Study On Forced Oscillation And Vortex-induced Vibration(VIV) Of Circular Cylinder Under Combined Uniform Flow And Oscillatory Flow At Low Reynolds Number

Many of the offshore engineering structures are cylindrical, such as marine risers, mooringlines. With the fast development of deepwater discovery, it needs longer and more reliableriser system and mooring system to ensure normal production, which requires a furtherresearch on cylindrical structure hydrodynamics and its fluid-solid coupling characteristicsunder complex flow. Vortex-induced vibration (VIV) will occur on cylindrical structures suchas risers under the action of flow. It is one of the major causes of structures fatigue damage.So research on forced oscillation and VIV of cylinder has important significance.This paper is a part of the National Natural Science Foundation Project "Research onvortex-induced vibration of deepwater riser considering the large displacementfluid-structure interaction"(No.51179179), aimes at understanding mechanism of forcedoscillation and VIV of cylindrical offshore engineering structures under uniform flow andoscillatory flow, which provides the reference and basic research at different flow situations.In this paper, the latest research on forced oscillation and VIV of cylinder and riser isintroduced and the basic parameters of cylinder vortex-induced vibrations are given. Studiedon the force, vibration displacement, vortex shedding, the phase between the force andvibration displacement and m ζimpact on the vortex-induced vibration of a cylinder underdifferent flow field (uniform flow and combined uniform flow and oscillatory flow) whenflow runs around. In this paper, cylinder forced oscillation mode, fluid-structure interaction,combined uniform flow and oscillatory flow and the separation of drag force and inertialforce are all achieved by programming. The main contents of this paper and the results aresummarized in the following points: (1) In this paper, previous research on forced oscillation and VIV of cylinder issummarized and introduced. The mechanism and basic parameters of cylinder VIV is givenand flow around cylinder under different flow field is summarized and analyzed.(2) The numerical methods and numerical control equation used in this paper is introduced.Meshing methods and three different types of dynamic mesh are compared. Then summarizethe characteristics and scope of the three dynamic mesh and select the best structured gridsand dynamic layer model.(3) Through the numerical simulation of oscillating cylinder in water at rest and comparedwith experimental results, find a higher accurate dynamic mesh movement method ofdynamic layer model and analyze and compare no convergence situation in CFD. It foundthat when the boundary of the computational domain is closed to the structure, the wall willaffect the flow field near the structure. In this situation, using a denser mesh could getnon-convergence of the calculation.(4) The model of forced oscillation cylinder is established. First, calculate the cylinder inuniform flow and analyze the vortex shedding and lift force. By comparison with theexperimental results, verify the reliability of the grid model. Secondly, transverse forcedoscillation cylinder at different frequency is numerical simulated and get the lift and dragcurve and vortex shedding."Beating" and "Phase Shift" is found and compared with theexperimental results.It is verified that the numerical simulation is accurate. Finally,transverse forced oscillation cylinder under combined uniform flow and oscillatory flow issimulated. In order to contrast with the case of uniform flow alone, using the same frequencyratio in uniform flow calculations. It found that when KC number of oscillatory flow issmaller (oscillatory flow Re=15, KC=0.56), the cylinder under combined flow is similarto that under uniform flow. However, when increasing the KC number (oscillatoryflow Re=100，K C=2.51), original phenomenon is lost. It is out of lock-in when frequencyratio is equal to0.85. There are no obvious period and frequency.(5) The elastic support cylinder model is established. First calculate the VIV of elasticsupport cylinder in uniform flow, completely simulate lock-in and transition phase,Experiment parameters by Anagnostoplulos&Bearman (1992)[137] are used in numericalcalculation in the paper. The calculated results are compared with the experimental results. It turns out that the calculated results can be good agreement with the experimental results. The"beating" and "phase shift" are also found in the calculation. Secondly, the lock-in range ofthe cylinder vibration system with small mass ratio is wider than that of large mass ratio. Theamplitude of oscillating cylinder is decrease when increaseζ. The third part research onvibration response, force analysis and vortex shedding wake flow field of elastic supportcylinder in combined uniform flow and oscillatory flow. Calculate a series of different KCnumber oscillatory flow combined with uniform flow. It found that the range of lock-inincrease under combined flow at large mass ratio. The more average velocity close to themiddle part of lock-in range (lock-in under uniform flow), the greater lock-in range increase.The range of lock-in increase further more under combined flow at small mass ratio than thelarge mass ratio. If the oscillatory flow keeps increasing, random vibration will occur on theelastic support cylinder. The amplitude of random vibration exceeds the maximum amplitudeof lock-in. The two votex near the cylinder shed at the same time from both the upper andlower circular cylinder at high mass ratio.The wake vortex shedding will appearlow-frequency oscillating along the horizontal direction in the2S mode at low mass ratio.There is a certain angle between the wakes and horizontal centerline.