Screech feedback loop and mode staging process of axisymmetric underexpanded jets

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.