昆虫翅悬停飞行中的三维空间旋涡结构

昆虫(果蝇)悬停飞行中,翅膀按照特定的拍动方式往复运动,产生非定常高升力维持身体的平衡.研究昆虫高升力机理,需要探索拍动翼运动引发的三维空间非定常流场的特性,尤其是三维空间非定常涡的发展变化过程.本文将氢气泡流动显示技术应用于动态模型实验,定性的观察拍动翼前缘涡(LEV)的发展破裂过程.并利用数字体视粒子图像测速,DSPIV(Digital Steroscopic Particle Image Velocimetry)技术,测得了拍动翼运动瞬时相位和相位平均的三维空间流场信息(速度向量场、截面涡量场、空间涡量场,以及三维空间流线),揭示了拍动翼展向流动的存在,并结合定性和定量方法多角度说明了前缘涡沿展向发展到破裂的流动结构,并说明了侧缘涡与前缘涡的相互影响.测量结果表明:在雷诺数960的情况下,拍动翼运动至相位时,翼面上前缘涡在距翼根约60%展长的位置发生破裂;翼根至破裂点之间,展向流动稳定,指向翼梢;破裂点以后,展向流改变方向,指向翼根.     

微型飞行器的仿生流体力学——昆虫前飞时的气动力和能耗

 用数值模拟方法研究了昆虫前飞时的气动力和需用功率。由N S方程的数值解提供速度场和压力场,从而得到涡量、气动力和力矩 (惯性力矩用解析方法计算 )。基于流场结构,解释了非定常气动力产生的原因;基于气动力和力矩,得到需用功率。悬停飞行中揭示出的 3个非定常高升力机制 (不失速机制,拍动初期的快速加速运动,拍动后期的快速上仰运动 )在前飞时仍然适用 (即使在快速前飞时,V∞ =2～ 2.5m/s,失速涡也不脱落 )。在低速飞行时 (V∞ ≈ 0.5m/s)平衡重量的升力既来自于翅膀的下拍运动也来自于上挥运动,并主要由翅膀的升力贡献;克服身体阻力的推力主要来自于翅膀的上挥运动,由翅膀的阻力贡献。在中等速度下 (V∞ ≈ 1.0m/s),升力主要来自于下拍运动,其中一半由翅膀升力贡献,一半由翅膀阻力贡献;推力主要来自上挥运动,也是一半由翅膀升力贡献,一半由翅膀阻力贡献。在快速飞行时 (V∞ ≈ 2.0m/s),升力主要来自于下拍运动,主要由翅膀阻力贡献;推力来自上挥运动,主要由翅膀升力贡献。悬停时,下拍和上挥做功同样大;前飞时,下拍做功较上挥大得多 :V∞ =0.5,1.0和 2.0m/s时,下拍做的功分别是上挥的 1.6,2.6和 3.5倍。     

机械蜻蜓悬停时的气动力实验研究

为研究蜻蜓等有两对翼的昆虫前后翼相互干扰下的气动力,设计并制作了一个可以模拟蜻蜓悬停的3种拍动模式的机电模型,并以此进行一系列实验,对前后翼相互作用的流场进行了观测。采用染色液流动显示的方法,定性观察了后翼前缘涡在各种不同拍动模式和不同翼间距离时的发展过程,对模型翼在各种拍动模式下的气动力进行了定量测量,并分别从流场结构和力学性能两方面对两翼间的干扰作用进行了分析讨论。     

NS-DBD激励控制非细长三角翼前缘涡仿真研究

通过在三角翼前缘施加纳秒脉冲介质阻挡放电（NS-DBD）激励唯象学模型，进行了47°后掠角钝前缘三角翼流动控制的仿真。分析了不同迎角下升力和阻力系数的变化、流场结构的变化、以及激励诱导旋涡的演化过程。研究表明:施加无量纲激励频率F+=1.44的NS-DBD激励后，可明显提高三角翼失速前后的升力系数；同时阻力系数也有所增加，变化趋势与实验结果一致。激励在前缘分离剪切层处诱导产生流向涡，改变了前缘剪切层结构，使其向内卷吸；激励后时均流场形成了明显的负压峰值，前缘涡附着线外移，吸力面回流区减小。      

蝴蝶翼面形状对绕流结构的影响

蝴蝶的扑翼飞行因其相对翼展较大、扑翼频率低等特点引起广泛关注,尤其是具有越洋迁徙特性的蝴蝶更是备受瞩目。本文以天堂鸟翼凤蝶和黑脉金斑蝶这两种长距离迁徙蝴蝶作为研究原型,利用氢气泡流动显示技术研究这两种不同外形的蝴蝶翼面绕流结构随迎角的变化。在相同的实验条件下,两种模型翼面绕流均出现前缘涡、翼尖涡等流动结构,且绕流结构随迎角的变化趋势基本相同。但由于翼面外形的不同,两种模型在前缘涡的形态、强度及后翼面翼尖绕流结构等方面表现出一定的差异。     

扑翼飞行高升力机制仿真及对比分析

扑翼飞行生物因其高效的飞行方式和灵活的机动性能,受到越来越多学者的关注和研究。针对扑翼飞行生物如何利用非定常空气力学,通过拍动翅膀实现高升力达到与自身重力平衡的问题,系统分析了扑翼飞行生物高升力机制,对每种高升力机制在扑翼飞行器样机上的应用进行了总结,并建立了高升力机制翼面拍动的运动学模型,对比分析了三种高升力机制的特点和差异。仿真分析得出每种运动学模型翼面拍动轨迹,可为扑翼飞行器翼面驱动机构的设计提供参考。     

''''W''''型无尾布局流动机理研究

基于NS方程数值模拟方法,研究了‘W’型无尾布局的流动机理。与参照前掠翼布局相比,‘W’布局优越的气动性能来源于其流动形态的变化:小迎角时,翼身融合升力体设计,使机体表面流动更为通畅,升力增加,机体部件干扰减小,部分补偿了因机身加宽,浸润面积增大带来的摩擦阻力,使总阻力没有明显增加。α≥6°,‘W’布局具有新的流动结构,机翼上表面流动由侧缘涡和前缘涡及其诱导的二次涡所控制,侧缘涡与前缘涡之间产生有利干扰,增强了对机翼表面流动的控制能力,不仅带来涡升力,而且有效控制了前掠翼根部流动分离,是其具有优越纵向气动性能的物理原因。‘W’布局新的流动结构为其横侧气动性能改善奠定了基础,为进一步完善布局设计提供了理论依据。     

微型旋转翼与拍动翼气动力特性的比较

 用拟压缩性法求解三维非定常不可压N-S方程，研究了旋转翼及拍动翼的气动力和流场。当昆虫正常悬停时，如果翅膀旋转，产生的平均升力以及气动效率并不比拍动时的差，甚至还略好，因此，厘米级微型飞行器可以采用旋转翼，这比拍动模式容易实现。另外，小迎角旋转时升阻比可达3.1（在15°迎角，雷诺数4 000情况下），气动效率较高而且升力系数不小，多个桨叶可以产生很大的升力。因此，在悬停时，多个桨叶小迎角旋转是厘米级微型飞行器的一个很好的方案。     

小展弦比飞翼跨声速典型流动特性研究

小展弦比飞翼标模为国内自主设计的融合体飞翼通用研究模型,前缘后掠角为65°,展弦比为1.54。风洞试验结果表明小展弦比飞翼标模在跨声速迎角4°开始出现非线性升力,在迎角12°至16°范围内会出现升力突然下降、俯仰力矩突然上扬的现象。为了分析该现象的机理,通过数值模拟的方法研究了小展弦比飞翼标模在马赫0.9时的流动特性,分析了前缘涡的产生、发展直至破裂的整个过程,结果表明:小展弦比飞翼标模在迎角4°开始出现涡升力;随着迎角增加,前缘涡逐渐向内侧移动,涡强和背风面激波的强度也逐渐增加,前缘涡与激波发生交叉干扰并达到一个平衡流态;当前缘涡与激波无法维持既有平衡时则会发生涡破裂,流场急剧变化以达到新的平衡,从而导致升力突然下降并产生抬头力矩增量。     

微型扑旋翼的运动测量与气动特性分析

采用实验测量和数值仿真相结合的方法研究了微型扑旋翼多自由度复杂运动及运动与气动特性之间的相互作用。基于机械式扑旋翼运动模型,使用高速摄影测量和数字图像处理的方法,测量了扑旋翼的运动与变形,并将此结果作为数值仿真的输入,进一步研究了翼的气动特性。结果表明:1)扑旋翼的拍动角近似三角波变化,旋转角随时间线性增加,俯仰角在初始迎角附近波动,并有三个波峰;2)弦向弯曲度的变化规律近似为三角波,沿展向不同位置有相位差异,展向扭转角的变化规律接近于梯形波;3)扑旋翼运动产生的翼尖涡、前缘涡与后缘涡形成了复杂的涡环,涡结构的变化决定了翼的瞬态气动力的产生;4)扑旋翼的柔性变形能够带来平均和峰值升力的提高。     

单自由度扑翼模型脱落涡及其升力机制的实验研究

采用色流实验与particle image velocimetry(PIV)测量相结合的方法,对单自由度扑翼模型所产生的脱落涡进行实验研究,得到以下结论:①在扑翼上扑或下扑的过程中,前后缘处产生连续的牛角形涡环,且脱落涡均存在展向运动.②翼翅表面脱落涡的交替产生,使得翼翅上下表面交替产生不对称分布的压力差.③研究对象产生升力的条件是攻角与来流速度均不为零.④扑翼上扑和下扑过程中,脱落涡环量最大值均出现在单行程的3/5处.⑤脱落涡环量与扑动速度成正比.⑥相比上扑过程,下扑过程中翼翅对流场做功更多,在一个扑动周期内,这多余的能量转换为扑翼升力.     

A review of bird-like flapping wing with high aspect ratio

Bird-like flapping-wing vehicles with a high aspect ratio have the potential to fulfill missions given to micro air vehicles, such as high-altitude reconnaissance, surveillance, rescue, and bird group guidance, due to their good loading and long endurance capacities. Biologists and aeronautical researchers have explored the mystery of avian flight and made efforts to reproduce flapping flight in bioinspired aircraft for decades. However, the cognitive depth from theory to practice is still very limited. The mechanism of generating sufficient lift and thrust during avian flight is still not fully understood. Moving wings with unique biological structures such as feathers make modeling, simulation, experimentation, and analysis much more difficult. This paper reviews the research progress on bird-like flapping wings from flight mechanisms to modeling. Commonly used numerical computing methods are briefly compared. The aeroelastic problems are also highlighted. The results of the investigation show that a leading-edge vortex can be found during avian flight. Its induction and maintenance may have a close relationship with wing configuration, kinematics and deformation. The present models of flapping wings are mainly two-dimensional airfoils or three-dimensional single root-jointed geometric plates, which still exhibit large differences from real bird wings. Aeroelasticity is encouraged to consider the nonignorable effect on aerodynamic performance due to large-scale nonlinear deformation. Introducing appropriate flexibility can improve the peak values and efficiencies of lift and thrust, but the detailed conclusions always have strong background dependence.     

扑翼的变刚度设计及其对升力和推力的影响

为提高扑翼飞行器的升力和推力以提高其飞行性能,运用生物扑翼的仿生原理,研究扑翼飞行器的扑翼升力和扑翼推力随扑翼刚度变化的机理。借鉴"变刚度关节机构和平面转动冗余并联机构通过调节张力改变刚度"这一机构学原理,运用变刚度关节机构相互串联实现扑翼在扑动方向上变刚度,同时运用平面转动冗余并联机构实现扑翼在扭转方向上变刚度。建立扑翼的扑动关节刚度和扭转机构刚度随预张力变化的模型,并通过实验和仿真验证扑翼扑动关节刚度和扭转机构刚度随预张力的变化。研究扑翼的升力和推力与扑翼刚度之间的关系,验证了通过调节扑翼刚度匹配其扑动频率可以提高扑翼的升力和推力。     

Lift performance enhancement for flapping airfoils by considering surging motion

The flapping motion has a great impact on the aerodynamic performance of flapping wings. In this paper, a surging motion is added to an airfoil performing pitching-plunging combined motion to figure out how it influences the lift performance and flow pattern of flapping airfoils. Firstly, the numerical methods are validated by a NACA0012 airfoil pitching case and a NACA0012 airfoil plunging case. Then, the E377m airfoil which has typical geometric characteristics of the bird-like airfoil is selected as the calculation model to study how phase differences φ₁ between surging motion and plunging motion affect the aerodynamic performance of flapping airfoils. The results show that the airfoil with surging motion has comprehensively better lift performance and thrust performance than the airfoil without surging motion when 15°< φ₁ < 90°. It is demonstrated that surging motion has a powerful ability to improve the aerodynamic performance of flapping airfoil by adjusting φ₁. Finally, to further explore how flapping airfoil improves lift performance by considering surging motion, the flapping motions of E377m airfoil with the highest lift coefficient and lift efficiency are obtained through trajectory optimization. The surging motion is removed in the highest lift case and highest lift efficiency case respectively, and the mechanism that surging motion adjusts the aerodynamic force is analyzed in detail by comparing the vortex structure and kinematic parameters. The results of this paper help reveal the aerodynamic mechanism of bird flight and guide the design of Flapping wing Micro Air Vehicles (FMAV).     

Recent progress in flapping wing aerodynamics and aeroelasticity

Micro air vehicles (MAVs) have the potential to revolutionize our sensing and information gathering capabilities in areas such as environmental monitoring and homeland security. Flapping wings with suitable wing kinematics, wing shapes, and flexible structures can enhance lift as well as thrust by exploiting large-scale vortical flow structures under various conditions. However, the scaling invariance of both fluid dynamics and structural dynamics as the size changes is fundamentally difficult. The focus of this review is to assess the recent progress in flapping wing aerodynamics and aeroelasticity. It is realized that a variation of the Reynolds number (wing sizing, flapping frequency, etc.) leads to a change in the leading edge vortex (LEV) and spanwise flow structures, which impacts the aerodynamic force generation. While in classical stationary wing theory, the tip vortices (TiVs) are seen as wasted energy, in flapping flight, they can interact with the LEV to enhance lift without increasing the power requirements. Surrogate modeling techniques can assess the aerodynamic outcomes between two- and three-dimensional wing. The combined effect of the TiVs, the LEV, and jet can improve the aerodynamics of a flapping wing. Regarding aeroelasticity, chordwise flexibility in the forward flight can substantially adjust the projected area normal to the flight trajectory via shape deformation, hence redistributing thrust and lift. Spanwise flexibility in the forward flight creates shape deformation from the wing root to the wing tip resulting in varied phase shift and effective angle of attack distribution along the wing span. Numerous open issues in flapping wing aerodynamics are highlighted.     

一种仿生扑翼飞行器的设计及动力学分析

为了提高仿生扑翼飞行器设计水平,弄清仿生扑翼飞行器的动力学特性、改善其飞行性能,设计了一种折展翼仿鸟扑动飞行样机,并对其动力学特性进行了分析。通过仿生方式设计了基于曲柄连杆的折展扑动机构,建立了扑翼机构的运动学模型,得到了相关运动学参数,并采用拉格朗日方法,推导出扑翼扑动时的动力学模型。基于条带方法对该扑翼的气动力、气动力矩载荷进行估算,分析了折展扑翼非对称扑动时翼翅气动力矩变化规律,结果表明:与一般直扑翼相比,折展翼在上扑阶段受到更小的阻力,因而扑翼扑动过程中能够获得更大的升力。基于ADAMS软件包,对扑动机构关键铰接位置的力学特性进行分析,为优化扑动机构、提高其结构强度提供重要参考。      

Unsteady characteristic research on aerodynamic interaction of slotted wingtip in flapping kinematics

The slotted wingtip structure of birds is considered to be the product of improving flight efficiency in the process of evolution. It can change the vortex structure of wingtip and improve aerodynamic efficiency. This paper reports a numerical investigation of slotted wing configuration undergoing bio-inspired flapping kinematics(consisting of plunging and in-line movement)extracted from a free-flying bald eagle wing. The aim is to eluci-date the collective mechanism of the flow generated by slo...     

Unsteady aerodynamics and flow control for flapping wing flyers

The creation of micro air vehicles (MAVs) of the same general sizes and weight as natural fliers has spawned renewed interest in flapping wing flight. With a wingspan of approximately 15 cm and a flight speed of a few meters per second, MAVs experience the same low Reynolds number (10⁴–10⁵) flight conditions as their biological counterparts. In this flow regime, rigid fixed wings drop dramatically in aerodynamic performance while flexible flapping wings gain efficacy and are the preferred propulsion method for small natural fliers. Researchers have long realized that steady-state aerodynamics does not properly capture the physical phenomena or forces present in flapping flight at this scale. Hence, unsteady flow mechanisms must dominate this regime. Furthermore, due to the low flight speeds, any disturbance such as gusts or wind will dramatically change the aerodynamic conditions around the MAV. In response, a suitable feedback control system and actuation technology must be developed so that the wing can maintain its aerodynamic efficiency in this extremely dynamic situation; one where the unsteady separated flow field and wing structure are tightly coupled and interact nonlinearly. For instance, birds and bats control their flexible wings with muscle tissue to successfully deal with rapid changes in the flow environment. Drawing from their example, perhaps MAVs can use lightweight actuators in conjunction with adaptive feedback control to shape the wing and achieve active flow control. This article first reviews the scaling laws and unsteady flow regime constraining both biological and man-made fliers. Then a summary of vortex dominated unsteady aerodynamics follows. Next, aeroelastic coupling and its effect on lift and thrust are discussed. Afterwards, flow control strategies found in nature and devised by man to deal with separated flows are examined. Recent work is also presented in using microelectromechanical systems (MEMS) actuators and angular speed variation to achieve active flow control for MAVs. Finally, an explanation for aerodynamic gains seen in flexible versus rigid membrane wings, derived from an unsteady three-dimensional computational fluid dynamics model with an integrated distributed control algorithm, is presented.     

Review of flow control mechanisms of leading-edge vortices

Vortex control concepts employed for slender and nonslender delta wings were reviewed. Important aspects of flow control include flow separation, vortex formation, flow reattachment, vortex breakdown, and vortex instabilities. The occurrence and relative importance of these phenomena strongly depend on the wing sweep angle. Various flow control methods were discussed: multiple vortices, control surfaces, blowing and suction, low-frequency and high-frequency excitation, feedback control, passive control with wing flexibility, and plasma actuators. For slender delta wings, control of vortex breakdown is achieved by modifications to swirl level and external pressure gradient acting on the vortex core. Effects of flow control methods on these two parameters were discussed, and their effectiveness was compared whenever possible. With the high-frequency excitation of the separated shear layer, reattachment and lift enhancement in the post-stall region is observed, which is orders of magnitude more effective than steady blowing. This effect is more pronounced for nonslender wings. Re-formation of vortices is possible with sufficient amplitude of forcing at the optimum frequency. Passive lift enhancement on flexible wings is due to the self-excited wing vibrations, which occur when the frequency of wing vibrations is close to the frequency of the shear layer instabilities, and promote flow reattachment.     

Flapping and flexible wings for biological and micro air vehicles

Micro air vehicles (MAVs) with wing spans of 15 cm or less, and flight speed of 30–60 kph are of interest for military and civilian applications. There are two prominent features of MAV flight: (i) low Reynolds number (10⁴–10⁵), resulting in unfavorable aerodynamic conditions to support controlled flight, and (ii) small physical dimensions, resulting in certain favorable scaling characteristics including structural strength, reduced stall speed, and low inertia. Based on observations of biological flight vehicles, it appears that wing motion and flexible airfoils are two key attributes for flight at low Reynolds number. The small size of MAVs corresponds in nature to small birds, which do not glide like large birds, but instead flap with considerable change of wing shape during a single flapping cycle. With flapping and flexible wings, birds overcome the deteriorating aerodynamic performance under steady flow conditions by employing unsteady mechanisms. In this article, we review both biological and aeronautical literatures to present salient features relevant to MAVs. We first summarize scaling laws of biological and micro air vehicles involving wing span, wing loading, vehicle mass, cruising speed, flapping frequency, and power. Next we discuss kinematics of flapping wings and aerodynamic models for analyzing lift, drag and power. Then we present issues related to low Reynolds number flows and airfoil shape selection. Recent work on flexible structures capable of adjusting the airfoil shape in response to freestream variations is also discussed.