Numerical Investigation On Vortex-induced Vibration Of Cylinder In Pulsating Flow And Suppression Methods

Vortex induced vibration(VIV)always occurs in offshore cylindrical structures under the action of flow loads,which leads to fatigue damage or even fracture,resulting in unpredictable losses.As a result,its security has been a difficult issue in offshore engineering.In an actual marine environment,waves and platform motions will induce relative oscillations between the cylinder structure and the fluid,resulting in more complex vortex-induced vibrations of the cylinder structure.At present,the research on vortex induced vibration of cylindrical structures mainly focuses on the traditional constant velocity inflow forms such as uniform flow and shear flow,and the research on the relative inflow change of pulsating flow is less.Therefore,this paper investigates the characteristics of vortex-induced vibrations of cylindrical structures in pulsating flows and methods for their suppression.In this paper,we first summarize the research background,the domestic and international studies of vortex-induced oscillations,numerical simulations,and suppression,introduce the basic theory of vortex-induced oscillations and the fluid structure coupling approach,and lay the foundation for subsequent research efforts.Secondly,a 2D numerical model of an elastically supported cylinder is developed to verify the mesh splitting and numerical results.The pulsating flow is used as the velocity conditional entry for the numerical simulations.The influence of the pulsation frequency,amplitude and complex pulsating flow on the vibrational response,trajectory and wake of the cylinder in the downstream and transverse flow directions is emphatically analyzed.It has been shown that the vibrational response properties of cylinders in pulsating flows are more complex than those in homogeneous flows,which can excite multi-mode vibrations of cylinders.The pulsation frequency mainly affects the vibrational frequency in the downstream and transverse directions,and the pulsation amplitude mainly affects the amplitude response in the downstream and transverse directions.The influence of low-frequency components is greater in complex pulsating flow,low pulsation frequencies and high pulsation amplitudes will increase the contingency of the cylinder amplitude response.Finally,a pulsating flow is chosen as the velocity condition.Based on the System Coupling module of the Ansys Workbench,the 2D numerical simulation is expanded to the 3D numerical simulation of single flexible cylinders,parallel cylinders,tandem cylinders and cylinders with fairings are numerically simulated.The results show that the drag coefficient of the flexible single cylinder is not significantly different for different confinement conditions,and the lift coefficient and amplitude are largest for hinge-hinge confinement.The lift coefficients and amplitudes are significantly reduced after adding the fixed constraints.For the parallel and tandem double cylinder,the effect is largest between two cylinders at small spacing ratio and decreases with increasing spacing ratio.For the cylinder with fairing,the hollow fairing has the best inhibition effect when θ= 45°,the average drag coefficient decreases by 73.10% and the root mean square value of transverse amplitude decreases by 58.27%,while the solid fairing and two-way hollow fairing have little improvement in inhibition effect.