风剪切对风力机叶片气动性能及尾迹形状的影响

风力机通常运行在非定常工况中,其气动性能及尾迹会随着工况的变化而变化. 风剪切是风力机长期所处的环境,它会影响到叶片气动载荷、尾迹形状、总体性能等,分析风剪切作用下的叶片气动性能对风力机的设计有重要意义.本文采用一种时间步进自由涡尾迹(free vortex wake, FVW)方法,耦合FVW方法与风剪切模型,计算不同风剪切因子作用下叶片的气动力系数、推力以及风轮后的尾迹形状变化,研究尾迹形状变化对风轮旋转平面诱导速度及风力机叶片气动性能的影响. 结果表明:在风剪切入流条件下, 随着风剪切因子的增大,风力机的气动力系数随时间做周期性波动的幅度加剧, 推力的平均值逐渐减小,尾迹倾斜程度增大, 尾迹在轮毂下方的倾斜程度更明显;尾迹形状的变化使风轮平面轴向诱导速度因子分布不均匀,同时使风力机的总体性能降低且偏离较大;倾斜尾迹相比于对称尾迹对风轮平面处的诱导影响有明显差别, 波动幅值增大,气动力系数在波谷处的偏差比波峰处大. 尾迹越倾斜,风轮旋转平面处的载荷不对称性越明显.      

一种风力机气动计算的全自由涡尾迹模型

采用全自由方式建立风力机尾流场的涡尾迹模型,引入“虚拟周期”的概念,并发展一种自适应松弛因子方法,从而改善了自由尾迹迭代的稳定性,提高了迭代收敛速度。利用建立的自由涡尾迹模型,计算了风力机叶片的尾流场结构、气动性能及叶片载荷,并与实验结果进行了对比分析。结果表明,尖速比越大,自适应松弛因子方法对缩小模型计算时间越有效;全自由涡尾迹模型能准确给出风力机尾流场的结构,包括尾迹的扩张以及叶尖涡和叶根涡的产生、发展和耗散的过程,风轮扭矩与实验数据吻合;叶片载荷分布的计算结果在低风速下与实验值基本一致,但是在大风速下差别较大,说明需要一个准确的失速模型。     

复杂工况下的大型风力机气动性能和尾迹研究Investigation of aerodynamic performance and wake of the large scale wind turbine in complicated conditions

在复杂工况下,大型风力机非定常特性会更严重,导致风力机气动性能变化和尾迹预测更加复杂。本文主要针对稳态偏航、动态偏航、风剪切和随机风速场等复杂工况,基于自由涡尾迹方法,嵌入复杂工况的模块,加入了动态失速模型和三维旋转效应模型修正,实现了复杂工况数值模拟计算,比较了不同复杂工况的气动载荷和尾迹形状。最后,得出了风力机在复杂工况下的气动性能、载荷和尾迹叶尖涡线特性,并计算出风力机在复杂工况下的气动载荷超调量和迟滞时间。对推进自由涡尾迹方法应用于风力机工程的大批工况载荷计算,提高大型风力机的载荷计算精度和设计水平等具有重要意义。     

复杂工况下的大型风力机气动性能和尾迹研究

在复杂工况下,大型风力机非定常特性会更严重,导致风力机气动性能变化和尾迹预测更加复杂。本文主要针对稳态偏航、动态偏航、风剪切和随机风速场等复杂工况,基于自由涡尾迹方法,嵌入复杂工况的模块,加入了动态失速模型和三维旋转效应模型修正,实现了复杂工况数值模拟计算,比较了不同复杂工况的气动载荷和尾迹形状。最后,得出了风力机在复杂工况下的气动性能、载荷和尾迹叶尖涡线特性,并计算出风力机在复杂工况下的气动载荷超调量和迟滞时间。对推进自由涡尾迹方法应用于风力机工程的大批工况载荷计算,提高大型风力机的载荷计算精度和设计水平等具有重要意义。     

单一水平轴风电机组尾迹的模拟方法与流动机理研究综述

为实现碳达峰、碳中和“3060”目标, 风能将在我国能源体系发挥重要作用. 风力机尾迹是影响风电性能和度电成本的关键因素, 需在风力机布置和控制设计中充分考虑. 本文首先介绍风力机尾迹的数值模拟方法, 包括解析模型、低阶模型、大涡模拟和来流湍流生成方法. 解析模型和低阶模型可快速计算风力机尾迹, 但依赖于模型参数, 且不能或不能准确预测尾迹湍流特性. 结合风力机参数化模型的大涡模拟可准确预测尾迹蜿蜒等湍流特征, 是流动机理研究的有力工具, 可为发展快速预测模型提供数据和理论支撑. 接着, 本文介绍了叶尖涡、中心涡和尾迹蜿蜒并讨论其产生机理. 对于湍流来流, 叶尖涡主要存在于近尾迹. 蜿蜒是远尾迹的主要特征, 影响下游风力机的来流特征. 尾迹蜿蜒的产生有两种机制: 来流大尺度涡和剪切层失稳. 数值和观测结果显示两种机制共同存在. 机舱和中心涡对尾迹蜿蜒有重要影响. 采用叶片和机舱的致动面模型可准确预测尾迹蜿蜒. 研究显示不同风力机尾迹间的湍流特征存在相似性, 为发展尾迹湍流的快速预测模型提供了理论依据. 当前研究多关注平坦地形上的风力机尾迹, 复杂地形和海洋环境下的大气湍流和风力机尾迹的机理复杂, 现有工程模型无法准确预测, 有待深入研究.      

自由涡尾迹方法中涡核尺寸对风力机气动计算的影响

涡核模型中的涡核尺寸对自由涡尾迹(free vortex wake,FVW)方法准确预估风力机气动特性至关重要,涡核尺寸包括初始涡核半径和由于耗散效应涡核半径在尾迹中的增长.FVW方法中涡线控制方程离散采用三步三阶预估校正格式,涡核模型采用经典Lamb-Oseen模型,并考虑了涡耗散效应和拉伸效应.首先,通过气动载荷和叶尖涡涡量平均值的分析得到初始涡核半径的取值范围;然后,根据叶尖涡耗散特性的分析,确定体现涡黏性耗散效应涡核半径增长的经验常数的取值;最后,分析了涡核尺寸对叶尖涡结构的影响,进一步验证初始涡核半径和涡黏性耗散经验常数的取值对风力机气动计算的影响.结果表明:当初始涡核半径大于50%弦长时,FVW方法收敛稳定且能准确预估风轮气动载荷;综合风轮气动载荷和叶尖涡耗散特性,初始涡核半径取60%到70%弦长为宜,且对应的涡黏性耗散经验常数取值也不同;风轮气动载荷和叶尖涡结构的准确预估主要受初始涡核半径影响,经验常数对其影响不大,而经验常数主要影响风轮下游尾流场叶尖涡的耗散特性.      

自由涡尾迹方法中涡核尺寸对风力机气动计算的影响

涡核模型中的涡核尺寸对自由涡尾迹(free vortex wake, FVW)方法准确预估风力机气动特性至关重要,涡核尺寸包括初始涡核半径和由于耗散效应涡核半径在尾迹中的增长. FVW方法中涡线控制方程离散采用三步三阶预估校正格式,涡核模型采用经典Lamb-Oseen模型,并考虑了涡耗散效应和拉伸效应.首先,通过气动载荷和叶尖涡涡量平均值的分析得到初始涡核半径的取值范围;然后,根据叶尖涡耗散特性的分析,确定体现涡黏性耗散效应涡核半径增长的经验常数的取值;最后,分析了涡核尺寸对叶尖涡结构的影响,进一步验证初始涡核半径和涡黏性耗散经验常数的取值对风力机气动计算的影响.结果表明:当初始涡核半径大于50%弦长时,FVW方法收敛稳定且能准确预估风轮气动载荷;综合风轮气动载荷和叶尖涡耗散特性,初始涡核半径取60%到70%弦长为宜,且对应的涡黏性耗散经验常数取值也不同;风轮气动载荷和叶尖涡结构的准确预估主要受初始涡核半径影响,经验常数对其影响不大,而经验常数主要影响风轮下游尾流场叶尖涡的耗散特性.     

均匀、振荡及组合流绕平板流动的尾迹研究

本文利用离散涡模型及改进的新生涡产生机制对三种不同来流绕平板的近尾迹进行数值研究。计算讨论了定常流中平板绕流流动的总体特性和近尾迹流场;对于简谐振荡来流,相应于K_c=2.0、4.0 和10.0 分别得到两种不同的尾迹形态。给出了小 K_c 数平板尾迹涡配对、运动的新模式而相应的阻力、惯性力系数计算比以前涡模拟结果更接近于 U 型管实验结果。对于流向组合来流本文模拟了涡锁定及其动力特性并于实验相符,给出了流向扰动对平板绕流流动的影响。     

基于Schmitz理论的风叶气动设计研究

应用Schmitz理论进行叶片气动设计,考虑了风力机叶片的气动损失,用Schmitz理论推导出风力机叶片的基本设计参数的计算公式,并考虑了风力机在启动和空载时风力机的实际工作点偏离了设计点,对叶片的气动性能参数进行了修正。通过对200kW风力机的算例表明:随着叶片半径的增大,入流角逐渐减小;叶片弦长先增大后减小,修正后得到的风力机在非设计点处的推力、驱动力矩、功率与实际风力机的特性规律相符。     

大型海上风力机尾迹区域风场分析

随着世界范围内海上风电场的不断兴起及海上风电场设计规模的日趋庞大,大型海上风力机尾迹区域风场特征的研究对于海上风电场的优化布局有着重要的指导意义。本文基于三维Navier-Stocks控制方程和适用于旋转流场分析的RNGk-ε湍流模型,采用滑移网格技术对美国可再生能源实验室(NREL)的5 MW海上风力机的性能及其尾迹区域的风场特征进行了较为系统的数值模拟。通过将不同风速下风力机输出功率的数值结果与NREL的设计参考数据进行对比,三维数值模型的有效性得到了很好地验证。此外,在此基础上进一步研究了大型海上风力机额定风速下及不同风轮转速下尾迹区域平均风速的分布特征,并得到了一系列具有参考价值的重要结论。     

Aeroelastic analysis of large horizontal wind turbine baldes

A nonlinear aeroelastic analysis method for large horizontal wind turbines is described. A vortex wake method and a nonlinear finite element method (FEM) are coupled in the approach. The vortex wake method is used to predict wind turbine aerodynamic loads of a wind turbine, and a three-dimensional (3D) shell model is built for the rotor. Average aerodynamic forces along the azimuth are applied to the structural model, and the nonlinear static aeroelastic behaviors are computed. The wind rotor modes are obtained at the static aeroelastic status by linearizing the coupled equations. The static aeroelastic performance and dynamic aeroelastic responses are calculated for the NH1500 wind turbine. The results show that structural geometrical nonlinearities significantly reduce displacements and vibration amplitudes of the wind turbine blades. Therefore, structural geometrical nonlinearities cannot be neglected both in the static aeroelastic analysis and dynamic aeroelastic analysis.     

火星科学实验室气动特性数值分析

针对火星着陆探测器进入-下降-着陆过程的高超声速进入阶段,求解三维流体动力学Navier-Stokes 方程与化学反应动力学模型,分析火星科学实验室进入火星大气时的化学非平衡效应、探测器周围的流场结构和气动特性在化学非平衡效应影响下的变化. 结果表明,CO₂ 在激波后大量分解,消耗大量能量;在化学非平衡效应影响下,探测器头部激波脱体距离大幅减小,尾迹旋涡运动减弱;化学非平衡效应影响下探测器升力系数变化不大,阻力系数高于完全气体,升阻比略低,配平攻角小于完全气体.     

Lift enhancement of airfoil and tip flow control for wind turbine

Two techniques that improve the aerodynamic performance of wind turbine airfoils are described. The airfoil S809, designed specially for wind turbine blades, and the airfoil FX60-100, having a higher lift-drag ratio, are selected to verify the flow control techniques. The flow deflector, fixed at the leading edge, is employed to control the boundary layer separation on the airfoil at a high angle of attack. The multi-island genetic algorithm is used to optimize the parameters of the flow deflector. The results indicate that the flow deflector can suppress the flow separation, delay the stall, and enhance the lift. The characteristics of the blade tip vortex, the wake vortex, and the surface pressure distributions of the blades are analyzed. The vortex diffuser, set up at the blade tip, is employed to control the blade tip vortex. The results show that the vortex diffuser can increase the total pressure coefficient of the core of the vortex, decrease the strength of the blade tip vortex, lower the noise, and improve the efficiency of the blade.     

Dynamic wind loads and wake characteristics of a wind turbine model in an atmospheric boundary layer wind

An experimental study was conducted to characterize the dynamic wind loads and evolution of the unsteady vortex and turbulent flow structures in the near wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind tunnel. In addition to measuring dynamic wind loads (i.e., aerodynamic forces and bending moments) acting on the wind turbine model by using a high-sensitive force-moment sensor unit, a high-resolution digital particle image velocimetry (PIV) system was used to achieve flow field measurements to quantify the characteristics of the turbulent vortex flow in the near wake of the wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions in the wake flow, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the wake flow in relation to the position of the rotating turbine blades. The effects of the tip-speed-ratio of the wind turbine model on the dynamic wind loads and wake flow characteristics were quantified in the terms of the variations of the aerodynamic thrust and bending moment coefficients of the wind turbine model, the evolution of the helical tip vortices and the unsteady vortices shedding from the blade roots and turbine nacelle, the deceleration of the incoming airflows after passing the rotation disk of the turbine blades, the TKE and Reynolds stress distributions in the near wake of the wind turbine model. The detailed flow field measurements were correlated with the dynamic wind load measurements to elucidate underlying physics in order to gain further insight into the characteristics of the dynamic wind loads and turbulent vortex flows in the wakes of wind turbines for the optimal design of the wind turbines operating in atmospheric boundary layer winds.     

Influence of pitching angle of incidence on the dynamic stall behavior of a symmetric airfoil

This study presents the influence of pitch angle of an airfoil on its near-field vortex structure as well as the aerodynamic loads during a dynamic stall process. Dynamic stall behavior in a sinusoidally pitching airfoil is usually analyzed at low to medium reduced frequencies and with the maximum angle of attack of the airfoil not exceeding 25°. In this work, we study dynamic stall of a symmetric airfoil at medium to high reduced frequencies even as the maximum angle of attack goes from 25° to 45°. The evolution and growth of the laminar separation bubble, also known as a dynamic stall vortex, at the leading edge and the trailing edge are studied as the pitch cycle goes from the minimum to the maximum angle of attack. The effect of reduced frequencies on the vortex structure as well as the aerodynamic load coefficients is investigated. The reduced frequency is shown to be a bifurcation parameter triggering period doubling behavior. However, the bifurcation pattern is dependent on the variation of the pitch angle of incidence of the airfoil.     

Independent research and development progress in large-scale wind turbine blade with coordinated aerodynamics,structure, and load

Abstract According to the three key elements in blade design process, i.e., aerodynamic design, structure design, and load prediction, the independent research and development (R&D) progress of blade design is summarized and analyzed. The calculational fluid dynamics (CFD) method, the vortex method, and the blade element momentum method (BEM) are described. Based on the widely used BEM method, the solutions for the blade design in low-speed wind area are obtained. A brief overview of the traditional design and analysis methods based on beam models is given. The defects of these methods used for simulating the structure of large-scale composite blade are analyzed. The application progress of the finite element method (FEM) used in the blade structure analysis is shown. The effects of load prediction on the blades and entire wind turbine are introduced. The progress in load forecasting is described. With the analysis of the relationship among these three key elements, it is concluded that developing a blade optimization design system with coordinated aerodynamics, structure, and load will truly meet the requirement of high efficiency and low cost. The main directions for further study are pointed out, e.g., high efficiency and low load airfoils, structural nonlinear finite element analysis, aerodynamic structure coupling research, and establishing different design standards. The aim is to establish a blade R&D system suitable for the conditions of wind resources in China and promote the development of wind power in the country.     

A nonlinear model for aerodynamic configuration of wake behind horizontal-axis wind turbine

Determination of the aerodynamic configuration of wake is the key to analysis and evaluation of the rotor aerodynamic characteristics of a horizontal-axis wind turbine.According to the aerodynamic configuration, the real magnitude and direction of the onflow velocity at the rotor blade can be determined, and subsequently, the aerodynamic force on the rotor can be determined. The commonly employed wake aerodynamic models are of the cylindrical form instead of the actual expanding one. This is because the influence of the radial component of the induced velocity on the wake configuration is neglected. Therefore, this model should be called a "linear model". Using this model means that the induced velocities at the rotor blades and aerodynamic loads on them would be inexact. An approximately accurate approach is proposed in this paper to determine the so-called "nonlinear" wake aerodynamic configuration by means of the potential theory,where the influence of all three coordinate components of the induced velocity on wake aerodynamic configuration is taken into account to obtain a kind of expanding wake that approximately looks like an actual one. First, the rotor aerodynamic model composed of axial(central), bound, and trailing vortexes is established with the help of the finite aspect wing theory. Then, the Biot-Savart formula for the potential flow theory is used to derive a set of integral equations to evaluate the three components of the induced velocity at any point within the wake. The numerical solution to the integral equations is found,and the loci of all elementary trailing vortex filaments behind the rotor are determined thereafter. Finally, to formulate an actual wind turbine rotor, using the nonlinear wake model, the induced velocity everywhere in the wake, especially that at the rotor blade,is obtained in the case of various tip speed ratios and compared with the wake boundary in a neutral atmospheric boundary layer. Hereby, some useful and referential conclusions are offered for the aerodynamic computation and design of the rotor of the horizontal-axis wind turbine.     

Thrust augmentation of flapping airfoils in low Reynolds number flow using a flexible membrane

The unsteady aerodynamic thrust and aeroelastic response of a two-dimensional membrane airfoil under prescribed harmonic motion are investigated computationally with a high-order Navier–Stokes solver coupled to a nonlinear membrane structural model. The effects of membrane prestress and elasticity are examined parametrically for selected plunge and pitch–plunge motions at a chord-based Reynolds number of 2500. The importance of inertial membrane loads resulting from the prescribed flapping is also assessed for pure plunging motions. This study compares the period-averaged aerodynamic loads of flexible versus rigid membrane airfoils and highlights the vortex structures and salient fluid–membrane interactions that enable more efficient flapping thrust production in low Reynolds number flows.     

Simple and efficient numerical methods for vortex sheet motion with surface tension

We present two simple and efficient explicit methods for the vortex sheet with surface tension. The first one is the standard point vortex method, which has been known to be unstable in the presence of surface tension, due to spurious growth of waves of high modes. We show, for the first time, that the standard point vortex method is able to calculate the vortex sheet motion with surface tension by employing a Fourier filtering. The second method is a modification of the Pullin method using central differences for numerical differentiations. This method is more convenient to implement than other spectral methods and is free from the aliasing instability. We give a linear stability analysis for the numerical methods and show results for the long‐time evolution of the vortex sheet. We also propose a new redistribution procedure to control point clustering, by setting limits of minimum and maximum distances between neighboring points. This procedure is found to be very efficient for long‐time computations of the explicit methods. Copyright © 2013 John Wiley & Sons, Ltd.