An experimental study on vortex induced motion of a tethered cylinder in uniform flow

At present, most research achievements are about the response of two or three degrees of freedom vortex induced vibration. Responses of vortex induced motions (VIM) of marine structures with 6-DOF remain to be further studied. In this paper, a model test of a tethered cylinder in uniform flow was carried out. The 6-DOF motion responses with different mass ratios were measured and the motion trajectories, amplitudes and frequencies were investigated. Results show that, the vibration amplitude of the cylinder increases with the reduced velocity at first, and then remain relatively stable. In the “lock in” range, the cross-flow vibration frequency increases with the reduced velocity, and the in-flow vibration frequency increases in a ladder type. Yaw motion was also observed in this experiment, and a strong correlation between yaw motion and cross-flow vibration was found. In the initial branch, yaw motion has only one dominant frequency, while in the “lock in” range, yaw motion appears to have two dominant frequencies, the major frequency is equal to the cross-flow vibration frequency and the minor frequency is half of the transverse vibration frequency. It is also found that in the range of mass ratio applied in this paper (0.272