Numerical simulations of the vortex-induced vibration (VIV) on a circular cylinder in an oscillatory flow were conducted using the direct-forcing immersed boundary (DFIB) method. VIV of structures is a practical engineering problem. Many engineers have devoted themselves to research on the prevention of VIV, which can cause severe damage to offshore construction equipment and structures. Fluctuating hydrodynamic force induces vibration of structures because of the vortex around the structure. This vibration causes structural failure due to the lock-in phenomenon. A phenomenon termed “springing” (a secondorder wave effect caused by superposition of the incident waves and the reflected waves or other wave systems) was also observed in this study. This phenomenon is more hazardous than the lock-in phenomenon because it causes a very high transverse frequency response that can result in serious structural damage. In springing, a vortex that is bounded near the cylinder appears; this vortex is termed a “bound vortex.” A stationary cylinder in oscillatory flow was used to so that the experimental and numerical results of the dynamic and velocity components at three cross-sectional areas could be verified and validated with published results. The DFIB method was then applied to simulate a moving circular cylinder in the transverse direction in an oscillatory flow with varying mass loading. The results of the varying mass ratio were discussed as well as the effects of the reduced velocity, and the corresponding lock-in region was determined. This proposed model can be used for predicting VIV of structures.

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