Numerical Simulation Of Vortex-induced Vibration Response Of Rough Cylinders

A large number of flexible cylindrical structures in the ocean engineering can undergo Vortex Induced Vibration(VIV)under the action of current.Vortex-induced vibration will cause structural fatigue or even damage,further resulting in huge economic losses.After a period of service,the surface roughness of the marine cylindrical structures will change the vortex-induced vibration response characteristics of the original structure.Therefore,the study of the vortex-induced vibration response characteristics of the rough surface structures is of great theoretical and practical significance for the accurate prediction of vortex-induced vibration.The main contents of this paper include:(1)The numerical simulation of two-degree-of-freedom Fluid-solid coupling vibration of the smooth cylinder is realized by embedding the UDF(User-Defined Function)module in the FLUENT software and using the dynamic grid method.Compared with the experimental results,the reliability of the numerical simulation method is proved.By using the equivalent sand roughness model,the vortex vibration of rough cylinders with four different orders of magnitude is simulated.The change laws of the parameters such as the resistance coefficient,the dimensionless displacement response,the whirlpool shedding frequency,the trajectory of the motion,the phase angle between the displacement and the vortex force,and the vortex release mode are analyzed and summarized in detail.The judgment and division basis of "Lock-in" phenomenon and different intervals are classified.The cause of the vortex discharge mode variations is deeply explored.Synthesize the vibration trajectory,frequency ratio and locking interval of the cylinder,the speed range is divided into initial branch,upper branch,lower branch and unlock region,so as to facilitate further research and analysis.(2)Through the numerical simulation of the rough cylinder,it is found that the small roughness does not affect the vortex-induced vibration response characteristics of the cylinder.As the roughness increases,the maximum vibration displacement of the cylinder in the CF direction decreases from 1.08D to 0.78D gradually,and the width of the lock-in region decreases from 6.5 to 5.0.In the lock-in region,the cylinder's vibration trajectory is a regular"8" shape,and the vortex discharge frequency is kept near the natural frequency of the structure.In the initial branch,upper branch and unlock region,the vortex-shedding pattern is 2S mode.In the lower branch,the vortex-shedding pattern is 2P mode.(3)When the cylinder's roughness is small(Smooth,Ks/D =1×10-4,Ks/D =5×10-3),the vibration displacement in the CF direction increases to the maximum value gradually with the increase of reduced velocity in the upper branch.When Vr enters into the lower branch from the upper branch,the phase angle between the vortex-induced force coefficient and the displacement response makes a 180-degree shift.The vibration amplitude in the CF direction jumps down obviously,and the vibration response frequency jumps directly from 0.9fn to about 1.25fn.The above changes cause the vortex-shedding pattern to change from 2S mode to 2P mode.When the roughness of the cylinder is large(Ks/D =1×10-2,Ks/D =2×10-2),the upper branch is reduced to a reduced velocity point,and the maximum displacement in the CF direction appears in the lower branch.In the changing process from upper branch to lower branch,the vibration amplitude in the CF direction increases,the frequency ratio changes from about 0.9fn to about 1.1fn,and the phase angle between the vortex-induced force and the displacement also changes from about 0° to about 180°.The vortex-shedding pattern in the lower branch shows two different 2P modes,which is mainly due to the decrease of Strouhal number.