Investigation of wake interactions between two side-by-side rigid cylinders near free surface at low Reynolds number

This study presents an experimental and numerical investigation of wake dynamics behind single and two side-by-side circular rigid cylinders submerged near a free surface at a low Reynolds number (Re=U∞D/ν=240), defined using diameter (D), free-stream velocity (U∞), and kinematic viscosity (ν). Using time-resolved particle image velocimetry and validated CFD (computational fluid dynamics) simulations, the effects of submergence depth (H) and inter-cylinder gap (G)—formulated as depth (H/D) and gap (G/D) ratio based on cylinder diameter—on wake structure, vortex shedding, and flow symmetry are systematically examined. For a single cylinder, free surface effects are significant only at shallow depths (H/D≤0.5), where the wake is elongated, deflected downward, and exhibits a reduced Strouhal number. At greater depths (H/D≥2), wake behavior approaches that of an unbounded flow. In contrast, the two-cylinder configuration exhibits strong wake interference and is highly sensitive to both H/D and G/D. At intermediate submergence (0.5≤H/D≤1.5), the free surface induces persistent upward deflection of the gap flow for G/D≤1, resulting in asymmetric wake patterns and altered shedding frequencies. At shallow submergence (H/D=0.25), the free surface dominates the dynamics, suppressing vortex shedding from the upper cylinder and reversing gap flow direction as spacing decreases. CFD analysis further reveals that the cylinder associated with the lower pressure region experiences relatively higher momentum flow, leading to the formation of a narrower wake. These findings highlight free surface as a key factor in wake interactions, with implications for small-scale aquatic and engineered systems operating near the water surface.