  Screech resonance is studied with experimentally validated large-eddy
simulation data for a 4:1 rectangular under-expanded jet at three nozzle
pressure ratios. The analysis uses spectral proper orthogonal decomposition
(SPOD) and spatial cross correlation to characterize the oppositely-traveling
waves in the jet at the screech fundamental frequency. The results support
recent theoretical framing of screech as absolute instability, and further
reveal the spatial separation of individual processes for screech generation.
From the leading-order SPOD mode, direct evidence of the guided jet mode being
the screech closure mechanism, not the external acoustic feedback, is observed.
A match in the spatial wavenumber suggests the guided jet mode is generated via
interactions between the Kelvin-Helmholtz wave and the shock cells. The energy
of the oppositely-moving waves shows spatially global and non-periodic behavior
of the coherent structures in the streamwise direction. The ratio of wave
energy identifies regions where distinct processes in screech generation take
place by comparing the rate of energy propagation in the downstream direction
to that of the upstream direction. The distinct regions correspond to initial
shear layer receptivity, sound emission, guided jet mode excitation and decay
of coherence. The leading-order SPOD mode also enables the approximation of
Lighthill's stress tensor and allows for accurate calculation of the far-field
screech tone amplitude with the acoustic analogy formulation. The current
findings provide insights on building a physics-based reduced order model for
screech amplitude prediction in the future.
