Numerical analysis of multi-modal vibrations of a vertical riser in step currents

This paper describes numerical simulations of the vortex-induced vibrations (VNs) of a long flexible riser in a step current. We consider the model vertical riser tested at the Delta Flume. The simulation is carried out by our in-house computational fluid dynamics (CFD) solver viv-FOAM-SJTU, which was coupled with the strip method and developed in OpenFOAM platform. The vibration modes in both in-line (IL) and cross-flow (CF) directions are accurately predicted. The numerically predicted maximum mean IL displacement and its location differed marginally from the experimental results. The good agreement between the numerical and experimental results proved that this solver was reliable for predicting the VIV response. A large number of numerical tests were then carried out to study the effects of various parameters on VIV responses further. Three main parameters are considered in this study: current velocity, top tension and mass ratio. The intrinsic relationship between the natural frequency and oscillating frequency was analyzed to explain the occurrence of the dominant mode. Based on the numerical results, the regular characteristics of the VIV response with the reduced velocity were pointed out. The curvatures and the maximum mean offset values were proportional to the squares of the reduced velocities.