Numerical Investigation of Vortex-Induced Vibrations in Flexible Risers Subjected to Bidirectional Shear Flow

To analyze the vortex-induced vibration (VIV) characteristics of marine flexible risers under the action of bidirectionally sheared flow, a numerical model of fluid-structure interaction (FSI) was first constructed using ANSYS and compared with the results of the existing literature to verify the reliability of the model. Then, the effects of different depth ratios on the VIV characteristics of the riser were deeply analyzed. The results indicate that slices near the top of the riser have more stable lift and drag coefficients, while those elsewhere fluctuate significantly. Moreover, the decrease in depth ratio reduces the multi-modal and broadband vibration phenomena of the riser, inducing transitions in its center-of-mass trajectory from unidirectional to antisymmetric modes in the in-line (IL) direction and amplifying the cross-flow (CF) displacements. Furthermore, compared with unidirectional shear flow, the fluid excitation in bidirectional shear flow produces opposite direction interference between the upper and lower regions, disrupting the stable lock-in state present in unidirectional flow. This leads to a reduction in the dominant vibration frequency of the structure and a weakening of its broadband characteristics.