树木风振特性试验研究与有限元分析
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摘要
风害是树木损伤的主要形式之一,每年由于强风导致城市内的古树、不健康的树木出现树枝坠落、树干弯曲、折断,甚至掘根倒伏等现象时有发生,对建筑设施和人群的安全构成了威胁,因此,进行树木的安全性评估就显得尤为重要。但是,目前国内外还没有评估树木安全性的有效检测装置。本文以毛白杨为研究对象,利用振动试验和数值模拟技术研究了树干、树枝和树根在强风作用下的振动特性,为研发能够快速、准确评估树木安全性的检测装置提供基础资料和理论依据。
本文的主要研究工作如下：
论文利用计算机辅助设计软件Pro/Engineer中的族表与UDF功能,基于参数化建模思路和分形理论,提出了一种快速建立树木结构几何模型的方法。采用该方法所建立的树木力学几何模型,在具有自相似、自仿射的分形结构特点的同时,还能够体现树木材料的各向异性。而且,通过修改相关参数可以建立不同种类树木的几何模型。
为了寻找一种简单有效的分析方法来研究树木在风荷载作用下的振动特性。本文以毛白杨树干和树枝作为试验对象,通过振动测试方法测量并计算了其自由振动时的自振频率和阻尼比。以离心风机作为人工风源,搭建了风振试验测试平台,对毛白杨施加强风,通过测量树木主干和树枝的运动轨迹以及同一时刻的风压强度,分析树枝对树木风振特性的影响。在分析试验结果和理论模型的基础上,把树枝与树干看作多自由度振动系统,将树木的弹性模量看作弹簧刚度,弹塑性变形以及空气阻尼看作阻尼系数,并考虑树枝与树干之间扭转刚度,初步建立了树木风振运动的微分方程。
基于流固耦合原理,利用有限元软件ANSYS模拟并分析了树木在强风作用下的风振运动特性。研究发现,单向流固耦合法计算树干胸径处的位移比双向流固耦合法的明显增大,相位提前了约10%,但振动周期减小。与试验值对比发现,双向流固耦合法能够更准确地模拟树木风振的运动特性,而单向流固耦合法能够较准确的估算树木风振运动时的最大应力。
最后,本文通过根系振动试验与有限元模拟分析,分别在自然状态、根土摩擦和固定端三种情况下测量了树干的自振频率。试验结果表明,在自然状态下树木自振频率大于根土摩擦,但小于固定端约束。基于试验结果和锚固理论,本文提出了根土刚性连接有限元模型,用以描述自然状态下根系与土壤间的关系。通过有限元仿真分析,并将仿真结果与试验结果进行了对比,发现根土刚性连接模型较其它两种模型能够更为准确的描述自然状态下根系与土壤间的关系。
Wind not only threatens the production rate of forests but also has a huge effect on the people who live in the city and their wealth. In order to provide a wind vibration theoretical basis of forest management and regulation after strong wind disaster, this paper study vibration characteristics of Chinese white poplars under strong wind.
This article research Chinese white poplars which3years old grows in nursery of Beijing Forestry University. Chinese white poplars are studied by wind vibration tests, numerical simulation and mechanical vibration theoretic analysis. Main research methods are as following:
1. The rapid modeling method for trees. This study attempts to provide three-dimensional structure model through fractal theory to study the mechanical properties of trees. Through the "family table" and "UDF library" which are the functionality of Pro/Engineer software, the results is to build a geometric structure of the model with three-dimensional and lays the foundation for analysis wind characteristic in the numerical simulation part.
2. Wind vibration test and numerical simulation about the crown of Chinese white poplars. This article sets up finite element model of the crown of single trees which not including leaves and FEM software ANSYS Workbench had been used to analyze. Transverse view mechanical model had been used to build up multiple level branches structure model. Used centrifugal pump fan as the manual wind source, half sine gust of wind had been made to replace natural wind. Acceleration transducers had been used to measure vibration frequency of main trunk and first level branches.
3. Established the model of trunk and crown, and simulated the dynamic characteristic by using finite element software ANSYS. The analysis results indicate that there is significant relationship between the distribution of branches and wind resistance mechanism of trees. Under strong wind, for the sake of decreasing the windward area of crown to avoid trees lodging, the stress and trunk and perpendicular to the direction of branches will be under the action of wind perpendicular to the direction of in-plane vibration occurred, when crown in perpendicular to the direction of in-plane asymmetric, trees will lateral movement, and so that the trunk root produce elliptic type rotary.
4. In order to research the influence of interaction relation between roots and soil to vibration performance of trees, the natural frequency of four Chinese white poplars had been measured through trunks vibration tests in three different constraint conditions which include natural growth and root-soil friction and fixed end. Then root-soil stiffness connection model, root-soil friction model and fixed end constrained model had been constructed to calculate the natural frequency in those conditions which were mentioned.
本文的主要研究工作如下：
论文利用计算机辅助设计软件Pro/Engineer中的族表与UDF功能,基于参数化建模思路和分形理论,提出了一种快速建立树木结构几何模型的方法。采用该方法所建立的树木力学几何模型,在具有自相似、自仿射的分形结构特点的同时,还能够体现树木材料的各向异性。而且,通过修改相关参数可以建立不同种类树木的几何模型。
为了寻找一种简单有效的分析方法来研究树木在风荷载作用下的振动特性。本文以毛白杨树干和树枝作为试验对象,通过振动测试方法测量并计算了其自由振动时的自振频率和阻尼比。以离心风机作为人工风源,搭建了风振试验测试平台,对毛白杨施加强风,通过测量树木主干和树枝的运动轨迹以及同一时刻的风压强度,分析树枝对树木风振特性的影响。在分析试验结果和理论模型的基础上,把树枝与树干看作多自由度振动系统,将树木的弹性模量看作弹簧刚度,弹塑性变形以及空气阻尼看作阻尼系数,并考虑树枝与树干之间扭转刚度,初步建立了树木风振运动的微分方程。
基于流固耦合原理,利用有限元软件ANSYS模拟并分析了树木在强风作用下的风振运动特性。研究发现,单向流固耦合法计算树干胸径处的位移比双向流固耦合法的明显增大,相位提前了约10%,但振动周期减小。与试验值对比发现,双向流固耦合法能够更准确地模拟树木风振的运动特性,而单向流固耦合法能够较准确的估算树木风振运动时的最大应力。
最后,本文通过根系振动试验与有限元模拟分析,分别在自然状态、根土摩擦和固定端三种情况下测量了树干的自振频率。试验结果表明,在自然状态下树木自振频率大于根土摩擦,但小于固定端约束。基于试验结果和锚固理论,本文提出了根土刚性连接有限元模型,用以描述自然状态下根系与土壤间的关系。通过有限元仿真分析,并将仿真结果与试验结果进行了对比,发现根土刚性连接模型较其它两种模型能够更为准确的描述自然状态下根系与土壤间的关系。
Wind not only threatens the production rate of forests but also has a huge effect on the people who live in the city and their wealth. In order to provide a wind vibration theoretical basis of forest management and regulation after strong wind disaster, this paper study vibration characteristics of Chinese white poplars under strong wind.
This article research Chinese white poplars which3years old grows in nursery of Beijing Forestry University. Chinese white poplars are studied by wind vibration tests, numerical simulation and mechanical vibration theoretic analysis. Main research methods are as following:
1. The rapid modeling method for trees. This study attempts to provide three-dimensional structure model through fractal theory to study the mechanical properties of trees. Through the "family table" and "UDF library" which are the functionality of Pro/Engineer software, the results is to build a geometric structure of the model with three-dimensional and lays the foundation for analysis wind characteristic in the numerical simulation part.
2. Wind vibration test and numerical simulation about the crown of Chinese white poplars. This article sets up finite element model of the crown of single trees which not including leaves and FEM software ANSYS Workbench had been used to analyze. Transverse view mechanical model had been used to build up multiple level branches structure model. Used centrifugal pump fan as the manual wind source, half sine gust of wind had been made to replace natural wind. Acceleration transducers had been used to measure vibration frequency of main trunk and first level branches.
3. Established the model of trunk and crown, and simulated the dynamic characteristic by using finite element software ANSYS. The analysis results indicate that there is significant relationship between the distribution of branches and wind resistance mechanism of trees. Under strong wind, for the sake of decreasing the windward area of crown to avoid trees lodging, the stress and trunk and perpendicular to the direction of branches will be under the action of wind perpendicular to the direction of in-plane vibration occurred, when crown in perpendicular to the direction of in-plane asymmetric, trees will lateral movement, and so that the trunk root produce elliptic type rotary.
4. In order to research the influence of interaction relation between roots and soil to vibration performance of trees, the natural frequency of four Chinese white poplars had been measured through trunks vibration tests in three different constraint conditions which include natural growth and root-soil friction and fixed end. Then root-soil stiffness connection model, root-soil friction model and fixed end constrained model had been constructed to calculate the natural frequency in those conditions which were mentioned.
引文
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