大型风力机叶片模态性能及振动分析

为研究大型风力机叶片铺层参数对叶片动态性能的影响、防止叶片发生共振、减小叶片挠度、改善叶片结构力学性能和提高风力机安全性,建立了5 MW风力机叶片的有限元模型,通过改变铺层材料和铺层角度实现不同的叶片结构,并对成型叶片进行了模态分析;采用CFD方法获得叶片表面载荷,分析不同风速下不同铺层结构叶片振动性能,结果表明:复合材料铺层角度能影响叶片固有频率,叶片低阶振型以挥舞和摆振为主,高阶模态出现扭转;增加0°铺层纤维比例可提高低阶固有频率,45°铺层能提高叶片抗扭能力;叶片振动位移沿叶片展向呈非线性增长,风速越大叶片挠度越大;碳纤维可有效提高叶片固有频率,减小叶片挠度。     

基于ONERA模型的柔性叶片气弹稳定性研究北大核心CSCD

为了探究大型风力机柔性叶片在挥舞-摆振耦合作用下的气弹稳定性,文章基于ONERA非线性气动模型建立了包括二维翼型非线性气动升阻力方程及其挥舞-摆振耦合运动方程的气弹模型。利用该气弹模型计算得到NREL 5 MW风力机叶中段DU35-A17翼型在叶片变桨前后的挥舞、摆振变形量变化曲线,并与FAST计算结果进行比较,以验证气弹模型的准确性。结果表明:在额定工况时,叶片出现z轴正方向、y轴负方向的弯曲变形;风力机未变桨时,挥舞、摆振变形量会随风速增大而增大;叶片变桨后,挥舞、摆振变形量会比额定工况下的变形量有所减少。由于计算得到的挥舞、摆振变形量曲线是收敛的,故叶片是气弹稳定的。该气弹模型为评估大型风力机柔性叶片气弹稳定性提供了新方法,计算得到的挥舞摆振位移数据为优化风力机结构参数、提升叶片气弹稳定性提供了数据参考。     

基于实测风场的沿海风力机叶片流固耦合特性分析

采用无人机测风手段实测获得某沿海风电场的风场特征，将所测量的风场参数应用到后续数值模拟中。以某2 MW风力机叶片全尺寸模型为研究对象，基于Workbench平台建立流固耦合系统进行计算，通过对比近尾流风速分布的数值模拟结果与实测结果来验证数值计算的准确性，进而计算额定风速下旋转叶片的挥舞变形和等效应力等响应。结果表明：风力机叶片的挥舞变形在0°方位角时最大，最大变形为1916.4 mm，接近静力加载试验最大挥舞工况结果 2299 mm；相同方位角下的挥舞变形沿着叶片展向呈非线性增大，0.60倍风轮半径处变形增长速度明显提高。叶片等效应力集中区位于大梁和前缘侧交界处，最大值出现在60°方位角下0.78倍风轮半径处，同时在风轮高度方向上呈非对称分布。     

水平轴风力机塔架的频率和振型特性实验研究

为了研究风力机塔架的振动特性,文章利用动态信号采集分析系统,对水平轴风力机塔架进行了实验模态分析和运行模态分析测试,得到了塔架静止与振动两种工况下的固有频率与模态振型,分析了塔架的振动特性。通过对风力机振动信号的频谱分析发现,风速小于10 m/s时,只能激励起塔架挥舞方向与摆振方向的二阶模态;通过对风力机塔架的模态分析发现,风力机发生振动,塔架固有频率与模态振型发生小幅度改变;随着风速和振动烈度的增大,塔架模态参数的变化幅度随之增大。该研究可以为风力机塔架优化设计提供借鉴。     

风电场风速对风机叶片性能影响的模拟分析

建立了风机叶片的三维数学模型,采用计算流体动力学软件Fluent对风机叶片流场进行了模拟分析,通过调用Static Structural模块进行叶片形变分析,并在Workbench平台下实现了流固耦合.基于该方法研究了不同风速下的流场分布情况和叶片变形情况.结果表明:本文计算值与文献[14]中的测量值吻合较好,两者最大误差不超过5%,验证了本文数学模型和计算方法的正确性;随着风速的增大,叶轮表面的速度值也在增大,气动压强分布的不均匀性更加明显;当风速由5m/s增大至12.5m/s时,叶片的最大形变量增大了0.91m,叶片形变量与风速呈非线性变化关系;挥舞是叶片的主要振动形式.     

风力机叶片动态特性与分形维数关系研究

为探究风力机叶片动态特性与分形维数之间的关系,基于湍流风谱模型Von Karman和NWTCUP生成2种风场,通过多体动力学软件FAST仿真出不同风速条件下的风力机叶片挥舞和摆振时域动态响应,采用计盒维数法求解其分形维数。用发电机输出功率验证分形维数描述风力机叶片动态特性的可行性。结果表明:挥舞时域动态响应的分形维数随风速的增大逐渐减小,而摆振则相反,当达到额定功率时挥舞和摆振的分形维数变化都较小;相同风速下不同湍流风谱模型的功率虽几乎相同,但其对应的分形维数却不同,表明不同的湍流风谱模型有其特定的分形维数。     

高风速及风突变对风力机柔性部件振动特性研究

风力机运行在复杂多变的自然环境之中,风是影响风力机气动特性和振动特性的最直接因素,高风速及风速突变将诱发风力机更强的气动载荷。为探究风力机柔性部件在高风速及突变湍流风作用下的振动特性,以NREL(美国国家能源部可再生能源实验室)实测数据为湍流风数据源,并添加相干结构描述风速突变,以NREL 1.5 MW近海桩柱式风力机为样机,建立基于Kane方法的风力机结构动力学模型,并使用假设模态离散化方法对其进行柔性化,而后将该模型与风场和气动力模型一起组成气-弹相互耦合系统动力学模型,分别研究了风力机叶片和塔架的结构动力学响应。结果表明:相干结构的添加可使基础湍流风具有更大的风突变以及更高的湍流强度;额定风速附近,叶尖位移体现为挥舞,切出风速附近,叶尖位移同时体现为挥舞和摆振;相干结构的添加使得叶片和塔架振动加速度成倍增加。     

基于气动弹性理论的风力机叶片耦合分析

针对风力机叶片,建立其结构动力学方程,推导分析了叶片旋转所产生的振动速度及其对来流的影响。基于BEM(Blade Element Momentum)理论,在风力机空气动力学基础上,建立了风力机的气动耦合分析模型。应用该模型,对某2MW风力机进行了计算分析,得到了叶片在额定工作风速下的振动变形、速度、加速度以及叶片沿展向的变形和载荷分布。充分考虑叶片的结构振动特性与来流风速的耦合效应,使得风力机空气动力学特性模型更加准确,对于风力机的设计和分析具有重要意义。     

随机风载荷下大型风力机叶片/机舱/塔架耦合动力学分析

由于风力机叶片所受风力来流的随机性和风力机结构的复杂性,大型风力机在随机风载荷下的动力学行为分析一直是风电行业急需解决的难题之一。利用MATLAB/Simulink对随机风速进行了模拟,通过柔性多体动力学方法建立了符合实际的风力机叶片/机舱/塔架耦合动力学方程。在随机风载荷下对目前国内1.5 MW主流风力机的叶片、塔架的动力学行为进行了实例分析,得到了10 min时序随机风载下的叶片挥舞位移、速度历程和塔架的位移、速度历程。分析结果表明,在随机风载下,风力机启动时叶片、塔架振动较为剧烈,随时间的增加叶片、塔架振动幅度逐渐减小,振动速度也呈减小趋势。该研究结果为我国风力机设计理论的完善和工程实践奠定了一定的基础。     

自适应垂直轴风力机翅叶夹角优化的数值模拟

在传统的Savonius型风力机的基础上,提出了一种具有自适应功能的柔性叶片垂直轴风力机,该风力机在达到一定风速时,会通过减小叶片的迎风面积减小受力,当风速降低时,又可以增大叶片迎风面积,获取更多风能,同时该风力机具有阻力型风力机的优点——启动力矩大,在低风速下便可启动。文章对该风力机进行简化,运用计算流体动力学对风力机进行三维数值计算,分析不同翅叶夹角在不同尖速比下风力机的转矩特性,从而得到最佳夹角,并分别对不同夹角工况下的压力场进行分析。结果表明,在翅叶夹角为15°时,风轮的转矩特性最好。     

Power,structural and noise performance tests on different wind turbine rotor blade designs

Most blades available for commercial-grade wind turbines incorporate a straight, span-wise profile and airfoil-shaped cross-sections. These blades are found to be very efficient at low and medium wind speeds compared with the potential energy that can be extracted. This paper explores the possibility of increasing the efficiency of the blades by modifying the blade design to incorporate a swept edge. The design intends to maintain efficiency at low to medium wind speeds by selecting the appropriate orientation and size of the airfoil cross-sections based on an oncoming wind speed and given constant rotation rate. The torque generated from a blade with straight-edge geometry is compared with that generated from a blade with a swept edge as predicted by CFD simulations. To validate the simulations, the experimental curve of the NTK500/41 turbine using LM19.1 blades is reproduced using the same computational conditions. In addition, structural deformations, stress distributions and structural vibration modes are compared between these two different turbine blade surfaces.     

切出风速下风力机变桨故障叶片气动特性及准静态结构响应分析

为探究大型水平轴风力机达到切出风速停机后变桨故障叶片的气动特性及准静态结构响应,基于计算流体力学方法对NREL 5 MW风力机变桨故障/成功叶片气动侧状态进行分析,并利用双向弱流固耦合及曲屈分析对典型方位角下变桨故障叶片展开研究。结果显示：切出风速下变桨故障叶片挥舞力矩平均值为变桨成功叶片的13.8倍,且前者的流场尾迹更为明显。此外,180°方位角变桨故障叶片较之0°方位角变桨故障叶片应力及叶尖位移分别减小29.8%和32.7%,一阶屈曲因子增加20.2%。     

The concept of a smart wind turbine system

A smart wind turbine concept with variable length blades and an innovative hybrid mechanical-electrical power conversion system was analyzed. The variable length blade concept uses the idea of extending the turbine blades when wind speeds fall below rated level, hence increasing the swept area, and thus maintaining a relatively high power output. It is shown for a typical site, that the annual energy output of such a wind turbine that could double its blade length, could be twice that of a corresponding turbine with fixed length blades. From a cost analysis, it is shown that the concept would be feasible if the cost of the rotor could be kept less than 4.3 times the cost of a standard rotor with fixed length blades. Given the variable length blade turbine system exhibits a more-or-less linear maximum power curve, as opposed to a non-linear curve for the standard turbine, an innovative hybrid mechanical-electrical power conversion system was proposed and tested proving the feasibility of the concept.     

Quantification of wind turbine energy loss due to leading-edge erosion through infrared-camera imaging,numerical simulations,and assessment against SCADA and meteorological data

Quantification of the performance degradation on the annual energy production (AEP) of a wind farm due to leading-edge (LE) erosion of wind turbine blades is important to design cost-effective maintenance plans and timely blade retrofit. In this work, the effects of LE erosion on horizontal axis wind turbines are quantified using infrared (IR) thermographic imaging of turbine blades, as well as meteorological and SCADA data. The average AEP loss of turbines with LE erosion is estimated from SCADA and meteorological data to be between 3% and 8% of the expected power capture. The impact of LE erosion on the average power capture of the turbines is found to be higher at lower hub-height wind speeds (peak around 50% of the turbine rated wind speed) and at lower turbulence intensity of the incoming wind associated with stable atmospheric conditions. The effect of LE erosion is investigated with IR thermography to identify the laminar to turbulent transition (LTT) position over the airfoils of the turbine blades. Reduction in the laminar flow region of about 85% and 87% on average in the suction and pressure sides, respectively, is observed for the airfoils of the investigated turbines with LE erosion. Using the observed LTT locations over the airfoils and the geometry of the blade, an average AEP loss of about 3.7% is calculated with blade element momentum simulations, which is found to be comparable with the magnitude of AEP loss estimated through the SCADA data.     

Classical flutter analysis of composite wind turbine blades including compressibility

For wind turbine blades with the increased slenderness ratio, flutter instability may occur at lower wind and rotational speeds. For long blades, at the flutter condition, relative velocities at blade sections away from the hub center are usually in the subsonic compressible range. In this study, for the first time for composite wind turbine blades, a frequency domain classical flutter analysis methodology has been presented including the compressibility effect only for the outboard blade sections, which are in the compressible flow regime exceeding Mach 0.3. Flutter analyses have been performed for the baseline blade designed for the 5‐MW wind turbine of NREL. Beam‐blade model has been generated by making analogy with the structural model of the prewisted rotating thin‐walled beam (TWB) and variational asymptotic beam section (VABS) method has been utilized for the calculation of the sectional properties of the blade. To investigate the compressibility effect on the flutter characteristics of the blade, frequency and time domain aeroelastic analyses have been conducted by utilizing unsteady aerodynamics via incompressible and compressible indicial functions. This study shows that with use of compressible indicial functions, the effect of compressibility can be taken into account effectively in the frequency domain aeroelastic stability analysis of long blades whose outboard sections are inevitably in the compressible flow regime at the onset of flutter.     

Damping of edgewise vibration in wind turbine blades by means of circular liquid dampers

This paper proposes a new type of passive vibration control damper for controlling edgewise vibrations of wind turbine blades. The damper is a variant of the liquid column damper and is termed as a circular liquid column damper (CLCD). Rotating wind turbine blades generally experience a large centrifugal acceleration. This centrifugal acceleration makes the use of this kind of oscillatory liquid damper feasible with a small mass ratio to effectively suppress edgewise vibrations. A reduced 2‐DOF non‐linear model is used for tuning the CLCD attached to a rotating wind turbine blade, ignoring the coupling between the blade and the tower. The performance of the damper is evaluated under various rotational speeds of the rotor. A special case in which the rotational speed is so small that the gravity dominates the motion of the liquid is also investigated. Further, the legitimacy of the decoupled optimization is verified by incorporating the optimized damper into a more sophisticated 13‐DOF aeroelastic wind turbine model with due consideration to the coupled blade‐tower‐drivetrain vibrations of the wind turbine as well as a pitch controller. The numerical results from the illustrations on a 5 and a 10MW wind turbine machine indicate that the CLCD at an optimal tuning can effectively suppress the dynamic response of wind turbine blades. Copyright © 2015 John Wiley & Sons, Ltd.     

Load alleviation of wind turbines by yaw misalignment

Vertical wind shear is one of the dominating causes of load variations on the blades of a horizontal axis wind turbine. To alleviate the varying loads, wind turbine control systems have been augmented with sensors and actuators for individual pitch control. However, the loads caused by a vertical wind shear can also be affected through yaw misalignment. Recent studies of yaw control have been focused on improving the yaw alignment to increase the power capture at below rated wind speeds. In this study, the potential of alleviating blade load variations induced by the wind shear through yaw misalignment is assessed. The study is performed through simulations of a reference turbine. The study shows that optimal yaw misalignment angles for minimizing the blade load variations can be identified for both deterministic and turbulent inflows. It is shown that the optimal yaw misalignment angles can be applied without power loss for wind speeds above rated wind speed. In deterministic inflow, it is shown that the range of the steady‐state blade load variations can be reduced by up to 70%. For turbulent inflows, it is shown that the potential blade fatigue load reductions depend on the turbulence level. In inflows with high levels of turbulence, the observed blade fatigue load reductions are small, whereas the blade fatigue loads are reduced by 20% at low turbulence levels. For both deterministic and turbulent inflows, it is seen that the blade load reductions are penalized by increased load variations on the non‐rotating turbine parts. Copyright © 2013 John Wiley & Sons, Ltd.     

Erosion modelling on reconstructed rough surfaces of wind turbine blades

Numerical simulations of rain droplet impacts on real rough surfaces of leading edges of wind turbine blades are presented. The effect of rough blade surface conditions during liquid impacts on the stress distribution in the protective coating is studied. Realistic rough surfaces of wind turbine blades, obtained from 3D reconstruction of real blades with photogrammetry, as well as artificially generated rough surfaces were introduced into finite element models of the droplet/blade coating interaction. Stress distributions in the protective coating with rough and flat surfaces were studied and compared. The results of the simulations suggest that roughness on the surface of the blade leads to increased stresses in the protective coating.     

Localization of wind turbine noise using a microphone array in wind tunnel measurements

An experimental investigation on scaled wind turbine models in a wind tunnel with a microphone array is presented. Our study focuses on the localization and quantification of aerodynamic noise sources on rotating wind turbine blades with the aim of identifying the contributing factors that have an impact on the source spectra. Therefore, wind tunnel measurements were conducted for three different blade geometries (NACA 4412 shape, Clark-Y shape, and sickle shape), five pitch angles between ?2° and +8° and five wind velocities between 5 and 13 ms^??1. For the localization of rotating sound sources with a microphone array, a rotating beamforming method based on the acoustic ray method is used. The Clean-SC deconvolution method was used to improve the resolution of the acoustic sources, and integrated spectra were calculated for the individual blades. The sound sources were localized at the wind turbine blades and assigned to the leading edge and trailing edge subregions. The results show a high dependency on the sound source distribution and the source strength with regard to the observed one-third octave bands, wind velocity, and blade geometry. Hence, the localization of rotating sound sources with a microphone array is a suitable method for the development of wind turbine blades that emit less noise.     

Representation of wind turbine blade responses in power production load cases by linear mode shapes

The wind turbine industry is designing large MW size turbines with very long blades, which exhibit large deflections during their operational life. These large deflections decrease the accuracy of linear models such as linear finite element and modal‐based models, in which the structure is represented by linear mode shapes. The aim of this study is to investigate the competence of the mode shapes to represent the large blade responses in normal operation load cases. For this purpose, blade deflections are projected onto the linear modal space, swept by mode shape vectors. The projection shows the contribution of each mode and the projection error. The blade deflections are calculated by a nonlinear aero‐servo‐elastic solver for power production fatigue load cases with normal turbulence. The mode shapes are calculated at the steady‐state deflected blade position computed at different wind speeds. Three reference turbine blades are used in the study to evaluate the effects of various blade design parameters such as length, stiffness, mass, and prebend. The results show that although the linear mode shapes can represent the flapwise and edgewise deflections accurately, axial and torsional deflections cannot be captured with good accuracy. The geometric nonlinear effects are more apparent in the latter directions. The results indicate that the blade deflections occur beyond the linear assumptions.