Evolution of Effective Source and Screech Mode in Compressible Starting Jet

High-speed jets have wide applications in many directions, such as material cutting, surface cleaning, and rocket launches. The steady situation of free jet has been widely studied, but transient evolution details have not been given, so the analysis of the starting process of the supersonic free jet is the main concern. The large-eddy simulation of supersonic underexpanded free jet with a nozzle pressure ratio of 2.7 is performed. Both transient and steady flow characteristics are in good agreement with the experiment. The overall flowfield is axisymmetric at first and becomes antisymmetric in the end. Short-time Fourier transform is employed to explore changes in screech frequency during the starting process of the jet. Variations in the position of acoustic source are given for the first time. Dynamic mode decomposition analysis is applied to study the screech mode changing. The wavenumber spectra and dispersion relations are utilized to indicate that the Kelvin-Helmholtz wavepacket interacts with different wavenumbers of the shock-cell structures at different stages of the starting jet.