Understanding twin-jet screech using a vortex-sheet model

A twin-jet vortex-sheet model is applied to study the axisymmetric screech modes, generated by simply converging nozzles, for low supersonic Mach numbers. This allows for the identification of the different waves supported by the flow for different conditions. Propagative regions for the upstream-propagating neutral guided jet mode are observed to vary both as a function of solution symmetry, SS and SA, and the jet separation, whilst bounding the screech tones. The A1 screech mode is observed to be cut-off sharply by the SS lower bound. Screech frequency predictions are performed using the wavenumbers obtained from the vortex-sheet model, which provided a good qualitative agreement with the experimental data. Additional A1 and A2 screech tones were observed in the acoustic spectrum as the jet separation was increased, pointing to the existence of new resonance cycles associated with the different symmetries of the flow. These additional tones exist simultaneously and are seen to merge together as increases leaving only a single A1 and A2 tone.