Numerical Simulation Of Vortex In-Duced Vibration Of Circular Cylinder In Different Oncoming Flow Forms

Vortex-induced vibration of circular cylinder has attracted the attention of many scholars.There are many influencing factors for the vortex-induced vibration of a cylinder,such as fluid viscosity,fluid flow status and the Reynolds number of flow field which has a close contact with the velocity of the flow.Presently,we often regard the velocity of the oncoming flow as uniform and constant when we study the problem of vortex-induced vibration of cylinder.However,in engineering area and our lifes,the flow velocity of the fluid which has a gradient is often in shear flow form.To explore the effect of the oncoming flow forms in vortex-induced vibration on the circular cylinder,numerical simulation were carried out in uniform oncoming flow and shear oncoming flow in this paper.A CIP(Constrained Interpolation Profile)method is applied with immersed boundary theory and multiphase flow theory to simulate the problem of fluid flow through a circu-lar cylinder and the problem of circular cylinder undergoing vortex-induced vibration in Cartesian coordinate system.The CIP method is also choosed due to its strong stability,high efficiency,high precise.To investigate the effect of the oncoming flow forms on parameters of the cylinder undergoing vortex-induced vibration.Firstly,we simulate the fluid flowing over a still cylinder in uniform flow and shear flow at low Reynolds number in this paper.After finding the effect of the flow forms on still cylinder,simulation were carried out on cylinder forced vibrating in uniform flow and shear flow to study the effect of vibrating frequency and amplitude.Through the analysis we know that the drag coefficient and lift coefficient increase with the increasing of frequency in lock in zone and change dramaticly at the endpoints of the lock in interval in uniform flow.The mean drag coefficient increases with the increasing of amplitude in lock in zone in uniform flow,and,the lift coefficient declines with it.When the amplitude is smaller,it hardly has effect on the lift and drag coefficient.When the amplitude is larger,with the increasing of shear ratio in the shear ratio range,the drag coefficient firstly declines and then increases,but the lift coefficient firstly increases,and then declines.The above conclusion shows that the oncoming flow forms also have effect on forced vibrating cylinder.In this paper we focus on the effect of flow forms on vortex-induced vibration of circular cyllinder after studying the effect on still cylinder and forced vibrating cylinder.Through the analysis we know that in uniform flow,the cylinder's displacement trajectory likes a '8' and have a better repeatibility in lock in zone.The phase angle between lift coefficient and transverse displacement change dramatically in the middle of the lock in zone in uniform flow.In the shear flow the displacement trajectory of the cylinder change from '8' to 'rain drop' shape with the increasing of shear ratio in the lock in zone,and the'rain drop' shape will be stretched if the shear ratio continue increasig.Due to space limitations,at last we just pay attention on the tandem cylinders in uniform flow to study the effect of combination of motion freedom on tandem cylinders undergoing vortex-induced vibration.Through the analysis we conclude that,when one cylinder vibrating in two degrees of motion freedom,another cylinder's motion freedom will have a big effect on the vortex-induced vibration of the tandem cylinders.The highlights of this work includes:(1)In this paper,it's the first time to study the effect of oncoming flow forms systematically through simulation of fluid flowing past still cylinder,forced vibrating cylinder and cylinder un-dergoing vortex-induced vibration.The simulation results include that oncoming flow forms will have a big effect on the flow field parameter ralated to the cylinder.(2)In this paper,we focuse on the effect of negligible motion freedom of tandem cylinders undergoing vortex-induced vibration in uniform flow.The results show that the motion freedom combination is an important factor which can have a big effect on the simulation results.