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。     

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。     

用规范型直接法研究立方非线性机翼的颤振

研究了在不可压缩流中具有立方非线性俯仰刚度的二元机翼的颤振问题。首先，对系统平衡点进行了特征值分析，得到在Hopf分岔点附近的颤振参数方程。然后，应用规范形直接法的Maple推导程序，计算得到Hopf分岔的规范形．并数值模拟验证了超、亚临界类型分岔情形下响应对初值的依赖性。最后．分析了线性和非线性刚度系数对颤振响应拓扑结构的影响。     

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。     

带有气动及结构非线性的二元机翼颤振分析

研究了翼型在低马赫数条件下的非定常气动特性,从翼型表面气流运动的角度对Leishman-Beddoes(L-B)模型进行了修正,并在此基础上建立了适合低马赫数颤振研究且带有气动及结构非线性的二元机翼气弹系统分析模型.对比低马赫数翼型气动载荷试验结果表明对L-B模型的修正是有效的,且机翼颤振试验结果亦验证了二元机翼气弹分析模型.研究结果表明:二元机翼气弹系统的失速颤振与初始变距角和来流速度密切相关,且耦合的三次非线性变距和浮沉刚度是造成系统呈现准周期运动的主要原因.      

长直机翼的颤振及混沌运动分析

采用非定常气动力并考虑几何非线性的影响,建立了长直机翼的气动弹性运动方程。运用伽辽金法对方程进行离散,通过数值模拟研究了机翼的颤振特性及混沌运动。结果表明:考虑几何非线性后,出现极限环振动的初始点与线性预测结果基本一致;不同机翼模型,机翼振动从收敛到混沌的过程不同,可由单个极限环振动经拟周期运动进入混沌,也可以由单个极限环到拟周期运动,再回到单环振动,然后经极限环的周期倍化进入混沌状态。     

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。     

小展弦比扭转机翼三维流场的数值模拟

以高速战斗机的气动特性分析为研究背景,对小展弦比带扭转机翼的亚跨超声速气动特性进行了数值分析,首先对机翼关键数据进行参数化,选取几何扭转角作为设计变量对机翼进行建模,然后利用数值模拟方法,对参数化建模后的机翼在不同扭转角下的气动力和流场结构进行了计算与分析,揭示了机翼不同扭转角对失速特性,机翼展向不同截面的压力系数分布的影响,同时,对最佳扭转角的机翼在不同马赫数下的三维流场进行了数值模拟,验证了小展弦比机翼在亚跨声速下的优越性能。     

复合材料超临界机翼跨音速颤振实验与数值计算研究

颤振课题是飞机设计过程中常常遇到的一个关键技术问题。以支线客机ARJ21超临界机翼颤振特性研究为背景,在俄罗斯TsAGI的T-106风洞中完成了该复合材料机翼跨音速颤振实验,基于N-S方程和无限插值方法（TFI）生成三维贴体运动网格对超临界机翼跨音速颤振进行了并行计算。结果表明：复合材料的超临界机翼在跨音速区域具有跨音速颤振＂凹坑＂现象;与风洞实验结果相比,有较好的一致性,为使用超临界机翼的运输类飞机跨音速颤振特性预计提供了一定的参考。     

考虑气动热的机翼颤振特性分析

为研究气动热对高超音速飞行器气动弹性特性的影响,首先对小展弦比机翼施加不同温度场,通过热应力分析理论获得不同温度载荷下机翼的热模态,然后应用二阶活塞理论计算非定常气动力,最后采用p-k法进行颤振计算。计算结果表明,气动加热后结构的模态特性和颤振特性均发生变化,由于温度效应降低了各阶固有频率,并且改变了它们之间的差距,从而导致机翼颤振速度降低。     

切尖三角翼极限环振荡的气动弹性模拟

 建立了高精度的气动弹性计算模型,对切尖三角翼风洞试验中的极限环振荡（LCO）现象进行数值模拟。结构部分引入大变形产生的几何非线性和塑性引起的材料非线性,气动部分采用Euler方程描述跨声速流动。结构/气动交界面精确匹配,并选取三维插值进行界面载荷传递。依据所建模型分析切尖三角翼的颤振和LCO,并与试验值进行比较。在小来流动压情况下,结构几何非线性引起了切尖三角翼的LCO,计算结果和试验值吻合较好。在大来流动压情况下,结构材料非线性导致了LCO幅值的急剧增大,其变化趋势与试验观察相符。研究结果显示,切尖三角翼的LCO不仅与结构几何非线性密切相关,而且受到结构材料非线性的显著影响。     

Stall flutter prediction based on multi-layer GRU neural network

The modeling of dynamic stall aerodynamics is essential to stall flutter, due to the flow separation in a large-amplitude pitching oscillation process. A newly neural network based Reduced Order Model (ROM) framework for predicting the aerodynamic forces of an airfoil undergoing large-amplitude pitching oscillation at various velocities is presented in this work. First, the dynamic stall aerodynamics is calculated by solving RANS equations and the transitional SST-γ model. Afterwards, the stall flutter bifurcation behavior is calculated by the above CFD solver coupled with structural dynamic equation. The critical flutter speed and limit-cycle oscillation amplitudes are consistent with those obtained by experiments. A newly multi-layer Gated Recurrent Unit (GRU) neural network based ROM is constructed to accelerate the calculation of aerodynamic forces. The training and validation process are carried out upon the unsteady aerodynamic data obtained by the proposed CFD method. The well-trained ROM is then coupled with the structure equation at a specific velocity, the Limit-Cycle Oscillation (LCO) of stall flutter under this flow condition is predicted precisely and more quickly. In order to predict both the critical flutter velocity and LCO amplitudes after bifurcation at different velocities, a new ROM with GRU neural network considering the variation of flow velocities is developed. The stall flutter results predicted by ROM agree well with the CFD ones at different velocities. Finally, a brief sensitivity analysis of two structural parameters of ROM is carried out. It infers the potential of the presented modeling method to depict the nonlinearity of dynamic stall and stall flutter phenomenon.     

考虑机翼几何非线性的气动弹性建模与分析

大展弦比柔性机翼结构重量轻、气动效率高,广泛应用于高空长航时无人机(UAVs)。飞行过程中,这类机翼在气动力作用下发生大变形,线性结构模型不再适用,需要建立考虑几何大变形的结构模型。采用牛顿力学方法推导了考虑结构几何非线性的机翼结构动力学模型,该方法推导过程简洁、物理意义明确,可以与Hodges基于哈密顿原理的推导方法相互补充,相互验证。为了能够更准确地求解大展弦比柔性机翼的非定常气动力,建立了能够考虑机翼三维效应且适用于机翼空间大变形的非定常气动力模型。基于建立的非线性结构模型和非定常气动力模型,采用松耦合方法建立了非线性气动弹性模型,并通过算例验证了气弹模型的准确性。研究结果表明,大展弦比柔性机翼颤振速度对来流迎角和机翼的展长均较为敏感;当来流速度大于颤振速度时,由于几何非线性,机翼振动并未发散而是形成稳定的极限环振荡(LCO);随着来流速度进一步增加,机翼再次穿过临界稳定点,由不稳定系统变为稳定系统,直到随着速度的增加系统再次达到临界稳定状态。     

Limit cycle oscillation control of wing with static output feedback control method

This paper presents a static output feedback controller (SOFC) for aeroelastic control of a cantilevered rectangular wing in low subsonic flow. For this purpose, an optimal formulation of this control method is developed, and a solution method is proposed for the related matrix equation. This optimal solution is obtained by solving combined Lyapunov and Riccati equations. At first these equations are transformed into a set of nonlinear algebraic equations and then are solved with iterative Newton–Raphson?s method. This solution method is applicable to the full state feedback case. The controller is designed to extend flutter boundary and suppress limit cycle oscillation (LCO) of a low aspect ratio rectangular nonlinear structural wing. This structural nonlinearity is given by Von Karman plate theory. Both full and reduced order aerodynamic models are examined based on the modified vortex lattice theory. Results show combination of SOFC with reduced order aerodynamic model would be an effective choice for aeroelastic stabilization, and this controller has a very comparable result with linear quadratic regulator (LQR).     

Nonlinear aeroelastic analysis of airfoils: bifurcation and chaos

Different types of structural and aerodynamic nonlinearities commonly encountered in aeronautical engineering are discussed. The equations of motion of a two-dimensional airfoil oscillating in pitch and plunge are derived for a structural nonlinearity using subsonic aerodynamics theory. Three classical nonlinearities, namely, cubic, freeplay and hysteresis are investigated in some detail. The governing equations are reduced to a set of ordinary differential equations suitable for numerical simulations and analytical investigation of the system stability. The onset of Hopf-bifurcation, and amplitudes and frequencies of limit cycle oscillations are investigated, with examples given for a cubic hardening spring. For various geometries of the freeplay, bifurcations and chaos are discussed via the phase plane, Poincaré maps, and Lyapunov spectrum. The route to chaos is investigated from bifurcation diagrams, and for the freeplay nonlinearity it is shown that frequency doubling is the most commonly observed route. Examples of aerodynamic nonlinearities arising from transonic flow and dynamic stall are discussed, and special attention is paid to numerical simulation results for dynamic stall using a time-synthesized method for the unsteady aerodynamics. The assumption of uniform flow is usually not met in practice since perturbations in velocities are encountered in flight. Longitudinal atmospheric turbulence is introduced to show its effect on both the flutter boundary and the onset of Hopf-bifurcation for a cubic restoring force.     

考虑间隙非线性的控制舵非线性气动弹性分析

间隙结构的气动弹性系统非线性颤振问题是飞行器气动弹性力学工程领域的研究热点和难点,研究 考虑间隙非线性的控制舵系统的气动弹性特性具有重要意义。基于最小状态拟合方法获得时域降阶气动力模 型,并通过Lagrange方程获得系统非线性气动弹性方程;对比分析三种不同非线性控制舵系统的极限环颤振 及非线性动力学响应特性,并与等效线化法和时域仿真的结果进行一致性对比。结果表明:俯仰和扑动弹簧刚 度的变化对系统颤振边界有显著影响,当俯仰和扑动两个方向同时含有间隙非线性时,系统在线性颤振速度内 存在倍周期、混沌等复杂非线性动力学现象。