Aerodynamic coupling and vibration modes in vortex-induced vibrations of side-by-side cylinder arrays

Wind-tunnel experiments were conducted to investigate vortex-induced vibrations (VIVs) of side-by-side arrays of two and three identical circular cylinders mounted on flexible beams. The gap-to-diameter ratio was varied over 0.025≤G/D≤4. At large spacings, the response approached that of an isolated oscillator: 1≤G/D≤4 for two cylinders and 2≤G/D≤4 for three cylinders. At smaller spacings, wake coupling modified the lock-in response and produced distinct collective vibration modes. In the two-cylinder array, the cylinders oscillated either synchronously or 180° out of phase. In the three-cylinder array, additional states appeared, including cases where the two outer cylinders moved together while the middle cylinder moved oppositely and cases where the middle cylinder remained nearly stationary while the outer cylinders vibrated 180° out of phase. With increasing reduced velocity, these states transitioned toward in-phase motion. Decreasing G/D below 0.5 substantially increased both vibration amplitude and lock-in bandwidth, with maximum amplitudes reaching approximately three and four times the isolated-cylinder value for the two- and three-cylinder arrays, respectively. These results demonstrate that spacing-controlled wake coupling can amplify VIV response and broaden the operational bandwidth of multi-cylinder oscillator arrays for flow-energy harvesting applications.