Study On Vortex Induced Vibration Characteristics Of Circular Cylinders Using Numerical Model

The vortex induced vibration(VIV)of cylinders caused by flow is a frequent Fluid-Solid Interaction(FSI)phenomenon in the field of ocean engineering.Such as deep-sea riser,which often works in hundreds of meters and even thousands of meters underwater.Under certain external excitation,VIV may occur in riser system,which can cause fatigue damage to the pipeline.With the exploitation of oil and gas resources moving towards the deep sea,the structural strength and stability requirements of the opposing pipe system are becoming more and more strict.In thise thesis,the characteristics and mechanism of VIV of circular cylinders are studied using numerical model,and the results are expected to lay the foundation for the development of vibration suppression and so on.Considering the computation load and engineering application,the Finite Volume Method(FVM)combined with Reynolds Average Numerical Simulation(RANS)was used to calculate the problems of VIV.By introducing turbulence model,the governing equations were closed.The fourth order Runge Kutta method was used to solve the vibration equation and the dynamic grid technology was used to deal with the moving boundary.Firstly,the numerical model and the method to handle with fluid solid coupling were verified.Considering the available experimental and numerical results,the flow around a fixed cylinder under the Reynolds number of3900 was simulated firstly.The numerical results were in good agreement with the reference data.In addition,numerical simulation of VIV at low Reynolds number was also given,and satisfactory results were also obtained.Using the verified model and method,the VIV of cylinders with single and two degree of freedom were studied respectively,and the normalized velocity ranged from2 to 14.The results show that for simulating VIV,compared with RNG k-?,Realizable k-?and Standard k-?turbulence models,the SST k-?model performances better,and it has obvious advantage in catching the shape of wake vortex shedding.As for the influence factors of VIV,the mass ratio(m~*)determines the locking interval length of the cylinder.Under the same conditions,the smaller the m~*is,the larger the locking interval will be.The mass-damping parameter(m~*?)determines the magnitude of the resonance amplitude when the cylinder is in lock-in.Under the same conditions,the smaller the m~*?is,the greater the amplitude will be.Under certain conditions,there is a big difference in the hydrodynamic force between a vibrating cylinder with single degree of freedom and two degrees of freedom.This indicates that in this case,the motion parallel to the direction of flow could not be ignored.Because the motion parallel to the flow may cause a new vortex shedding mode which is called as“2T”mode,the vibration response can goes into the super-upper branch.In the velocity range of the study,the response frequency of the downstream vibration is almost 2times as big as that of the transverse vibration,and this may be one of the reasons causing the fatigue damage of the risers.As the mass ratio changes from small to large,the influence of the downstream vibration on the transverse vibration decreases gradually.The critical mass ratio where the response and vortex shedding mode of two degrees of freedom vibration become quite different from those of single degree of freedom vibration is about 3.5.