Numerical Simulation Of Fluid-structure Interaction For Two Cylinders In Axial Flow

Fluid-structure interaction(FSI)in nuclear reactors is closely related to the safety of nuclear installations.Fluid axial flow in the core will causes small amplitude vibration of fuel rods.Due to the tight arrangement of fuel rods,axial flow FSI will also cause damage to fuel rods.After several horrific nuclear safety accidents,people pay attention to the problems of axial flow FSI gradually.The FSI of rod bundle that contains a elastic cylinder and a rigid cylinder arranged side by side is simulated.This present paper studied the turbulent intensity T_u(=0.7%-5.0%)effects on the vibration amplitude,mode and dimensionless frequency of elastic cylinder in the flow velocity range of 2.04–9.67.In this paper,Fluent and ANSYS mechanical solver are used to solve the axial flow FSI problem of an elastic cylinder and a rigid cylinder arranged side by side based on System Coupling module.The fluid control equations are arbitrary Lagrangian-Euler(ALE)N-S equations,and the dynamic equation of cylinder is Euler-Bernoulli beam equation.Large-eddy simulation(LES)model and wall-adapted local eddy-viscosity(WALE)subgrid-scale model are adopted presently to simulate the turbulence flow.The numerical simulation results,the dependence of frequency on velocity,and predicted critical velocity are in good agreement with experiment results.With increasing U,the elastic cylinder undergoes subcritical vibration,buckling,buckling and flutter mixed vibration and flutter in turn.In the sub-critical vibration stage,that the fluctuation in the incident flow interacts with the elastic cylinder with the increase of T_u,gives rise to large fluctuation of pressure and unsteady fluid force,thus resulting in an increase of vbration amplitude.The increase in T_u level could affect vibration mode to change from second-order mode to first-order mode when U is low(3.49),but does not affect the value of vibration frequency,thereby buckling instability isn't induced by increasing T_u.It is remarkable that at low T_u(0.7%)and high subcritical U(?5.59),the asymmetry of flow field makes equilibrium position of the vibration close to rigid cylinder.As T_u increases up to 2.9–5.0%,the vibration surrounds initial zero point again.When elastic cylinder developes into buckling,the buckling and flutter mixed vibration does not appear with increasing T_u upto 2.0%,however,the cylinder buckles faster.In the mixed vibration stage,the vibration mode of cylinder develops from first order buckling to mixed vibration of first order buckling and second order flutter.The flutter vibration surrounds the buckling dynamic equilibrium point which coincident with nonlinear theory,and the flutter vibration postpones to take place when T_u increases from 0.3%to 1.5%.The elastic cylinder flutters completely when increase U further,the flutter vibration surrounds the initial zero point with ring orbit.The flutter frequency slightly rises when the T_u increases from 0%to 1.5%,but the vibration amplitude,mode and equilibrium point almost keep constant.