Numerical Simulation Of Vortex Induced Vibration Of A Circular Cylinder With Two DOFs Based On OpenFOAM

The riser is a pipeline connecting the submarine and the engineering structure.When the viscous fluid flows through the vertical tube,the vortex induced vibration(VIV)will be generated.When the vortex shedding frequency is close to the natural frequency,the"locking" phenomenon will lead to the increase of the amplitude of the riser.Many scientific units begin to use OpenFOAM to carry out numerical simulation of VIV.This paper contains:First,the OpenFOAM was used to verify the reliability of improved SST.Second,the VIV simulation was carried out using three kinds of initial conditions(acceleration,uniform and deceleration).Compared with different initial conditions,the characteristics of VIV are different.Third,the effects of Reynolds number on the vibration characteristics of a cylinder under multi boundary conditions are studied.1st,SST,LES and DES model are used to simulate the flow around a cylinder.The results show:the SST main defects are the sudden drop of lift.To solve this defect,the energy transfer terms are added to the K transport equation.By comparing the results of two SST models,it is showed that:the improved SST model is more accurate than the original SST model.Three dimensional DES is used to simulate the flow around a cylinder,the unfolded length is ?D,2?D and 3?D.The results showed that:when angle in 60°???90°,the pressure along the cylindrical surface decreased along the axial direction,when angle is?? 60°,the pressure declines sharply from the middle to both ends.U rapid increases in the distance in ?D/2 range,smally increases from ?D/2 to 3?D/2 range.V gradually increases along the direction of the distance.With the increase of the length,the drag and ST number increase slightly and the lift coefficient decreases.Secondly,the simulation is carried out by ways of acceleration,deceleration and uniform velocity in the "numerical pool".The results show that the hysteresis affects the amplitude significantly.Three initial conditions captured the "lock" phenomenon.The vortex model shows the "inertia" property,i.e.,when the velocity is increased,the vortex will tend to be smaller.When the force of the reduced velocity is greater than the maximum capacity of maintaining the original vortex model,the vortex model will leap.In the hysteresis,with the same Ur,the vortex model is different with different initial condition,which leads to the"hysteresis".In the hysteresis,there is a critical value between 0.025 and 0.05,larger than the critical value,the lift and the displacement will be shifted in advance.Finally,the effects of Reynolds number on the characteristics of the VIV with multi boundary conditions(low mass ratio,high mass ratio,frequency ratio)were studied.The amplitude,frequency,hydrodynamic coefficient,motion trajectory and the vortex shedding mode are analyzed.Results show:1)The Reynolds number will increase the amplitude of vibration and lengthen the upper branch and the vibration locking frequency is closer to the natural frequency.2)Reynolds has little effect on trajectory and vortex shedding mode in initial and lower branch,has tremendous influence on trajectory in the upper branch.3)For fNx/fNy = 2 cylinder,in the locked range,lift coefficient showing first harmonic frequency and three order harmonic frequency,with the increase of Reynolds number,the three frequency gradually increased.In summary,the numerical simulation of the VIV of a cylinder based on OpenFOAM is studied.The rule of vortex induced vibration of cylinder under different boundary conditions is given.This paper provides a reference for the study of the VIV in engineering.