Study On Excitation Vibration Performance And Vibration Control Of Integral Wake Flow Of Parallel Double Sling Cables

With the continuous increase of the span of the suspension bridge,the length of the suspension bridge also increases.Each suspension point of suspension bridge cable is usually composed of two or four parallel strands arranged in parallel.The sling cable has the characteristics of small stiffness and large slenderness ratio,and there is still pneumatic interference effect in the adjacent sling cable,so in practical engineering,the large amplitude vibration of the sling cable occurs from time to time.At present,the understanding of the mechanism of cables vibration is not clear enough,and there is no systematic vibration reduction theory of double cables vibration.In this paper,the overall vibration characteristics and vibration initiation mechanism of double cables are studied by wind tunnel test of vibration measurement and force measurement by double cables segment model,and the vibration control of double cables is analyzed by theory and test respectively.The main research contents and conclusions are as follows:(1)Based on the rigid model of smooth double-cylinder segments with D=44mm,the elastic suspension segment model vibration test of parallel double-suspension cables was carried out with 3D spacing.The wind speed was 0~30m/s,the Reynolds number was 0~90411,and the Sc number was 6.4.The experimental results show that the vibration of the double cables occurs greatly in the range of 6°~10°,and the vibration trajectory of the double cables is an ellipse dominated by transverse wind vibration,and the long axis direction of the ellipse does not change with the wind speed.This characteristic is different from the wake induced vibration motion characteristics of the middle and downstream cable strands without separator.In the range of 6°~10°,the critical wind speed increases with the increase of wind attack angle,and the larger the wind attack angle is,the more rapid the amplitude increases.The critical wind speed induced by the whole wake of the double cables increases with the increase of the damping ratio of the structure,and the curve of the critical reduced wind speed with the damping ratio of the structure is obtained by fitting.(2)The whole force measurement test of the double cables model shows that the overall lift coefficient of the double cables varies violently in the range of1°~10°wind attack angle under 3D spacing,and the peak value occurs when the wind attack angle is 6°.The resistance coefficient increases gradually in the range of0°~24°wind attack angle.According to the measured lift resistance coefficient,thecorresponding galloping force coefficient can be obtained from the classical galloping theory of single degree of freedom,and then it can be qualitatively obtained that the unstable region is 7°~ 10°.The unstable region is in good agreement with the unstable region obtained by elastic suspension vibration measurement test.The critical wind speed obtained by classical galloping theory is obviously higher than the actual wind speed when the damping ratio is more than 0.32%,and obviously lower than the actual wind speed when the damping ratio is less than 0.32%.It is shown that the critical wind speed calculated by the classical galloping theory is unsafe at a large structural damping ratio.(3)Based on the experimental vibration response signal,the aerodynamic damping ratio is identified according to the nonlinear theory of single degree of freedom galloping vibration.The values are all negative,and the absolute value of aerodynamic negative damping is proportional to the wind speed.The absolute value of the aerodynamic negative damping ratio in the critical state of vibration initiation is obviously larger than that of the theoretical aerodynamic negative damping,and the deviation rate increases with the increase of the damping ratio of the structure.It is shown that the two-degree-of-freedom coupling of transverse wind direction and downstream wind direction is more likely to occur than the single-degree-of-freedom galloping phenomenon.(4)The optimal parameter expression is obtained by pneumatic optimization,and the influence of parameter deviation on the damping effect of the structure is obtained,and the damping effect under different optimal parameters is compared.The control effect of LT-TMD(rod mass tuned damper)on wake induced vibration and vibration reduction of double cables is analyzed quantitatively.The maximum allowable total damping of LT-TMD is the highest under the optimal parameters,that is to say,the damping effect is the best.When the damping ratio deviates,the damping effect will have a peak value with the change of frequency ratio.The curve of damping ratio with the change of frequency ratio is similar to that of optimal damping ratio,and it changes violently near the peak value.With the increase of mass ratio,the optimal damping ratio and frequency ratio increase,and the maximum allowable total damping increases rapidly,but the increasing trend slows down.When the mass ratio is between 0 and 0.02,the increase of mass ratio is more effective to improve the damping effect.Therefore,it is suggested that the mass ratio of TMD in practical engineering should be selected between 0.005 and 0.02.(5)Through the wind tunnel test of double cables vibration control,it is foundthat the damping effect of LT-TMD under the optimal parameter design is obvious,and the critical reduction wind speed increases to about 5 times.Under the optimal parameters,the damping effect of LT-TMD is the best.After deviating from the optimal damping ratio,the influence of LT-TMD on the damping effect of double cables are obvious,and the greater the deviation degree,the worse the damping effect.The frequency ratio has a great influence on the damping effect of LT-TMD.When the frequency ratio deviates from the optimal frequency ratio,the damping effect of LT-TMD decreases obviously,and the greater the degree of offset optimal frequency ratio,the worse the damping effect.