弹性耦合对直升机复合材料桨叶稳定性的影响

研究了弹性耦合对复合材料桨叶动特性和气弹稳定性的影响, 所采用的结构模型考虑了剪切变形、剖面面外翘曲变形和复合材料弹性耦合。推导出同时考虑剪切和翘曲影响的小应变、中等变形梁的应变2位移关系,并构造出21 个自由度梁单元, 应用Hamilton 原理推导出桨叶运动的有限元方程。在此基础上, 对三种不同构型的复合材料桨叶进行固有频率计算和气弹稳定性分析。计算结果表明: 尽管复合材料弹性耦合对桨叶固有频率的影响非常小, 但却改变了固有振型分布, 使桨叶挥舞-摆振-扭转运动之间存在耦合; 弹性耦合对桨叶气弹稳定性有很大的影响; 正的挥舞2扭转耦合使得摆振一阶稳定性增加, 负的挥舞2扭转耦合却使摆振一阶稳定性下降。      

Particle swarm optimization approach for multi-objective composite box-beam design

This paper presents a multi-agent search technique to design an optimal composite box-beam helicopter rotor blade. The search technique is called particle swarm optimization (‘inspired by the choreography of a bird flock’). The continuous geometry parameters (cross-sectional dimensions) and discrete ply angles of the box-beams are considered as design variables. The objective of the design problem is to achieve (a) specified stiffness value and (b) maximum elastic coupling. The presence of maximum elastic coupling in the composite box-beam increases the aero-elastic stability of the helicopter rotor blade. The multi-objective design problem is formulated as a combinatorial optimization problem and solved collectively using particle swarm optimization technique. The optimal geometry and ply angles are obtained for a composite box-beam design with ply angle discretizations of 10°, 15° and 45°. The performance and computational efficiency of the proposed particle swarm optimization approach is compared with various genetic algorithm based design approaches. The simulation results clearly show that the particle swarm optimization algorithm provides better solutions in terms of performance and computational time than the genetic algorithm based approaches.     

A parametric study of skin laminates used in the composite layup of large-scale wind turbine blades

To reach an optimal design solution for the composite layup of large-scale wind turbine blades, subjected to various design load conditions, while, fulfilling numerous design requirements, is a challenging task to accomplish. Since, a large-scale blade is a slender beam structure, therefore, its thin composite layup can be assumed to be under plane stress condition. Consequently, a parametric study of the skin laminates used in the blade composite layup, is conducted to explore and identify the possible design improvements. The results show that the use of off-axis fiber angles of the skin laminate lower than the conventional 45° are more favorable to achieve higher laminate stiffness, strength, bucking stability, fatigue resistance, and bend-twist coupling value, thereby, demonstrating the potential improvements to the existing composite layup design of large-scale wind turbine blade.     

Impact damage tolerance of laminated composite helicopter blades

The paper is concerned with the study of the damage resistance of laminated composite helicopter blades subjected to impact loading. Dynamic stress intensity factors are determined for composite laminate and separate layers using combined theoretical and experimental approach. The effect of the projectile size on the damage tolerance of composite blade is investigated and tolerable sizes of the defects are estimated.     

GFRP风电叶片段结构强度三维有限元分析

叶片结构强度是决定风电机组寿命和全寿命周期度电成本的关键因素。本文选取美国SNL 100 m玻璃纤维增强树脂复合材料（GFRP）风电叶片,根据关键截面的几何和复合材料铺层构造建立了叶片段三维有限元实体模型。分析了叶片段结构的非线性屈曲、胶接界面脱粘及复合材料失效的耦合行为。结果表明:挥舞载荷下叶片段首先出现复合材料失效,然后是胶接界面脱粘,最后出现结构胶失效;而摆振载荷下,叶片段最先出现非线性屈曲,接着依次出现复合材料失效、胶接脱粘和结构胶失效,且尾缘屈曲形变是胶接脱粘的驱动因素。      

Prediction of material coupling effect on structural damping of composite beams and blades

The present paper investigates the effect of material coupling on static and modal characteristics of composite structures. Incorporation of stiffness and damping coupling terms into a beam formulation yields equivalent section stiffness and damping properties. Building upon the damping mechanics, an extended beam finite element is developed capable of providing the stiffness and damping matrices of the structure. Validation cases on beams and blades demonstrate the importance of all stiffness and damping terms. Numerical results validate the predicted effect of material coupling on static characteristics of composite box-section beams. The effect of the full coupling damping matrices on modal frequencies and structural modal damping of composite beams is investigated. Box-section beams and small blade models with various ply angle laminations at the girder segments are considered. Finally, the developed finite element is applied to the prediction of the modal characteristics of a 19 m realistic wind-turbine model blade.     

Development of a High‐fidelity Experimental Substructure Test Rig for Grid‐scored Sandwich Panels in Wind Turbine Blades

This paper outlines high‐fidelity experimental substructure testing of sandwich panels which constitute the aerodynamic outer shell of modern wind turbine blades. A full‐scale structural experimental and numerical characterisation of a composite wind turbine blade has been conducted. The development of a full‐scale numerical model is detailed, and the necessary experimental set‐up is described. Further, the numerical and experimental results obtained are compared, and an idealised set of boundary conditions for a chosen blade substructure is presented. From this, the development of a test rig suitable for representing the established loading and boundary conditions is presented, and some preliminary experimental results are discussed. The work provides a road map for developing high‐fidelity experimental substructure tests, which in more generic terms are applicable to similar developments of substructure tests for composite wind turbine blades. Furthermore, recommendations on the use of grid‐scored sandwich structures in wind turbine blades are presented, which outline the sensitivity in terms of quasi‐static strength to the established loading conditions.     

叶片局部损伤对大型水平轴风力机静动态特性影响的仿真分析

基于参数化整机模型研究风力机的静态和动态特性,以及叶片局部损伤对单叶片和整机的静动态特性影响规律,揭示叶片局部损伤与这些特性的关系。使用ANSYS的APDL语言,实现MW级水平轴风力机复合材料叶片的参数化建模;采用公共几何元素、连接单元和自由度耦合等方法,组建包含叶片、轮毂、塔架等部件的整机有限元模型。计算分析了重力、离心力和风压力共同作用下正常整机和包含叶片局部损伤的整机在迎风位置时的静态响应。比较了正常和损伤状态下根部固定单叶片和装入整机中叶片的动态应变响应。研究表明:对根部固定的单叶片,风轮面内的动态应变响应随迎风面损伤程度的增大而减小;对整机中的叶片,垂直风轮面的动态应变响应随迎风面损伤程度的增大而增大。研究方法和结论对大型水平轴风力机叶片损伤的在线监测具有重要 意义。     

大型水平轴风力发电机桨叶稳定性研究

大型水平轴风力发电机桨叶为流-刚-柔耦合的周期时变多体系统。本文暂未考虑风载荷,分析了重力载荷和桨叶预锥角、转速等因素的变化对稳定性的影响。力学建模中,考虑了桨叶挥舞、摆振、扭转和轴向运动以及根部铰的挥舞、摆振和变矩等刚体运动。利用有限元法形成5节点18自由度的刚-柔混合梁单元模型,应用Hamilton原理建立桨叶动力学方程,求得对应的摄动方程,采用Newmark隐式积分方法求解。根据Floquet理论判断运动稳定性,计算了相关转换矩阵的特征值。结果表明预锥角对桨叶运动稳定性影响不容忽视。在通常的工况下,桨叶能够稳定地运转。     

GFRP复合材料叶片摆振运动表面位移与层间断裂韧性响应

风机GFRP复合材料叶片摆振运动时产生的层间滑动裂纹是叶片破坏的主要诱因之一,应力强度因子K是层间断裂韧性的重要参量,也是表面裂缝安全性评估的重要指标之一。本文在试验基础上提出了由GFRP复合材料叶片表面位移推导K值的新方法,通过试验验证其理论正确性,试验与仿真对比证明了通过叶片摆振运动表面位移来研究层间断裂韧性响应的方法是可行的,为GFRP复合材料风机叶片的强度预测提供了新的思路和方法。     

Aeroelastic response and stability analysis of composite rotor blades in forward flight

The aeroelastic phenomena of a composite rotor blade are investigated using a finite element method. The aeroelastic equations of motion of the rotor blade are formulated using a large deflection-type beam model that has no artificial restrictions on the magnitudes of displacements and rotations due to the degree of nonlinearity. The sectional elastic constants of a composite box beam including warping deformations are determined from the refined cross-sectional finite element method. The nonlinear, periodic blade steady response is obtained using a time finite element method on full finite element equation. The aeroelastic stability of a rotor blade is investigated by linearizing the dynamics of the rotor blade about the nonlinear equilibrium position. Numerical results of the steady equilibrium deflections and the aeroelastic modal damping are presented for various configurations of composite rotor blades and are compared with those obtained from a previously published modal analysis using a moderate deflection-type beam theory.     

A coupled aeroelastic damage progression model for wind turbine blades

The prediction of damage progression in composite wind turbine blades, especially under dynamic aeroelastic conditions, is usually a cumbersome multi-step process with significant manual user intervention. In this paper a novel approach is presented where the different components of this process – dynamical structural analysis under varying aerodynamic and deterministic loads, and damage progression – are integrated into one reduced-order model capable of predicting the occurrence and progression of damage in real time. Key to this integration is the use of an effective one-dimensional model of the turbine blade known as thin-wall beam model, which allows for the reconstruction of a three-dimensional stress field of a volume given by the blade. This stress field can then be used to assess damage and locally modify the structural properties to account for the presence of damage, leading to a reduced load carrying capacity. The model was previously tested in its components, demonstrating a good agreement of the predicted structural and static damage progression behaviour compared to detailed high-order finite-element models of the same blade. Once validated, the model was applied to severe load cases and the potential for real-time predictions of damage progression was demonstrated.     

风力机叶片挥舞—摆振气弹稳定性分析

根据Euler-Bernoulli梁理论和粘弹性材料的Kelvin-Voigt理论建立风力机叶片挥舞—摆振耦合非线性动力学方程。将位移视为静态位移和动态位移的叠加,进而将非线性动力学方程线性化为动态位移的线性方程,得到叶片耦合振动特征方程。使用基于加权残值的Galerkin方法求解特征方程,分析叶片气弹稳定性,讨论风速、安装角、耦合效应和材料阻尼对叶片颤振稳定性和非线性自激振动行为的影响。结果表明:摆振方向易出现不稳定振动,通过设置安装角,利用挥舞—摆振耦合可以控制不稳定振动,但当安装角太大时,挥舞—摆振耦合会引起不稳定振动。     

风力机复合材料柔性叶片的颤振分析

将风力机叶片简化为薄壁复合材料封闭截面弯扭耦合变形梁,基于Hamilton原理并结合变分渐进法（VAM）,建立风力机叶片的气动弹性力学模型。结构模型包括材料各向异性,截面翘曲,离心载荷,科里奥利加速度,以及预锥角和预扭转角的影响。气动载荷采用叶素动量理论和准定常气动力理论进行描述。将位移按广义坐标进行模态展开,采用Galerkin法导出系统的质量、刚度和阻尼矩阵,采用特征值技术进行叶片颤振性能的数值求解。针对周向反对称刚度配置（CAS）叶片进行数值近似计算,揭示了入流比、预扭转角和纤维铺层角等参数对风力机叶片颤振性能的影响。     

Effects of Bend–Twist Coupling on Composite Propeller Performance

An investigation of a conventional propeller, made from composite materials, was conducted in which propeller characteristics were studied under quasi-static aerodynamic loading. Emphasis was placed on understanding the effects of bend-twist coupling of composite laminates on propeller performance. The classical blade element theory of propellers was used to calculate propeller characteristics and the aerodynamic force distribution acting on the propeller. The finite-element method was used to calculate the resulting deformation of the propeller blades. A simple algorithm was used in which propeller blade deformation and resulting changes in loading conditions were calculated repeatedly until equilibrium between deformation and loading was achieved. Results show that improvements in propeller performance are possible using the bend-twist coupling of composite laminates. Additionally, this study shows that the numerical approach developed by the authors is well suited for analyzing the performance characteristics of composite propellers.     

A computational procedure for prebending of wind turbine blades

Prebending of wind turbine blades constitutes a viable engineering solution to the problem of tower clearance, that is, ensuring that during wind turbine operation there is sufficient distance between the rotor blades and the tower to avoid collision. The prebent shape of the blade must be such that when the turbine rotor is subjected to wind and inertial loads, the blades are straightened into their design configuration. In this paper, we propose a method for accurate prediction of the prebent shape of wind turbine blades. The method relies on a stand‐alone aerodynamics simulation that provides the wind loads on a rigidly spinning rotor, followed by a series of structural mechanics simulations to determine the stress‐free prebent shape of the blade. This procedure involves only one‐way coupling between the fluid and structural mechanics, which avoids the challenges of solving the coupled fluid–structure interaction problem. The proposed methodology, which has no limitations on the blade geometry and structural modeling, is successfully applied to prebending of a 63‐m offshore wind turbine blade. Copyright © 2011 John Wiley & Sons, Ltd.     

Local Buckling Prediction for Large Wind Turbine Blades

Local buckling is a typical failure mode of large scale composite wind turbine blades. A procedure for predicting the onset and location of local buckling of composite wind turbine blades under aerodynamic loads is proposed in this paper. This procedure is distinct from its counterparts in adopting the pressure distributions obtained from Computational Fluid Dynamics (CFD) calculations as the loads. The finite element method is employed to investigate local buckling resistance of the composite blade. To address the mismatch between the unstructured CFD grids of the blade surface and the finite shell elements used during the buckling analysis, an interpolation code is developed, allowing mapping the pressure computed by using CFD to the finite element model. With the well documented National Renewable Energy Laboratory phase VI wind turbine blade, the procedure is demonstrated to be capable of yielding satisfactory results. Comparison with results obtained by using the blade surface pressure distributions calculated using a simple method is also conducted.     

A study on structural design and analysis of small wind turbine blade with natural fibre(flax) composite

In recent years, there has been a growing interest in the use of naturally sourced fibres for use in composites design and manufacture. In this work, a structural design on 1-kW-class horizontal axis wind turbine blade using natural flax fibre composite is performed. The structural design results of flax/epoxy composite blade are compared with the design results of glass/epoxy composite blade. In order to evaluate the structural design of the composite blade, the structural analysis was performed by the finite element method. Through the structural analyses, it is confirmed that the designed blade using natural composite is acceptable for structural safety, blade tip deflection, structural stability, resonance possibility and weight.     

不同湍流模型下旋转叶片气固耦合动力学特性的研究

本文对轴流式压气机内旋转叶片在不同湍流模型的气固耦合作用下的动力特性进行了研究。利用FSI（fluid-structure interaction）的双向顺序耦合算法,采用CFX和ANSYS对压气机内气体和旋转叶片分别计算,并通过迭代求解,获得叶片在气流作用下的动力学响应。为了研究耦合场内对叶片动力特性的影响,本文对某一级转子叶片进行流固耦合计算,由于在模拟流场计算中,不同的湍流模型对计算结果影响较大,采用3种湍流模型：标准κ-ε湍流模型(standard κ-ε)、重正化群κ-ε湍流模型（renormalization group κ-ε,RNG κ-ε）和大涡湍流LES模型（Large eddy simulation）,获得了叶片在瞬态流场变化下的动力特性,并比较了不同湍流模型对计算的影响,为工程上对压气机内转子工作特性的分析提供一个可行的方法。     

Obituary: Johannes Boersma (1937–2004), a remarkable mathematician

Problems involving the modeling and free vibration of pre-twisted rotating blades made of functionally graded materials (FGMs) and operating in a high-temperature field are considered. The blade, mounted on a rigid hub, is modeled as a thin-walled beam that incorporates the warping restraint and the pre-twist effects. As a result of the latter feature, an extension-twist elastic coupling is induced. Consistent with the concept of the FGM structures, the two constituent materials, ceramic and metal, experience a continuous variation across the beam wall thickness, and, as a result, the adverse effects featured by the standard laminated structures, such as delamination/debonding, are precluded to occur. Numerical results highlighting the effects of the extension-twist elastic coupling considered in conjunction with the volume fraction of the two constituent phases and of the thermal degradation of material properties on eigenfrequencies are presented, and pertinent conclusions are outlined. Comparisons of predictions, as well as validations of results against those obtained in some special cases, which are available in the specialized literature, are also supplied. In addition to a better understanding of the implication of incorporation of FGMs, the results of this research can be instrumental toward the reliable design of advanced turbomachinery blades that operate in a high-temperature environment. Professor Liviu Librescu died a heroic death on April 16, 2007, while protecting the lives of his students in a Virginia Tech classroom. This paper was virtually accepted for publication with laudatory reviews two weeks before his death.