Study On The Sharp Change Of Lift Force Of A Cylinder With Transverse Vibration In The Steady Flow

As an important part of deep-sea oil and gas exploitation equipment,marine riser structures are vulnerable to being damaged by vortex-induced vibration(VIV)under complex sea conditions.VIV caused by water flow is the greatest cause for structural fatigue of risers.Hence,the prediction of the deep-sea risers response characteristics in a complex marine environment caused by VIV accurately is of great significance.Taking effective measures to reduce the damage degree of risers will improve the fatigue life of the marine riser and enhance economic benefits.As one of the research branches of vortex induced vibration of riser,forced vibration of riser has important reference value for predicting vortex induced vibration of riser.Based on the innovative project of high-tech ship numerical tank of the Ministry of industry and information technology(MIIT)"Research and development of vortex-induced motion/vortex-induced vibration subsystem",transverse forced oscillation of risers in uniform flow at typical subcritical Reynolds number is numerically simulated in this paper.The lift coefficient and the associated vortex shedding modes of the riser structure undergoing forced vibration are investigated,which will serve as a reference for predicting the VIV of the marine riser.In this paper,the effects of the flow around a rigidly fixed circular cylinder and the influence of the amplitude ratio of the transverse forced oscillation at subcritical Reynolds number(Re=10~5)are investigated numerically.The contents contain the analysis of numerical calculation accuracy of turbulent model selection,the two-dimensional(2D)and three-dimensional(3D)numerical calculation of transverse forced oscillation of a circular cylinder in a uniform flow environment.Also,the correlation between the fluctuating lift coefficient and the vortex shedding modes is discussed.By analyzing the critical physical parameters related to the forced oscillation of the circular cylinder,the effects of the non-dimensional oscillation frequency f_y/f_s and amplitude A_y/D on the vibration response are emphatically investigated.The extraordinary phenomenon of the lift coefficient and drag coefficient is analysed and discussed.The numerical simulation results demonstrate that the dynamic responses of the three kinds of oscillation frequency modes are different.The oscillation frequency and amplitude are under a profound impact on the fluctuating lift coefficient of the circular cylinder.With the increase of the forced oscillation amplitude,the fluctuating lift coefficient of the circular cylinder decreases abruptly.The phase difference of the lift coefficient controlled by the natural shedding frequency of the cylinder and the forced oscillation frequency occurs,which makes the energy transfer between fluid and the cylinder reverse.The vortex shedding model reflects the abrupt change of the fluctuating lift coefficient.The"2P"model reflects that the lift coefficient keeps a small value and the change is gradual when the amplitude of the cylinder lies in the intermediate range.Compare to the two-dimensional forced oscillation,the jump phenomenon of the lift coefficient for the three-dimensional case is delayed.There is a certain difference between the three-dimensional situation and the two-dimensional situation.Under the same"2P"mode,the lift does not show the attenuation phenomenon in the usual"2P"mode due to the influence of the distribution of three-dimensional axial vortex.