Numerical investigation on competitive mechanism between internal and external effects of submarine pipeline undergoing vortex-induced vibration

This work aims to model and explore the competitive mechanism between the internal flow effect (IFE) and external current effect (ECE) on the cross-flow vortex-induced vibration (VIV) of a free-span submarine pipeline transporting an axial internal flow. Considering the coupling between the structure and fluids, the partial differential equations of pipeline system are described by Euler-Bernoulli beam theory coupled with the wake oscillator model. To obtain the VIV response of pipeline at different internal flow and external current velocities, the generalized finite difference method (GFDM) and Houbolt method (HM) are employed for spatial and temporal discretizations for equations, respectively. The results indicates that when the IFE is not accounted, the higher-order VIV modes of the pipe are successively excited by ECE. Of interest is that coupling flutter phenomenon between even-order and adjacent modes will occur. Whereas, when the fluid inside the pipe flows, which means the IFE occurrence to be observed, the coupled flutter phenomenon disappears along with significant enlargement of the amplitude in even-order modes. Meanwhile, the mode transition is associated with internal flow velocity and a continuous change in the external flow velocity.