| Marine riser is one of the important transportation components which connecting the seabedand ocean platform in marine engineering, and it is a kind of elongated structure, The stressand response mechanism of the riser under the ocean environment load is very complicated,Vortex-induced vibration is a typical phenomenon of dynamic response of the riser, on theone hand the phenomenon complicate the stress of structure, on the other hand make thestructure produces fatigue and reduce the reliability. Therefore, the mechanism andcharacteristics of vortex-induced vibration research has been at the forefront of oceanengineering research hot topic. In recent years, with the progress of computer technology andthe improvement of software and hardware, foreign scholars began to try to directly simulatethree-dimensional Marine riser vortex-induced vibration.This paper adopts numerical calculation method, on the general application platform,combining CFD method and FEM method, simulating two-dimensional elastic cylindervortex-induced vibration in a series of frequency ratio (structural natural vibration frequencyand the ratio of fixed frequency of vortex flow around cylinder release) and compared withsingle degree of freedom and two degrees of freedom of two-dimensional on vortex-inducedvibration characteristics. On this basis, this paper also simulate three dimensional vortex-induced vibration, comparing three-dimensional flexible riser vortex-induced vibrationcharacteristics in different flow conditions, and compared with two-dimensional vortex-induced vibration characteristics. This paper USES the FLUENT software for flow fieldsolver, LES method was used to simulate2d and3d flow field of vortex-induced vibration; atthe same time, ANSYS is used as a solver of riser structure dynamic response, the finiteelement method was used to simulate the dynamic response of a riser. This paper based on theWorkbench platform, through a new transient strong coupling method to realize two-wayfluid-structure coupling a solid coupling surface data exchange of seamless transfer.Based on the two-dimensional vortex-induced vibration simulation, due to the doubledegree of freedom model considers in-line displacement response, by comparing thetransverse amplitude of single degree of freedom and double degree of freedom model, rootmean square of the transverse amplitude and the phase-switch of the frequency ratio, and soon, have proved the importance significance of the double degree of freedom model to studyvortex-induced vibration characteristics.This paper study the two dimensional vortex-induced vibration at different frequency ratioand mainly discusses the following questions: (1) both Single degree of freedom and two degrees of freedom model produced "(in phase)"and "(out-phase)","(beat)","(the lock-in)","(phase-switch)" phenomenon, but doubledegree of freedom model experience "phase switch","beat" phase of the frequency ratio issmall in comparison with single degree of freedom.(2) Successfully captured wake vortex phenomenon at different stages, in the "lock" stage ismainly two rows of vortex wake, while in other stage are mainly in the form of a singlerow of vortex; In the case of lower frequency showed "2p" wake vortex, in the case ofhigher frequency display "2s" wake vortex.(3) Found the flow speed around cylinder induced by cylindrical transverse vibration and theposition of the vortex release is the root cause of the phase difference between vortex-induced vibration lateral displacement response and the lift force.(4) On the following low exploratory explanation respectively:(a) For the cause of the switch"double row vortex" and "single vortex" in the stage between the "lock" stage and "beat"stage;(b) the causes of the lift coefficient of root mean square difference between the1DOF and2DOF at the different frequency ratio;(c) the reason why the law of averagedrag coefficient of the1DOF and1DOF at the different frequency ratio showed anddescribed in this article.(5) Summarize some very meaningful vortex-induced vibration characteristics:(a) minimumroot mean square of lift coefficient at "phase switch" stage;(b) phase difference betweenlift coefficient and lateral displacement response jump in nearly180;(c)In the "lock"stage root mean square of lateral displacement is larger;(d)"beat" stage performancecharacteristic of the multi-frequency vibration; in the larger frequency range, a vortexrelease frequency locking on structural vibration frequency;(f)In some frequency, thevortex-induced vibration of the frequency of drag coefficient is not the two times of thefrequency of lift coefficient.Based on the above research, I further simulated three dimensional (3D) flexible riser VIVdirectly. Comparing the numerical simulation results with other researches’ experimental data,this research validated the feasibility of fluid-structure coupling scheme and the rationality ofthe grid and the boundary conditions. Being through the study of the flexible riser VIVcharacteristics in different flow conditions, and analysis of the structure characteristics ofresponse of the3D flexible riser, it is concluded that the3D vibration response characteristics(including trajectory) of flexible riser has obvious differences with2D VIV characteristics. Italso can capture the phenomenon of switching the adjacent-order vibration modessuccessfully, and observe the characteristics of “traveling wave" and "standing wave" on the both ends of the riser. This research also analyzed the frequency of the structure vibrationresponse characteristics and found that the3D flexible riser VIV shows the multi-frequencyvibration phenomenon and the vibration response characteristics of broadband spectrum at ahigh speed flow condition. Finally, this research analyzed the flexible riser wake vortex forms,and distinguished vortex forms at the different locations of the riser: in the middle of the riser,mainly shows the "2P" or "P+S" shape, on both ends of the riser, mainly the "2S" form.After comparative analysis riser wake vortex in different speed flow conditions, this researchfound that under low flow velocity, the riser does not appear strong3D effect; however, underthe high flow velocity, riser shows a strong sense of3D effect. |
PDF Full Text Request