An sequential optimization and aeroelastic constraint transformation method for strength-aeroelastic comprehensive design

With the development of modern aircraft technology, aeroelasticity plays a more and more crucial role in aircraft structural design. However, low efficiency of present aeroelastic analysis and optimization methods makes it difficult to apply in engineering practice. This paper presents a sequential optimization and aeroelastic constraint transformation method (SOACTM) for comprehensive design of airplane wings with strength and aeroelastic constraints. Optimization with structural strength constraint and aeroelastic constraint is transformed into a serial of cycles of decoupled structural strength sub-optimizations and aeroelastic sub-optimizations based on sequential optimization strategy. In structural strength sub-optimization, structural strength constraint is translated along its normal direction to make optimal design point satisfying aeroelastic constraint. And the goal of aeroelastic sub-optimization is to find the translational distance of structural strength constraint. Aeroelastic constraint is transformed to equivalent structural strength constraint via above approach. In this way, number of aeroelastic analyses in SOACTM is less than that in traditional optimization method and total computational time decreases. SOACTM is verified based on two examples. Traditional optimization method is applied for the sake of validation. The results demonstrated the accuracy and efficiency of SOACTM for wing comprehensive optimization considering both structural strength and aeroelastic constraints.     

基于流形切空间插值的折叠翼参数化气动弹性建模

变体飞行器的气动弹性力学建模是当前先进飞行器设计的研究热点和难点. 然而传统的气动弹性动力学建模方法对于具有结构参变特性的变体飞行器气动弹性力学研究存在建模效率低、计算复杂等问题. 本研究提出了一种基于流形切空间插值的可折叠式变体机翼参数化气动弹性建模方法. 首先, 该方法建立若干个典型折叠角下的折叠翼结构有限元模型, 通过流形切空间插值方法建立折叠翼参数化结构动力学模型. 其次, 采用偶极子网格法得到参数化非定常气动力模型, 进而建立气动和结构相互耦合的折叠翼参数化气动弹性模型. 为了验证该参数化建模方法在折叠翼气动弹性分析中的准确性, 本文以一小展弦比折叠翼为研究对象, 从折叠翼自由振动时的参变模态特性、颤振边界预测两方面进行了算例验证, 并与直接计算方法进行了对比, 进一步验证了参数化气动弹性模型的有效性. 研究结果表明, 该参数化气动弹性模型对上述两类问题的计算精度与直接计算方法一致, 并且有着计算效率更高的优势.      

Static Finite Element Validation of a Flexible Micro Air Vehicle

The flexible-wing approach has proven to be a successful method for designing micro air vehicles. The wing’s passive deformation under wind loads can allow for gust rejection, delayed stall, or improved longitudinal stability. As such, an accurate structural model of the flexible wing can provide greater understanding of the aforementioned phenomena. This paper seeks to formulate a static finite element wing model, with a particular emphasis on accuracy. The wing is broken into three different types of elements: beams, plates, and membranes. Individual element types are characterized and validated by constructing simple structures from the appropriate material, and then comparing experimental and numerical deformation fields. Experimental results are found through a visual image correlation system. The elements are then combined to form the complete wing model, which is also validated through experiments. The resulting finite element model is found to be very accurate, able to predict the complicated structural response of a composite wing. Due to observations made during standard wind tunnel testing, the structural response of a typical membrane MAV wing in steady level pre-stall flight is thought to be quasi-static. As such, the finite element model formulated in this work will be indispensable towards future numerical static aeroelastic optimization research efforts aimed at improving the efficiency, agility, and sensitivity of practical micro air vehicles.     

直升机旋翼/机体动稳定性研究进展

首先对直升机旋翼/机体动不稳定性问题的种类进行了简要概述,包括旋翼挥舞/变距、变距/摆振、挥舞/摆振和挥舞/摆振/变距耦合等孤立旋翼动不稳定性问题,以及直升机地面共振和空中共振等旋翼/机体耦合动不稳定性问题,然后分别从气动力与结构的高精度数值模型、动稳定性的计算分析方法和实验模型测试3 个方面详细介绍了直升机旋翼/机体动不稳定性问题的研究现状,并着重讨论了直升机旋翼/机体动稳定性分析技术最近的主要研究方向:耦合CFD(computational fluid dynamics)/CSD(computational structuraldynamics) 的直升机旋翼气弹动稳定性分析、复合材料旋翼动稳定性分析及其材料不确定性影响、带减摆器的旋翼/机体动稳定性分析和先进直升机构型的旋翼/机体动稳定性分析,最后对直升机旋翼/机体动稳定性研究的发展趋势进行了展望.     

考虑几何非线性的复合材料机翼气动弹性分析

本文研究结构几何非线性与气动力非平面效应对大展弦比复合材料机翼的气动弹性行为的影响．将非线性有限元法与曲面涡格法结合，计算机翼静气动弹性变形；通过曲面偶极子格网法结合静气动弹性平衡位置处的结构切线刚度，建立气动弹性方程并求解得到机翼颤振速度．针对板模型机翼，分析了迎角对机翼几何非线性气动弹性特性的影响．结果表明：本文复合材料板模型机翼的颤振形式不受水平弯曲模态影响，属于经典弯扭颤振；在几何非线性的影响下，机翼扭转频率随结构变形增大而明显减小，颤振速度随迎角增大而减小．     

SIZE OPTIMIZATION FOR CANTILEVER I-BEAM BASED ON VARIATIONAL METHOD

以变分方法为工具,研究了分布载荷下工字型悬臂梁的尺寸优化问题,给出了在强度和位移约束下梁缘条沿展向的宽度函数,并简要介绍了优化准则法和多岛遗传算法. 以平直机翼为工程实例,将之简化为一个工字型悬臂梁,分别采用该3 种方法进行优化设计. 结果表明变分法不仅效率高,而且给出了尺寸分布的函数,这对飞行器概念设计阶段的机翼结构设计具有较大的指导意义.     

计算机辅助结构优化设计软件MCADS的开发与应用

工程结构优化设计，是将有限元等数值分析方法和数学规划等最优化方法结合起来、优选改进结构设计方案的一种先进技术，它使得工程师不再局限于凭经验和试凑的方法去设计，而是可以科学地高效率地选择最佳设计方案，因而也是结构分析和ＣＡＤ技术向更高层次的发展。本文介绍的ＭＣＡＤＳ是一个通用性的计算机辅助结构优化设计软件，它吸收了结构优化理论研究的成果并采用了面向工程结构设计的实用化技术，具备较强的功能和适用性，已作为商品化软件转让并在许多工业部门得到成功的应用．     

Enhancing the optimization of material distributions in composite structures using gradient architectures

Many structural components encounter service conditions and hence, material performance requirements, which vary from location to location within the component resulting in a composite structure. It has been shown that abrupt transitions in material properties within a composite structure can cause concentrations of deformation, which are mitigated by gradually varying the microstructure and/or composition of materials in a gradient architecture. Structural optimization techniques, such as the homogenization method, have yet to take full advantage of gradient architectures. Many of these structural optimization techniques employ robust mathematical techniques, such as genetic algorithms (GAs), with finite element simulations to optimize material distributions in composite structures through computationally intensive stochastic, global explorations of the design search space defined by a multitude of design variables associated with the discrete representation of the composite structure. Using gradient architectures, it is demonstrated that GAs can be enhanced for composite structures by constraining the design search space through a reduction in the number of design variables, thereby substantially reducing the computational effort. For more complex material distributions, a “material gradient optimization method” is proposed that produces multiple gradient architecture constraints with more optimal solutions than obtained without using these constraints, but whose uniqueness will be dependent upon the number of layers used in the finite element simulations.     

基于非均匀梁模型的二维柔性机翼固有振动分析

变体飞行器通过光滑连续的结构变形改变气动特性,从而提高飞行器的飞行性能,具有很大的应用前景.由于这类新概念飞行器主要通过改变自身结构形状以获得最佳工作性能的需求,因此具有变形大、质量轻等特点,较容易发生结构振动响应.本文研究了一种以柔性变后缘作为变体形式的二维柔性机翼等效建模方法,基于非均匀梁模型假设,建立了该柔性翼的动力学模型.通过利用Frobenius方法得到解析解及固有频率,并用有限元方法进行对比验证,发现前4阶固有频率的误差均在1%以内,每阶固有频率对应的振型一致.通过3D打印工程塑料ABS和硅胶蒙皮材料制备了柔性机翼结构件,并通过动态测量法和拉伸试验分别测定了打印材料和硅胶蒙皮材料的杨氏模量,搭建振动响应实验平台对制备的柔性机翼试验件进行振动试验.对比发现模型振动试验获得的基频与理论模型结果一致,并与有限元方法误差在3%以内.本文通过理论分析和实验验证,建立了二维柔性机翼等效建模方法,研究结果将为柔性变后缘结构动力学特性分析及其控制应用方面提供理论支持.     

Aeroelastic stability of a cantilevered plate in yawed subsonic flow

The aeroelastic stability of cantilevered plates with their clamped edge oriented both parallel and normal to subsonic flow is a classical fluid–structure interaction problem. When the clamped edge is parallel to the flow the system loses stability in a coupled bending and torsion motion known as wing flutter. When the clamped edge is normal to the flow the instability is exclusively bending and is referred to as flapping flag flutter. This paper explores the stability of plates during the transition between these classic aeroelastic configurations. The aeroelastic model couples a classical beam structural model to a three-dimensional vortex lattice aerodynamic model. The aeroelastic stability is evaluated in the frequency domain and the flutter boundary is presented as the plate is rotated from the flapping flag to the wing configuration. The transition between the flag-like and wing-like instability is often abrupt and the yaw angle of the flow for the transition is dependent on the relative spacing of the first torsion and second bending natural frequencies. This paper also includes ground vibration and aeroelastic experiments carried out in the Duke University Wind Tunnel that confirm the theoretical predictions.     

An efficient implementation of aeroelastic tailoring based on efficient computational fluid dynamics-based reduced order model

Current and future trends in the aerospace industry leverage on the potential benefits provided by lightweight materials that can be tailored to realize desired mechanical characteristics when loaded. For aircraft design, the deployment of aeroelastic tailoring is hindered by the need to re-compute, for any possible modification of the structure, the dependence of the aerodynamic field on the underlying structural properties. To make progress in this direction, the work presents a rapid computational fluid dynamics based aeroelastic tool which is built around a reduced order model for the aerodynamics that is updated for any modification of the structure by using the structural dynamics reanalysis method. The aeroelastic tailoring tool is demonstrated in transonic flow for the AGARD 445.6 wing, suitably modified with composite materials. It was found that the proposed method provides accurate engineering predictions for the aeroelastic response and stability when the structure is modified from the baseline model.     

An aeroelastic model for composite rotor blades with straight and swept tips. Part I: Aeroelastic stability in hover

An analytical model for predicting the aeroelastic behavior of composite rotor blades with straight and swept tips is presented. The blade is modeled by beam type finite elements along the elastic axis. A single finite element is used to model the swept tip. The non-linear equations of motion for the finite element model are derived using Hamilton's principle and based on a moderate deflection theory and accounts for: arbitrary cross-sectional shape, pretwist, generally anisotropic material behavior, transverse shears and out-of-plane warping. Numerical results illustrating the effects of tip sweep, anhedral and composite ply orientation on blade aeroelastic behavior are presented. It is shown that composite ply orientation has a substantial effect on blade stability. At low thrust conditions, certain ply orientations can cause instability in the lag mode. The flap-torsion coupling associated with tip sweep can also induce aeroelastic instability in the blade. This instability can be removed by appropriate ply orientation in the composite construction.     

Direct flutter and limit cycle computations of highly flexible wings for efficient analysis and optimization

The usefulness of flutter as a design metric is diluted for wings with destabilizing (softening) nonlinearities, as a stable high-amplitude limit cycle (subcritical) may exist for flight speeds well below the flutter point. It is thus desired to design aeroelastic structures such that the post-flutter behavior is as benign (i.e., supercritical) as possible, among the other constraints commonly considered in the optimization process. In order to account for these metrics in an accurate and efficient manner, direct tools are utilized to first locate the Hopf-point (flutter speed), and then to obtain a nonlinear perturbation solution via the method of multiple scales. The latter scheme provides a scalar variable whose sign and magnitude dictate the nature of the limit cycle. The accuracy of these methods is demonstrated with a high-aspect-ratio highly flexible wing, modeled with nonlinear beam finite elements and the ONERA dynamic stall tool. Stiffness and inertial design variables are allowed to vary spatially throughout the wing, in order to conduct gradient-based optimization of the limit cycle under flutter and mass constraints. The resulting wing structure demonstrates strongly supercritical behavior, as well as several design conflicts between linear (flutter) and nonlinear (limit cycles) sensitivities, which are not present in the uniform baseline wing.     

基于POD和代理模型的静气动弹性分析方法

静气动弹性问题考虑弹性结构与定常气动力间的相互耦合作用,对飞行器的性能和安全具有显著的影响.在现代飞行器设计阶段,计算流体力学(CFD)/计算结构力学(CSD)直接耦合方法是精确考察静气动弹性影响的重要手段.然而,基于CFD技术的气动力仿真手段在耦合过程中计算量大且耗时长,难以满足设计阶段的需求.因此,为了兼顾计算精度与效率,文章采用本征正交分解(POD)和Kriging代理模型相结合的模型降阶方法,替代CFD求解过程并耦合有限元分析(FEA)方法,建立了高效、准确的静气动弹性分析框架.相较于传统的以模态法为主的静气动弹性分析方法,该方法能够解决更为复杂的静气动弹性问题以及提供静气动弹性变形过程中的气动分布载荷.针对典型三维跨声速HIRENASD机翼模型开展的马赫数、迎角变化的算例验证表明:由建立的静气动弹性分析方法与CFD/CSD直接耦合方法计算得到机翼翼梢处的静变形量间的相对误差在5%以内;同时该方法预测静平衡位置处的气动分布载荷的误差在5%以内,静气动弹性分析的计算效率至少提升了6倍.     

一种用于振动疲劳试验研究的典型翼面模拟件

为开展飞机典型翼面结构振动疲劳问题及相关试验技术的研究,参考某型飞机垂尾翼面设计了其模拟件。在有限元分析软件Abaqus中建立其有限元模型,进行该模拟件的动力学分析和优化,使其模拟某飞机垂尾的前三阶模态。结合模态振型分析,给出一种确定单点振动载荷最优加载点的方法。窄带随机试验结果表明,模拟件达到了设计要求,可以用于某飞机垂尾翼面振动疲劳问题及相关试验技术的研究。     

基于卷积神经网络和松鼠优化算法的机翼结构混合优化设计#br#

针对大展弦比机翼的结构轻量化优化设计,提出了一种高效的布局和尺寸混合优化方法．在CFD/CSD气动弹性计算的基础上,对不同的结构变量进行统一编码,使用一维卷积神经网络建立代理模型,并使用松鼠优化算法建立了混合优化模型进行搜索寻优．以某型太阳能无人机的机翼结构优化为例,优化结果表明翼肋的布局变量和翼梁的尺寸变量之间存在着耦合关系,使用松鼠优化算法相比于遗传算法节省了35 %~45 %的计算成本,且混合优化后的结构比原始结构减重4.1 %,验证了该方法的有效性．     

遗传算法在随机参数刚架结构概率优化设计中的应用

对随机参数刚架结构在随机荷载作用下基于概率的优化设计进行了研究。同时考虑结构的物理参数和作用荷载等的随机性；建立以杆截面积为设计变量、结构质量均值极小化为目标函数、具有刚度和强度可靠性约束的优化设计数学模型；通过可靠性约束等价显式化处理和引入罚函数，将原概率约束优化问题转化为无约束优化问题，利用遗传算法求解。算例表明：文中提出的模型和方法是合理有效的。     

Benchmark case studies in structural design optimization using the force method

A structural optimization algorithm is developed for truss and beam structures under stress–displacement or frequency constraints. The algorithm combines the mathematical programming based on the Sequential Quadratic Programming (SQP) technique and the finite element technique based on the Integrated Force Method. A new approach based on the single value decomposition technique has been developed to derive the compatibility matrix required in the force method. Benchmark case studies illustrate the procedure and allow the results obtained to be compared with those reported in the literature. It is shown that the computational effort required by the force method is significantly lower than that of the displacement method and in some cases such as structural optimization problems with multiple frequency constraints, the analysis procedure (force or displacement method) significantly affects the final optimum design and the structural optimization based on the force method may result in a lighter design.     

基于网格退化和重组技术的类桁架拓扑优化方法

发展了一种基于有限元网格退化和重组技术的类桁架拓扑优化方法,可在不改变设计域包络的情况下(如维持流型、艺术外观和附加功能等)对结构进行减重。首先,分别对二维问题和三维问题发展相应的有限元网格退化算法,并在刚度等效的意义上对网格进行重组,建立了具有杆元拓扑特征的有限元模型。其次,以全局种子网格的长度尺寸和杆元横截面积为优化变量,构造了域内双层驱动拓扑优化问题,得到具有最优体分比的杆元拓扑结构。数值算例表明,所提方法可获得新型式的结构拓扑优化方案,并可将结构拓扑优化理论推向工程化应用。     

Computational aeroelastic investigation of a transonic limit-cycle-oscillation experiment at a transport aircraft wing model

Aeroelastic measurements of a three-dimensional wing model, the so-called Aerostabil wing, were conducted in the Transonic Windtunnel Göttingen. This clean, backward-swept wing allowed the experimental investigation of limit cycle oscillations in a certain transonic parameter range. In this paper, a detailed insight into the observed physical phenomena, especially the measured limit cycle oscillations, is presented by means of CFD–CSM coupled simulations. These simulations on the basis of a detailed structural finite element model reveal the specific properties of the Aerostabil wing and furthermore allow investigating the unstable behavior of this windtunnel model for transonic flow settings. The aerodynamic characteristics include a two-shock system and large flow separation areas, further increasing the complexity of the aeroelastic problem. A structural single degree-of-freedom system is used for the prediction of the experimental stability range and the limit cycle oscillation investigations. Due to the good agreement of simulation and experiment the limit cycle oscillations can be explained by means of nonlinear aerodynamic effects.