摘要
树叶的形状重构和减阻能力在太阳能帆板、机翼结构、仿生天线设计和新型发电技术等方面具有应用价值.紫藤萝羽状复叶垂直悬挂于风洞中,在风速0～25 m/s范围内进行正面和反面迎风测试.发现存在前期稳定、中间过渡和后期稳定3个阶段以及5个临界风速.在前期阶段叶轴随风速弯曲变化剧烈,出现小叶分层飞翼和分层多形状稳定.过渡阶段出现叶轴大幅低频振动和部分小叶小幅高频振动两种不稳定形式.后期出现两层或单一整体稳定,横截面形状分为锥形、楔形和U形.随着风速增大,复叶宽度减小,小叶层数逐步减少,直至出现流线形单一整体.随着雷诺数增大,复叶阻力系数先是快速下降,后又缓慢地趋于常数.复叶Vogel负指数绝对值|α|随小叶数目的增大而增大.反面迎风时|α|比正面迎风时大,但随着小叶数目增加两者趋于一致.当复叶旋涡脱落频率与叶轴固有频率接近时,叶轴出现大幅振动.理论分析得到叶轴振动的第二临界风速V_2/(E/ρ)^(1/2)是b/l和d/l的函数,其中E,ρ, d和l分别为叶轴弹性模量、密度、直径和长度,b为变形后的复叶宽度,并由实验数据得到了其变化图.
The capability of reconfiguration of tree leaves is of significance in the designs of solar panel,aerofoil,bionic antenna,and wind power generation tree.The leaf was clamped at the base end of the rachis and vertically suspended in the center of wind tunnel test section,and tested with its front and back surface facing on-coming stream respectively at step-by-step increasing wind speed from 0 to 25 m/s.Results show that the changing process of the leaf can be divided into three stages:earlier steady,intermediate transition,and later steady,and critical wind speeds are observed.In earlier stage,the downstream bending curvature of the rachis increases rapidly with wind speed,and multi-layer wing steady and multi-layer multi-shape steady states exist.In intermediate stage,large amplitude low frequency rachis vibration,and small amplitude high frequency lobules vibration are observed.In later steady stage,two-layer structure or single streamlined body of conic,or wedge or U-shape cross section can be found.As wind speed increasing,the number of lobule layers and the width of the compound leaf b decrease,until the single streamlined body formed.As Re increasing,the drag coeffcient of the leaf decreases rapidly at first,then slowly approaching to a constant.The absolute value of the negative Vogel component|α|decreases with the increase of lobule number of the leaf.|α|of the leaf with its back surface is larger than that with its front surface facing wind,but they tend to converge with the increase of lobule number.Rachis vibration occurs if the frequency of vortex shedding from the leaf is close to the natural frequency of the rachis.The second critical wind speed V2/√E/P,at which the rachis vibration begins,is shown to be the function of b/l and d/l,where E;P;d;and l are respectively elastic module,mass density,diameter and length of the rachis,b is width of the deformed leaf,and a figure about this function is drawn using experimental data.
作者
俞科杰
邵传平
Yu Kejie;Shao Chuanping(Institute of Fluid Measurement and Simulation,China Jiliang University,Hangzhou 310018,China)
出处
《力学学报》
EI
CSCD
北大核心
2019年第1期245-262,共18页
Chinese Journal of Theoretical and Applied Mechanics
基金
国家自然科学基金资助项目(11172286)
关键词
树叶
形状重构
振动
临界风速
阻力
Vogel指数
tree leaves
reconfiguration
vibration
critical wind speed
drag
Vogel component
