Abstract Acoustic measurements and time-resolved schlieren visualizations are performed on underexpanded jets issuing from a convergent nozzle to investigate the screech feedback loop and the mode staging phenomenon. In the present experiments, the nozzle pressure ratio ranges from 2.00 to 4.00. The instability mode of the jet successively experiences the axisymmetric A1 and A2 modes, the flapping B mode and the helical C mode with the increase of the pressure ratio. Dynamic mode decomposition is conducted on the schlieren images to extract the flow structures associated with different screech tones. When the jet transforms from the B mode to the C mode, a secondary axisymmetric u mode is identified. The results of wavelet analyses show that multiple screech modes can coexist during the staging processes from the A2 mode to the B mode and from the B mode to the C mode. The coexistence state of screech modes is studied based on the modal analysis results. The high amplitude parts associated with the B mode move upstream during the transformation process from the A2 mode to the B mode and move downstream during the transformation process from the B mode to the C mode.

中文翻译：

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轴对称欠扩射流的尖叫反馈回路及模态分级过程

摘要 声学测量和时间分辨纹影可视化是对从收敛喷嘴发出的欠膨胀射流进行的，以研究尖叫反馈回路和模式分级现象。在本实验中，喷嘴压力比的范围为 2.00 到 4.00。射流的失稳模态随着压力比的增加依次经历轴对称A1和A2模态、扑动B模态和螺旋C模态。对纹影图像进行动态模式分解以提取与不同尖叫音调相关的流动结构。当射流从 B 模式转变为 C 模式时，识别出二次轴对称 u 模式。小波分析结果表明，在从A2模式到B模式和从B模式到C模式的分期过程中，多种尖叫模式可以共存。基于模态分析结果研究了尖叫模式的共存状态。与B模式相关的高振幅部分在从A2模式到B模式的转换过程中向上游移动，在从B模式到C模式的转换过程中向下游移动。