Numerical simulation of vortex-induced vibration of a full-scale flexible riser at supercritical Reynolds number in uniform and linear shear flow

This study employs a large eddy simulation with a three-dimensional strip-theory-based method on the self-developed platform (HRAPIFS) to investigate vortex-induced vibrations(VIV) of a full-scale (300 m) riser, experiencing both uniform flow ( Re = 10 6 ) and linear shear flow conditions (from subcritical to supercritical Re = 10 5 ∼ 10 6 ). The computational model has been rigorously verified through the flow around a fixed cylinder entering the supercritical region and the VIV response of a flexible riser in uniform and linear shear flows. This study provides an in-depth investigation of this practically significant yet rarely studied problem with substantial engineering importance, yielding several new findings. First, traveling wave responses are observed in both flow conditions. VIV response in uniform flow exhibits 8th mode dominance (1.56 Hz) and shows multimodal behavior combining 1st (0.1Hz, corresponding to subcritical St) and 7th modes (1.3 Hz) in linear shear flow. Second, the present study demonstrates that lock-in happens almost along the whole span (with positive excitation force coefficient) in uniform flow, while the low-velocity regions ( z < 170 m) in shear flow exhibit negative excitation force coefficients along with clockwise trajectories. The dominant vortex shedding frequency progressively shifts from subcritical ( St = 0.2 ) to critical ( St = 0.3 ) features before synchronizing with the structural vibration frequency in high-velocity zones. Third, trajectory analysis reveals elliptical patterns in both cases due to identical vibration frequencies in the cross-flow and in-line directions, contrasting with subcritical figure-eight patterns, attributed to the dramatic change in the hydrodynamic responses entering the critical region.