Flow-induced vibrations of two staggered cylinders with a moderate spacing and varying incident angles at subcritical Reynolds numbers

Two-degree-of-freedom (2DOF) flow-induced vibrations (FIV) of a stagger-arranged circular cylinder pair with a constant centre-to-centre spacing ratio P/D = 6.0 and varying incident angles (a) are numerically studied. The two cylinders can oscillate freely in both the in-line and cross-flow directions. The Reynolds number range Re = 1470-10320 belongs to the subcritical flow regime. Two identical cylinders with mass ratios of m* = 2.6 and mass-damping parameters of (m* +C-a) zeta = 0.013 are adopted in the present simulation. Four incident angles ranging from 0 to with an increment of 30 are considered to investigate the effect of a on the FIV responses, hydrodynamic characteristics and wake patterns of the two cylinders at this specific P/D. For small incident angles (i.e., a = 0 and 30), the influence of Cylinder II on the FIV of Cylinder I is almost negligible. In contrast, owing to the complicated wake-cylinder and wake-wake interactions, large-amplitude vibrations are found for Cylinder II at high reduced velocities (V-r). The wake patterns suggest that the periodic vortex coupling arisen from the synchronous movements between the upstream vortices and Cylinder II contributes to maintaining a vibration mode that resembles wake-induced flutter (WIF). For large incident angles of alpha = 60 and 90, the two cylinders oscillate similarly and their response amplitudes experience decline at lower V-r values. The analyses of the wake characteristics demonstrate that the restrained wake widths and the stronger proximity interference between the two cylinders lead to the vibration attenuation.