蜻蜓滑翔时柔性褶皱前翅气动特性分析

蜻蜓是自然界优秀的飞行家,滑翔是其常见且有效的飞行模式.蜻蜓优异的飞行能力来源于其翅膀的巧妙结构,褶皱是蜻蜓翅膀上最为显著的结构之一,不仅提高了翅膀的刚度,还改变了其气动特性,而飞行过程中柔性翅膀会产生变形是蜻蜓翅膀的另一特性.为揭示蜻蜓在滑翔时,柔性褶皱前翅的变形,探究褶皱和柔性的共同作用对其气动特性的影响,基于逆向工程,依据前人的测量数据和研究成果,通过三维建模软件建立了蜻蜓三维褶皱前翅的计算流体力学(computational fluid dynamics,CFD)模型和计算结构力学(computational structural mechanics,CSD)模型,并通过模态分析验证了此模型有足够的精度.基于CFD方法和CFD/CSD双向流固耦合计算方法分别对蜻蜓滑翔飞行时刚性和柔性褶皱前翅的气动特性进行了数值模拟,结果表明,柔性褶皱前翅受气动载荷后,翅脉和翅膜产生形变,柔性前翅上下表面压力差相较于刚性前翅减小了,从而其升力和阻力也减小了,而在大攻角时,变形后的前缘脉诱导出比刚性前翅更强的前缘涡.因此在攻角小于10?时刚性前翅的气动特性优于柔性前翅,继续增大攻角,柔性前翅的气动特性则优于刚性前翅.前翅受载后气动响应时间短,翅尖的变形最大,仅仅产生了垂直于翅膀所在平面方向上的变形,而没有发生扭转,翼根处受到应力最大,褶皱上凸部分承受蜻蜓滑翔时前翅的主要载荷.     

蜻蜓非均匀柔性前翼扑动飞行的气动性能研究

为了探究柔性对于蜻蜓前翼在扑动向前飞行时的气动性能, 本文根据蜻蜓前翼的实际参数建立蜻蜓前翼模型, 提出了两种柔性分布方式即均匀柔性分布和沿蜻蜓前翼弦向的变柔性分布. 本文通过STAR-CCM+软件, 首先采用重叠网格和双向流固耦合技术, 用于实现蜻蜓前翼的扑动流固耦合, 其次通过改变蜻蜓前翼固体区域的杨氏模量函数从而实现蜻蜓前翼的两种不同柔性分布. 结果表明, 在均匀柔性分布条件下, 柔性翼在杨氏模量较小时的升力系数和阻力系数曲线的变化规律滞后于刚性翼半周期并且给飞行增加阻力, 但是随着杨氏模量的逐渐增加即柔性逐渐减小, 蜻蜓前翼受到的阻力减小, 获得的推力增加且推力给予蜻蜓前飞的动量增量、加速度以及时均推力系数先增加后减小. 在合理的非均匀柔性分布条件下, 柔性翼显著提高推力系数峰值和时均推力系数, 在扑动前飞时, 给予蜻蜓前翼较大的动量增量以及加速度. 两种柔性分布方式的蜻蜓前翼与刚性翼对比之下, 蜻蜓前翼在柔性为非均匀柔性分布时可以获得更好的气动性能.      

昆虫飞行的空气动力学

昆虫是最早出现、数量最多和体积最小的飞行者. 它们能悬停、跃升、急停、快速加速和转弯, 飞行技巧十分高超. 由于尺寸小, 因而翅膀的相对速度很小, 从而进行上述飞行所需的升力系数很大. 但昆虫翅膀的雷诺数又很低. 它们是如何在低雷诺数下产生高升力的, 是流体力学和生物学工作者都十分关心的问题. 近年来这一领域有了许多研究进展. 该文对这些进展进行综述, 并对今后工作提一些建议. 因2005 年前的工作已在几篇综述文章有了详细介绍, 该文主要介绍2005 年以来的工作. 首先简述昆虫翅的拍动运动及昆虫绕流的基本方程和相似参数; 然后对2005 年之前的工作做一简要回顾. 之后介绍2005 年后的进展, 依次为: 运动学观测; 前缘涡; 翅膀柔性变形及皱褶的影响; 拍动翅的尾涡结构; 翼/身、左右翅气动干扰及地面效应; 微小昆虫; 蝴蝶与蜻蜓; 机动飞行. 最后为对今后工作的建议.      

昆虫飞行的空气动力学

昆虫是最早出现、数量最多和体积最小的飞行者,它们能悬停、跃升、急停、快速加速和转弯,飞行技巧十分高超,由于尺寸小,因而翅膀的相对速度很小,从而进行上述飞行所需的升力系数很大,但昆虫翅膀的雷诺数又很低。它们是如何在低雷诺数下产生高升力的,是流体力学和生物学工作者都十分关心的问题,近年来这一领域有了许多研究进展。该文对这些进展进行综述,并对今后工作提一些建议。因2005年前的工作已在几篇综述文章有了详细介绍,该文主要介绍2005年以来的工作。首先简述昆虫翅的拍动运动及昆虫绕流的基本方程和相似参数;然后对2005年之前的工作做一简要回顾。之后介绍2005年后的进展,依次为:运动学观测;前缘涡;翅膀柔性变形及皱褶的影响;拍动翅的尾涡结构;翼/身、左右翅气动干扰及地面效应;微小昆虫;蝴蝶与蜻蜓;机动飞行。最后为对今后工作的建议。     

蜻蜓翅膀的力学研究进展

本文对蜻蜓翅膀的力学问题研究进展进行了综述.蜻蜓之所以成为自然界最优秀的飞行者之一,是因为其翅膀的特殊结构.翅结和翅痣的作用,是在飞行中平衡翅膀前后两部分和消除翅膀振颤.常用的翅膀模型有四种:概念模型、物理模型、简单分析模型和数值分析模型.其中前三种模型过于简化,数值模型虽然可以进行三维数值模拟,但是忽略了翅膀的材料组成.本文还分析了蜻蜓翅膀的飞行机制和力学特点,认为蜻蜓的飞行力学分析必须考虑非定常流的影响;介绍了翅膀的弹性模量、硬度和抗弯刚度的测量方法,指出翼展方向的抗弯刚度(EI)与翼展长度的三次方(L3)成正比,翼弦方向的抗弯刚度(EI)与翼弦长度的平方(D2)成正比;最后提出了对蜻蜓翅膀力学的研究展望和有待于进一步解决的问题.     

乘波体前体/进气道一体化设计研究

为了研究乘波体几何外形参数和飞行参数对前体/进气道一体化设计的影响,采用理论分析和数值模拟相结合的方法,以马赫数Ma=6和攻角α=0为设计状态、进气道总压恢复系数和前体阻力系数为目标函数,对乘波体前体/进气道进行了优化设计,并在此基础上研究了攻角、马赫数、前缘半径、前体宽度对气动参数的影响。结果表明:该乘波体前体/进气道构型具有良好的攻角特性,总压恢复系数比基准构型提高17.79%,阻力系数比基准构型降低78.5%,符合高超声速飞行器高升力、低阻力的要求,且非常适合小攻角高超声速巡航飞行;为了得到较高升阻比的前体,在前缘半径R≤2mm的范围内进行流场反设计时,可以将设计马赫数的取值比预期低一些。     

鸟类羽毛在气流中变形的力学特性研究

鸟类羽毛在飞行中的物理性质是仿生力学关心的重要问题之一.基于CFD/CSD数值模拟方法研究了羽毛微结构在气流作用下的变形和力学特征,揭示了鸟类静止时羽毛蓬松、而在飞行状态下紧贴皮肤表面保持表面光滑的物理机制.首先,通过对鸟类羽毛在显微镜下的观察,将羽毛分解成典型简单微结构以模仿羽枝单元,从而对羽毛外形和结构进行建模,之后,采用CFD/CSD方法分析比较了两种典型羽枝模型结构(片状和枝状羽枝单元)的变形和力学特征,最后,基于上述片状羽枝模型进一步研究了来流方向对羽枝变形的影响机理及多根排列羽枝的变形和力学特征.结果表明:在一定风向的范围内,羽毛在气流下都具有保持紧贴皮肤表面的变形趋势,这种紧贴壁面的趋势只有在气流与羽轴几乎垂直时才会改变;在来流侧滑角为45°时,羽枝沿皮肤表面法向下压的变形最为显著,尖端位移达原始高度的约97%;多根排列的羽枝在顺流方向气动载荷逐渐下降,与迎风首根羽枝最大差距约11%.此研究工作对于理解鸟类飞行时羽毛的力学特性有明确的学术价值.     

方形截面弹俯仰振荡对滚转特性的影响

飞行器在大气环境中飞行时,经常受阵风等的干扰,引发非指令的自激振荡,威胁飞行安全.通过建立刚体六自由度运动方程和N-S方程的松耦合求解技术,研究强迫俯仰振荡过程对滚转特性的影响.针对背风区涡流形态及横侧向气动特性复杂的方形截面飞行器,数值模拟研究了其不同攻角下的静态滚转气动特性、自由滚转运动特性,以及俯仰振荡时不同振荡速率对滚转气动和运动特性的影响.结果表明,此飞行器在静态时临界攻角约为13°,当攻角小于临界攻角时,滚转方向是静不稳定的,诱发快速滚转运动;当攻角大于临界攻角时,滚转方向是静稳定,其滚转运动是收敛的.研究发现,俯仰振荡一般会降低飞行器滚转方向静稳定或静不稳定的量值,增强滚转方向的动态稳定性.在俯仰振荡的影响下,即使滚转方向是静不稳定的,如果俯仰振荡的频率足够大,飞行器的滚转运动也可能是收敛的.     

蜻蜓翅膀宏细观结构的实验观察与力学分析

0引言 蜻蜓翅膀是由纵脉和横脉组成的一个凹凸相间的立体网状结构(如图1所示).翅膀重量不到0.005 g,翅膜厚度只有2-3 μm,它每秒却可拍翅30-50次,冲刺飞行速度高达40 m/s.蜻蜒可以实现扑翼、滑行、悬停等飞行,蜻蜓的高超飞行能力与它的翅膀特殊结构有关.本论文主要研究蜻蜓翅膀的宏细观结构及其力学性能.     

两串列扑翼的相位差对平均推力影响机理的实验研究

在一个低雷诺数的循环水洞中,实验研究了前后翅翼之间的相位差对两串列扑翼平均推力的影响.利用一个三分量的Kistler 压力传感器来测量扑翼的瞬时力;利用一个数字粒子测速仪系统(TSI DPIV) 来测量扑翼的前缘涡以及其周围的流场. 当相位差从0° 增加到360°,前翅的平均推力随着相位差正弦变化;前翅平均推力的增加是由于后翅的前缘涡和滞止区域增加了前翅的有效攻角. 后翅平均推力曲线有一个明显的V 字形低谷.低谷处较小的平均推力是由于前翅的脱落涡抑制了后翅前缘涡的形成并且减小了其有效攻角.当间距为0.5倍弦长相位差约为290°时,前后翅翼平均推力系数的合值能达到最大值0.667,明显大于两倍的单翼平均推力系数(2×0.255).      

Structural Analysis of a Dragonfly Wing

Dragonfly wings are highly corrugated, which increases the stiffness and strength of the wing significantly, and results in a lightweight structure with good aerodynamic performance. How insect wings carry aerodynamic and inertial loads, and how the resonant frequency of the flapping wings is tuned for carrying these loads, is however not fully understood. To study this we made a three-dimensional scan of a dragonfly (Sympetrum vulgatum) fore- and hindwing with a micro-CT scanner. The scans contain the complete venation pattern including thickness variations throughout both wings. We subsequently approximated the forewing architecture with an efficient three-dimensional beam and shell model. We then determined the wing’s natural vibration modes and the wing deformation resulting from analytical estimates of 8 load cases containing aerodynamic and inertial loads (using the finite element solver Abaqus). Based on our computations we find that the inertial loads are 1.5 to 3 times higher than aerodynamic pressure loads. We further find that wing deformation is smaller during the downstroke than during the upstroke, due to structural asymmetry. The natural vibration mode analysis revealed that the structural natural frequency of a dragonfly wing in vacuum is 154 Hz, which is approximately 4.8 times higher than the natural flapping frequency of dragonflies in hovering flight (32.3 Hz). This insight in the structural properties of dragonfly wings could inspire the design of more effective wings for insect-sized flapping micro air vehicles: The passive shape of aeroelastically tailored wings inspired by dragonflies can in principle be designed more precisely compared to sail like wings —which can make the dragonfly-like wings more aerodynamically effective.     

褶皱幅度对仿生翼型滑翔气动特性的影响

蜻蜓翅膀具有独特的褶皱状形貌．研究者们致力于利用仿生学原理,设计在低雷诺数条件下具有更优气动性能的褶皱翼型．本文采用计算流体力学方法,求解二维不可压Navier-Stokes方程组,探讨了四种翼型（平板翼型、流线翼型、小幅度褶皱翼型和大幅度褶皱翼型）的气动表现．在低雷诺数条件下得到以下结果：(1) 较小幅度的褶皱结构有利于增加升力和减小阻力．(2) 雷诺数变化时褶皱翼型的升力系数呈非线性变化;在特定雷诺数区间,幅度相近的褶皱翼型会发生相对气动优势的转变．(3) 褶皱结构内的回流区通过减小粘性阻力,使得翼型总阻力下降．(4) 翼型前缘的极小区域会产生脉冲高升力,对升力表现产生较大影响．这些结果表明,调整褶皱幅度是实现褶皱翼型气动优化的有效方案．     

The effects of corrugation and wing planform on the aerodynamic force production of sweeping model insect wings

The effects of corrugation and wing planform (shape and aspect ratio) on the aerodynamic force production of model insect wings in sweeping (rotating after an initial start) motion at Reynolds number 200 and 3500 at angle of attack 40° are investigated, using the method of computational fluid dynamics. A representative wing corrugation is considered. Wing-shape and aspect ratio (AR) of ten representative insect wings are considered; they are the wings of fruit fly, cranefly, dronefly, hoverfly, ladybird, bumblebee, honeybee, lacewing (forewing), hawkmoth and dragonfly (forewing), respectively (AR of these wings varies greatly, from 2.84 to 5.45). The following facts are shown. (1) The corrugated and flat-plate wings produce approximately the same aerodynamic forces. This is because for a sweeping wing at large angle of attack, the length scale of the corrugation is much smaller than the size of the separated flow region or the size of the leading edge vortex (LEV). (2) The variation in wing shape can have considerable effects on the aerodynamic force; but it has only minor effects on the force coefficients when the velocity at r ₂ (the radius of the second moment of wing area) is used as the reference velocity; i.e. the force coefficients are almost unaffected by the variation in wing shape. (3) The effects of AR are remarkably small: when AR increases from 2.8 to 5.5, the force coefficients vary only slightly; flowfield results show that when AR is relatively large, the part of the LEV on the outer part of the wings sheds during the sweeping motion. As AR is increased, on one hand, the force coefficients will be increased due to the reduction of 3-dimensional flow effects; on the other hand, they will be decreased due to the shedding of part of the LEV; these two effects approximately cancel each other, resulting in only minor change of the force coefficients.     

The effects of corrugation and wing planform on the aerodynamic force production of sweeping model insect wings

The effects of corrugation and wing planform (shape and aspect ratio) on the aerodynamic force production of model insect wings in sweeping (rotating after an initial start) motion at Reynolds number 200 and 3500 at angle of attack 40° are investigated, using the method of computational fluid dynamics. A representative wing corrugation is considered. Wing-shape and aspect ratio (AR) of ten representative insect wings are considered; they are the wings of fruit fly, cranefly, dronefly, hoverfly, ladybird, bumblebee, honeybee, lacewing (forewing), hawkmoth and dragonfly (forewing), respectively (AR of these wings varies greatly, from 2.84 to 5.45). The following facts are shown. (1) The corrugated and flat-plate wings produce approximately the same aerodynamic forces. This is because for a sweeping wing at large angle of attack, the length scale of the corrugation is much smaller than the size of the separated flow region or the size of the leading edge vortex (LEV). (2) The variation in wing shape can have considerable effects on the aerodynamic force; but it has only minor effects on the force coefficients when the velocity at r ₂ (the radius of the second moment of wing area) is used as the reference velocity; i.e. the force coefficients are almost unaffected by the variation in wing shape. (3) The effects of AR are remarkably small: when AR increases from 2.8 to 5.5, the force coefficients vary only slightly; flowfield results show that when AR is relatively large, the part of the LEV on the outer part of the wings sheds during the sweeping motion. As AR is increased, on one hand, the force coefficients will be increased due to the reduction of 3-dimensional flow effects; on the other hand, they will be decreased due to the shedding of part of the LEV; these two effects approximately cancel each other, resulting in only minor change of the force coefficients. The project supported by the National Natural Science Foundation of China (10232010 and 10472008) and Ph. D. Student Foundation of Chinese Ministry of Education (20030006022) The English text was polished by Keren Wang.     

低雷诺数下柔性翼型气动性能分析

基于流固耦合方法对吸力面5%至95%弦长处为三段柔性结构的NACA0012翼型绕流进行了数值模拟,研究了不同弹性模量下柔性翼型的气动性能和结构响应.结果表明:在大攻角下,翼面变形影响着翼型表面的非定常流场,起到延缓失速和提高升力的作用;失速后柔性翼的升力系数下降得较为缓慢,且柔性越大,升力系数下降得越平缓;适当减小弹性模量能够提高翼型的气动性能,然而弹性模量过小反而不利于翼型气动性能的提升,并且翼面会产生大幅度的振动.     

A simulation-based study on longitudinal gust response of flexible flapping wings

Winged animals such as insects are capable of flying and surviving in an unsteady and unpredictable aerial environment. They generate and control aerodynamic forces by flapping their flexible wings. While the dynamic shape changes of their flapping wings are known to enhance the efficiency of their flight, they can also affect the stability of a flapping wing flyer under unpredictable disturbances by responding to the sudden changes of aerodynamic forces on the wing. In order to test the hypothesis, the gust response of flexible flapping wings is investigated numerically with a specific focus on the passive maintenance of aerodynamic forces by the wing flexibility. The computational model is based on a dynamic flight simulator that can incorporate the realistic morphology, the kinematics, the structural dynamics, the aerodynamics and the fluid–structure interactions of a hovering hawkmoth. The longitudinal gusts are imposed against the tethered model of a hovering hawkmoth with flexible flapping wings. It is found that the aerodynamic forces on the flapping wings are affected by the gust, because of the increase or decrease in relative wingtip velocity or kinematic angle of attack. The passive shape change of flexible wings can, however, reduce the changes in the magnitude and direction of aerodynamic forces by the gusts from various directions, except for the downward gust. Such adaptive response of the flexible structure to stabilise the attitude can be classified into the mechanical feedback, which works passively with minimal delay, and is of great importance to the design of bio-inspired flapping wings for micro-air vehicles.