Wall confinement and wake interference in vortex-induced vibration of two near-wall staggered cylinders

Vortex-induced vibration (VIV) of cylindrical structures near seabed critically impacts fatigue life and safety of subsea pipeline. This study investigates the VIV of two staggered cylinders near a wall to explore the coupled effects of wall confinement and wake interference, which has rarely been reported in previous research. By employing 2D (two-dimensional) direct numerical simulations, the effects of incidence angles (α = 0°–90°), gap-to-diameter ratios (G/D = 0.4, 0.9), and reduced velocities (Ur= 3–8) are systematically analyzed at a Reynolds number of Re = 200. Notably, we introduced a novel classification of seven distinct vibration modes (including suppression, sinusoidal, and various beat modes) and twelve wake patterns (e.g., E-state, S-I, and 2S-I) in the phase plane of Ur-α. Key findings reveal that for G/D = 0.4, increasing the incidence angle reduces vibration amplitudes and narrows the lock-in regime, with the upstream cylinder exhibiting soft lock-in behavior while the downstream cylinder is amplified by wake-induced excitation. In contrast, a larger gap (G/D = 0.9) diminishes wall effects, resulting in a broader range of full frequency synchronization for both cylinders and sharper responses in lift fluctuations. Special phenomena, such as secondary vibration amplification, modal transitions, and frequency switching, are observed at some conditions, highlighting their sensitivity to geometric configuration and flow parameters. Additionally, the strong interconnection between vibration and wake modes are clarified, which are governed by the flow confinement, intensity of wake interference, and vortex synchronization. These findings provide critical insights for designing and optimizing subsea pipelines near walls, enabling improved structural reliability and reduced damage in marine engineering applications.