Identification Of Hydrodynamic Coefficients For A Flexible Cylinder Undergoing Vortex-induced Vibration

Flexible cylinder structures,such as marine risers,free spanning pipelines and mooring lines of floating platform,could suffer from vortex-induced vibrations(VIV)when exposed to current flows.VIV is an important factor which may cause serious fatigue damage on marine structures.Till now,a large number of research works have been done on the VIV characteristics of cylinders and some excellent findings have been found.However,the hydrodynamic characteristics are still unknown and needed to conduct further investigation.Nowadays,the prediction on VIV of cylinders is mainly based on empirical models.And the hydrodynamic coefficients from forced oscillating VIV experiments of rigid cylinders are generally employed in the VIV prediction models.Some unexpected prediction errors are usually found.In this paper,the coupling between cross-flow(CF)and in-line(IL)vibrations was taken into account and a fluid force model was proposed.Lift coefficients,impulse drag coefficients and added mass coefficients were obtained by means of the least square method.In order to further investigate the hydrodynamic coefficients on a flexible cylinder undergoing VIV,a series of experimental tests on VIV of flexible cylinders were conducted in a towing tank.The characteristics of response and hydrodynamic coefficients on vertical and yawed flexible cylinders were studied.Based upon the analytical and experimental results,the following conclusions could be drawn:(1)The hydrodynamic coefficients of flexible cylinders share similar trend with those of rigid cylinders.A remarkable increase of lift and impulse drag coefficients appears when the second dominant mode is excited.In addition,the correlation of added mass coefficients and response displacements becomes much obvious with increasing response frequency.The energy dissipation appears with the case of large response displacement and small response frequency.This could be attributed to the self-excitation and self-limitation of VIV.(2)The displacement amplitude of the yawed flexible cylinder shows insignificant decrease,but higher mode can be more easily excited as the yaw angle is up to a large value.(3)With the increase of yaw angle,lift and impulse drag coefficients have a remarkable decrease.In addition,yaw angle has an obvious effect on added mass coefficients,and the average value of added mass coefficients with a=45° is generally larger than that of vertical flexible cylinders.The corresponding conclusions improve the understanding of hydrodynamic of cylinder undergoing VIV.The research work in this paper is of momentous significance to theoretical knowledge and engineering application.