Time-resolved refractive index matched particle image velocimetry measurements of unsteady flow dynamics in bi-cross channel-cavities

This study experimentally investigates the vortex dynamics and flow characteristics in bi-cross channel-cavity flows with varying cavity lengths (H/De=1, 2, 3) at a Reynolds number of 10 000, employing time-resolved refractive index matched particle image velocimetry. The analysis integrates time-averaged velocity fields, Reynolds shear stress distributions, spectral proper orthogonal decomposition (SPOD), and transient event statistics to elucidate the interplay between cavity geometry and flow organization. Key findings reveal distinct modulation mechanisms: shallow cavities (H/De=1) exert a global influence, manifested by expansive separation area in the mainstream region (>0.5 De) and a unique γ-shaped separation near the outlet-recirculation interface. In contrast, deeper cavities (H/De=2, 3) have localized effects, with circulation confined within the cavity and the adjacent region entrance; deep cavities exhibit dual recirculation structures constrained by cavity walls. SPOD analysis indicates that shallow cavities exhibit low buffer capacity, leading to overflow merging with the mainstream and generating low-speed patches, while deeper configurations prioritize enhanced shear layer near stagnation regions. The transient event correlations reveal phase lags between vortex shedding and cavity inflow, with deep cavities showcasing enhanced shear interactions near stagnation regions due to reduced vortex motion angles. These findings underscore the critical role of cavity depth in balancing global coherence and localized turbulence, providing direct insights for optimizing cooling channel designs in high-precision lithography systems. Specifically, the stabilization of heat transfer and suppression of flow-induced vibrations are achievable through tailored cavity geometries. This work establishes benchmark data for validating numerical models targeting vortex dynamics and shear layer variation in complex cavity flows.