摆动方式对水翼输入功率的影响

利用投影浸入边界法,研究了不同主动摆动方式对水翼输入功率的影响。计算模型采用二维NACA0012翼型,雷诺数Re=800。最大摆角θ0=75°,摆动频率f的变化范围为0.1Hz~0.2Hz,非正弦摆动参数β的变化范围为1~3。首先,研究摆动频率f和非正弦摆动参数β对平均输入功率的影响,研究发现,平均输入功率随着f和β值的增加而增加,但当f0.16 Hz,β2.5时,平均输入功率急剧增加。其次,在摆动频率f=0.16 Hz时,研究不同非正弦摆动参数β下力矩系数、输入功率系数以及升阻力系数随时间的变化规律,研究发现,随着β值的增加,峰值输入功率也逐渐增加,而且β值影响峰值输入功率出现的位置。最后,研究不同β值下,变化的尾流发展对输入功率的影响,认为水翼上下表面产生的后缘涡与升阻力有关系,水翼上表面的负涡对水翼摆动产生阻力,而下表面的负涡对水翼摆动产生升力,从而对水翼摆动的输入功率产生影响。     

An analytic approach to theoretical modeling of highly unsteady viscous flow excited by wing flapping in small insects

Numerous studies on the aerodynamics of insect wing flapping were carried out on different approaches of flight investigations, model experiments, and numerical simulations, but the theoretical modeling remains to be explored. In the present paper, an analytic approach is presented to model the flow interactions of wing flapping in air for small insects with the surrounding flow fields being highly unsteady and highly viscous. The model of wing flapping is a 2-D flat plate, which makes plunging and pitching oscillations as well as quick rotations reversing its positions of leading and trailing edges, respectively, during stroke reversals. It contains three simplified aerodynamic assumptions: (i) unsteady potential flow; (ii) discrete vortices shed from both leading and trailing edges of the wing; (iii) Kutta conditions applied at both edges. Then the problem is reduced to the solution of the unsteady Laplace equation, by using distributed singularities, i.e., sources/sinks, and vortices in the field. To validate the present physical model and analytic method proposed via benchmark examples, two elemental motions in wing flapping and a case of whole flapping cycles are analyzed, and the predicted results agree well with available experimental and numerical data. This verifies that the present analytical approach may give qualitatively correct and quantitatively reasonable results. Furthermore, the total fluid-dynamic force in the present method can be decomposed into three parts: one due to the added inertial (or mass) effect, the other and the third due to the induction of vortices shed from the leading-and the trailing-edge and their images respectively, and this helps to reveal the flow control mechanisms in insect wing flapping. The project supported by the National Natural Science Foundation of China (10072066) and the Chinese Academy of Sciences (KJCX-SW-LO4, KJCX2-SW-L2)     

仿生扑翼飞行机器人翅型的研制与实验研究

模仿昆虫和小鸟飞行的扑翼飞行机器人将举升、悬停和推进功能集于一个扑翼系统,与固定翼和旋翼完全不同,因此研究只能从生物仿生开始。生物飞行的极端复杂性使得进行完整和精确的扑翼飞行分析非常复杂,因此本文在仿生学进展基础上,通过一些合适的假设和简化,建立了仿生翅运动学和空气动力学模型,并以此为基础研制了多种翅型。研制了气动力测量实验平台,对各种翅型进行了实验研究。实验结果表明,研制的翅型都能产生一定的升力,其中柔性翅具有较好的运动性能和气动性能,并且拍动频率和拍动幅度对升力有较大影响。     

扑翼柔性及其对气动特性的影响

以往对扑翼气动特性的研究基本上都是基于简单的匀速刚性模型,但是通过大量观察不同飞鸟的扑翼动作发现,该模型与鸟翼的实际扑动还有很大差别。鸟翼不但上扑段和下扑段所需时间不同,而且在扑动过程中,鸟翼的形状无论沿弦向或展向都存在着相当大的柔性变形。本文在原有匀速刚性模型的基础上,加入了扑动速率变化和形状变化的影响,得出新的变速柔性扑翼分析模型,使之更接近鸟翼柔性扑动的真实情况。通过对比计算发现,柔性变形对扑翼的升力与推力都有着显著影响,如果控制得当,柔性变形能大大改善扑翼的气动性能。     

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.      

小型飞行器空气动力学

对小型飞行器设计中涉及的空 气动力学问题进行了综述.描述了雷诺数和展弦比对固定翼飞行器的设计以及飞行 性能的影响.在低雷诺数飞行范围,翼型上边界层的特性对飞行器的设计尤为关键. 本文讨论了大量有关层流边界层(包括层流分离泡影响)的实验,作为例子,列举了几 个此飞行雷诺数范围的小型低空无人驾驶飞行器(UAVs).此外,对扑动翼推进的理论 模型进行了简述;其范围涵盖了早期的准定常附着流模型,以及后来计及非定常尾涡、 流动分离以及气动弹性等效应的模型.文中还介绍了那些与理论互补并最终导致扑 翼机设计成功的实验.     

Binuclear trans‐Bis(β‐iminoaryloxy)palladium(II) Complexes Doubly Linked with Pentamethylene Spacers: Structure‐Dependent Flapping Motion and Heterochiral Association Behavior of the Clothespin‐Shaped Molecules

The synthesis, structure, and solution‐state behavior of clothespin‐shaped binuclear trans‐bis(β‐iminoaryloxy)palladium(II) complexes doubly linked with pentamethylene spacers are described. Achiral syn and racemic anti isomers of complexes 1 – 3 were prepared by treating Pd(OAc)₂ with the corresponding N,N′‐bis(β‐hydroxyarylmethylene)‐1,5‐pentanediamine and then subjecting the mixture to chromatographic separation. Optically pure (100 % ee) complexes, (+)‐anti‐ 1 , (+)‐anti‐ 2 , and (+)‐anti‐ 3 , were obtained from the racemic mixture by employing a preparative HPLC system with a chiral column. The trans coordination and clothespin‐shaped structures with syn and anti conformations of these complexes have been unequivocally established by X‐ray diffraction studies. ¹H NMR analysis showed that (±)‐anti‐ 1 , (±)‐anti‐ 2 , syn‐ 2 , and (±)‐anti‐ 3 display a flapping motion by consecutive stacking association/dissociation between cofacial coordination planes in [D₈]toluene, whereas syn‐ 1 and syn‐ 3 are static under the same conditions. The activation parameters for the flapping motion (ΔH^≠ and ΔS^≠) were determined from variable‐temperature NMR analyses as 50.4 kJ mol^?1 and 60.1 J mol^?1 K^?1 for (±)‐anti‐ 1 , 31.0 kJ mol^?1 and ?22.7 J mol^?1 K^?1 for (±)‐anti‐ 2 , 29.6 kJ mol^?1 and ?57.7 J mol^?1 K^?1 for syn‐ 2 , and 35.0 kJ mol^?1 and 0.5 J mol^?1 K^?1 for (±)‐anti‐ 3 , respectively. The molecular structure and kinetic parameters demonstrate that all of the anti complexes flap with a twisting motion in [D₈]toluene, although (±)‐anti‐ 1 bearing dilated Z‐shaped blades moves more dynamically than I‐shaped (±)‐anti‐ 2 or the smaller (±)‐anti‐ 3 . Highly symmetrical syn‐ 2 displays a much more static flapping motion, that is, in a see‐saw‐like manner. In CDCl₃, (±)‐anti‐ 1 exhibits an extraordinary upfield shift of the ¹H NMR signals with increasing concentration, whereas solutions of (+)‐anti‐ 1 and the other syn/anti analogues 2 and 3 exhibit negligible or slight changes in the chemical shifts under the same conditions, which indicates that anti‐ 1 undergoes a specific heterochiral association in the solution state. Equilibrium constants for the dimerizations of (±)‐ and (+)‐anti‐ 1 in CDCl₃ at 293 K were estimated by curve‐fitting analysis of the ¹H NMR chemical shift dependences on concentration as 26 M ^?1 [K_D(racemic)] and 3.2 M ^?1 [K_D(homo)], respectively. The heterochiral association constant [K_D(hetero)] was estimated as 98 M ^?1, based on the relationship K_D(racemic)=1/2 K_D(homo)+1/4 K_D(hetero). An inward stacking motif of interpenetrative dimer association is postulated as the mechanistic rationale for this rare case of heterochiral association.     

Thrust enhancement due to flexible trailing-edge of plunging foils

Drag reduction for hydrofoils is studied through thrust generation on foils plunging at low Strouhal numbers in order to simulate the action of the ocean waves. Force, deformation and flow field measurements are presented for a partially flexible plunging foil in water tunnel experiments. The foil is predominantly rigid with a short flexible trailing-edge plate of length: L=0.1c, 0.2c, or 0.3c. Using flexible plates, whose natural structural frequency is much higher than the frequency of the plunge oscillations, increases thrust compared to the rigid case. Flexibility is generally more effective for larger lengths of the flexible plate and smaller plunge amplitudes. The maximum observed is therefore for the largest length and smallest amplitude studied: L=0.3c and a=0.1c and equates to 28% more thrust than the rigid case. Optima are observed in the non-dimensional rigidity (λ) versus flap angle amplitude (δ, which is a measure of the relative deformation) parameter space. These occur at λ≈2 and δ≈7–13° for a wide range of flexible plate length and plunge amplitude. Whilst a satisfactory explanation of why there is an optimal flap amplitude remains unavailable, the case of optimal flap angle amplitude results in increased trailing-edge vortex circulation, giving a stronger reverse Kármán vortex street and thus a stronger time-averaged jet.     

A flow control mechanism in wing flapping with stroke asymmetry during insect forward flight

A theoretical modeling approach as well as an unsteady analytical method is used to study aerodynamic characteristics of wing flapping with asymmetric stroke-cycles in connection with an oblique stroke plane during insect forward flight. It is revealed that the aerodynamic asymmetry between the downstroke and the upstroke due to stroke-asymmetrical flapping is a key to understand the flow physics of generation and modulation of the lift and the thrust. Predicted results for examples of given kinematics validate more specifically some viewpoints that the wing lift is more easily produced when the forward speed is higher and the thrust is harder, and the lift and the thrust are generated mainly during downstroke and upstroke, respectively. The effects of three controlling parameters, i.e. the angles of tilted stroke plane, the different downstroke duration ratios, and the different angles of attack in both down- and up-stroke, are further discussed. It is found that larger oblique angles of stroke planes generate larger thrust but smaller lift; larger downstroke duration ratios lead to larger thrust, while making little change in lift and input aerodynamic power; and again, a small increase of the angle of attack in downstroke or upstroke may cause remarkable changes in aerodynamic performance in the relevant stroke.The project supported by the National Natural Science Foundation of China (10072066, 90305009) and the Chinese Academy of Sciences (KJCX-SW-L04, KJCX2-SW-L2)The English text was polished by Ron Marshall.     

Enclosure enhancement of flight performance

We use a potential flow solver to investigate the aerodynamic aspects of flapping flights in enclosed spaces. The enclosure effects are simulated by the method of images. Our study complements previous aerodynamic analyses which considered only the near-ground flight. The present results show that flying in the proximity of an enclosure affects the aerodynamic performance of flapping wings in terms of lift and thrust generation and power consumption. It leads to higher flight efficiency and more than 5% increase of the generation of lift and thrust.