大展弦比复合材料机翼的非线性颤振分析

大展弦比机翼在气动力作用下产生较大变形,颤振速度和颤振频率随之发生明显变化,线性理论难以获得比较合理的解答。综合考虑了结构几何非线性、气动非线性和材料各向异性对机翼运动状态的影响,将复合材料机翼建模为非线性薄壁单闭室截面梁,建立机翼的运动方程,并使用小扰动分析的方法得到机翼在平衡位置附近的振动方程。采用Theodorsen非定常气动理论构建气动模型,获得机翼在平衡位置附近的非线性颤振方程,并利用v-g法判定机翼颤振稳定性。通过算例演示了一些非线性颤振的特点,讨论了铺层角、展弦比、机翼线密度等参数对颤振速度的影响,并与线性理论得到的结果进行对比。     

侧向随动力作用下大展弦比柔性机翼的稳定性

 随动力能够诱发弹性结构发生颤振失稳。以侧向随动力和集中质量分别模拟发动机推力和外挂质量,考虑机翼垂直弯曲-扭转刚度比、集中质量大小、侧向随动力和集中质量的位置以及机翼后掠角和上反角的影响,研究了受侧向随动力作用的大展弦比柔性机翼的气动弹性稳定性。数值模拟所采用的大展弦比柔性机翼非线性气动弹性模型耦合了几何精确完全本征运动梁模型和ONERA动失速非定常气动力模型,该模型考虑了几何非线性、动失速和材料各向异性。模拟结果表明,侧向随动力对机翼颤振可以具有稳定作用,其具体表现依赖于若干变参数的影响,如：减小机翼垂直弯曲-扭转刚度比;发动机吊舱靠近翼根布置;使发动机推力作用点在法向上与机翼弹性轴靠近;单纯的集中质量避免布置在柔性机翼中部,且布置在机翼弹性轴之前或下方,这些设计或布置均有利于提高带发动机吊舱/有效载荷外挂的柔性机翼的气动弹性稳定性。     

大展弦比大柔性机翼载荷分布求解的一种方法

大展弦比大柔性机翼在气动力作用下产生较大的弯曲和扭转变形,会引起明显的气动载荷重新分布。基于一种只具有2个广义转角自由度的梁单元模型,提出了一种大展弦比大柔性机翼载荷重新分布的新方法。该方法将大柔性机翼弯曲变形的几何非线性问题转化为线性问题,同时,基于弯曲变形结果,可在局部坐标系下进行机翼扭转变形求解,避免了整体坐标系下扭转变形的几何非线性问题。综合来看,该方法可将具有明显几何非线性效应的大展弦比大柔性机翼的载荷重新分布问题转化为线性问题,且计算量小,效率高,非常适合工程实用。通过与大柔性悬臂梁解析解的对比,验证了本文方法的正确性和有效性。     

大展弦比复合材料机翼失速颤振分析

研究了大展弦比复合材料机翼在较大迎角状态下的失速颤振特性,探讨了结构几何非线性和由复合材料剪裁产生的刚度耦合效果对机翼失速颤振特性的影响.首先,将复合材料机翼建模为转角和位移均可为有限值的非线性薄壁单闭室截面Euler梁,并在综合考虑结构几何非线性、气动非线性和材料各向异性对机翼运动状态的影响的基础上,建立机翼的运动微分方程.然后,使用小扰动分析的方法得到机翼在平衡位置附近的振动方程,采用ONERA半经验的非定常失速气动力模型,获得机翼在平衡位置附近的非线性失速颤振分析方程.最后,利用谐波平衡法求解并判定机翼颤振稳定性.通过算例,首先验证了算法的正确性,然后研究了几何非线性对失速颤振的影响,并讨论不同的复合材料铺层方式导致机翼失速特性的改变.     

考虑翼吊发动机推力的机翼颤振分析

分析了带翼吊发动机的大展弦比机翼颤振特性,其中考虑了机翼大变形和发动机推力的作用。采用梁单元对双梁式机翼和发动机吊舱进行了结构建模,采用修正的片条理论计算曲面定常气动力,修正的Theodorsen方法计算曲面非定常气动力,发动机推力始终沿发动机轴线。研究机翼在不同攻角时的定常气动力及发动机推力联合作用下固有振动和颤振特性的变化情况。结果表明,发动机推力对机翼静变形和固有频率改变不大,但对颤振临界速度的变化量不容忽视,有可能成为大型运输机机翼设计的限制因素。     

基于本征方程的柔性机翼非线性颤振分析

柔性机翼在气动载荷作用下常常会产生较大的变形,颤振特性会随之发生变化,针对此问题线性理论常常难以进行合理的预测。以几何精确本征梁模型建立了机翼的运动方程,耦合ONERA-EDlin非线性气动模型,建立了柔性机翼的非线性气动弹性分析模型。利用Newton-Raphson和Backward-Differentiation-Formula(BDF)分别求解机翼的静态变形和动态响应,基于机翼平衡位置附近的线性化方程来判断系统的稳定性,进而确定颤振临界速度。通过算例验证了模型的准确性,并分析了不同刚度、后掠角、机翼安装角等参数对颤振速度的影响。     

大展弦比机翼的非线性气弹响应分析

高空长航时飞行器将在未来军事与民用领域具有广阔的应用前景,这类飞行器具有大展弦比、大柔度机翼,在对该类型结构的气弹响应分析中,必须要考虑机翼的非线性因素.目前,对大型的工程结构MSC.NASTRAN提供了线性颤振、线性气弹的计算功能,但无法进行非线性结构的气弹响应分析.本文在非线性运动方程中引入ONERA气动力的线性项,利用DMAP语言在NASTRAN非线性瞬态响应分析程序中加入相关气动力模块,实现了大型有限元机翼的非线性气弹响应分析.文中给出了算例,结果验证了该法分析大展弦比机翼结构气弹响应的有效性,具有重要的工程应用价值.     

柔性飞机非线性气动弹性与飞行动力学耦合静、动态特性

高空长航时(HALE)飞机结构细长、具有柔性,在常规飞行条件下可发生结构大变形、气动失速以及结构低频振动与刚体运动耦合,这些现象显著影响其静、动态特性.基于几何精确完全本征运动梁模型、ONERA动失速气动力模型和六自由度刚体运动模型,建立了考虑几何非线性、动失速和材料各向异性等因素的大展弦比柔性飞机非线性气动弹性与飞行...     

大变形状态机翼振动试验与气动弹性分析

本文针对结构大变形引起的几何非线性对于结构动力学特性的影响作了计算和试验研究,设计了一个试验,用于体现大展弦比平板机翼结构大变形对于动力学特性的改变。并计算分析了这种大变形对于该机翼气动弹性颤振特性的影响,并参考了一个双梁骨架结构机翼的计算分析结果。经分析发现几何非线性因素对于这两种机翼结构的影响体现在:第一,使得固有模态中的扭转模态频率大幅降低;第二,使得颤振特性发生变化。     

低速流场中柔性悬臂板的后颤振响应

建立了一个新的非线性气动弹性模型,对低速流场中柔性悬臂板的后颤振响应特性进行了分析。建模中考虑了结构几何非线性、气动力非线性以及两者之间的强耦合效应。通过实验数据对所建立的气动弹性模型进行了验证。发现在低速流场中柔性悬臂板可能会以周期加倍的方式进入混沌运动。结构几何非线性效应和翼尖涡引起的非定常气动力效应对柔性悬臂板的结构响应有显著影响,而尾涡变形引起的非定常气动力对结构运动的影响较小。还研究了不同耦合算法的差异,给出了小展弦比大柔性结构非线性气动弹性数值仿真时耦合策略的选择依据。     

基于CFD/CSD的非线性气动弹性分析方法

提出了一种基于计算流体力学/计算结构动力学(CFD/CSD)的非线性气动弹性分析方法,并应用于切尖三角翼的非线性颤振和极限环振荡(LCO)研究。该方法将非线性有限元(FEM)和CFD计算相结合,并辅以高精度的界面插值,能够分析结构和气动非线性共存的气动弹性问题。结构部分以四边形平板壳元为基础,采用更新的拉格朗日(UL)方法分析结构大变形引起的几何非线性问题。气动部分以Navier-Stokes(N-S)方程作为控制方程,采用CFD方法计算跨声速气动力。机翼的非线性颤振计算表明了方法的有效性。最后应用该方法研究了切尖三角翼的LCO现象,其计算精度明显优于已有结果。     

Gust response analysis and wind tunnel test for a high-aspect ratio wing

A theoretical nonlinear aeroelastic response analysis for a flexible high-aspect ratio wing excited by harmonic gust load is presented along with a companion wind tunnel test. A multidisci-plinary coupled numerical calculation is developed to simulate the flexible model wing undergoing gust load in the time domain via discrete nonlinear finite element structural dynamic analysis and nonplanar unsteady vortex lattice aerodynamic computation. A dynamic perturbation analysis about a nonlinear static equilibrium is also used to determine the small perturbation flutter bound-ary. A novel noncontact 3-D camera measurement analysis system is firstly used in the wind tunnel test to obtain the spatial large deformation and responses. The responses of the flexible wing under different static equilibrium states and frequency gust loads are discussed. The fair to good quanti-tative agreements between the theoretical and experimental results demonstrate that the presented analysis method is an acceptable way to predict the geometrically nonlinear gust response for flex-ible wings.     

基于非定常气动力辨识技术的气动弹性数值模拟

选择离散型输入输出差分模型,运用最小二乘方法进行非定常气动力建模,并将辨识得到的降阶模型用于气动弹性的数值模拟。1个马赫数下的颤振临界点的计算仅需调用一次非定常流场求解器。计算精度保持与非定常欧拉方程计算方法相当的同时计算效率提高了1~2个量级。计算了跨声速具有S型颤振边界的气动弹性标准算例－Isogaiwing和三维气动弹性标模算例AGARD445.6,辨识模型计算边界与非定常Euler方程计算结果吻合。证明非定常气动力辨识技术可以提供高效的高精度的气动弹性分析。     

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.     

Numerical studies of a non-linear aeroelastic system with plunging and pitching freeplays in supersonic/hypersonic regimes

The flutter and post flutter of a two-dimensional double-wedge lifting surface with combined freeplay and cubic stiffness nonlinearities in both plunging and pitching degrees-of-freedom operating in supersonic/hypersonic flight speed regimes have been analyzed. In addition to the structural nonlinearities, the third-order piston theory aerodynamics is used to evaluate the unsteady non-linear aerodynamic force and moment. Such model accounts for stiffness and damping contributions produced by the aerodynamic loads. Responses involving limit cycle oscillation and chaotic motion are observed over a large number of parameters that characterizes the aeroelastic system. Results of the present study show that the freeplay in the pitching degree-of-freedom and soft/hard cubic stiffness in the pitching and plunging degrees-of-freedom have significant effects on the LCOs exhibited by the aeroelastic system in the supersonic/hypersonic flight speed regimes. The simulations also show that the aeroelastic system behavior is greatly affected by physical structural parameters, such as the radius of gyration and the frequency ratio, especially in post-flutter regimes, when accounting for all system nonlinearities. It has been shown that at high Mach numbers the non-linear aerodynamic stiffness yields detrimental effects from the aeroelastic point of view, while the damping one do not.     

Transonic flutter computations for the NLR 7301 supercritical airfoil

A numerical investigation of the transonic steady-state aerodynamics and of the two-degree-of-freedom bending/torsion flutter characteristics of the NLR 7301 section is carried out using a time-domain method. An unsteady, two-dimensional, compressible, thin-layer Navier–Stokes flow-solver is coupled with a two-degree-of-freedom structural model. Fully turbulent flows are computed with algebraic or one-equation turbulence models. Furthermore, natural transition is modeled with a transition model. Computations of the steady transonic aerodynamic characteristics show good agreement with Schewe's experiment after a simplified accounting for wind-tunnel interference effects is used. The aeroelastic computations predict limit-cycle flutter in agreement with the experiment. The computed flutter frequency agrees closely with the experiment but the computed flutter amplitudes are an order of magnitude larger than the measured ones. This discrepancy is likely due to the omission of the full wind-tunnel interference effects in the computations.