Numerical analysis of Vortex-Induced Vibration for flexible risers under steady and oscillatory flows

Vortex-Induced Vibration (VIV) of flexible risers under unsteady flows is receiving increasing attention over the recent years. In this paper, an alternative time domain force-decomposition model for flexible risers is proposed to predict VIV response under both steady and oscillatory flows. Non-dimensional frequency range of [0.125, 0.25] is deemed as lock-in region. When lock-in occurs, the riser will be synchronized onto its own natural frequency closest to the non-dimensional frequency of 0.17. The hydrodynamic forces are time-varying and will be updated at each time step according to the riser's response amplitude and frequency. Firstly, the adopted lock-in region is verified well for uniform and sheared flow cases. Next, the same numerical model is also validated against experimental measurements when expanding to oscillatory flow conditions. VIV response with different KC numbers and maximum reduced velocities presents quite individual features, which can be reasonably explained from the VIV mechanism level. Then, the comparisons of VIV response between uniform and oscillatory flows are discussed and analyzed in essence. Finally, another large-scale riser is simulated under the designed oscillatory flows, and some new conclusions different from the small-scale risers are obtained.