Model Test Investigation On Vortex-induced Motions Of A Tethered Buoyancy Can

In marine engineering field,research on the VIV(vortex-induced vibration)phenomenon and the mechanism of VIV of cylindrical structures,such as risers and subsea pipelines,is gradually tending to be perfect.However,research on the VIM(vortex-induced motion)phenomenon of Spar?TLP and buoyancy can which just has been discovered in recent years is insufficient.VIM will increase the fatigue damage shorten the comprehensive fatigue life of mooring chain and riser connected to the working platforms.So correctly describing and understanding VIM characteristics is very important for the design of marine structures ?the mooring system and riser.It is also of great significance for ensuring the economic benefit and safe operation of on service marine structures.This paper carried out a model test research on the vortex-induced motion(VIM)response of a tethered buoyancy can,three buoyancy can models of different sizes were made and towing tests were done in the towing tank to measure the six-DOF motions under different flow velocities.Mainly attention was paid to the Cross-Flow(CF)direction?In-Line(IL)direction and yaw motion response,such as the time-displacement curve,the trajectory,frequency and motion amplitude.Firstly,the VIM characteristics along IL direction and CF direction were analyzed.We find that when the reduced velocity Ur is small,the motion is mainly along the IL direction,while the CF direction motion is smaller and irregular,so the X-Y motion trajectory seems to be rather messy.When the reduced velocity gets larger,both the IL direction and CF direction motion time-displacement curves gets much smooth,and the motion trajectory shows significant “8” figure.The motion frequency neither equals to the nature motion frequency nor equals to the stationary cylinder vortex shedding frequency,it almost increases linearly with the reduced velocity reduced velocity,it also can be found that several different reduced velocities with the same motion frequency in same cases.The IL direction motion frequency is twice the CF direction motion frequency,one time CF direction motion frequency and some lower frequency components also appear in some experiment conditions.When the reduced velocity Ur is small,the CF direction motion frequency also appears to be multiple-frequency,but when Ur gets lager it appears only one motion frequency.Analyzes the dimensionless motion RMS amplitude versus reduced velocity,find the RMS amplitude of the small-scale buoyancy can maintain at a high level over the entire experiment reduced velocities.For the middle-scale buoyancy can,the IL direction RMS amplitude increases rapidly with reduced velocity at first and then remained at a high level,the CF direction RMS amplitude appears "Twin Peaks" phenomenon,and the two peak RMS amplitudes almost equals,the CF direction RMS amplitude remains the same when Ur ?10.Though the large-scale buoyancy can we can see that the CF direction RMS amplitude is quite small compared to the IL direction when Ur ?3.The IL direction RMS amplitude appears a peak when Ur ?3,and then as the reduced velocity continues to increase,the IL direction RMS amplitude appears to decrease at first and then increase rapidly,while the CF direction RMS amplitude increases to a certain value and remain stable.Secondly,the characteristics of yaw was analyzed,we find that the frequency of yaw is single when the reduced velocity is small,while when reduced velocity gets larger it appears two frequencies,both increase as the reduced velocity increases,we can also find the frequency keeps the same with several different reduced velocities in same cases.Compare yaw with IL and CF direction motion,we find there is a close relationship between them.We also analyzed the motion amplitude of yaw versus reduced velocity.Finally,the VIM drag coefficient was calculated and compared with the resistance coefficient of flow around a stationary cylinder.Find that the VIM drag coefficient is about twice the resistance coefficient of flow around a stationary cylinder.In summary,this paper mainly studied the VIM of a tethered buoyancy can at different flow velocities by model test,and provides some references for the further study on VIM of cylindrical structure.