Generation of Screech Tones at the 2nd Harmonic for a Rectangular Jet*

Supersonic jet screech is a form of aero-acoustic resonance observed in non-perfectly expanded jets. Downstream-traveling hydrodynamic waves and upstream-traveling disturbances in the jet form a constructive feedback loop which produces high-intensity tones. The upstream-traveling waves comprise of both external acoustic waves and a guided jet mode. The fundamental screech tone is believed to result from interaction between the Kelvin-Helmholtz instability waves and shock cells. It has also been observed that the guided jet mode plays an important role in closing the feedback loop at the fundamental (Wu et al., 2021). The mechanisms responsible for producing higher harmonic screech tones, however, are less well understood. We perform high-fidelity LES simulations of jet screech from a 4:1 rectangular nozzle. SPOD is used to analyze the role of the upstream-traveling guided mode in closing the feedback loop at the second harmonic frequency. Analysis of the spatial wavenumbers also suggests that self-interaction of the guided jet mode at the fundamental frequency could produce the second harmonic guided mode. The performance of phased acoustic source models based on this theory are assessed for their abilities to reproduce the directivity patterns of the second harmonic.