Numerical Simulation Of The Vortex-Induced Vibration Of Steel Truss Arch Bridge Hangers

The development trend of modern bridge leads structural components to longer, softer, thinner, and lower damping ratio. The problems caused by vortex-induced vibration are increasingly concerned by structural engineers. Vortex-induced vibration occurred in the lower wind speed causes fatigue damage to the bridge structures and affects the durability. It brings great difficulties to the maintenance.The computational fluid dynamics software FLUENT 6.3 was further developed and the functions were expanded in this project. The code of Newmark method was embedded in user defined function (UDF) of FLUENT 6.3. It established the fluid-solid coupling model of vortex-induced vibration in two dimensional incompressible uniform cross flow. The vortex-induced vibration of the hangers at Nanjing Dashengguan Bridge was simulated and calculated. The maximum amplitude and lock-in wind speed domains were determined. The section geometry with good aerodynamic performance was designed to reduce the vortex-induced vibration together with the devices of higher damping ratio. This project has very important practical value. The content and main conclusions as follows:1.This project studied the methods to improve aerodynamic performance. In the case of rectangular sectioned cylinder with sharp, chamfered and rounded corners , the vorticity and lift force become smaller , the maximum amplitude and lock-in domains decrease when the wind blow along the bridge.2.This project studied the methods to suppress the vortex-induced vibration. The optimization of hanger section geometry was considered first, then the influence of damping devices was considered. Structural damping ratio affect both vibration amplitude and lock-in wind speed domains. As the damping ratio increases, the maximum amplitude of the vortex-induced vibration and lock-in wind speed domains decrease. When the damping ratio increases to a certain value, the lock-in wind speed domains disappear.3.This project studied the effect of wind angle on aerodynamic characteristics. The lift force coefficient for a chamfered rectangle hanger decreases when the approaching wind angle increases. The lift force coefficient for a rounded rectangle hanger changes nonlinearly with the change of wind angle.4.This project studied the interactions between two hangers in flow. For a single chamfered rectangle hanger, the maximum amplitude reached 1 m when the most unfavorable wind approachs along the bridge. If the front hanger wake effect takes into account in a two-hanger system, the maximum amplitude of the rear hanger reduces to 25 cm. The wake effect of the front hanger to the rear hanger decreases when the approaching wind angle increases. When the wind angle reaches to 20 degrees, it is similar to a single hanger in flow.5.This project studied the hangers in fluctuating wind field. When fluctuating wind field has a short period and high frequency, the lift force coefficient peak is a constant and the fluctuating wind field corresponds to only one primary frequency. When fluctuating wind field has a long period and low frequency, the lift force coefficient peak changes periodically and the fluctuating wind field corresponds to several primary frequencies. The maximum amplitude of the resulting vortex-induced vibration is smaller than that casused by homogeneous flow.6.This project studied a three-dimensional model of vortex-induced vibration. With the softwares WORKBENCH and CFX a three-dimensional fluid-solid coupling model of vortex-induced vibration was esteblished to analyze and calculate the amplitudes when vortex-induced vibration begin. The force achieved from two-dimensional vortex-induced resonance vibration at steady-state was used as harmonic load applied on the three-dimensional hungers for harmonic response analysis. The resulting maximum amplitude coincides with that from two-dimensional vortex-induced vibration.Through the results from the wind tunnel test and simulated calculations, we obtained the following conclusions:The numberical simulation results are in good agreement with the wind tunnel test results. It is difficult to fully suppress vortex-induced vibration only by optimizing the section geometry. It is necessary to apply the devices to increase the damping ratio. The damping ratio demanded to fully suppress vortex-induced vibration gradually become smaller along with the change of the shape of the hanger section from rectangle to chamfered rectangle, and to rounded rectangle. Considering the number of damping ratio demanded, the rectangular sectioned cylinder with rounded corners is better than rectangular sectioned cylinder with chamfered corners. Considering manufacture convenience, the rectangle hangers with damping devices are the best plan.