Cross-flow vortex-induced vibration of a flexible fluid-conveying riser undergoing external oscillatory flow

Cross-flow (CF) vortex-induced vibration (VIV) of a flexible riser considering both internal flow and external oscillatory flow is numerically investigated with consideration of combining the structural model with semi-empirical hydrodynamic force model by using Finite Element Method. The accuracy of the applied model is firstly examined by comparing the numerical results with the experimental data, which proves that the model can reproduce typical characteristics of CF VIV of a flexible riser undergoing external oscillatory flow. Then CF VIV of a flexible fluid-conveying riser subjected to external oscillatory flow is studied while the non-dimensional internal flow velocity and density ratio between internal and external flows are changed. The results show that regardless of the non-dimensional internal flow velocity and density ratio, typical VIV features of a flexible riser, such as intermittent VIV, amplitude modulation, hysteresis, mode and frequency transitions as well as standing and travelling wave responses, can be captured with variation of external oscillatory flow velocity. Moreover, VIV developing process, including building-up, lock-in and dying-out, can be detected for CF VIV. With the increase of the non-dimensional internal flow velocity and density ratio, high mode response can be effortlessly triggered for CF VIV, which is accompanied with occurrence of new vibrating frequencies. In addition, the vibrating frequency of CF VIV decreases while the non-dimensional internal flow velocity and density ratio are increased.