模型昆虫翼作非定常运动时的气动力特性

基于Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程的数值解,研究了一模型昆虫翼在小雷诺数（Ｒｅ＝１００）下作非定常运动时的气动力特性,这些运动包括：翼启动后的常速转动,快速加、减速转动,常速转动中快速上仰（模拟昆虫翼的上挥或下拍、翻转等运动）有如下结果：在小雷诺数下,模型昆虫翼以大攻角（α＝３５°）作常速转动运动时,由于失速涡不脱落,可产生较大的升为系数。其机理是：翼转动时,翼尖附近（该处线速度大）上翼面压强比翼根附近（该处线速度小）的小得多,因而存在展向压强梯度,同时存在着沿展向的离心力,此展向压强梯度和离心力导致的展向流动在失速涡的轴向方向,其可避免失速涡脱落。模型昆虫翼在快速加、减速转动和快速上仰运动中,虽然雷诺数小,但由于在短时间内产生了大涡量,也可产生十分大的气动力,例如在快速上仰运动中,升力系数可大于１０。     

昆虫拍翼方式的非定常流动物理再探讨

基于提出的理论模化方法来探讨昆虫拍翼方式的非定常流动物理. 以悬停飞行为 例,通过对拍翼运动的分析,不仅解释了昆虫利用高频拍翼的方式为何能够克服低雷诺数带 来的气动局限性(St \gg 1/Re),而且还指出高升力产生和调节的3个流动 控制因素：(1) 由于拍翼的变速运动即时引起了流体动力响应,这种附加惯性效应 可产生瞬时的高升力; (2) 保持前缘涡不脱离翼面有助于减少升力的下降; (3) 增大后缘涡的强度并加速其脱离后缘能够有效地提高升力.     

等速上仰翼型分离流动结构的研究

本文通过在翼型上游和翼表面边界层内放置产生氢气泡的铂丝的方法,清楚地显示了上仰翼型分离剪切层的结构。揭示了在不同的翼型转动角速度范围内,存在三种分离流结构。研究了失速涡,剪切涡及起动涡随时间的演变,它们之间的相互作用和转动角速度等参数的影响,分离剪切层的流动显示结果,结合翼型上气动力与流场中涡量矩的关系的理论,定性地解释了上仰翼型产生非定常高升力的原因。     

基于涡流发生器的翼型失速流动控制及雷诺数效应影响研究

针对所设计的三角形涡流发生器开展用于翼型失速流动控制的风洞实验研究,重点讨论涡流发生器几何参数、方向角、安装位置及实验雷诺数等因素对翼型失速流动控制的影响。实验结果表明:涡流发生器作用下,在干净翼失速迎角后能够形成一个升力几乎不随迎角变化的相对稳定的高升力状态,抑制了失速流动的发生,与此同时阻力大幅下降;本文所设计的涡流发生器方向角过大时会削弱翼型失速流动控制的效果;同一涡流发生器作用下雷诺数过大其失速流动控制效果会急剧恶化,第一种涡流发生器控制翼型失速的雷诺数有效范围略宽于第二种涡流发生器。     

等速上仰翼型动态失速现象研究

翼型大迎角绕流的静态失速将造成升力突降和气动性能急剧恶化，但利用非定常运动所产生 的动态失速效应，可以大大地延缓气流分离和失速现象的发生. 采用Rogers发 展的双时间步Roe格式，求解拟压缩性修正不可压N-S方程. 数值模拟了低雷诺数 ($Re=4.8 \times 10^{4}$)条件下NACA0015翼型作等速上仰($\alpha =0^{\circ} \sim 60^{\circ}$)的动态失速过程，同Walker的试验结果比 较，验证了计算结果的正确性. 研究了该过程中主涡、二次涡和三次涡的发展，升 力系数随攻角变化，以及不同上仰速度对动态失速效应所造成的影响.     

微型飞行器低雷诺数空气动力学

微型飞行器(MAVs)设计绝不是常规飞行器在尺度上的简单缩小,面临许多技术难题.其中微型飞行器低雷诺数空气动力学是其最为根本的技术瓶颈之一,也是当前受到广泛关注的热点之一.本文紧密结合微型飞行器技术,对这一领域中所面临的低雷诺数空气动力学问题和近两年来该方向国内一些新的进展进行了较为详细的介绍.按照MAVs飞行方式和结构特性进行分类,简单介绍微型飞行器研究中的低$Re$数空气动力学问题.首先介绍了二维和三维固定翼低雷诺数空气动力学问题:包括层流分离泡,翼型升力系数小攻角非线性效应,静态迟滞效应,以及低$Re$数小展弦比机翼气动特性.第2,介绍了拍动翼低雷诺数空气动力学方面的研究工作.包括前人提出的昆虫低$Re$数下获得高升力的多种非定常拍动翼飞行机制:Wagner效应、Weis-Fogh效应(clap-and-fling)、延迟失速效应(delayedstall)、Kramer效应(rotational forces)、尾迹捕获效应(wakecapture)、附加质量效应(addedmass)等.以及国内学者近几年在拍动翼方面取得的一些研究成果.第3,介绍了柔性翼低雷诺数气动问题.研究表明柔性翼对于固定翼微型飞行器提高抗阵风能力,拍动翼微型飞行器产生足够的升力和推力.最后简单介绍了可变形翼(morphingwing)微型飞行器方面的一些研究工作,指出微型飞行器技术可以通过采用可变形翼设计,突破众多的技术瓶颈.另一方面,可变形翼概念可以通过在低成本,低速的MAVs上进行飞行试验,获得非常好的验证平台.      

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.      

昆虫飞行的高升力机理

对近年来关于昆虫产生非定常高升力的研究进行了综述和归纳.这方面的工作对生物学研究和微型飞行器等微型机械的仿生设计有重要意义.研究表明:果蝇等昆虫翅膀的拍动运动可产生很大的非定常升力,其平均值是定常值的2~3倍,足够平衡昆虫的重量,并有较大的富余用于机动飞行;产生高升力有三个因素:一是拍动开始阶段翅的快速加速运动,二是拍动中的不失速机制,三是拍动结束阶段翅的快速上仰运动.人们从能耗的角度考察了这些非定常高升力机制的正确性和可行性.当作悬停飞行的果蝇用以上机制产生平衡其重量的升力时,其比功率(支持单位身体质量所需的功率)约为29W/kg, 生化/机械效率约为17%. 这些值与人们基于对昆虫肌肉力学特性的研究所预估的值接近.果蝇前飞时,其比功率随速度变化的曲线是一J形曲线,而不是象飞机或鸟的那样是一U形曲线;这与人们基于昆虫新陈代谢率的测量数据所推断的结果一致.对于蜻蜒等(功能上)有前、后两对翅膀的昆虫,有以下初步结果:翅的下拍主要产生升力,上挥主要产生推力;下拍时的平均升力系数可达2~3,十分大,上挥时的平均推力系数可达1~2, 也很大,它们主要由非定常效应产生;前、后翅的相互干扰并未起增大升力和推力的作用,反而有一定的不利作用.      

基于三路舵机驱动的扑翼飞行模型气动特性研究

扑动而形成非定常气动现象是扑翼飞行过程中产生高升力的主要原因。本文以Ellington实验的鹰蛾翅膀为原形,设计扑翼实验及数值计算模型。通过压差传感器对翅膀模型上翼面固定位置进行测压,分析前缘涡的产生及脱落情况(考虑动压效应)。测量上下翼面固定位置处的压差,揭示扑翼飞行中产生高升力的主要原因。利用烟风洞观察扑翼模型周围流场结构及特殊涡产生变化情况。另外,根据Ellington提供的升力关系式估算了扑翼模型在一个周期内的平均升力。最后,基于三维欧拉方程对扑翼飞行气动特性进行数值模拟,计算结果与实验吻合良好。     

Large aerodynamic forces on a sweeping wing at low Reynolds number

The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically. After an initial start from rest, the wing is made to execute an azimuthal rotation (sweeping) at a large angle of attack and constant angular velocity. The Reynolds number (Re) considered in the present note is 480 (Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root). During the constant-speed sweeping motion, the stall is absent and large and approximately constant lift and drag coefficients can be maintained. The mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows. Soon after the initial start, a vortex ring, which consists of the leading-edge vortex (LEV), the starting vortex, and the two wing-tip vortices, is formed in the wake of the wing. During the subsequent motion of the wing, a base-to-tip spanwise flow converts the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength. This prevents the LEV from shedding. As a result, the size of the vortex ring increases approximately linearly with time, resulting in an approximately constant time rate of the first moment of vorticity, or approximately constant lift and drag coefficients. The variation of the relative velocity along the wing span causes a pressure gradient along the wingspan. The base-to-tip spanwise flow is mainly maintained by the pressure-gradient force. The project supported by the National Natural Science Foundation of China (10232010)     

A study on the mechanism of high-lift generation by an airfoil in unsteady motion at low reynolds number

The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re=100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and , the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short “vortex street” in front of the airfoil and the “vortex street” induces a “wind”; against this “wind” the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect. The project supported by the National Natural Science Foundation of China (19725210)     

Lift force on rotating sphere at low Reynolds numbers and high rotational speeds

The lift force on an isolated rotating sphere in a uniform flow was investigated by means of a three-dimensional numerical simulation for low Reynolds numbers (based on the sphere diameter) (Re<68.4) and high dimensionless rotational speeds (Г5). The Navier-Stokes equations in Cartesian coordinate system were solved using a finite volume formulation based on SIMPLE procedure. The accuracy of the numerical simulation was tested through a comparison with available theoretical, numerical and experimental results at low Reynolds numbers, and it was found that they were in close agreement under the above mentioned ranges of the Reynolds number and rotational speed. From a detailed computation of the flow field around a rotational sphere in extended ranges of the Reynolds number and rotational speed, the results show that, with increasing the rotational speed or decreasing the Reynolds number, the lift coefficient increases. An empirical equation more accurate than those obtained by previous studies was obtained to describe both effects of the rotational speed and Reynolds number on the lift force on a sphere. It was found in calcttlations that the drag coefficient is not significantly affected by the rotation of the sphere. The ratio of the lift force to the drag force, both of which act on a sphere in a uniform flow at the same time, was investigated. For a small spherical particle such as one of about 100μm in diameter, even if the rotational speed reaches about 10^6 revolutions per minute, the lift force can be neglected as compared with the drag force.     

Reynolds number effects on leading edge vortex development on a waving wing

The waving wing experiment is a fully three-dimensional simplification of the flapping wing motion observed in nature. The spanwise velocity gradient and wing starting and stopping acceleration that exist on an insect-like flapping wing are generated by rotational motion of a finite span wing. The flow development around a waving wing at Reynolds number between 10,000 and 60,000 has been studied using flow visualization and high-speed PIV to capture the unsteady velocity field. Lift and drag forces have been measured over a range of angles of attack, and the lift curve shape was similar in all cases. A transient high-lift peak approximately 1.5 times the quasi-steady value occurred in the first chord length of travel, caused by the formation of a strong attached leading edge vortex. This vortex appears to develop and shed more quickly at lower Reynolds numbers. The circulation of the leading edge vortex has been measured and agrees well with force data.     

Lift enhancement through flexibility of plunging wings at low Reynolds numbers

In this paper the combined effect of two mechanisms for lift enhancement at low Reynolds numbers are considered, wing oscillations and wing flexibility. The force, deformation and flow fields of rigid and flexible low aspect ratio (AR=3) and high aspect ratio (AR=6) wings oscillating at a fixed post-stall angle of attack of 15° and amplitude of 15% of chord are measured. The force measurements show that flexibility can increase the time-averaged lift coefficient significantly. For low aspect ratio wings the maximum lift coefficient across all Strouhal numbers was C_l=1.38 for the rigid wing as opposed to C_l=2.77 for the flexible wing. Very similar trends were observed for the high aspect ratio wings. This increase is associated with significant deformation of the wing. The root is sinusoidally plunged with small amplitude but this motion is amplified along the span resulting in a larger tip motion but with a phase lag. The amount it is amplified strongly depends on Strouhal number. A Strouhal number of Sr_c=1.5 was selected for detailed flow field measurements due to it being central to the high-lift region of the flexible wings, producing approximately double the lift of the rigid wing. For this Strouhal number the rigid wings exhibit a Leading Edge Vortex (LEV) ring. This is where the clockwise upper-surface LEV pairs with the counter-clockwise lower-surface LEV to form a vortex ring that self-advects upstream and away from the wing's upper surface. Conversely the deformation of the flexible wings inhibits the formation of the LEV ring. Instead a strong upper-surface LEV forms during the downward motion and convects close to the airfoil upper surface thus explaining the significantly higher lift. These measurements demonstrate the significant gains that can be achieved through the combination of unsteady aerodynamics with flexible structures.     

可变形儒可夫斯基翼型亚音速非定常气动力的研究

对于翼面变形速度远小于来流速度情况下的儒可夫斯翼型亚音速绕流问题,通过仿射变换将可压缩流动转换成不可压缩流动,将解析解和离散涡方法相结合计算变形机翼的不可压缩流动速度场,再利用逆变换得到变形机翼的亚音速流动速度场,进而分析非定常气动力特性,建立变形机翼的准定常升力系数和非定常附加升力系数在可压缩和不可压缩两种状态下的简单近似对应关系。计算结果显示变形机翼的非定常气动升力近似等于准定常计算结果叠加上虚拟质量力导致的非定常附加升力,该非定常附加升力随翼型变形速率呈线性关系,由机翼当前时刻飞行姿态、翼型及其变形速率确定,与具体变形历史过程无关。低来流马赫数时虚拟质量力导致的非定常效应显著,高亚音速流动时准定常升力起主导作用。同时还分析了不同马赫数下机翼往复变形过程中升力的变化特性,指出尽管高亚音速变形机翼的气动升力近似等于准定常气动升力,但不能忽视非定常附加升力的影响,非定常附加升力将导致完成往复变形需要外界输入正比于Ma∞/[(1-Ma2∞)]的功。     

低雷诺数翼型蒙皮主动振动气动特性及流场结构数值研究

针对低雷诺数(Re)翼型气动性能差的特点,文章通过对翼型柔性蒙皮施加主动振动的方法,提高翼型低Re下的气动特性,改善其流场结构.采用带预处理技术的Roe方法求解非定常可压缩Navier-Stokes方程,对NACA4415翼型低Re流动展开数值模拟.通过时均化和非定常方法对比柔性蒙皮固定和振动两种状态下的升阻力气动特性和层流分离流动结构.初步研究工作表明在低Re下柔性蒙皮采用合适的振幅和频率,时均化升阻力特性显著提高,分离泡结构由后缘层流分离泡转变为近似的经典长层流分离泡,分离点后移,分离区缩小.在此基础上,文章更加细致研究了柔性蒙皮两种状态下单周期内的层流分离结构及壁面压力系数分布非定常特性和演化规律.蒙皮固定状态下分离区前部流场结构和压力分布基本保持稳定,表现为近似定常分离,仅在后缘位置出现类似于卡门涡街的非定常流动现象.柔性蒙皮振动时从分离点附近开始便产生分离涡,并不断向下游移动、脱落,表现为非定常分离并出现大范围的压力脉动.蒙皮振动使流体更加靠近壁面运动,大尺度的层流分离现象得到有效抑制.      

Experimental investigation on aerodynamic performance of a flapping wing vehicle in forward flight

The aerodynamic performance of a flexible membrane flapping wing has been investigated here. For this purpose, a flapping-wing system and an experimental set-up were designed to measure the unsteady aerodynamic forces of the flapping wing motion. A one-component force balance was set up to record the temporal variations of aerodynamic forces. The flapping wing was studied in a large low-speed wind tunnel. The lift and thrust of this mechanism were measured for different flapping frequencies, angles of attack and for various wind tunnel velocities. Results indicate that the thrust increases with the flapping frequency. An increase in the wind tunnel speed and flow angle of attack leads to reduction in the thrust value and increases the lift component. The aerodynamic and performance parameters were nondimensionalized. Appropriate models were introduced which show its aerodynamic performance and may be used in the design process and also optimization of the flapping wing.