Tower Wake-induced Vibration Of Long Sling Near The Suspension Bridge Tower

To interpret the vibration mechanism of suspension bridge slings which is near the bridge tower, this thesis refines the analysis model from actual suspension bridges where such phenomenon has been observed, and use CFD numerical simulation as well as wind tunnel test for relevant research.First, the string theory and finite element analysis are applied to analyze first-order free vibration characteristics of slings around the bridge tower in two suspension engineering, obtaining first-order free vibration frequency range of most suspension bridges.Secondly, a numerical simulation of two-dimensional flow is conducted to study the flow characteristics around slings, rectangular-section cylinders with side ratio of1.4:1, and the aerodynamic interference between rectangular towers and slings. The effects of wind speed and yaw angle are examined for some cases. The results show that the difference of aerodynamic characteristics between single sling and four-string sling is very little, and that the possibility of vortex-excited resonance of a sling by shedding vortex itself is very low. For the B/D=1.4rectangular-section, there is a rapid rise in the Strouhal number at small angles of yaw reaching the value of0.185and then a subsequent plateau region for a wide range of angles of yaw before a quick decrease at75degree. Also, the analysis indicates that the wake of two rectangular towers have dominant influence on slings around bridge towers, curbing the shedding vortex of slings, controlling the aerodynamic characteristics of slings.Finally, the tower-sling aeroelastic segment wind tunnel experiments are carried out in XNJD-1wind tunnel to achieve the recurrence of vibration of slings around bridge tower and to study the vibration responses of slings on both sides of tower. The wind tunnel tests show that, at certain yaw angles, intense vibrations occur on relevant slings while the wind speed grows. At this time, the vibration frequency of the sling, the shedding frequency of vortex behind tower and first-order free vibration frequency of this sling appear to be the same value. This verifies the correctness of numerical simulation analysis, while coming to the end of the mechanism of tower wake-induced vibration of slings around bridge tower: because the sling around bridge tower is strongly interfered by tower wake, so, at a certain wind velocity, the sling can be stimulated at the same frequency as its first-order vibration frequency, leading to the resonance. Further, the experiments also show that, for a wide range of yaw angle, the relevant sling vibrates obviously at a certain velocity due to the tower wake. And the motion trajectory of slings is substantially an ellipse. Also, the second-order vibration of the sling caused by tower wake is observed in the wind tunnel test.