Effects of gap ratio on flow-induced vibration of two rigidly coupled side-by-side cylinders

Vortex-induced vibration of two elastically mounted and rigidly coupled side-by-side circular cylinders in uniform flow is investigated using Direct Numerical Simulations (DNS). The two cylinders are allowed to vibrate in the cross-flow direction only. The aim of this study is to investigate the effect of gap ratio between the two cylinders on the vibration and wake flow. To achieve this, simulations are performed for gap ratios of 0.5, 1 and 3 and a wide range of reduced velocities from 1 to 20. The Reynolds number, the mass ratio and the damping ratio are 1000, 2 and 0, respectively. For small gap ratios of 0.5 and 1, the range of reduced velocity with high vibration amplitude is divided into a lock-in regime and a subharmonic regime. The subharmonic regime is a regime where the vibration frequency is half the dominant frequency of each individual cylinder's lift coefficient. In the subharmonic regime, the dominant frequency components of the two cylinders do not contribute to the vibration because they are out of phase with each other. In the lock-in and subharmonic regimes, the wake is vortex shedding flow instead of biased gap flow for small gap ratios of G=0.5 and 1. The combination of lock-in and subharmonic regimes at G=0.5 and 1 are wider than the lock-in regime of a single cylinder. The weak interference between the two cylinders make the lock-in regime of G=3 the same as that of a single cylinder. A single vortex street wider than that of a single cylinder is found for G=0.5 and two vortex streets are found for G=3. For G=1, vortex shedding happens but clearly defined vortex street does not exist because strong interaction between the vortices causes very fast dissipation of the vortices.