Some Insights into the Screech Tone of Under-Expanded Supersonic Jets Using Dynamic Mode Decomposition

Jet screech is an intense pure tone which has attracted decades of research interest due to its possible detrimental effect to engineering structures. Its modes and closure mechanisms have been investigated analytically, experimentally and numerically, however, there are still outstanding questions on the generation and propagation of instabilities in the near-field region. Recent studies have identified that the instabilities also travel inside the jet potential during the screech process to form the complete feedback loop. Using dynamic mode decomposition on a 3D pressure near-field from large eddy simulation results, the present study examines the viability of the modal decomposition to provide further insights into the screech modes and its associated characteristics and investigates the effect of temperature mixing in jet screech. The results show that the modal decomposition approach identifies very well the helical structure of the screech mode. Furthermore, a method is proposed to reveal the temporal evolution of the dynamic screech mode. It was found that the bulk behaviour of the pressure field at screech frequency propagates backward towards the nozzle exit.