡过程
Figure 6. Shock oscillation process of underexpanded impinging jet, NPR=3.5
图 7 NPR=3.5欠膨胀冲击射流反馈声波在喷嘴出口附近对剪切层的激励过程
Figure 7. Excitation process of the upstream-propagating acoustic wave on the shear layer near the nozzle exit
图 8 欠膨胀冲击射流噪声频谱图对比
Figure 8. Noise spectra of the modified nozzle and original nozzle under two operating conditions
图 9 欠膨胀冲击射流瞬时纹影图对比
Figure 9. Comparison of instantaneous schlieren images of underexpanded impinging jet
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