Linear Stability Analysis of Guided Jet Modes in High Aspect Ratio Rectangular Screeching Jets

A comprehensive analysis of modes supported in an aspect ratio 4:1 rectangular screeching jet is performed using linear stability analysis (LSA). The governing equations are linearized around 2D cross-sectional base flow profiles obtained from large-eddy simulation (LES) data generated by Wu et al. (2022). 'Flapping' and 'wagging' guided jet modes and trapped duct modes are identified through LSA, and their stability characteristics are evaluated as functions of frequency and streamwise location. The band-of-existence of the flapping guided jet mode shifts to lower frequencies as the LSA is marched downstream and only includes the screech frequency in a narrow region downstream. Upstream of this region, the guided jet mode may become a free-stream acoustic mode, and downstream it is predicted to be evanescent. The pocket of neutral stability of the flapping guided jet mode aligns well with the location of peak guided jet mode amplitude in the LES for three operating conditions. Eigenfunctions and eigenvalues associated with the flapping guided jet mode are also shown to have good agreement with the guided jet mode present in the LES.