Research On The Response Of Incoming Attack Angle To Vortex-induced Vibration Of Cylinder System

Vortex induced vibration(VIV),as a complex two-way fluid structure interaction problem in ocean engineering,has been widely concerned by researchers in recent years.When the water flows through the non streamline structures such as marine riser and submarine pipeline,due to the separation of shear layer,there will be vortex shedding on its back flow surface.The shedding of vortex will cause the structure to be subjected to periodic force and further induce vibration.When the reduced velocity is in the synchronous range,the natural frequency of the structure is equal to the vortex discharge frequency,and the structure will produce large vibration and obtain more fluid kinetic energy;when the reduced velocity is outside the synchronous range,the structure vibration is out of tune,and the vibration frequency conforms to the St-Re relationship,the amplitude is small,and the vibration energy is small.Vortex induced vibration(VIV)is essentially a feedback regulation process between fluid and structure.At present,the research on VIV mainly includes experimental research and numerical simulation.In this paper,In this paper,based on the UDF vibration control program of Fluent and the overset grid technology,the vortex induced vibration mechanism of the cylinder system with gyratory dart shaped control rod and tandem double cylinders under different inflow angles is discussed.(1)It is found that the simulation data are in good agreement with the previous experimental data,which verifies the accuracy of UDF program and numerical model.In the CFD simulation,the phenomena of "lock","beat","8" vibration trajectory are successfully reproduced.Finally,the mechanism behind the above phenomena is analyzed from the perspective of phase change and wake mode.(2)The vortex induced vibration of a cylinder with an additional control rod is studied by two-dimensional numerical simulation.Firstly,the grid independence of the numerical model of vortex induced vibration(VIV)under the condition of ? = 75° and Ur=14 is verified,and the VIV characteristics of the cylinder system under different attack angles and reduced velocities are studied by using the model.(3)When ? = 30°,Compared with the single cylinder,the vibration response and locking interval of the cylinder are reduced,showing obvious vibration suppression characteristics.It is found that when the angle of attack is from 45° to 75° the vibration response of the cylinder increases with the increase of the angle of attack,and the vibration mode of the cylinder changes from vortex induced vibration to soft galloping;when the angle of attack is from 90° the vibration response of the cylinder decreases,but it is still weak,it is larger than the single cylinder in the same reduced velocity range.(4)In order to have a deeper understanding of the vibration mechanism of the cylinder system with additional boomerang control rod,Through the variational mode decomposition(VMD),the frequency decomposition and feature extraction of the lift time history of the cylinder under galloping condition are carried out,and the energy capture efficiency of the cylinder is analyzed from the angle of time-frequency energy and information entropy,which further clarifies the inducing mechanism of the cylinder from vortex vibration to galloping.(5)For reynolds number Re = 0 ? 7000,reduced velocity Ur=2 ? 13 and angle of attack ? = 0°? 90° are studied.The simulation results show that: when the reduced velocity Ur = 2 ? 3,the lateral displacement difference between the upstream and downstream cylinders increases with the increase of the angle of attack of the incoming flow,and the wake mode of the cylinder undergoes the evolution process of single bluff body,2S(double single vortex mode)and 2S(in-phase synchronous or inverse synchronous mode),and the amplitude of the cylinder is small;(6)When Ur = 4 ? 8,the upstream and downstream cylinders enter the locking range,and the amplitude reaches the maximum.Different from high angle of attack(?= 60°?90°),the amplitude is larger due to the strong vortex fusion effect between cylinders at low angle of attack(?= 0°? 30°).Especially,under the control of the shielding effect between the columns,the maximum amplitude of the cylinder starts at a larger reduced velocity at a small angle of attack;when Ur = 9 ? 13,the upstream and downstream cylinders at a large angle of attack are out of the locking range,and the amplitude tends to be the same as that of a single cylinder.On the contrary,the vibration frequency of the cylinder at small angle of attack is close to the vortex shedding frequency,and the unilateral promotion of the upstream cylinder wake leads to the large amplitude of the upstream and downstream cylinders.The research results can provide a reference for the relevant marine engineering design.