Measurements on a wind turbine wake: 3D effects and bluff body vortex shedding

The velocity field in the wake of a two‐bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two‐component hot wire. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the freestream direction (the yaw angle was varied from 0° to 20°). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side, showing the potential of controlling the wake by yawing the turbine. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low‐frequency fluctuation both in the wake and in the flow outside the wake. This fluctuation was found both with and without freestream turbulence and also with a yawed turbine. The frequency expressed as a Strouhal number was shown to be independent of the freestream velocity or turbulence level, but the low frequency was only observed when the tip speed ratio (or equivalently the drag coefficient) was high. The shedding frequency changed also with the yaw angle. This is in agreement with the idea that the turbine sheds structures as a bluff body. The phenomenon, noticeable in all the velocity components, was further investigated using two‐point cross‐correlations of the velocity signals. Copyright © 2005 John Wiley & Sons, Ltd.     

Parametric dependencies of the yawed wind‐turbine wake development

Yaw misalignment is currently being treated as one of the most promising methods for optimizing the power of wind farms. Therefore, detailed knowledge of the impact of yaw on the wake development is necessary for a range of operating conditions. This study numerically investigates the wake development behind a single yawed wind turbine operating at different tip‐speed ratios and yaw angles using the actuator‐line method in the spectral‐element code Nek5000. It is shown that depending on the tip‐speed ratio, the blade loading varies along the azimuth, resulting in a wake that is asymmetric in both the horizontal and vertical directions. Large tip‐speed ratios as well as large yaw angles are shown to decrease the vertical asymmetry of the yaw‐induced counter‐rotating vortex pair. Both parameters have the effect that they increase the spanwise force induced by yaw relative to the wake rotation. However, while the strength of the counter‐rotating vortex pair in the far wake increases with yaw angle, it is shown to decrease with the tip‐speed ratio. The vertical shift in the wake center is found to be highly dependent on the yaw angle and the tip‐speed ratio. These detailed insights into the yawed wake are important when optimizing potential downstream turbines.     

基于流固耦合偏航下风力机尾迹湍流特征的研究

偏航状态下风力机叶片与流场之间相互作用会导致风力机近尾迹流场的湍流特征变化,采用双向流固耦合对不同偏航工况下水平轴风力机近尾迹流场进行数值模拟研究,获得不同偏航角下尾迹湍流特征演化规律。结果表明：随着偏航角的增大,正偏航侧会出现“速度亏损圆环”,且此圆环的范围呈扩大趋势;偏航角的增大对叶根处速度亏损影响最大,对叶尖处速度亏损影响最小,与正偏航侧相比,负偏航侧的速度亏损值减为约1/2;随着偏航角的增大,正负偏航侧的湍流强度变化呈不对称性,正偏航侧对湍流耗散的影响程度较负偏航侧大;涡流黏度越来越小,且在偏航10°涡流黏度相对于偏航5°减小约1/2,沿着轴向叶尖涡的管状环涡结构变得不稳定,出现明显耗散,且在偏航15°之后涡结构的耗散破裂程度越来越剧烈,进而对风力机气动噪声产生较大影响。     

垂直轴风力机气动特性与涡脱落模态分析

垂直轴风力机运行过程中,叶片上下表面边界层与剪切层的相互作用使风力机下游尾迹形成周期性涡结构,这种尾迹涡结构对风力机空气动力学特性具有重要影响。基于此,该文采用计算流体力学方法对不同工况下垂直轴风力机尾迹涡结构展开研究,利用快速傅里叶变换与相空间轨迹分析不同尖速比下风力机叶片涡脱落现象和尾迹涡结构,并通过分形维数研究转矩与尾迹流场速度变化。结果表明：风力机尾迹涡结构随尖速比变化呈现不同特征,当尖速比为3.6时,风力机尾迹两侧呈规则性反向脱落涡模态;低尖速比垂直轴风力机尾迹具有明显的混沌特性,且随尖速比的增加混沌特性逐渐减弱;随着尖速比的增加,风力机转矩与下游速度分形维数不断降低,且当尖速比为3.6时,风力机下游速度分形维数仅为1.07。     

An experimental study on the effects of winglets on the tip vortex interaction in the near wake of a model wind turbine

An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction.     

水平轴风力机近尾迹流场结构的实验研究

在风洞开口实验段,应用PIV锁相周期采样技术测试风力机近尾迹速度场,通过分析速度场和涡量场,得到近尾迹流场结构特征。近尾迹中存在具有形态特征强烈的叶尖涡结构向下游不断传播。由风轮旋转轴向外,近尾迹的结构组成依次为中央尾迹区、叶尖涡诱导效应区和外部主流区。在叶尖涡诱导效应区内,涡流诱导效应使流场中存在明显的速度增益区和速度亏损区,且增益区和亏损区关于叶尖涡核中心对称。在研究区域内,叶尖涡向下游运动的轴向位移与尖速比成反比,径向位移与尖速比成正比,使叶尖涡诱导效应区影响范围随尖速比的增加径向扩展、轴向缩小。     

An experimental and computational assessment of blockage effects on wind turbine wake development

An experiment was conducted to evaluate the initial wake expansion in scaled wind turbine tests as a means to guide future wake interference studies. Five scaled wind turbine rotors with different diameters were designed for testing in a closed‐loop water channel to evaluate the effects of blockage on the initial wake expansion behind a wind turbine. The initial wake expansion was assessed by using quantitative dye visualization to identify the propagation of tip vortices downstream of the rotor. The thrust coefficient developed by the scaled models was recorded using a six‐component balance and was correlated to the downstream wake expansion. The rotors used in the experiment were operated at a tip speed ratio of 6, a Reynolds number based on the tip speed and tip chord of approximately 23,000 and resulted in blockage values that ranged from 6% to 25%. Dye visualization indicated that the initial wake expansion downstream of a rotor was narrowed and that tip vortex pairing behaviour was modified because of increasing blockage. Blockage effects were significant and resulted in a wake that was more than 50% narrower when blockage was 25% compared with the observed expansion with 10% blockage. A computational simulation was conducted with the Generalized Unsteady Vortex Particle (GENUVP) discrete vortex method code using the rotor in freestream conditions and was compared with the experiments. The magnitude of the wake expansion in the freestream computations was similar to the wake expansion in the experiment when blockage was less than 10%. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization of the unsteady flow in the nacelle region of a modern wind turbine

A three‐dimensional Navier–Stokes solver has been used to investigate the flow in the nacelle region of a wind turbine where anemometers are typically placed to measure the flow speed and the turbine yaw angle. A 500 kW turbine was modelled with rotor and nacelle geometry in order to capture the complex separated flow in the blade root region of the rotor. A number of steady state and unsteady simulations were carried out for wind speeds ranging from 6 m s^?1 to 16 m s^?1 as well as two yaw and tilt angles. The flow in the nacelle region was found to be highly unsteady, dominated by unsteady vortex shedding from the cylindrical part of the blades, which interacted with the root vortices from each blade, generating high‐velocity gradients. As a consequence, the nacelle wind speed and the nacelle flow angle were found to vary significantly with the height above the nacelle surface. The nacelle anemometry showed significant dependence on both yaw and tilt angles with yaw errors of up to 10 degrees when operating in a tilted inflow. Copyright © 2010 John Wiley & Sons, Ltd.     

Wind tunnel investigation on the effect of the turbine tower on wind turbines wake symmetry

In wind farms, the wake of the upstream turbines becomes the inflow for the downstream machines. Ideally, the turbine wake is a stable vortex system. In reality, because of factors like background turbulence, mean flow shear, and tower‐wake interaction, the wake velocity deficit is not symmetric and is displaced away from its mean position. The irregular velocity profile leads to a decreased efficiency and increased blade stress levels for the downstream turbines. The object of this work is the experimental investigation of the effect of the wind turbine tower on the symmetry and displacement of the wake velocity deficit induced by one and two in‐line model wind turbines (,D= 0.9 m). The results of the experiments, performed in the closed‐loop wind tunnel of the Norwegian University of Science and Technology in Trondheim (Norway), showed that the wake of the single turbine expanded more in the horizontal direction (side‐wall normal) than in the vertical (floor normal) direction and that the center of the wake vortex had a tendency to move toward the wind tunnel floor as it was advected downstream from the rotor. The wake of the turbine tandem showed a similar behavior, with a larger degree of non‐symmetry. The analysis of the cross‐stream velocity profiles revealed that the non‐symmetries were caused by a different cross‐stream momentum transport in the top‐tip and bottom‐tip region, induced by the turbine tower wake. In fact, when a second additional turbine tower, mirroring the original one, was installed above the turbine nacelle, the wake recovered its symmetric structure. Copyright © 2017 John Wiley & Sons, Ltd.     

水平轴风力机尾迹流场PIV实验研究

在水平轴风力机模型不同尖速比条件下,利用PIV粒子图像测速技术对风轮尾迹流场进行了测量。采用锁相平均测量技术,获得了风轮尾迹流场的瞬时速度场、时均速度场、涡量场等有关定量信息,为准确计算风力机的流场、载荷和气动特性等提供了依据。实验结果表明:风轮叶片尾缘后侧的尾迹中存在轴向速度亏损区。尾迹在叶片尾缘生成后,随即发生膨胀。直到风轮下游2倍弦长以后,尾迹低速区逐渐衰减,轴向速度不断增加,尾迹区同时发生收缩现象。风轮尾迹涡从叶片尾缘脱落后,在向下游发展传播过程中,尾迹涡的涡心所形成的运动轨迹是与风轮叶片旋转方向相反的螺旋线,涡量数值随着螺旋线向风轮下游的延伸而减小。由于风力机叶片数少,相邻叶片之间的尾迹基本上不存在互相干扰的现象。     

Application of a LES technique to characterize the wake deflection of a wind turbine in yaw

When a wind turbine works in yaw, the wake intensity and the power production of the turbine become slightly smaller and a deflection of the wake is induced. Therefore, a good understanding of this effect would allow an active control of the yaw angle of upstream turbines to steer the wake away from downstream machines, reducing its effect on them. In wind farms where interaction between turbines is significant, it is of interest to maximize the power output from the wind farm as a whole and to reduce fatigue loads on downstream turbines due to the increase of turbulence intensity in wakes. A large eddy simulation model with particular wind boundary conditions has been used recently to simulate and characterize the turbulence generated by the presence of a wind turbine and its evolution downstream the machine. The simplified turbine is placed within an environment in which relevant flow properties like wind speed profile, turbulence intensity and the anisotropy of turbulence are found to be similar to the ones of the neutral atmosphere. In this work, the model is used to characterize the wake deflection for a range of yaw angles and thrust coefficients of the turbine. The results are compared with experimental data obtained by other authors with a particle image velocimetry technique from wind tunnel experiments. Also, a comparison with simple analytical correlations is carried out. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical investigation of unsteady aerodynamics of a Horizontal‐axis wind turbine under yawed flow conditions

In the present study, unsteady flow features and the blade aerodynamic loading of the National Renewable Energy Laboratory phase VI wind turbine rotor, under yawed flow conditions, were numerically investigated by using a three‐dimensional incompressible flow solver based on unstructured overset meshes. The effect of turbulence, including laminar‐turbulent transition, was accounted for by using a correlation‐based transition turbulence model. The calculations were made for an upwind configuration at wind speeds of 7, 10 and 15 m/sec when the turbine rotor was at 30° and 60° yaw angles. The results were compared with measurements in terms of the blade surface pressure and the normal and tangential forces at selected blade radial locations. It was found that under the yawed flow conditions, the blade aerodynamic loading is significantly reduced. Also, because of the wind velocity component aligned tangent to the rotor disk plane, the periodic fluctuation of blade loading is obtained with lower magnitudes at the advancing blade side and higher magnitudes at the retreating side. This tendency is further magnified as the yaw angle becomes larger. At 7 m/sec wind speed, the sectional angle of attack is relatively small, and the flow remains mostly attached to the blade surface. At 10 m/sec wind speed, leading‐edge flow separation and strong radial flow are observed at the inboard portion of the retreating blade. As the wind speed is further increased, the flow separation and the radial flow become more pronounced. It was demonstrated that these highly unsteady three‐dimensional aerodynamic features are well‐captured by the present method. Copyright © 2012 John Wiley & Sons, Ltd.     

A study of the near wake structure of a wind turbine comparing measurements from laboratory and full-scale experiments

Wake flow measurements have been performed using the technique of particle image velocimetry (PIV) at stations downstream from a model wind turbine rotor, and evaluated against experimental data from two full-scale machines. Comparisons include both mean velocity and turbulent intensity cross-wake profiles at a range of tip speed ratios. The application of PIV to the study of wind turbine wakes is described in detail, including the steps required to ensure appropriate and accurate simulation of the flow field conditions. The results suggest that the PIV method is a potentially useful tool in the investigation of detailed wake flow, though significant differences are observed between wake velocity deficits at full- and model scale. These are discussed with regard to scale effect, the influence of terrain, model similarity, and the phenomenon of wake meandering and effective cross-wake smoothing.     

沿海滩涂风力机近尾流风速分布特性研究

为获得风力机近尾流风速在垂直方向和水平方向的变化规律,提出一种测量风力机近尾流区风速的实测方法。针对某沿海滩涂风电场,采用2台搭载风速仪的无人机对近尾流区进行测量。结果表明：垂直方向,尾流和来流风速比值在1.0D~2.5D处（D为风轮直径）随着高度的增加呈先减小后增大的趋势,在轮毂中轴线处存在最小值0.53~0.68;风速比值沿轮毂中轴线呈非对称分布。0.5D处风速比值分别在上下风轮处存在2个极小值0.56和0.50。水平方向,风速比值在1.0D~3.0D处沿径向距离从左向右呈先减小后增大的趋势,在轮毂中轴线处存在最小值（0.54~0.78）;风速比值沿轮毂中轴线呈对称分布,随着风轮下游距离的增加呈扩张趋势。最后给出用于A类风场风力机下游尾流风速剖面的预测公式。     

Measurements and modelling of the power performance of a model floating wind turbine under controlled conditions

This paper describes power performance measurements undertaken on a model floating wind turbine installed on a tension‐leg platform (TLP) in a wind/wave generator facility. Initially, the surge of the platform was measured under different rotor and wave conditions. The surge behaviour depended considerably on the rotor tip speed ratio and the wave frequency and amplitude. High‐frequency data sampling techniques were then used to derive the instantaneous power coefficient and tip speed ratio directly from the measurements, together with the surge velocity of the floating system. The power measurements were compared with those predicted by three independent numerical models, two of which are based on the blade element momentum approach and the third involving a lifting‐line free‐wake vortex model. The fluctuations of the power coefficient with time predicted by the three models were in close agreement; however, these were all significantly larger than those derived from the rotor shaft torque measurements. This was found to be due to the limitations of the torque measurement technique. Although being accurate in measuring the time‐averaged torque, the sensor was incapable of measuring the high‐frequency low‐amplitude fluctuations in the rotor shaft torque induced by the TLP surge. This was confirmed using an alternative experimental technique involving hot‐wire near‐wake measurements. The study also investigated the influence of the platform surge motion on the time‐averaged power coefficients. Both the measurements and the free‐wake vortex model revealed marginal deviations in the time‐averaged power coefficients when compared with those obtained for a fixed, non‐surging rotor. Copyright © 2014 John Wiley & Sons, Ltd.     

Parabolic RANS solver for low‐computational‐cost simulations of wind turbine wakes

A numerical framework for simulations of wake interactions associated with a wind turbine column is presented. A Reynolds‐averaged Navier‐Stokes (RANS) solver is developed for axisymmetric wake flows using parabolic and boundary‐layer approximations to reduce computational cost while capturing the essential wake physics. Turbulence effects on downstream evolution of the time‐averaged wake velocity field are taken into account through Boussinesq hypothesis and a mixing length model, which is only a function of the streamwise location. The calibration of the turbulence closure model is performed through wake turbulence statistics obtained from large‐eddy simulations of wind turbine wakes. This strategy ensures capturing the proper wake mixing level for a given incoming turbulence and turbine operating condition and, thus, accurately estimating the wake velocity field. The power capture from turbines is mimicked as a forcing in the RANS equations through the actuator disk model with rotation. The RANS simulations of the wake velocity field associated with an isolated 5‐MW NREL wind turbine operating with different tip speed ratios and turbulence intensity of the incoming wind agree well with the analogous velocity data obtained through high‐fidelity large‐eddy simulations. Furthermore, different cases of columns of wind turbines operating with different tip speed ratios and downstream spacing are also simulated with great accuracy. Therefore, the proposed RANS solver is a powerful tool for simulations of wind turbine wakes tailored for optimization problems, where a good trade‐off between accuracy and low‐computational cost is desirable.     

基于叶片变形偏航下风力机叶片绕流流场的研究

针对偏航工况下风力机叶片与流场之间的相互作用而产生的变形影响叶片绕流流场问题,基于叶片变形对不同偏航工况下水平轴风力机叶片绕流流场进行双向流固耦合数值计算,分析偏航工况对风力机叶片变形和表面应力的影响,在此基础上研究不同偏航工况对叶片绕流流场的影响.结果表明,不同叶片上的变形和应力呈现不均匀性,且随偏航角增大,不均匀性...     

Flow characterization in the near‐wake region of a horizontal axis wind turbine

An experimental study is conducted to investigate the flow dynamics within the near‐wake region of a horizontal axis wind turbine using particle image velocimetry (PIV). Measurements were performed in the horizontal plane in a row of four radially distributed measurement windows (tiles), which are then patched together to obtain larger measurement field. The mean and turbulent components of the flow field were measured at various blade phase angles. The mean velocity and turbulence characteristics show high dependency on the blade phase angle in the near‐wake region closer to the blade tip and become phase independent further downstream at a distance of about one rotor diameter. In the near‐wake region, both the mean and turbulent characteristics show a systemic variation with the phase angle in the blade tip region, where the highest levels of turbulence are observed. The streamlines of the instantaneous velocity field at a given phase allowed to track a tip vortex which showed wandering trend. The tip vortices are mostly formed at r/R > 1, which indicates the wake expansion. Results also show the gradual movement of the vortex region in the axial direction, which can be attributed to the dynamics of the helical tip vortices which after being generated from the tip, rotate with respect to the blade and move in the axial direction because of the axial momentum of the flow. The axial velocity deficit was compared with other laboratory and field measurements. The comparison shows qualitative similarity. Copyright © 2015 John Wiley & Sons, Ltd.     

Wake impacts on downstream wind turbine performance and yaw alignment

Aerodynamic wake interaction between commercial scale wind turbines can be a significant source of power losses and increased fatigue loads across a wind farm. Significant research has been dedicated to the study of wind turbine wakes and wake model development. This paper profiles influential wake regions for an onshore wind farm using 6 months of recorded SCADA (supervisory control and data acquisition) data. An average wind velocity deficit of over 30% was observed corresponding to power coefficient losses of 0.2 in the wake region. Wind speed fluctuations are also quantified for an array of turbines, inferring an increase in turbulence within the wake region. A study of yaw data within the array showed turbine nacelle misalignment under a range of downstream wake angles, indicating a characteristic of wind turbine behaviour not generally considered in wake studies. The turbines yaw independently in order to capture the increased wind speeds present due to the lateral influx of turbulent wind, contrary to many experimental and simulation methods found in the literature. Improvements are suggested for wind farm control strategies that may improve farm‐wide power output. Additionally, possible causes for wind farm wake model overestimation of wake losses are proposed.Copyright © 2012 John Wiley & Sons, Ltd.     

Wake meandering effects on floating wind turbines

As more floating farms are being developed, the wake interaction between multiple floating wind turbines (FWTs) is becoming increasingly relevant. FWTs have long natural periods in certain degrees of freedom, and the large‐scale movement of the wake, known as wake meandering, occurs at very low frequencies. In this study, we use FAST.Farm to simulate a two‐turbine case with three different FWT concepts: a semisubmersible (semi), a spar, and a tension leg platform (TLP), separated by eight rotor diameters in the wind direction. Since wake meandering varies depending on the environmental conditions, three different wind speeds (for all three concepts) as well as two different turbulence levels (for the semi) are considered. For the below‐rated wind speed, when wake meandering was most extreme, yaw motion standard deviations for the downstream semi were approximately 40% greater in high turbulence and over 100% greater in low turbulence when compared with the upstream semi. The low yaw natural frequency (0.01 Hz) of the semi was excited by meandering, while quasi‐static responses resulted in approximately 20% increases in yaw motion standard deviations for the spar and TLP. Differences in fatigue loading between the upstream and downstream turbines for the mooring line tension and tower base fore‐aft bending moment mostly depended on the velocity deficit and were not directly affected by meandering. However, wake meandering did affect fatigue loading related to the tower top yaw moment and the blade root out‐of‐plane moment.