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。     

变厚度翼型的非定常气动力模型降阶及影响分析

建立了基于Kriging代理和递归算法的变厚度翼型气动力降阶模型,将滤波的高斯白噪声作为输入信号,采用计算流体力学(CFD)方法获得不同厚度下翼型的非定常气动力并将其作为降阶模型的训练样本.该降阶模型不仅大大提高了非定常气动力的计算效率,而且其预测得到的非定常响应的精度不低于92.01％.通过蒙特卡洛法的变厚度翼型全局灵敏度分析表明,俯仰运动是影响气动力波动的主要因素,俯仰与沉浮位移的耦合对气动力影响很小.考虑翼型变厚度时对翼面压力及分离点的影响,本文建立了压强系数的降阶模型.通过与CFD结果的对比,得到其精度为99.9995％,验证了降阶模型的正确性和有效性.     

多段翼型缝翼流动速度的定常和非定常特性研究

用热线风速仪研究多段翼型前缘缝翼在不同条件下流动速度的定常性和非定常性.结合多段翼型定常流动Navier-Stokes方程数值模拟的结果,分析了迎角、后缘襟翼参数(偏角、缝道宽度、搭接量)对缝翼定常和非定常流动速度的影响规律.研究结果表明:在缝翼后缘处,流动分为缝道加速流动区、缝翼尾流区、缝翼上表面以上的主流区;缝翼尾流区流动速度非定常性主要表现在中低频率范围(2kHz以下),而缝道加速流动区和缝翼上表面以上的主流区流动非定常性常表现出高频特性(2kHz以上);在失速前随迎角增加,或者当襟翼偏角从20°向30°增加时,缝道流动加速,槽区涡减小;缝翼槽区涡形成和振荡是中低频率范围流动非定常性的机理,而缝翼鼻尖脱落涡是缝翼槽区涡振荡的激励因素.     

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.     

相对弯度对低雷诺数流动中翼型动态气动力特性的影响

以固定翼微型飞行器为研究背景,研究了相对弯度对低雷诺数流动中翼型动态气动力特性的影响规律。采用Roe迎风差分格式和双时间步迭代方法,数值求解拟压缩性修正不可压Navier-Stokes方程组,给出了数值算法与实验数据的对比验证。以翼型弦长为特征长度,在Re=500~50000情况下,选取不同最大相对弯度和不同最大相对弯度位置的翼型,计算了其等速上仰时的动态气动力,结果表明后者对气动力的影响比较显著,把最大弯度位置布置在翼型弦向40%的地方要比布置在30%和50%两处所获得的动态升阻比大。     

振动叶栅非定常气动力的数值分析及应用

利用双时间方法结合运动网格求解Navier-Stokes方程，完成了振动叶栅非定常气动力的数值分析。运用流动显示和频谱分析技术，研究了振动叶栅非定常绕流与气动力的特征。根据能量法原理，对振动叶栅的气动弹性稳定性进行了分析。     

蜻蜓扑翼前飞时弯曲褶皱翼气动特性分析

蜻蜓翅膀具有独特的褶皱结构,其横截面在扑翼飞行中常呈现弯曲变形.本文设计了具有弯度的褶皱翼型,并综合考虑弯度大小及分布的影响;基于Fluent软件,采用重叠网格方法对其扑翼前飞的气动性能进行分析.首先,与平板褶皱翼相比,正弯度褶皱翼可以显著提高升力,在对弯曲大小和分布的合理设计下,如m=4％,p=0.2时,可实现在扑翼中同时提高升力和推力.其次,弯曲褶皱翼型的升力随着扭转角的增加而增加,但是较大扭转角会降低推力,此时可通过后移最大弯曲位置改善推力.因此,在特定条件下,合适的弯度大小及分布可以较大幅度地改善褶皱翼型的气动力性能.     

一种高空高超声速非定常气动力近似模型

针对高空高马赫数飞行环境和强黏性干扰的物理特性, 在当地流活塞理论的基础上引入有效外形修正, 发展了黏性修正当地流活塞理论, 结合定常N-S方程解给出了高空高马赫数下针对该方法的有效外形的判据, 并通过数值算例对该判据进行了验证.通过对典型尖头薄翼和典型钝头翼的一系列二维非定常算例, 将该方法与一阶活塞理论、基于欧拉(Euler)方程的当地流活塞理论和非定常N-S方程数值解进行了对比. 结果显示在高度为40~70 km、马赫数为10~20范围内, 通过该方法计算得到的非定常气动力与非定常N-S方程数值解吻合较好, 明显优于活塞理论和基于Euler方程的当地流活塞理论.该方法克服了传统的活塞理论和当地流活塞理论不能用于高空高马赫数这类强黏性效应情况的弊端, 在较宽的马赫数、攻角、飞行高度范围内都有良好的适用性, 同时其计算效率远高于非定常N-S方程.     

运动激波扫过运动平板的非定常传热问题

本文研究运动激波扫过平板边界层后边界层随时间的演化。首先将霍华斯变换推广到非定常情形并引入相似参数将非定常问题化为有两个前缘的边界层问题。这时抛物型方程是奇异的。在激波附近用奇异摄动法求出级数解,然后用逆风格式数值积分将解延拓到平板前缘。     

叶轮机中振动叶片非定常气动力的研究

采用“单一叶片振动方法”代替传统的“全部叶片振动方法”，用线性化的方法推导出计算振动叶片非定常气动力（引起叶片弯曲振动）及力矩（引起叶片扭转振动）方程。通过输入叶型及气流参数，它可以方便有效地估计出作用在振动叶片上的非定常气动力和力矩，以及振动叶片不稳定工况发生的条件和范围。并在氟里昂超音速风洞上进行了实验测量，结果表明，理论计算结果与实验测试结果符合较好。     

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)     

Fluid forces on a very low Reynolds number airfoil and their prediction

This paper presents the measurements of mean and fluctuating forces on an NACA0012 airfoil over a large range of angle (α) of attack (0-90°) and low to small chord Reynolds numbers (Re_c), 5.3 × 10³-5.1 × 10⁴, which is of both fundamental and practical importance. The forces, measured using a load cell, display good agreement with the estimate from the LDA-measured cross-flow distributions of velocities in the wake based on the momentum conservation. The dependence of the forces on both α and Re_c is determined and discussed in detail. It has been found that the stall of an airfoil, characterized by a drop in the lift force and a jump in the drag force, occurs at Re_c ? 1.05 × 10⁴ but is absent at Re_c = 5.3 × 10³. A theoretical analysis is developed to predict and explain the observed dependence of the mean lift and drag on α.     

应用当地流活塞理论的大攻角升力面颤振气动力表达式

本文应用当地流活塞理论,给出了弹性的夺动翼面的非定常压分分布以及用模态坐标的广义气动力系数表达式。同时提供了配套的当地流参数计算公式和采用样条函数计算广义气动力系数数值积分的表达式。     

AERODYNAMIC STIFFNESS AND DAMPING EFFECTS IN THE ROTATIONAL GALLOPING OF A RECTANGULAR CROSS-SECTION

The galloping oscillation with a single rotational degree of freedom is investigated for a prismatic beam with a rectangular cross-section. For this particular geometric configuration, the effect of the unsteady aerodynamic forces can be understood as a combination of aerodynamic stiffness and damping terms, with the latter being the most relevant for the instability behaviour. A theoretical prediction of both these effects is made for this type of galloping behaviour, based on a quasi-steady aerodynamic approach. Comparison to experimental results obtained in wind tunnel experiments reveals significant shortcomings of the quasi-steady approach in predicting, in particular, the damping effect. For the stiffness effect, the agreement is much better.     

超临界翼型跨音速特性实验研究

针对新设计的超临界翼型,采用风洞实验方法验证和评估了其气动特性。在增压连续式跨音速风洞(NF-6风洞)开展了超临界翼型跨音速特性的实验研究,验证了该翼型设计的压力分布曲线特点。激波位置和波后压力平台区长度表明设计结果和实验结果基本一致,揭示了超临界翼型跨音速的气动特性;阻力发散马赫数达到期望的设计指标,探讨了雷诺数对该翼型气动特性的影响。最后采用升华法实现了翼型表面流动特性的显示。结果表明转捩点约在16%弦长位置。     

Application of a Virtual-Boundary Method for the Numerical Study of Oscillations Developing Behind a Cylinder Near A Plane Wall

The conditions of onset and the character of the oscillations developing behind a circular cylinder located above a plane wall (screen) in a flow with a velocity profile of the boundary layer type are studied numerically. The dependence of the critical Reynolds number (at which a steady flow regime in the wake behind the cylinder is replaced by an oscillatory regime) on the cylinder-wall gap and the free-stream boundary layer thickness is found.     

对称翼型低雷诺数小攻角升力系数非线性现象研究

采用Rogers发展的三阶Roe格式,求解非定常不可压N-S方程,时间方向为二阶精度双时间步方法, 数值模拟了对称翼型SD8020低雷诺数(Re=40000,100000)条件下,流场层流分离涡结构和升力系数随攻角的变化．同试验比较证明了数值模拟的正确性．通过对数值模拟时均化流场结果的详细分析,发现对称翼型在小雷诺数0°攻角附近出现的层流分离泡,其内部结构和演化规律都不同于经典层流分离泡模型,从而提出了一种后缘层流分离泡模型．并应用该模型对对称翼型小攻角低雷诺数流场特性以及升力系数非线性效应的形成机理进行了研究和解释．     

Flow field characteristics study of a flapping airfoil using computational fluid dynamics

The flow field of a flapping airfoil in Low Reynolds Number (LRN) flow regime is associated with complex nonlinear vortex shedding and viscous phenomena. The respective fluid dynamics of such a flow is investigated here through Computational Fluid Dynamics (CFD) based on the Finite Volume Method (FVM). The governing equations are the unsteady, incompressible two-dimensional Navier-Stokes (N-S) equations. The airfoil is a thin ellipsoidal geometry performing a modified figure-of-eight-like flapping pattern. The flow field and vortical patterns around the airfoil are examined in detail, and the effects of several unsteady flow and system parameters on the flow characteristics are explored. The investigated parameters are the amplitude of pitching oscillations, phase angle between pitching and plunging motions, mean angle of attack, Reynolds number (Re), Strouhal number (St) based on the translational amplitudes of oscillations, and the pitching axis location (x/c). It is shown that these parameters change the instantaneous force coefficients quantitatively and qualitatively. It is also observed that the strength, interaction, and convection of the vortical structures surrounding the airfoil are significantly affected by the variations of these parameters.