Three-dimensional shear layer dynamics in a supersonic rectangular multi-stream jet

Three-dimensional (3-D) spectral proper orthogonal decomposition (SPOD) and resolvent analysis are performed to investigate the flow characteristics of a supersonic rectangular multi-stream jet. SPOD reveals low-frequency broadband and high-frequency tonal features, highlighting the complex dynamics of the rectangular jet flow. The tonal peak is attributed to two-dimensional Kelvin-Helmholtz (KH) instability arising from the mixing between the core (Mach 1.6) and bypass (Mach 1) streams. In contrast, the broadband content originates from the upper and lower shear layers (USL and LSL), formed as the jet mixes with the ambient air. Bispectral mode decomposition reveals broadband nonlinear energy transfer within the USL and LSL through triadic interactions. Tri-global resolvent is employed to examine the input-output dynamics in these shear layers, with energy trends aligning with SPOD findings. The resolvent modes identify 3-D KH-type amplification mechanisms in the initial shear-layer regions of both the USL and LSL. At low frequencies, streamwise vortices emerge from the nozzle corner, contributing to downstream axis-switching in the rectangular jet. Resolvent wavemaker analysis confirms a self-sustaining vortex mechanism near the corner.