Numerical Simulation On Key Parameters Of Vortex Induced Vibration Of Cylinder Based On An Improved Turbulence Model

Vortex-induced vibration(VIV)is a key factor causing fatigue damage of the marine pipelines.The accurate description of the VIV characteristics is helpful to the accurate prediction of fatigue,so as to ensure the stability and safety of pipelines.The vortex-induced vibration of marine pipelines is extremely complex.To better understand the problem and causes of vortex-induced vibrations,one simplification is to analyze elastically mounted rigid cylinder.And the numerical model which can be used to predict the VIV response of a flexible riser in time-domain is mainly based on trip theory.Therefore,the research on the VIV of rigid cylinder is the key to the prediction of VIV of flexible pipeline,and is the main means to understand the mechanism of vortex-induced vibration.In recent decades,a large number of researches on the VIV of rigid cylinder have been carried out.Many classical phenomena and conclusions have been obtained.However,the basic mechanisms and the influence parameters have not been fully understood.In this paper,based on the open source CFD software OpenFOAM,a numerical simulation of vortex induced vibration of rigid cylinder was carried out by means of RANS with an improved turbulence model.The effects of natural frequency ratio,mass damping ratio,Reynolds number and degree of freedom on VIV were studied.The main contents are as follows:The simulation of flow past a circular cylinder in the subcritical regime(around 150-300<Re<1.4×105)by means of RANS with different turbulence models had the problems that laminar boundary layer transition occured too early and viscous coefficient was excessively predicted in the near-wake region of the cylinder,which would lead to the deviation of pressure coefficient and separation Angle.To solve the above problems,an improved turbulence model was proposed by correcting the production and dissipation items in the turbulent kinetic energy control equation,based on the characteristics of SST k-co turbulence model and Launder and Sharma's low Reynolds number k-? turbulence model(LSKE).The basic feasibility of the improved turbulence model was verified by studying the law of the wall,mesh convergence and sensitivity.And the accuracy and application Reynolds number range of the model were analyzed by the simulation of flow over a circular cylinder.A numerical simulation of transverse vortex induced vibration of circular cylinder was carried out in the subcritical regime using the improved turbulence model.Three typical response branches including the "upper branch" were successfully simulated for low-mass damping ratio system.The accuracy of the improved turbulence model was verified by comparing with the experimental data and simulation results of other turbulence models.The influence of mass ratio,damping ratio and Reynolds number on the transverse VIV were analyzed,and the patterns that the vibration amplitude,frequency,force coefficient,phase between force and displacement and vortex shedding mode change were identified.The maximum transverse amplitude of 1.79D which was in better agreement with the experimental data and larger lock-in region were obtained in the TrSL3 regime.The variation of the maximum transverse amplitude with mass damping ratio and Reynolds number was discussed through comparison with Govardhan and Williamson's modified Griffin plot.Based on the improved turbulence model,a numerical simulation of 2-dof VIV in the subcritical regime was conducted.The super-upper response branch which agreed well with the experimental data was obtained.And the accuracy of the improved turbulence model was verified by comparing with the experimental data and simulation results of other turbulence models.The hysteresis under different inlet conditions was successfully simulated and the impact parameters were analyzed.The influence of mass ratio and damping ratio on the vibration amplitude,vibration frequency,lift and drag force coefficient are discussed in the TrSL2 regime.A wider super-upper branch with larger amplitude was observed in the TrSL3 regime,and the effect of Reynolds number on the 2-dof VIV was studied in the subcritical regime.A numerical simulation of the 2-dof VIV was carried out in a wider range of natural frequency ratio using the improved turbulence model.A "double peak" phenomenon similar to the experimental results appeared after the natural frequency ratio increase to 2.0 and more in the TrSL2 regime.The shape of the amplitude curve,the vibration frequency,additional mass and force coefficient changes greatly.The lift and drag force respectively showed third harmonic and fourth harmonic or even higher harmonic components at the reduced velocities with larger amplitude,and the third harmonic force increases and shifts to larger reduced velocities as the natural frequency ratio increases.