Numerical analysis of the effect of flaps on the tip vortex of a wind turbine blade

Two oscillating flaps located close to the tip and at mid span are used to excite the unstable modes of the tip vortex system of a wind turbine blade. The two flaps are deflected in opposite directions such that the root bending moment of the wind turbine blade remains almost unchanged. To investigate the mechanism of how and to what extent the deflection of the flaps influences the tip-vortex system, Large-Eddy Simulations in the Arbitrary Lagrangian-Eulerian formulation in a rotating frame of reference are performed at an averaged chord based Reynolds number of 300,000. Periodic boundary conditions are applied in the circumferential direction such that the flow over only one of the three blades of the wind turbine needs to be computed. A subsequent simulation of the trailing tip-vortex system is performed to analyze the evolution of the disturbed tip vortex. These simulations use a far-wake model based on the parameters obtained from the wind turbine simulation as inflow condition for the wake flow field. The comparison of the flow without and with oscillating flaps shows that the tip-vortex core is displaced by approximately 5% of the rotor radius by the flap motion. The root bending moment and torque at the root of the blade with flaps vary sinusoidally. Due to the compensation by the middle span flap, the difference of the root bending moment and torque is found to be less than 5% compared to the case without moving flaps. The simulations of trailing tip vortex show considerably earlier breakdown of the excited system, which proves the concept to excite instabilities in the vortex system by oscillating flaps successful.     

Supercritical and subcritical dynamic flow-induced instabilities of a small-scale wind turbine blade placed in uniform flow

There is a growing interest in extracting more power per turbine by increasing the rotor size in offshore wind turbines. As a result, the turbine blades will become longer and therefore more flexible, and a flexible blade is susceptible to flow-induced instabilities. In order to design and build stable large wind turbine blades, the onset of possible flow-induced instabilities should be considered in the design process. Currently, there is a lack of experimental work on flow-induced instabilities of wind turbine blades. In the present study, a series of experiments were conducted and flow-induced instabilities were observed in wind turbine blades. A small-scale flexible blade based on the NREL 5 MW reference wind turbine blade was built using three-dimensional printing technique. The blade was placed in the test section of a wind tunnel and was subjected to uniform oncoming flow, representing the case of a parked wind turbine blade. The blade׳s tip displacement was measured using a non-contacting displacement measurement device as the oncoming wind speed was increased. At a critical wind speed, the blade became unstable and experienced limit cycle oscillations. The amplitude of these oscillations increased with increasing wind speed. Both supercritical and subcritical dynamic instabilities were observed. The instabilities were observed at different angles of attack and for blades both with and without a geometric twist. It was found that the blade twist had a significant influence on the observed instability: a blade without a twist experienced a strong subcritical instability.     

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.     

基于CFD技术的垂直轴风力机动态尾流特性研究

利用CFD软件对麦克马斯特大学垂直轴风力机进行不同叶尖速比下的数值模拟,计算结果与风洞试验数据吻合良好。近场尾流中,与单叶片的风力机模拟结果比较,上游叶片产生并向下游延伸的旋涡影响下游运行轨道上叶片的升阻力特性,不仅使叶片扭矩输出峰值降低,而且峰值产生的时间延迟。对垂直轴风力机叶片叶梢进行修改,模拟结果显示,叶片扭矩输出峰值不变,但是谷值有所降低,修改后风力机沿风向推力幅值降低明显;远场尾流中,采用风速轮廓线原理,以瑞典的法尔肯贝里市200kW垂直轴风力机为原型,按照真实的空间排布进行数值模拟。模拟结果显示,上游风力机上下两端处产生较为集中的远场尾流,影响下游风力机叶片下半段的气动性能,下游风力机功率输出降低明显。     

基于仿生设计的风力发电机叶片力学性能的实验研究

根据风力机的基本理论和相似理论设计了一个翼型为SG6050,半径为1m的小型风力机叶片。运用结构仿生学原理,对所设计的风力机叶片进行了仿生物中轴铺层设计。通过模态实验与应变实验,比较了传统设计与仿生设计两种不同风力机叶片的力学性能。模态实验结果表明,基于仿生设计的叶片的前六阶固有频率比传统叶片的前六阶固有频率减少约8％;两种叶片的固有频率均满足设计要求;仿生设计的叶片几乎不会改变叶片的动态特性。而应变实验表明,仿生设计的叶片在各种工况下的应变均大于传统的叶片约10％～20％。新设计的叶片具有较好的柔性,有效减小了叶片的应力,提高了叶片的疲劳寿命。     

Progresses in application of computational fluid dynamic methods to large scale wind turbine aerodynamics

The computational fluid dynamics (CFD) methods are applied to aerodynamic problems for large scale wind turbines. The progresses including the aerodynamic analyses of wind turbine profiles, numerical flow simulation of wind turbine blades, evaluation of aerodynamic performance, and multi-objective blade optimization are discussed. Based on the CFD methods, significant improvements are obtained to predict two/three dimensional aerodynamic characteristics of wind turbine airfoils and blades, and the vortical structure in their wake flows is accurately captured. Combining with a multi-objective genetic algorithm, a 1.5 MW NH-1500 optimized blade is designed with high efficiency in wind energy conversion.     

Improved non-dominated sorting genetic algorithm （NSGA）-II in multi-objective optimization studies of wind turbine blades

The non-dominated sorting genetic algorithm (NSGA) is improved with the controlled elitism and dynamic crowding distance. A novel multi-objective optimization algorithm is obtained for wind turbine blades. As an example, a 5 MW wind turbine blade design is presented by taking the maximum power coefficient and the minimum blade mass as the optimization objectives. The optimal results show that this algorithm has good performance in handling the multi-objective optimization of wind turbines, and it gives a Pareto-optimal solution set rather than the optimum solutions to the conventional multiobjective optimization problems. The wind turbine blade optimization method presented in this paper provides a new and general algorithm for the multi-objective optimization of wind turbines.     

Numerical Simulation of the 3D Laminar Viscous Flow on a Horizontal-axis Wind Turbine Blade

The aim of this paper is to describe the methodology followed in order to determine the viscous effects of a uniform wind on the blades of small horizontal-axis wind turbines that rotate at a constant angular speed. The numerical calculation of the development of the three-dimensional boundary layer on the surface of the blades is carried out under laminar conditions and considering flow rotation, airfoil curvature and blade twist effects. The adopted geometry for the twisted blades is given by cambered thin blade sections conformed by circular are airfoils with constant chords. The blade is working under stationary conditions at a given tip speed ratio, so that an extensive laminar boundary layer without flow separation is expected. The boundary layer growth is determined on a non-orthogonal curvilinear coordinate system related to the geometry of the blade surface. Since the thickness of the boundary layer grows from the leading edge of the blade and also from the tip to the blade root, a domain transformation is proposed in order to solve the discretized equations in a regular computational 3D domain. The non-linear system of partial differential coupled equations that governs the boundary layer development is numerically solved applying a finite difference technique using the Krause zig-zag scheme. The resulting coupled equations of motion are linearized, leading to a tridiagonal system of equations that is iteratively solved for the velocity components inside the viscous layer applying the Thomas algorithm, procedure that allows the subsequent numerical determination of the shear stress distribution on the blade surface.     

Aeroelastic analysis of a rotating wind turbine blade using a geometrically exact formulation

In this paper, an aeroelastic analysis of a rotating wind turbine blade is performed by considering the effects of geometrical nonlinearities associated with large deflection of the blade produced during wind turbine operation. This source of nonlinearity has become more important in the dynamic analysis of flexible blades used in more recent multi-megawatt wind turbines. The structural modeling, involving the coupled edgewise, flapwise and torsional DOFs, has been performed by using a nonlinear geometrically exact beam formulation. The aerodynamic model is presented based on the strip theory, by applying the principles of quasi-steady and unsteady airfoil aerodynamics. Compared to the conventional steady aerodynamic model, the presented model offers a more realistic consideration of fluid–structure interactions. The resulting governing equation, expanded up to the third-order terms, is analyzed by using the reduced-order model (ROM). The ROM is developed by employing the coupled mode shapes of a cantilever blade under free loading condition. The specifications of the 5MW-NREL wind turbine are used in the simulation study. After verifying the ROM results by comparing them with those of the full FEM model, the model is used in additional static, modal and transient dynamics analyses. The results indicate the important effect of geometrical nonlinearity, especially for larger structural deformations. Moreover, nonlinear analyses reveal the important effects of torsion induced by lateral deformations. It is also found that the governing equation is more efficient, and sufficiently accurate, when it is developed by using the second-order kinetic terms, third-order potential terms and the second-order aerodynamic terms together with third-order damping. Finally, the effects of nonlinearities on the flutter characteristics of wind turbine blades are evaluated through frequency and dynamic analyses.     

Nonlinear modeling of tuned liquid dampers (TLDs) in rotating wind turbine blades for damping edgewise vibrations

Tuned liquid dampers (TLDs) utilize the sloshing motion of the fluid to suppress structural vibrations and become a natural candidate for damping vibrations in rotating wind turbine blades. The centrifugal acceleration at the tip of a wind turbine blade can reach a magnitude of 7–8g. This facilitates the use of a TLD with a relatively small fluid mass and with feasible geometric dimensions to mitigate the lightly-damped edgewise vibrations effectively. In the present paper, modal expansions are carried out directly on the velocity field and the free surface of the sloshing liquid in the rotating coordinate system. A formulation has been proposed leading to coupled nonlinear ordinary differential equations, which have been obtained through the Galerkin variational approach together with the modal expansion technique. Two models, with one sloshing mode and three sloshing modes, have been studied in the numerical simulation. It is shown that the one-mode model is able to predict the sloshing force and the damped structural response accurately, since the primary damping effect on the structure is achieved by the first sloshing mode of the fluid. Although it is unable to predict the fluid free-surface elevation equally well, the one-mode model can still be utilized for the design of TLD. Parametric optimization of the TLD is carried out based on the one-mode model, and the optimized damper effectively improves the dynamic response of wind turbine blades.     

风机叶片涂层雨蚀研究

研究了风电叶片在雨水冲击下的失效过程,利用有限元方法建立雨水冲击的数值模型,分析了雨滴的冲击角度、直径、涂层性能等对涂层受力的影响,并对双雨滴及四雨滴耦合模型进行了研究。结果表明:风雨耦合流场中叶片最大载荷在叶尖处,涂层的冲击受力与雨滴冲击角度和雨滴直径的大小呈正相关。低模量涂层能大幅降低涂层受冲击时的拉伸应力,小幅增加其压缩应力。同一平面内的双雨滴耦合时,拉伸应力受影响的范围较大,压缩应力受影响的范围较小。不同高度差下的双雨滴耦合中,随着高度差的增大,材料拉伸和压缩应力都呈现先增后减的趋势,其中压缩应力随高度差的变化幅度明显大于拉伸应力。四雨滴耦合时压缩应力对间距的变化更加敏感。涂层失效的过程主要为:涂层表面拉应力疲劳产生微裂纹;液压渗透引起裂纹扩展,涂层质量减少,表面粗糙度增加;表面小范围破坏后形成的凹面结构促进涂层的失效。     

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。     

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.     

叶片转动对涡轮间隙内部流动影响的数值研究

机匣与叶片的相对转动是影响涡轮叶顶间隙流动的重要冈素之一.对LISA 1.5级轴流涡轮间隙内部流动的数值计算结果表明:叶片转动对涡轮间隙流动有阻塞作用.叶片静止时,由于阻塞作用消失,导致间隙入口速度增大,间隙流鼍增加,并且通过间隙的流体全部卷起形成间隙涡.同时在叶片顶部吸力面侧前缘、中部各形成一个间隙涡,使得间隙流动损失增加.而且转速下降会加剧动叶出口截面气流过偏/偏转不足现象.同时叶片静止时,间隙前部各个弦长截面内静压自间隙入口开始一直呈增加趋势,直到叶片尾缘附近截面,间隙截面内静压才逐渐稳定.     

Investigation of horizontal-axis wind turbine (HAWT) blade threedimensional rotational effect based on field experiments

Field experiments are performed on a two-bladed 33 kW horizontal-axis wind turbine (HAWT). The pressures are measured with 191 pressure sensors positioned around the surfaces of seven spanwise section airfoils on one of the two blades. Three-dimensional (3D) and two-dimensional (2D) numerical simulations are performed, respectively, on the rotor and the seven airfoils of the blade. The results are compared with the experimental results of the pressure distribution on the seven airfoils and the lift coefficients. The 3D rotational effect on the blade aerodynamic characteristics is then studied with a numerical approach. Finally, some conclusions are drawn as follows. From the tip to the root of the blade, the experimental differential pressure of the blade section airfoil increases at first and then decreases gradually. The calculated 3D result of the pressure distribution on the blade surface is closer to that of the experiment than the 2D result. The 3D rotational effect has a significant impact on the blade surface flow and the aerodynamic load, leading to an increase of the differential pressure on the airfoils and their lift coefficient than that with the 2D one because of the stall delay. The influence of the 3D rotational effect on the wind turbine blade especially takes place on the sections with flow separation.     

Experimental study of the effect of slotted blades on the Savonius wind turbine performance

This study investigates the effect of Reynolds number on the performance of Savonius wind turbine with slotted blades. The turbine performance investigation was based on the torque coefficient( Ct), power coefficient( Cp), and tip speed ratio( TSR). The experiment used two number of blade configuration, blade overlap ratio of 10%, 12.5% and 20%, slotted position of 15%, 20%, 25% and 35%, and also slotted gap width of 3 mm, 5 mm, 7 mm, and 9 mm. The wind speed carried out in this experiment are 5.94 m/s, 6.46 m/s, 6.99 m/s, and 7.27 m/s, which are generated from the fan blowers as a wind source. The Savonius turbine with 10% overlap ratio shows the best performance. The highest Cp obtained is 0.138 by the variation of a 3 mm gap with Re of 1.44 × 10~4 and 0.526 TSR.     

Application of bamboo laminates in large-scale wind turbine blade design

From the viewpoint of material and structure in the design of bamboo blades of large-scale wind turbine, a series of mechanical property tests of bamboo laminates as the major enhancement materials for blades are presented. The basic mechanical characteristics needed in the design of bamboo blades are briefly introduced. Based on these data, the aerodynamic-structural integrated design of a 1.5 MW wind turbine bamboo blade relying on a conventional platform of upwind, variable speed, variable pitch, and doubly-fed generator is carried out. The process of the structural layer design of bamboo blades is documented in detail. The structural strength and fatigue life of the designed wind turbine blades are certified. The technical issues raised from the design are discussed. Key problems and direction of the future study are also summarized.     

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.     

串列风力机尾流干扰的研究简

广义致动盘方法是通过引入体积力代替叶片的致动盘技术与三维Navier-Stokes 方程相结合来获得风力机周围流场信息的一种方法. 该方法避免了花费大量网格与计算资源去求解风力机叶片的附面层,从而可以把更多的网格与计算资源用于风力机尾流流场的模拟,非常适合用于风力机尾流流场的研究. 以NH1500风力机为计算模型,将常规CFD (computational fluid dynamics) 方法与广义致动盘方法计算所得的叶片载荷分布进行比较,以验证广义致动盘方法的可行性. 然后使用广义致动盘方法对风场中串列风力机进行数值模拟,研究串列风力机之间间距变化时,上游风力机产生的尾流对下游风力机的干扰影响.     

风速不确定性对风力机气动力影响量化研究

风力机气动力学一直是国内外研究的热点课题之一.目前相关研究大都是基于确定性工况条件, 但因风力机常年工作在自然来流复杂环境,风速随机波动致使风电系统呈现不确定性, 对电网稳定性带来巨大挑战,因此进行不确定风速条件下风力机气动力学研究具有重要意义.为揭示不确定性对风力机流场影响机理并明确其对气动力的影响程度,本文提出一种风力机不确定空气动力学分析方法,基于修正叶素动量理论和非嵌入式概率配置点法,建立水平轴风力机不确定性空气动力学响应模型; 以NREL Phase VI S809风力机叶轮为研究对象, 基于该模型提取风力机输出随机响应信息,量化不确定风速对风力机风轮功率、推力、叶片挥舞弯矩和摆振弯矩的影响程度;通过分析流动诱导因子不确定性在叶片展长方向上的分布规律,揭示不确定因素在风力机本体上的传播机制,为风电系统设计及应用提供理论依据和重要参考. 结果表明,风速波动对风力机功率和气动力影响显著,高斯风速标准差由0.05倍增大至0.15倍均值,功率和推力最大波动幅度分别由13.44%和8.00%增大至35.11%和22.02%,叶片挥舞弯矩和摆振弯矩最大波动幅度分别由7.20%和12.84%增大至19.90%和33.49%.来流风速不确定性导致叶片根部位置气流明显波动,可以考虑在该部分采取流动控制措施降低叶片对风速不确定性的敏感程度.