The flow-induced vibration (FIV) of a cylinder cluster is often observed in subsea pipelines, marine risers and high-voltage transmission lines et al. In previous studies, the motion of the cylinder clusters was often modeled as the equation of motion of a particle, which has ignored the torsion freedom. In the present work, the FIV of a two-tandem cylinder system was numerically studied under the consideration of torsional freedom. The ratio of the torsional natural frequency to the translational natural frequency was fixed as 12.5 in this paper. The ratio of gap between the two cylinders to the diameter of the cylinder was modified, g*=0.5, 1, 2 and 3, to investigate the effect of the shear layer interference around different cylinders on the FIV. It was found that the frequencies of the FIV of the cylinder system can lock to both the translational natural frequency and the torsional natural frequency. Except g*=2, the torsional motions of the cylinder system have magnified its peak translational vibration amplitude. For g*=0.5,1 and 2, the upstream cylinder plays a leading role in the cross-flow vibration of the cylinder system. For g*=3, the downstream cylinder is playing a leading role. Due to the torsional motion of the cylinder system, the two cylinders rotate with respect to their centers. The Magnus effect make the separation point of the shear layers move upstream and the lifts on the cylinders are magnified. The smaller the gap ratio is, the more obvious the Magnus effect is.
When a fluid flows past a bluff body, vortices are shed from the bluff body alternately and cause fluctuant forces on it, generating vortex-induced vibrations (VIVs). The VIVs of a single cylinder have been extensively studied. The key effects of reduced velocity Ur, Reynolds number Re, mass ratio m*, and damping ratio ξ on VIVs have been discussed (Xu-Xu et al., 2016 and Kumar et al., 2018). Williamson and Govardhan (2004) have reviewed the research on the VIV of the single cylinder. The vibration frequency lock to the natural frequency of the cylinder system in the neighborhood of Ur=5, and the range of the lock-in regime varies following Re, m* and ξ. For high m*, the vibration divide into three branches, initial branch, low branch and desynchronization branch. For low m*, the new branch, upper branch, can be observed.
