Experimental study on flow-induced vibration of flexible twin cylinders and its suppression using passive-suction-jet control

This study experimentally investigates interference-induced vibrations between flexible twin cylinders in nine staggered arrangements with L/D = 3, 4, 5 and T/D = 0, 1, 2 combinations. Dominant vibration modes, phase differences, and frequency characteristics were analyzed, along with the validation of a passive-suction-jet control method. At high wind speeds, besides higher-order vortex-induced vibration (VIV), three types of large-amplitude vibration were identified: wake-induced vibration (WIV), wake-induced flutter (WIF), and LSTR (laminar separation and turbulent reattachment) flow pattern-induced horizontal vibration (LHV)—the latter being observed for the first time in flexible experiments. During LHV, the downstream cylinder exhibited first-mode dominated horizontal motion while its vertical motion was characterized by higher-order modes, with poor coherence between the cylinders. The frequency-locking phenomenon was observed in both WIF and WIV, and proved essential for WIF occurrence. At close spacing, the cylinders primarily displayed vertical approximately anti-phase VIV, which was caused by shear layer interactions. The optimal control scheme, which employs pipe installations of 3/4 span length, effectively suppresses all three large-amplitude vibration types and mitigates various orders of VIV (except for anti-phase VIV in tandem arrangements).