Numerical simulation of vortex-induced vibrations of inclined flexible risers subjected to uniform current

The vortex-induced vibrations (VIVs) of a flexible riser at four different inclination angles (θ) subjected to a uniform current were numerically simulated by using three-dimensional Large-Eddy-Simulation (LES). A weak coupling method was adopted for addressing the fluid-structure interaction, where the fluid domain was calculated by the commercial code ANSYS Fluent, while the structural response was calculated by the mode superposition method. The sensitivity of the vibration to grid refinement was checked, and the numerical simulation accuracy was validated by comparison to experimental results. The effects of the θ on the VIV of the flexible cylinder were investigated. The cross-flow vibration is found to decrease with increasing θ, while the in-line vibration does not show the same variation. Figure-eight shaped orbital trajectories are observed in four different θ cases. If θ attains a specific value, the vortex shedding separates into two different modes (i.e., straight vortex and hairpin vortex) along the flexible cylinder. With increasing θ, the hairpin vortex tubes gradually become the dominant vortex shedding mode.