Experimental analysis of three-dimensional coherent structures associated with the screech-resonance mechanism in underexpanded jets: Spatiotemporal super-resolution and dynamic-mode-decomposition analyses of density fields

Three-dimensional (3D) coherent structures associated with screech tones, along with their correlations and resonance mechanisms, were investigated in underexpanded jets at a nozzle pressure ratio of 2.52 using the 3D spatiotemporal super-resolution reconstruction (3DSSR) method and dynamic mode decomposition analysis. Both approaches successfully captured coherent structures associated with the dominant flapping-mode (B mode) and the secondary axisymmetric-mode (a2 mode) screech tones, while the 3DSSR method additionally provided their temporal evolution with high resolution. The two oppositely rotating helical structures forming the B-mode flapping structure exhibited similar flow characteristics but were negatively correlated, leading to azimuthal transitions occurring in the B mode. Furthermore, the correlation between the helical structures of the B mode and the axisymmetric structure of the a2 mode was examined. One helical structure showed a positive correlation with the axisymmetric structure, while the other displayed a negative correlation. Consequently, the a2-mode axisymmetric structure attained low amplitude when the B-mode flapping structure occurred and high amplitude when the B-mode helical structure dominated. Finally, the screech-resonance mechanisms of the a2 and B modes were investigated using the dispersion relations of the upstream-propagating guided-jet modes and the classical feedback model. The results demonstrated that the screech-feedback loops for both modes are closed by upstream-propagating the guided-jet modes generated by the downstream-propagating Kelvin–Helmholtz wavepacket and the shock-cell structure.