Coherence-to-multiscale transition of shear-layer vortex shedding in a closed side-branch junction

This paper investigates a coherence-to-multiscale transition of shear-layer vortex shedding in a closed side-branch junction and quantifies how geometric perturbations modify (i) the spectral distribution of pressure fluctuations across frequencies and (ii) the spatial footprint of the dominant-frequency oscillations. Large-eddy simulations are analyzed using spectral proper orthogonal decomposition (SPOD) for 16 geometries with side-branch diameters D side = 24, 32, 40, and 48 mm and lip fillet-chamfer diameters D cham = 0, 10, 20, and 30 mm. After statistically steady states are reached, we characterize the dominant shedding frequency, the temporal standard deviation of pressure fluctuations, the leading SPOD-mode energy fraction and its spatial footprint, and the time-averaged shear-layer length and thickness. Increasing the geometric dimensions reduces the dominant shedding frequency and amplifies pressure fluctuations, while the main-frequency energy fraction decreases and the dominant-response region becomes markedly more spatially dispersed. These trends indicate that the shear layer evolves from a dominant narrowband shedding state, with a strong leading SPOD mode at the shedding frequency, to a broader-band state in which energy is distributed among higher-order modes and broadband fluctuations. This reduction in single-frequency dominance may lower the propensity for lock-in when a flow-acoustic feedback pathway is present. An equivalent characteristic length derived from the shear-layer statistics accounts for the combined effects of the side-branch and lip fillet-chamfer diameters. The resulting Strouhal numbers range from 0.9 to 1.3, supporting a common convective time-scale interpretation of the dominant shedding frequency across the investigated geometries.