Wind Tunnel Investigations Into The Wake Induced Oscillations Of Parallel Cables Coupled By Rigid Separators

The hangers are the key components of the suspension bridges,and they are usually made of the conventional wire strand ropes or the relatively new parallel wire strands(PWS)with PolyEthylene sheath with circular or nearly circular cross sections.Hangers are commonly arranged in pairs.Due to the interference effects among cylinders,the aerodynamics of the bridge hangers becomes more complicated and is quite different from that of a single isolated one,making them more prone to wind-induced vibrations.Interlocking individual hangers with rigid separators is an effective method and is actually widely used to mitigate hanger oscillations.This method can effectively reduce the relative motion(clash of hanger ropes)between the strands,but the global oscillation of coupled parallel hangers by rigid separators still can not be eliminated completely.In this paper,the vibration behavior and the starting mechanism of the global oscillation of the coupled parallel hangers are investigated through the wind tunnel test.The main research contents are as follows:(1)The global oscillations of the coupled parallel hangers under varying center-to-center spacing of hangers and wind attack angles are evaluated by spring-mounted section model tests.It is shown that the large amplitude global oscillations occur at certain spacing and wind attack angle.The orbit of large-amplitude oscillations is elliptical,and directions of major axis of the elliptical orbit remains unchanged for varying wind velocity which is different from wake-induced vibration of leeward hangers for uncoupled parallel hangers.Based on the measured data of vibration,the empirical formula for evaluating critical wind speed is obtained.The effect of structural damping on the global oscillation of the parallel hangers is discussed:the damping ratio has a significant effect on the critical wind speed,it increases with the damping ratio;but the damping ratio has no obvious effect on the amplitude of the parallel hangers.(2)Through the parallel circular cylinder model,the spatial distributions of the mean drag coefficient CD and the mean lift coefficient CL of the model are obtained.The results show that when the spacing is less than 3.8D,the lift coefficient changes drastically around wind attack angle of 9?11°,and there is a predominant peak.The drag coefficient of the global model is less than the single cylinder at 0° angle of attack,and there is obvious negative resistance effect.(3)Based on the quasi-steady galloping theory of single degree of freedom(SDOF),the instability zone is qualitatively discriminated.The resulting instability zone agrees with the instability zone obtained from model tests.However,the critical wind speed obtained by the classical SDOF galloping theory is significantly higher than that of the actual wind speed,indicating that the critical wind speed from the classical galloping theory is overestimated.(4)The overall damping of the coupled parallel hangers undergoing large amplitude oscillations are evaluated from measured responses,which is shown to negative.The negative damping increases with wind velocity.The absolute value of the identified aerodynamic damping is obviously larger than the aerodynamic damping predicted by the classical SDOF galloping model(Den Hartog Model),implying that the instability of the coupling of the two degree of freedom is more likely to occur than the single degree of freedom.(5)The measurements of pressure distributions of parallel cylinders show that there is a critical spacing between the tandem parallel cylinders.And there is a critical angle of attack except the critical spacing in staggered arrangement.The shear layer of the forward cylinder reattachment switch to generate vortex shedding in the critical spacing,resulting in forming a huge lift peak and drag step.