【作者】 詹昊；

【导师】 方秦汉； 李万平；

【作者基本信息】 华中科技大学 ， 桥梁与隧道工程， 2009， 博士

【摘要】 现代结构向长、柔、细、弱阻尼方向发展，涡激振动问题越来越受到结构工程师的关注。涡激振动发生的风速较低，频度大，会对结构造成疲劳损伤，影响耐久性，给养护带来极大麻烦。本文对计算流体力学软件FLUENT自主进行了二次开发，扩展了FLUENT的功能。将振动方程的数值方法Newman法代码嵌入FLUENT软件中，建立了非定常钝体绕流粘性不可压缩流体的流固耦合数值模型，使之可以进行涡激振动仿真计算。本文对南京大胜关大桥吊杆进行涡激振动仿真计算，求出其最大振幅，锁定风速区间，设计出具有良好气动性能的截面并辅以增加阻尼装置以减小涡激振动的影响，具有非常重要的工程实用价值。本研究工作内容和主要结论为：1．研究改善结构气动力性能的方法。顺桥向来风时，长方形截面，切角长方形截面，圆角长方形截面的涡脱落规模依次减小，升力依次减小。其中圆角长方形截面的最大振幅和锁定区间均小于切角长方形截面。2．分析比较了抑制涡激振动的方法。首先考虑优化结构截面形状，在此基础上考虑增加结构阻尼。通过增加结构阻尼的方法可以抑制涡激振动。结构阻尼不但对涡激振动振幅大小有影响，也会对锁定风速区间有影响。随着阻尼的增加，结构最大振幅减小，所俘获的涡脱频率的范围较小，锁定区间的长度相应较小，当阻尼增大到一定值后，锁定风速区间消失。3．分析了风偏角对结构气动性能的影响。单根杆件，对于切角长方形截面，顺桥向来风时升力系数最大，随着风偏角的增加，升力系数有减小的趋势。横桥向来风时升力系数最小。对于圆角长方形截面，攻角的变化对升力系数的影响比较复杂，不是一种单调线性趋势。4．研究了两根杆件气流的相互影响。对于切角长方形截面，在单根杆件分析中，顺桥向来风是最不利风向，最大振幅达1m。但考虑了前面杆件尾流的影响后，最大振幅减小到25cm。随着风偏角的增加，后面杆件受前面杆件尾流影响有减小趋势，当风偏角达到20度后，与单根杆件绕流情况相似。5．分析了脉动风场对结构的影响。相对结构固有频率，当脉动风场周期较小，频率较大时。升力系数峰值恒定不变。对应一个主要频率；当脉动风场周期较大，频率较小时。升力系数峰值呈周期变化，对应多个主要频率，其涡激振动最大振幅小于均匀来风。6．将二维涡激共振达到稳态振幅后的涡激力作为简谐荷载加到三维杆件模型上进行了谐响应分析，得到与二维涡激振动相吻合的最大振幅。运用了软件WORKBENCH和CFX建立三维柱体流固耦合模型，进行吊杆三维涡激振动初步分析，计算得到起振风速时的振幅。通过风洞试验和仿真计算可以得到以下结论：本文数值仿真结果与节段模型风洞试验吻合良好。仅通过优化截面的形状截面来完全抑制涡激振动是很困难的，需辅以增加阻尼装置的措施。对于长方形截面，切角长方形截面和圆角长方形截面完全抑制涡激振动需增加的阻尼数量逐渐减小。单从增加阻尼的数量来看，圆角长方形截面方案优于切角长方形截面方案。若从制造便利考虑，长方形截面加阻尼装置方案最好。更多还原

【Abstract】 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.更多还原