飞行器的升力、阻力及升力与环量定理的起源

本文介绍了空气动力学中几个基本概念与定律的起源。其中,升力与阻力分别是空气对物体作用力的两个方向上的分量,它们均是由空气与物体的相对运动而产生的,并与该运动速度的平方成正比。库塔儒可夫斯基升力环量定理给出了翼型升力与翼型绕流之间的关系,开启了20世纪早期各国对翼型性能的研究。同时,鉴于理想流体圆柱绕流无阻力的理论结果与实验观察存在的矛盾开始激发人们对黏性流体运动的研究兴趣,并由此诞生了纳维斯托克斯方程组。而后普朗特提出边界层概念,巧妙解决了局部流动与整体流动的关系问题。针对大展弦比直机翼,普朗特又提出了基于升力线假设的升力线模型,并根据翼型气动数据得到三维机翼的气动性能。     

DERIVATION OF LIFT,DRAG AND THE KUTTA--JOUKOWSKI THEOREM FOR AIRCRAFTS 1)

本文介绍了空气动力学中几个基本概念与定律的起源。其中,升力与阻力分别是空气对物体作用力的两个方向上的分量,它们均是由空气与物体的相对运动而产生的,并与该运动速度的平方成正比。库塔儒可夫斯基升力环量定理给出了翼型升力与翼型绕流之间的关系,开启了20世纪早期各国对翼型性能的研究。同时,鉴于理想流体圆柱绕流无阻力的理论结果与实验观察存在的矛盾开始激发人们对黏性流体运动的研究兴趣,并由此诞生了纳维斯托克斯方程组。而后普朗特提出边界层概念,巧妙解决了局部流动与整体流动的关系问题。针对大展弦比直机翼,普朗特又提出了基于升力线假设的升力线模型,并根据翼型气动数据得到三维机翼的气动性能。     

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.      

具有脱体激波的跨音速流场计算

本文从跨音速高级近似扰动速势方程出发,研究了低超音速来流中基本的二维钝头翼型的绕流特性和计算方法,给出了适用的远场边界条件和差分格式。对NACA 0012翼型进行了零升力和有升力条件下的初步计算。计算表明,本文计算方法能得到与有关理论结果~[2]和风洞试验数据~[8]相接近的收敛解。     

颗粒流动力学及其离散模型评述

颗粒流是由众多颗粒组成的具有内在相互作用的非经典介质流动. 自然界常见颗粒流都是密集流, 颗粒间接触形成力链, 诸多力链相互交接构成支撑整个颗粒流重量和外载荷的网络, 其局部构型及强度在外载荷下演化, 是颗粒流摩擦特性和接触应力的来源.本文介绍球形颗粒间无粘连作用时的Hertz法向接触理论和Mindlin-Deresiewicz切向接触理论. Campbell依据是否生成较为稳定的力链把颗粒流分为弹性流和惯性流两大类, 其中弹性-准静态流和惯性-碰撞流分别对应准静态流和快速流, 作为两种极端流动情况通常处理成连续体, 分别采用摩擦塑性模型和动理论予以描述, 但是表征接触力链的颗粒弹性参数并不出现这两个模型和理论框架中, 如何进一步考虑颗粒弹性参数将非常困难. 目前离散动力学方法逐渐成为复现其复杂颗粒流动现象、提取实验不可能获得的内部流动信息进而综合起来探索颗粒流问题的一种有效工具, 其真实性强于连续介质理论的描述. 软球模型对颗粒间接触力简化处理, 忽略了切向接触力对法向接触力及其加载历史的依赖, 带来了法向和切向刚度系数如何标度等更艰难的物理问题, 但由于计算强度小而广泛应用于工程问题中. 硬球模型不考虑颗粒接触变形, 因而不能描述颗粒流内在接触应变等物理机理, 仅适用于快速颗粒流, 这不仅仅是由于两体碰撞的限制. 因此基于颗粒接触力学的离散颗粒动力学模型是崭新的模型,适用于准静态流到快速流整个颗粒流态的模拟, 可以细致考虑接触形变及接触力的细节,建立更为合理的颗粒流本构关系, 进而有力的促进颗粒流这一非经典介质流动的研究.      

并列圆柱绕流的格子Boltzmann数值模拟

采用非均匀不可压格子Boltzmann模型对低雷诺数下并列圆柱绕流进行了数值模拟,给出了数值计算结果,分析了间距g对圆柱尾流及升力、阻力的影响,并在此基础上得到了4种尾迹模式.此外,研究了流场的初始扰动对流动分岔现象的影响,发现在适当的扰动下可以很快得到同步同相的尾流.对Re=160和200下圆柱的升、阻力进行了对比,结果表明升力和阻力受间距g的影响大于雷诺数.     

昆虫飞行——非常规空气动力学

<正> 许多证据表明,昆虫飞行的机理与飞机及直升机的机理大不相同。zanker&Gotz测出了被系住的果蝇所产生的瞬时力。并发现这些力不能用常规空气动力学理论来解释。这些力也是这些果蝇使用不一般的方法来产生升力的证据。 在飞机稳态飞行时,空气在机翼上方流动比下方快。这时绕机翼周围有一纯环流。正是具有附体环流的机翼在空气中的运动产生了升力。可是,如果机翼从静止开始加速,那么它必须移动比它本身宽度长几倍的距离,才会有环流绕流机翼而产生足够的升力以使飞机达到稳态飞行。这一现象叫做Wagner效应。     

气液两相流中旋涡诱发圆柱振动时的脉动升力研究

一定条件下 ,垂直上升矩形截面管内的气液两相流横向冲刷水平布置的圆柱时 ,圆柱在与来流垂直的水平方向上受到了脉动升力的作用。本文在 1.6× 10 4到 6 .0×10 4的 Re数范围和 0到 0 .32的来流含气率范围内 ,进行了实验研究。实验中 ,流体工质为由空气和水混合而成的泡状流 ,采用弹性梁 -电阻应变式小力值传感器来测量圆柱受到的脉动升力 ,利用计算机对传感器的输出信号进行连续采集 ,采集频率为5 0 0 Hz。对由实验得到的脉动升力数据进行自功率谱分析 ,得出在两相流中只有在小于 0 .10～ 0 .12的来流含气率范围内 ,圆柱后部才会形成涡街 ,且此时的 Strouhal数随来流含气率的增大而增大 ;由脉动升力数据的均匀方根值算出脉动升力系数 C′L发现小 Re数时 C′L 随来流含气率的增大而增大 ,而在大 Re时 C′L 随来流含气率的增大先稍下降后再增大。     

简化Navier-Stokes方程组(SNSE)的几个精确解

对不可压二维驻点流、三维驻点流和旋转圆盘附近的流动等三种流动情况,本文给出简化Navier-Stokes方程组(SNSE)及其精确解。表明:文献[1]理论的SNSE的精确解,在三种流动情况下均与完全Navier-Stokes方程组(NSE)的精确解完全一致;文献[3]SNSE的精确解的速度解与完全NSE精确解的速度解一致,但压力解在三种流动情况下均与完全NSE精确解的压力解不同。文献[3]SNSE精确解给出的压力分布相对与完全NSE精确解给出的压力分布的最大相对误差为100％。     

可变形儒可夫斯基翼型非定常气动力的研究

对于翼面变形法向运动速度远小于来流速度的儒可夫斯基机翼,将解析解和离散涡方法相结合计算变形机翼的流场及非定常气动力,较详细地分析了变形机翼升力系数的准定常计算方法的误差来源,并给出修正方法.计算结果显示脱落涡尾迹对升力系数和机翼绕流环量的影响很小,变形机翼升力系数准定常计算方法的误差丰要来源于流体非定常运动引起的虚拟质量力,该非定常附加升力仅与当前时刻飞行姿态及翼犁形状和变形速率有关,与具体的变形历史过程无关,变形机翼的升力近似等于准定常计算结果叠加上相应的虚拟质量力.     

Qualitative features of high lift hovering dynamics and inertial manifolds

Hovering aerodynamics, such as that practiced by dragonflys, hummingbirds, and certain other small insects, utilizes special patterns of vorticity to generate high lift flows. Such lift as we measure it computationally on the airfoil surface is in good agreement with downstream thrust measured in the physical laboratory. In this paper we examine the qualitative signatures of this dynamical system. A connection to the theory of inertial manifolds, more specifically the instance of time-dependent slow manifolds, is initiated. Additional interest attaches to the fact that in our compact computational domain, the forcing is on the boundary. Because of its highly oscillatory nature, in this dynamics one proceeds rapidly up the bifurcation ladder at relatively low Reynolds numbers. Thus, aside from its intrinsic interest, the hover model provides an attractive vehicle for a better understanding of dynamical system attractor dynamics and inertial manifold theory.The authors appreciate grants of NAS computational resources at the NASA Ames Research Laboratories with the support of the NASA Lewis Research Laboratory.     

On the high-frequency response of unsteady lift and circulation: A dynamical systems perspective

In this paper, we consider the unsteady aerodynamics of a two-dimensional airfoil as a dynamical system whose input is the angle of attack (or airfoil motion) and output is the lift force. Based on this view, we discuss the evolution of lift and circulation from a purely dynamical perspective through step response, frequency response, transfer function, etc. In particular, we point to the relation between the high-frequency gain of the transfer function and the physics of the development of lift and circulation. Based on this view, we show that the circulatory lift dynamics is different from the circulation dynamics. That is, we show that the circulatory lift is not lift due to circulation. In fact, we show that the circulatory–non-circulatory classification is arbitrary. By comparing the steady and unsteady thin airfoil theory, we show that the circulatory lift possesses some acceleration (added-mass) effects. Finally, we perform simulations of Navier–Stokes equations to show that a non-circulatory maneuver in the absence of a free stream induces viscous circulation over the airfoil.     

On the role of the lateral lift force in poly-dispersed bubbly flows

An extensive experimental database comprising air–water as well as steam-water upwards vertical pipe flows for a pressure up to 6.5 MPa was used to investigate the effect of the lateral lift force on turbulent poly-dispersed flows with medium or high gas volume fraction. It was clearly shown that the lift force plays an important role also in such flows. Several effects such as bubble coalescence and breakup as well as fast rising large bubbles which push small bubbles towards the pipe wall superpose the effect of the lift force but can be separated from this effect. The critical bubble diameter, at which the lift force changes its sign, predicted by using Tomiyama’s correlation agrees well with experimental data obtained for turbulent air–water and steam-water flows with medium and high void fraction and a broad spectrum of bubbles sizes. The values for this critical bubble diameter are confirmed by the experimental data within the frame of the uncertainty of the data. Consequences of the action of the lateral lift force on flow structures in different flow situations are discussed. From the investigations it can be concluded that the lift force including the bubble size dependent change of its sign should be considered in a proper numerical 2D or 3D-simulation on flows in which bubbles in the range of several millimeters are present.     

Numerical study of the lift force influence on two-phase shock tube boundary layer characteristics

The importance of the lift force acting on the dispersed phase in the boundary layer of a laminar gas-particle dilute mixture flow generated by a shock wave is investigated numerically. The particle phase is supposed to form a continuum and is described by an Eulerian approach. The ability of the Eulerian model to simulate particle flows and the importance of the two-way coupling are proven by comparison with experimental data as well as with the numerical results from schemes based on a Lagrangian approach. The models used for the lift force are discussed through comparisons between numerical and experimental results found in the literature. Some results about the formation of a dust cloud are numerically reproduced and show the major role of the lift force. Simulations of two-dimensional two-phase shock tube flows are also performed including the lift force effects. Although the wave propagation is weakly influenced by the lift force, the force modifies substantially the dynamics of the flow near the wall. Received 17 February 2000 / Accepted 30 November 2000     

On the relation between the global modes and the spectra of drag and lift in periodic wake flowsSur la relation entre les modes globaux et les spectres des forces hydrodynamiques dans un sillage périodique

In this Note we are interested in the relation between the symmetry properties of the global mode envelopes in wake flows and the spectra of the drag and lift forces. We consider the “impulse” formula for the hydrodynamic force and show that the drag force consists of contributions from the even harmonics, and the lift force of contributions from the odd harmonics, only. Our argument explains this well-known empirical fact and is also supported by the computational evidence we provide. Finally, we identify the unsteady wake flows, both controlled and uncontrolled, as belonging to a broader family of “streaming flows”. To cite this article: B. Protas, J.E. Wesfreid, C. R. Mecanique 331 (2003).     

Influences of Lorentz force on the hydrofoil lift

In this paper, Lorentz forces are proved to be able to suppress separation in flows over hydrofoils. Furthermore, a differential equation of pressure distributions on the hydrofoil surface is derived, from which it is found that BVF （boundary vortex flux） σ is a suitable criterion for describing the lift coefficient variations during the electromagnetic control process. According to our numerical results, the periodic variations of lift for a hydrofoil at an attack angle of 17 ° are analyzed and its inherent mechanism is discussed in detail with the concept of BVE On the other hand, the effects of Lorentz force on the hydrofoil＇s lift are investigated both experimentally and numerically for different magnitudes and locations.     

Two-phase flows: Constitutive equations for lift and Brownian motion and some basic flows

Constitutive relations for the lift force on the particulate phase and the effect of Brownian motion are presented. These constitutive relations are derived subject to three new principles of constitutive equations. The effects of lift and Brownian motion in basic parallel flows are considered in order to determine the importance and the consequences of these effects. The relation of the Brownian motion model involving momentum balance to the diffusive model of particle motions is studied. Dimensional and scaling arguments are given.     

近空间高超声速飞行器气动特性研究的若干关键问题

在30$\sim$70km空域机动飞行的高超声速飞行器的优点是可以耦合利用所处空域的空气产生的升力和高速飞行的离心力进行远距离机动滑翔飞行,具有重要的实用价值.尽管过去数十年在高超声速流动研究方面取得显著进展,但在设计研究近空间远程滑翔的高超声速飞行器方面仍然存在许多挑战,特别是对特定飞行条件下的流动机理了解不清楚.本文介绍了作者研究团队在开展近空间高超声速飞行器有关的关键气动问题方面的研究进展,主要包括:建立了近空间高超声速飞行的流动模型,发展了系统的相关计算空气动力学方法,针对高空高速飞行条件下稀薄气体效应和真实气体效应的耦合作用影响研究了合适的滑移边界条件,考虑了不同组分存在条件下的温度、速度和压力的滑移效应影响;提出了飞行器气动外形的动态优化方法,获得了可工程实用化的高升阻比飞行器气动外形;建立了高速飞行器动稳定性理论,在实现高超声速飞行器动态稳定飞行方面取得重大进展;最后讨论了高超声速飞行器设计中进一步需要关注的若干关键技术和科学问题、可能解决的途径及其所涉及的学科发展方向.      

Steady vortex force theory and slender-wing flow diagnosis

The concept vortex force in aerodynamics is systematically examined based on a new steady vortex-force theory (Wu et al., Vorticity and vortex dynamics, Springer, 2006) which expresses the aerodynamic force (and moment) by the volume and boundary integrals of the Lamb vector. In this paper, the underlying physics of this theory is explored, including the general role of the Lamb vector in nonlinear aerodynamics, its initial formation, and its relevance to the total-pressure non-uniformity. As a typical example, the theory is applied to the flow over a slender delta wing at a large angle of attack. The highly localized flow structures with high Lamb-vector peaks are identified in terms of their net contribution to various constituents of the total aerodynamic force. This vortex-force diagnosis sheds new light on the flow control and configuration optimization. The project supported partly by the National Natural Science Foundation of China (10572005).     

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。