Internal Acoustic Mode in Rectangular Jet Screech

Experimentally validated high-fidelity LES of rectangular under-expanded jets are used to study the feedback involved in screech generation. The leading SPOD modes at the screech fundamental frequency are extracted from pressure and transverse velocity fluctuations in the jet minor-axis plane, showing both the coherent structures inside the jet plume and the acoustic waves outside the jet. Spatial cross correlation with the SPOD data shows that upstream-travelling external acoustic waves and the internal acoustic mode coexist at the resonant and neighboring frequencies. However, the spatial amplification relative to nozzle exit reference for the internal mode is much larger at the resonant frequency. On the other hand, the spatial amplitude of the external mode is similar at resonant and non-resonant frequencies. The cross-correlation analysis is also applied to two other screech cases at different NPR values and similar results are obtained. The local mode shape of the internal mode agrees well with the theoretical model by Tam & Norum (1992).