Theoretical Study Of Eddy Current Damper And Its Application In Vertical Vortex-induced Vibration Control Of Bridges

Eddy current damper is a type of energy dissipation device for vibration control by utilizing the eddy current principle.It has the advantages of simple structure,good durability,little maintainence requirement and easy adjustment.However,the energy dissipation density of the eddy current damper is quite small compared with the traditional passive energy dissipation devices,and this is the main reason why the eddy current damper can not be applied on vibration control of large-scale civil engineering structures.In order to deeply understand the damping performance of the eddy current damper,improve its energy dissipation density and promote its application in the field of civil engineering,a system study of the damping characteristics of the plane eddy current damper was carried out and a novel ball screw type axial eddy current damper was developed in this paper.In addition,the application of the plane eddy current damper and ball screw type axial eddy current damper in suppressing the vertical vortex-induced vibration of long-span bridges were studied.The main contents and achievements are as following:1.Theoretical analysis of the plane eddy current damper was carried out.A theoretical formula of the eddy current damping force was derived for the plane eddy current damper working at low velocity.Based on this theoretical formula,a dimensionless parametric analysis of the damping performance of the plane eddy current damper was conducted.The results showed that the primary back iron and the secondary back iron can significantly increase the energy dissipation density of the plane eddy current damper.In addition,the most suitable dimension of the conductive plate,the optimal magnetic pole shape and the optimal arrangement of the permanent magnets were obtained.2.Electromagnetic finite-element analysis of the plane eddy current damper was conducted.The influences of the electromagnetic characteristics of the back irons and the relative magnetic permeability of the permanent magnets on the damping charactersistics of the plane eddy current damper were revealed.Through analyzing the force-velocity charactersistics of the plane eddy current damper,a nonlinear mathematical model of the plane eddy current damper was proposed.Based on this mathematical model,the effectiveness of the edd current tuned mass damper(ECTMD)was examined.The results show that under common working condition,the force-velocity nonlinearity of the plane eddy current damper is sufficiently low so that the ECTMD can be designed as the same with the traditional linear TMD.3.A novel ball screw type axial eddy current damper was developed.Parametric analysis of the nonlinear axial force-velocity relation was carried out employing the electromagnetic finite-element method.The influence of the design parameters of the conductive disk and the permanent magnets on the maximum axial damping force,the critical velocity and the initial damping coefficient were obtained.Based on the parametric analysis,a mathematical model was proposed for the ball screw type axial eddy current damper and a method for improving the damping performance of this device at high velocity was provided.Moreover,a reduced-scale ball screw type axial eddy current damper was manufactured and tested by experiments.4.The application of ECTMD in suppression of the vertical vortex-induced vibration(VVIV)of long-span bridges was investigated.The design methodology of the multiple eddy current tuned mass dampers(MECTMD)was proposed,demonstrating the advantages of the MECTMD in control of the VVIV of long-span bridges.Through wind tunnel tests of a bridge deck section model,the effectiveness of the ECTMD in mitigation of the VVIV were evaluated,revealing the influence of the damping ratio and frequency of the ECTMD on its control effectiveness.Moreover,the allowable value of the VVIV of long-span bridges was discussed,and the results show that it is mainly determined by the vibration comfort of people working on the bridge and the insight distance requirement for vehicle safety.5.The application of tuned viscous mass damper(TVMD)and rotary inertia double tuned mass damper(RIDTMD)in suppression of VVIV of long-span bridges was studied.The semi-emperical nonlinear model proposed by Scanlan was used to model the vortex-induced force.Based on this model the optimal parameters of the TVMD and RIDTMD were obtained and their control robustnesses were analyzed.The results show that the TVMD and RIDTMD have the advantages of the three-element control theory and perform better than the TMD.In addition,the vortex-induced reponses of the bridge-TVMD system and the bridge-RIDTMD system under the simple Hopf burfication were computed,respectively.The results indicate that both TVMD and TMD can effectively reduce the amplitude response of the VVIV of bridges under simple Hopf burfication.