Study On Vortex-Induced Vibration Of Cylindrical Structures With NES And Its Broadband Vibration Absorption Characteristic

Cylindrical structure is a common structure in wind and ocean engineering.When the structure experiences an incoming flow,it will inevitably generate a certain degree of vibration due to the action of fluid force.Vortex induced vibration（VIV）is a phenomenon from the result of aerodynamic forces acting on the structure due to the vortex shedding in the wake of a bluff body,after the fluid flows through it.Under the action of fluctuating aerodynamic loads,‘lock in’ phenomenon might occur from the synchronization of the frequency of vortex formation with the frequency of bluff body vibration.Within the lock-in regime,the large-amplitude vibration can give rise to fatigue damage with the obvious decrease in the lifespan of the slender structure,and could also lead to sudden structure failure.Cylindrical structure are found in many engineering applications,such as marine riser,wind turbine tower,high-rise buildings and cables,which are sensitive to external load.Such structures usually have strict requirements for the structural stability,but they also have the characteristics of long natural vibration period and small damping.Therefore,the VIV effect on such cylindrical structures is obvious,and it can disturb the normal operation of the system.Supported by the National Natural Science Foundation of China（No.12002301）and the Natural Science Foundation of Jiangsu Province（No.BK20190871）,the VIV suppression of related cylindrical structures in wind engineering and ocean engineering is studied in this paper.Nonlinear energy sink（NES）is a passive vibration absorber without preferential suppression frequency which can realize a targeted energy transfer（TET）.Alien from the narrow frequency bandwidth of tuned mass dampers（TMD）,and tuned liquid dampers（TLD）,it has a broadband excitation for harvesting energy from primary structure and dissipating energy by its damper in a broad frequency range.Extensive theoretical and experimental research on NES has been carried out in the fields of civil,aerospace and machinery engineering,and rich achievements have been obtained.For some flexible cylindrical structures bearing large range of inflow speed,the excitation frequency from strong wind is wide,and the vibration dominated by second-order mode may occur.Moreover,the parameters of such systems are usually time-varying,and the linear damper with narrow frequency bandwidth has certain limitations.Therefore,a NES with broadband vibration absorption characteristic is added to the cylindrical structure in this pape,to study the suppression effect and mechanism of NES on VIV.After adding the NES,the dynamic behavior of the structure is much more complex.The establishment of a model,predicting the VIV of the cylindrical structure with a vibration absorber,can be used to further guide the engineering practice,and will also provide a reference for the research and development of similar vibration absorbers.The main work of this paper is as follows:（1）Two kinds of NES are added to the cylindrical structure respectively,to evaluate their effect on the suppression of two-dimensional VIV.The suppression mechanism and effect of NES with different structural parameters are explored by different numerical simulation methods;（2）Based on the transfer matrix method of multibody system（MSTMM）,the rotor,flexible tower and wind turbine model are established respectively.The accuracy of the model is verified by comparing with the finite element model（FEM）model and experimental results.Combining the flexible tower model with Van der Pol wake oscillator model,the VIV model of the flexible tower is established based on the strip theory,and the vibration characteristics of the flexible tower in subcritical Reynolds number are studied;（3）Taking the flexible tower structure of wind turbine as an example,a passive vibration absorber T-NES is installed inside the three-dimensional tower.Based on MSTMM,the body dynamics equation of three-dimensional flexible cylinder structure under the action of a passive vibration absorber is deduced.Combined with CFD method,the unidirectional fluid structure coupling vibration prediction is realized,and the effects of T-NES with different structural parameters on the vibration of flexible tower is explored.