Influence of inflow conditions on turbine loading and wake structures predicted by large eddy simulations using exact geometry

A large eddy simulation was performed on an National Renewable Energy Laboratory (NREL) phase VI wind turbine (10 m diameter), using the exact blade geometry, to determine the influence of different inflow conditions on the aerodynamic loadings and the near wake characteristics. The effects of the three inflow conditions, uniform inflow, linear wind shear and linear wind shear with turbulence, are investigated. Wind shear causes periodic variations in power and aerodynamic loading with an additional force component exerted along the lateral direction. Significant separation occurs in the high wind region on the suction side of the blades, resulting in unstable loading in off‐design inflow conditions. Because of the shear effect between the near‐blade tip vortex and ambient flow, the strong vortex core in the helical structure dissipates and transforms into a continuous vorticity sheet when x/D > 1.5. The combination of inflow turbulence and wind shear enhances the turbulence generation mechanism in the near wake, where energy is withdrawn from large wake structures and converted into energy of small‐scale structures. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

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.     

Wind turbine performance in shear flow and in the wake of another turbine through high fidelity numerical simulations with moving mesh technique

We present numerical simulations of two horizontal axis wind turbines, one operating under the wake of the other, under an incoming sheared velocity profile. We use a moving mesh technique to represent the rotation of the turbine blades and solve the unsteady Reynolds averaged Navier–Stokes equations with a shear stress transport k ? ω turbulence model. Temporal evolution of the lift and drag coefficients of the front turbine show a phase shift in the periodic cycle due to the non‐uniform incoming free stream velocity. Comparisons of the lift and drag coefficients for the back turbine with the unperturbed behaviour of the front demonstrate the complex non‐linear interactions of the blades with the wake, with a significant decrease in overall performance and two peaks at specific points in the cycle associated with local angle of attack modification in the wake. The vorticity field in the near wake shows tilting of the vortex lines in the wake due to the shear and a faster diffusion of the tip vortical signature compared with the uniform free stream velocity case. Observations of the wake–wake interaction show good agreement with recent studies that use different methodologies. Copyright © 2012 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.     

Validation of the actuator line method using near wake measurements of the MEXICO rotor

The purpose of the present work is to validate the capability of the actuator line method to compute vortex structures in the near wake behind the MEXICO experimental wind turbine rotor. In the MEXICO project/MexNext Annex, particle image velocimetry measurements have made it possible to determine the exact position of each tip vortex core in a plane parallel to the flow direction. Determining center positions of the vortex cores makes it possible to determine the trajectory of the tip vortices, and thus the wake expansion in space, for the analyzed tip speed ratios. The corresponding cases, in terms of tip speed ratios, have been simulated by large‐eddy simulations using a Navier–Stokes code combined with the actuator line method. The flow field is analyzed in terms of wake expansion, vortex core radius, circulation and axial and radial velocity distributions. Generally, the actuator line method generates significantly larger vortex cores than in the experimental cases, but predicts the expansion, the circulation and the velocity distributions with satisfying results. Additionally, the simulation and experimental data are used to test three different techniques to compute the average axial induction in the wake flow. These techniques are based on the helical pitch of the tip vortex structure, 1D momentum theory and wake expansion combined with mass conservation. The results from the different methods vary quite much, especially at high values of λ. Copyright © 2014 John Wiley & Sons, Ltd.     

Stability analysis of the tip vortices of a wind turbine

The aim of the present paper is to obtain a better understanding of the stability properties of wakes generated by wind turbine rotors. To accomplish this, a numerical study on the stability of the tip vortices of the Tjaereborg wind turbine has been carried out. The numerical model is based on large eddy simulations of the Navier–Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies, the flow is disturbed by inserting harmonic perturbations, giving rise to spatially developing instabilities. The results show that the instability is dispersive and that growth arises only for some specific frequencies and type of modes, in agreement with previous instability studies. The result indicates two types of modes; one where oscillations of neighboring vortex spirals are out of phase and one where oscillations in every vortex spiral in phase. The mode with spirals out of phase results in the largest growth with the main extension of the disturbance waves in radial and downstream directions. The out‐of‐phase disturbance leads to vortex pairing once the development leaves the linear stage. The study also provides evidence of a relationship between the turbulence intensity and the length of the near wake. The relationship, however, needs to be calibrated against measurements. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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。     

雾霾之后的反思

新年过后的第二个周末,浓重的雾霾已在全国多个城市肆虐,这让人们的心情变得糟糕。数据显示,截至1月13日零时,全国有33个城市的部分监测点PM2.5浓度超过300微克/立方米,个别城市出现PM2.5"爆表",比如北京的PM2.5浓度最高达到950微克/立方米。环保专家称,如此严重的空气质量污染,可以说已近人类所能承受的极限。于是,人们看到了政府有关方面发布的紧急预案,比如通知市民减少户外活动,要求学校停止户外体育锻炼,这体现了政府的责任意识。但我们是满足于制定完美的灾情应对预案,还是谋求从根本上消除灾难?     

A wind‐tunnel study of the wake development behind wind turbines over sinusoidal hills

In the present work, the wake development behind small‐scale wind turbines is studied when introducing local topography variations consisting of a series of sinusoidal hills. Additionally, wind‐tunnel tests with homogeneous and sheared turbulent inflows were performed to understand how shear and ambient turbulence influence the results. The scale of the wind‐turbine models was about 1000 times smaller than full‐size turbines, suggesting that the present results should only be qualitatively extrapolated to real‐field scenarios. Wind‐tunnel measurements were made by means of stereoscopic particle image velocimetry to characterize the flow velocity in planes perpendicular to the flow direction. Over flat terrain, the wind‐turbine wake was seen to slowly approach the ground while it propagated downstream. When introducing hilly terrain, the downward wake deflection was enhanced in response to flow variations induced by the hills, and the turbulent kinetic energy content in the wake increased because of the speed‐up seen over the hills. The combined wake observed behind 2 streamwise aligned turbines was more diffused and when introducing hills, it was more prone to deflect towards the ground compared to the wake behind an isolated turbine. Since wake interactions are common at sites with multiple turbines, this suggested that it is important to consider the local hill‐induced velocity variations when onshore wind farms are analysed. Differences in the flow fields were seen when introducing either homogeneous or sheared turbulent inflow conditions, emphasizing the importance of accounting for the prevailing turbulence conditions at a given wind‐farm site to accurately capture the downstream wake development.     

Numerical investigation of wind turbine wake development in directionally sheared inflow

Turbines in wind farms are subject to complex mutual aerodynamic interactions, which in detail depend upon the characteristics of the atmospheric boundary layer. Our two objectives with this paper were to investigate the impact of directionally sheared inflow on the wake development behind a single wind turbine and to analyse the impact of the wakes on the energy yield and loading of a downstream turbine, which is exposed to partial and full wake conditions. We performed simulations with a framework based on a coupled approach of large‐eddy simulation and an actuator line representation of an aeroelastic turbine model. Our results show that directionally sheared inflow leads to a non‐symmetrical wake development, which transfers to distinct differences in the energy yield and loading of downstream turbines of equal lateral offsets in opposite direction. Therefore, the assumption of wakes being axisymmetrical could lead to notable deviations in the prediction of wake behaviour and their impact on downstream turbines for atmospheric inflow conditions, which include directional shear. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical simulations of wake characteristics of a wind turbine in uniform inflow

The wake of a wind turbine operating in a uniform inflow at various tip speed ratios is simulated using a numerical method, which combines large eddy simulations with an actuator line technique. The computations are carried out in a numerical mesh with about 8.4·10⁶ grid points distributed to facilitate detailed studies of basic features of both the near and far wake, including distributions of interference factors, vortex structures and formation of instabilities. Copyright © 2009 John Wiley & Sons, Ltd.     

The velocity of the tip vortex of a horizontal-axis wind turbine at high tip speed ratio

An equation is derived for the streamwise velocity of the tip vortex of a horizontal-axis wind turbine as the pitch of the vortex tends to zero. The equation is applicable at high tip speed ratios provided the vortex core remains of constant size and there is no flow along the vortex axis. Under these conditions, the vortex velocity is the average of the velocity in the wake and the external wind speed. This result appears to conflict with the computational need to have the vortex velocity approach the wind speed in the high thrust region. It is suggested that the conflict could be resolved by considering the axial flow within the tip vortex.     

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。     

Cylindrical vortex wake model: skewed cylinder,application to yawed or tilted rotors

A vortex system consisting of a bound vortex disk, a root vortex and a vortex cylinder is presented and applied for skewed wake situations. Both the longitudinal and tangential components of vorticity of the cylinder are considered. A subset of this system leads to a model, which is commonly used in Blade Element Momentum method codes for yawed conditions. Here, all the components of the full vortex system are analyzed in view of extending Blade Element Momentum models. The main assumptions of the current study are a constant uniform circulation, an infinite number of blades, an un‐expanding wake shape and a finite tip‐speed ratio. The investigation remains within the context of inviscid potential flow theory. The model is derived for horizontal‐axis rotors in general, but results are presented for wind‐turbine applications. For each vortex element, the velocity components in all directions are computed analytically or semi‐analytically for the entire domain. Simplified engineering models are provided to ease the evaluation of velocities in the rotor plane. The predominant velocity components are assessed. Copyright © 2015 John Wiley & Sons, Ltd.     

基于预定涡尾迹法的风力机性能研究

以三叶片水平轴风力机为研究对象,建立了尾迹扩张模型,研究了叶片尾迹流动结构。涡面模型叶片无量纲环量的计算结果与经典环量数据对比,结果吻合较好。计算了根尖涡模型风轮面诱导速度随尾迹扩张系数的变化关系,得出了尖速比对气动性能的影响:推力系数与环量及尖速比成正比;对于不同环量,功率系数存在最佳尖速比。分析了尾迹扩张模型对气动性能的影响:推力系数和功率系数与尾迹扩张系数成正比,尾迹收敛参数对其影响甚微。     

The near wake of a model horizontal-axis wind turbine at runaway

This paper completes a series which describes measurements within two chord lengths of the blades of a small horizontal-axis wind turbine over a wide range of operating conditions. Prior to the present experiment, the turbine was rebuilt to allow operation at its runaway point, where no power is produced. Runaway can be viewed as the upper limit on wind turbine performance at which thrust and wake expansion are maximised. The measurements, which approximate the mean and fluctuating velocity fields seen by an observer rotating with the blades, were obtained from a stationary X-probe hot-wire anemometer by the technique of phase-locked averaging. It is shown conclusively that there is negative (power-producing) angular momentum extracted from the wake, but a balancing positive angular momentum resides in the tip vortices. The mean velocity through the blades increases significantly with radius, in contrast to the near-constant velocity when the turbine is producing its maximum power. Comparisons with conventional blade calculations suggest that the circulation in the wake is related to the difference between the circumferential components of the lift and drag, rather than the magnitude of the lift as is often assumed. Within the range and accuracy of measurement, the pitch of the tip vortices is constant and proportional to the inverse of the tip speed ratio.     

Performance and near wake measurements of a model horizontal axis wind turbine

The performance characteristics and the near wake of a model wind turbine were investigated experimentally. The model tested is a three‐bladed horizontal axis type wind turbine with an upstream rotor of 0.90 m diameter. The performance measurements were conducted at various yaw angles, a freestream speed of about 10 m s ^?1, and the tip speed ratio was varied from 0.5 to 12. The time‐averaged streamwise velocity field in the near wake of the turbine was measured at different tip speed ratios and downstream locations. As expected, it was found that power and thrust coefficients decrease with increasing yaw angle. The power loss is about 3% when the yaw angle is less than 10° and increases to more than 30% when the yaw angle is greater than 30°. The velocity distribution in the near wake was found to be strongly influenced by the tip speed ratio and the yaw angle. At the optimum tip speed ratio, the axial velocity was almost uniform within the midsection of the rotor wake, whereas two strong peaks are observed for high tip speed ratios when the yaw angle is 0°. As the yaw angle increases, the wake width was found to be reduced and skewed towards the yawed direction. With increasing downstream distance, the wake velocity field was observed to depend on the tip speed ratio and more pronounced at high tip speed ratio. Copyright © 2011 John Wiley & Sons, Ltd.