Numerical Simulation Of Vortex-induced Vibration Of A Circular Cylinder In Oscillating Flow

As a key part of the deep-sea oil production system,which connects seabed and offshore platform,the safety of marine risers needs to be fully guaranteed.Under the action of the ocean current,vortex shedding is easy to occur on both sides of the riser.Due to the asymmetry,the cylinder will be subjected to lateral forces,resulting in vortex-induced vibrations(VIV)of the riser in both the cross-flow and in-flow directions.Vortex shedding is a regular periodic phenomenon,when the vortex shedding is synchronized with the structure's vibration,i.e.a mechanism referred to as lock-in,the VIV exhibits large amplitude,leading to significant fatigue damage.The leakage of oil and gas resources caused by the riser damage may cause serious harm.In the actual marine environment,the motion of the waves and the top platform will cause a relative oscillating flow between the riser and the fluid,which can affect the vortex-induced vibration of the riser.Therefore,the study using steady flow and other traditional methods cannot reflect the characteristics of vortex-induced vibration of the riser.In addition,considering the combined effects of ocean currents and waves,it is necessary to conduct research on the characteristics of vortex-induced vibration of risers under the superposition of steady and oscillating flow.In this paper,a two-dimensional numerical model of vortex-induced vibration of marine riser in oscillatory flow is established by using the RNAS equations in conjunction with the SST k-?turbulence model.Firstly,the numerical model is validated using the experimental and numerical data in literatures.Then,based on Fluent software,numerical research was carried out on the vortex-induced vibration characteristics of the cylinder under the oscillating flow and the superposition of the steady and oscillating flow,and then discusses the VIV displacement,frequency,response amplitude,hydrodynamic force and shedding vortex.The main research contents and conclusions are as follows:(1)Based on the established numerical model of the vortex-induced vibration of the cylinder,the mesh sensitivity?turbulence model and flow field characteristics were verified.The results show:the height of the first layer of the model is 0.001D and the number of nodes near the wall is 200,which can ensure the accuracy and efficiency of calculation;the SST k-?turbulence model is more accurate than the RNG k-?turbulence model in capturing and presenting the dynamic changes of the flow field;the two oscillating flow grid implementation methods proposed in this paper are feasible.(2)The numerical simulation of the cross-flow vortex-induced vibration of elastically supported cylinders in an oscillating flow with a KC number of 25?50 and 75,and a reduction in U_rfrom 2 to 18,the results show that the oscillatory flow can lead to multi-mode vibration of the cylinder as expected,and larger KC number corresponds to more obvious multi-mode feature.When in the lock-in range,the amplitude of the dominant mode is significantly larger than other excited modes.With the increase of the KC number,the lock-in range and the U_rnumber corresponding to the maximum amplitude tend to larger in the oscillating flow.After the flow reveals,the previously shed vortices would be convected to pass the cylinder from the upside and downside alternatively,amplifying the hydrodynamic force of the cylinder.(3)The numerical simulation of the cross-flow vortex-induced vibration of an elastically supported cylinder under the superposition of a steady flow and an oscillating flow with a KC number of 25,a velocity ratio of 0.2?0.4?0.6 and 0.8,and a reduction in U_rfrom 2 to 25 is simulated.The results show that the superimposed flow can excite the multi-modal vibration of the cylinder,and the multi-modal characteristics are affected by the proportions of the steady flow and the oscillating flow.When a=0.4 and 0.6,the locking interval of the cylinder increases significantly.After the locking interval ends,the average amplitude ratio of the cylinders seems to decrease gradually as the flow velocity ratio increases.In addition,the"beat frequency phenomenon"was found in both the force and displacement curves of the cylinder.In the superimposed flow,the cylinder has different vortex shedding modes and numbers in the same period,and when the oscillating flow is dominant,there will be vortices that fall off in the first half of the cycle and pass through the cylinder again,causing the force and displacement of the cylinder to increase suddenly Large.This situation will increase the contingency of the cylinder's force and displacement conditions,which is disadvantageous for practical engineering applications.