An Investigation of the Separate Flow and Stall-delay for Horizontal-Axis Wind Turbine

The flow characteristics and stall delay phenomenon of a stall regulated wind turbine rotor due to blade rotation in steady state non-yawed conditions are investigated. An incompressible Reynolds-averaged Navier-Stokes solver is applied to carry out the separate flow cases at high wind speeds from 11 m/s to 25 m/s with an interval of 2 m/s. The objective of the present research effort is to validate a first-principles based approach for modeling horizontal axis wind turbines (HAWT) under stalled flow conditions using NREL/Phase VI rotor data. The computational results are compared with the predicted values derived by a new stall-delay model and blade element momentum (BEM) method. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

气动、结构、载荷相协调的大型风电叶片自主研发进展

从叶片设计的3个关键环节（气动设计、结构设计和载荷评估）出发,对叶片自主研发进展进行了总结分析．在气动设计方面,概述了计算流体动力学（computational fluid dynamics, CFD）方法、涡方法和叶素动量（blade element momentum, BEM）方法,并依据工程中广泛应用的BEM方法,指出了低风速区风电叶片的解决思路;在结构设计方面,简要概述了基于梁模型的传统设计分析方法,分析了其在大型复合材料叶片薄壳结构上的不足,并对有限元方法（finite element method, FEM）在叶片结构分析中的应用进展进行了介绍;在载荷评估方面,介绍了其对叶片和整机其它部件的影响,阐述了载荷预估方面的工作进展．然后,通过分析3个关键环节之间的相互关系,得到如下结论:建立气动、结构和载荷相协调的叶片优化设计体系,才能真正满足高效低成本的需要．最后,指明了需要进一步研究的主要方向,即高效低载翼型研究,结构非线性有限元分析,气动-结构耦合研究,设计标准制定．最终目标是建立适合中国风资源特点的叶片研发体系,推动我国风电产业发展.     

Stall-delay modelling of wind turbine blades

Most aerodynamic design tools for horizontal-axial wind turbines are based on the blade-element momentum theory (BEM). Due to the nature of this theory, the design tools need 2-D steady sectional lift and drag curves as an input. In practice, flow over a wind turbine rotor blade is neither two-dimensional nor steady, and is affected by rotation. Pioneering experiments have identified a consequence: at inboard rotor blade sections stall is delayed. This so-called Himmelskamp effect [1] gives a larger lift than predicted and, as a result, a higher power and loading than expected. Consequently, an aerodynamic model is needed to explain and predict sectional lift and drag under rotating conditions. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Design and veri?cation of airfoil families for large-size wind turbine blades?

For the design of wind turbine blades, the use of a family of specially tailored airfoils is particularly important. The dedicated airfoils can dramatically improve the capability of capturing wind power, reduce the structural weight to save the cost of manufacturing and transportation, and lower the inertial loads as well as the loads due to gust. An overview of the world-wide wind turbine airfoil families developed since 1990’s is presented, such as the S series, the DU series, the Ris? series, and the FFA series. The design and wind-tunnel tests of the Northwestern Polytechnical University (NPU) airfoil family for megawatt-size wind turbines, called the NPU-WA series, are summarized. All tests for the NPU-WA series are carried out in the NF-3 low-speed wind-tunnel with a two-dimensional (2D) test section of 1.6 m?0.8 m and at the Reynolds number ranging from 1.6?106 to 5?106 . The research activities for further improving the NPU-WA airfoils towards lower roughness sensitivity are also reviewed. The development of the new NPU-WA series dedicated for multi-megawatt wind turbines is discussed.     

A numerical investigation on the dynamic stall of a vertical axis wind turbine

In the last years, for home user, the wind turbine with vertical axis (VAWT) began to be more attractive due benefits in exploitation. In terms of aerodynamics, when the wind speed approaches the speed of operation (low value of tip speed ratio -TSR) the blade airfoil exceeds the critical angle of incidence for static conditions. Angle of incidence varies quickly across blade and the blade works in dynamic stall condition. The goal of the present work is to investigate the two-dimensional dynamic stall phenomenon around the NACA 0012 airfoil at relatively low Reynolds. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of horizontal-axis wind turbine (HAWT) blade three-dimensional rotational e?ect based on ?eld 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 coe?cients. The 3D rotational e?ect 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 di?erential pressure of the blade section airfoil increases at ?rst 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 e?ect has a signi?cant impact on the blade surface ?ow and the aerodynamic load, leading to an increase of the di?erential pressure on the airfoils and their lift coe?cient than that with the 2D one because of the stall delay. The in?uence of the 3D rotational e?ect on the wind turbine blade especially takes place on the sections with ?ow separation.     

Dynamic modeling and analysis of wind turbine drivetrain considering the effects of non-torque loads

With the increase of the rotor diameter and the deterioration of operating conditions, modern wind turbines suffer from more and more significant time-varying non-torque loads, which increases the burden of turbine structures especially the gearbox. Based on an aeroelastic loose coupling approach and assembly of the finite element method, an integrated drivetrain coupling analysis model including blade module, aerodynamic module, and gearbox module is established in this study. This proposed model is validated by comparing the calculation results with previous literature. Taking National Renewable Energy Laboratory 5-MW wind turbine as the research object, the gearbox vibration responses, gear meshing forces and bearing forces under non-torque loads caused by blade gravity, wind shear (WS), tower shadow (TS) and yawed inflow are studied in detail. Results show that the y-direction displacements of the gearbox, sun gear 1, sun gear 2 and gear are larger than those in x-direction because F_y or M_x generated by blade gravity, WS and TS dominates the non-torque loads. The non-torque loads lead to a non-uniform planet load sharing especially for the planetary gear stage 1. Because of the fluctuations of non-torque loads, not only the rotation frequencies of the corresponding carrier but also the multiple frequencies of the carrier 1 are observed in the frequency spectrums. The non-torque loads are mainly borne by carrier 1 bearings. Except for the blade gravity, the bearing forces caused by other unsteady inflows have obvious fluctuations.     

Dynamic Stall Measurements

Forces acting statically on an airfoil in laminar flow are well known. Under turbulent conditions dynamic effects (like dynamic stall) take place. There are different theoretical models to describe dynamic stall, but very little experimental data. We present dynamic stall measurements using an FX 79-W-151A airfoil in a closed test section of a wind tunnel with laminar inflow. Dynamic stall was induced by pitching the foil sinusoidally. Lift forces were measured using the pressure distribution on the wind tunnel walls. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

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 ?nite element method (FEM) are coupled in the approach. The vortex wake method is used to predict wind turbine aero-dynamic 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 signi?cantly 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.     

三叶片垂直轴水轮机结构动力响应分析

提出采用改进离散涡和几何精确梁理论混合方法对三叶片垂直轴水轮机进行结构动力响应分析.相比传统的有限元方法,该方法具有求解速度快、建模简单、计算精确等优点.在模态分析中,计算了不同叶片高度下,水轮机叶片和整体的前五阶固有频率,分析了水轮机半径大小和叶片高度对固有频率的影响,结果显示:随着尺寸的增加,叶片和整体固有频率显著减小,整体固有频率更易受到半径大小的影响.在瞬态分析中,考虑了离心载荷和叶片的水动力载荷,得到在工作状况下,旋转一周过程中叶片的最大变形曲线;分析了在不同H/R（叶片高度和半径的比值）的情况下的叶片强度问题,结果显示:当H/R大于3.0时,叶片强度将会失效.     

大型水平轴风力机叶片气动弹性计算

给出了一种考虑几何非线性的大型风力机静、 动气动弹性一体化计算方法．采用涡尾迹方法进行风力机气动载荷计算．建立风力机风轮的三维壳模型．沿周向平均风力机叶片载荷并加载到结构模型进行非线性静气动弹性分析．基于动力学小扰动假设, 在静平衡构型下进行动力学线性化, 计算风轮固有振动特性．继而结合非定常涡尾迹方法计算风力机动气动弹性响应．计算了NH 1500叶片考虑几何非线性的静气动弹性位移和动气动弹性响应．结果表明,大型风力机叶片几何非线性较为明显地减小静气动弹性位移,同时降低动气动弹性的响应幅值．大型风力机气动弹性响应计算需要考虑几何非线性     

A dynamic wind farm aggregate model for the simulation of power fluctuations due to wind turbulence

An important aspect related to wind energy integration into the electrical power system is the fluctuation of the generated power due to the stochastic variations of the wind speed across the area where wind turbines are installed. Simulation models are useful tools to evaluate the impact of the wind power on the power system stability and on the power quality. Aggregate models reduce the simulation time required by detailed dynamic models of multiturbine systems.In this paper, a new behavioral model representing the aggregate contribution of several variable-speed-pitch-controlled wind turbines is introduced. It is particularly suitable for the simulation of short term power fluctuations due to wind turbulence, where steady-state models are not applicable.The model relies on the output rescaling of a single turbine dynamic model. The single turbine output is divided into its steady state and dynamic components, which are then multiplied by different scaling factors. The smoothing effect due to wind incoherence at different locations inside a wind farm is taken into account by filtering the steady state power curve by means of a Gaussian filter as well as applying a proper damping on the dynamic part.The model has been developed to be one of the building-blocks of a model of a large electrical system, therefore a significant reduction of simulation time has been pursued. Comparison against a full model obtained by repeating a detailed single turbine model, shows that a proper trade-off between accuracy and computational speed has been achieved.     

Numerical bifurcation analysis of static stall of airfoil and dynamic stall under unsteady perturbation

By the finite element method combined with Arbitrary-Lagrangian-Eulerian (ALE) frame and explicit Characteristic Based Split Scheme (CBS), the complex flows around stationary and sinusoidal pitching airfoil are studied numerically. In particular, the static and dynamic stalls are analyzed in detail, and the natures of the static stall of NACA0012 airfoil are given from viewpoint of bifurcations. Following the bifurcation in Map, the static stall is proved to be the result from saddle-node bifurcation which involves both the hysteresis and jumping phenomena, by introducing a Map and its Floquet multiplier, which is constructed in the numerical simulation of flow field and related to the lift of the airfoil. Further, because the saddle-node bifurcation is sensitive to imperfection or perturbation, the airfoil is then subjected to a perturbation which is a kind of sinusoidal pitching oscillation, and the flow structure and aerodynamic performance are studied numerically. The results show that the large-scale flow separation at the static stall on the airfoil surface can be removed or delayed feasibly, and the ensuing lift could be enhanced significantly and also the stalling incidence could be delayed effectively. As a conclusion, it can be drawn that the proper external excitation can be considered as a powerful control strategy for the stall. As an unsteady aerodynamic behavior of high angle of attack, the dynamic stall can be investigated from viewpoint of nonlinear dynamics, and there exists a rich variety of nonlinear phenomena, which are related to the lift enhancement and drag reduction.     

Three‐dimensional boundary layer on wind turbine blades

This paper describes a three‐dimensional analysis of the laminar boundary layer that develops on the blades of an horizontalaxis wind turbine. The main aim was to investigate a fundamental phenomenon: the effect of rotation on the blade boundary layer of a wind turbine in conjunction with the widely observed phenomenon of stall‐delay. The separation position in retarded flows with pressure gradients is calculated and compared for the rotation and non‐rotation cases. It is concluded that the stall is linearly postponed due to the Coriolis force and the separation point is delayed as a result of increasing rotation speed or decreasing blade spanwise position. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamic stall measurements on airfoils

We present a new method for measuring the lift forces acting on an airfoil segment under dynamic stall conditions. Thereby the pressure distribution over the wind tunnel walls is detected by an adequate number of pressure sensors. The airfoil is rotated by a stepping motor with angular velocities up to 300°/s. Measurements with Reynolds number up to Re = 700 000 can be performed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations of Passive Flow Control Devices for Vertical Axis Wind Turbines

The purpose of these investigations is to study the possibilities to improve performances of a typical vertical axis wind turbines (VAWT), of Darrieus-type, using passive devices (mounted “on” or “inside” the blades of VAWT) to control dynamic flow separation (dynamic stall phenomenon). The passive devices considered for investigations are the Gurney flap, the slot or thin channel, and the turbulence promoters. Studies are performed numerically using computational fluid dynamics and where it is possible the results are compared with existent experimental or numerical data. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

An efficient envelope-based Branch and Bound algorithm for non-convex combined heat and power production planning

Combined heat and power (CHP) production is universally accepted as one of the most energy-efficient technologies to produce energy with lower fuel consumption and fewer emissions. In CHP technology, heat and power generation follow a joint characteristic. Traditional CHP production is usually applied in backpressure plants, where the joint characteristic can often be represented by a convex model. Advanced CHP production technologies such as backpressure plants with condensing and auxiliary cooling options, gas turbines, and combined gas and steam cycles may require non-convex models. Cost-efficient operation of a CHP system can be planned using an optimization model based on forecasts for heat load and power price. A long-term planning model decomposes into thousands of single-period models, which can be formulated in the convex case as linear programming (LP) problems, and in the non-convex case as mixed integer programming (MIP) problems.     

翼涡干扰前缘开孔被动控制数值研究

开孔方法是一种简单的流动被动控制方法.为找到一种有效降低桨涡干扰效应的被动控制方法,以NACA 0012翼型作为研究对象,建立了4种前缘开孔的模型.在不同来流速度、涡的强度和干扰距离条件下,对4种前缘开孔模型和无孔的基准翼型进行了二维平行桨涡干扰（翼涡干扰）数值模拟,对比了升力系数的变化.结果表明:前缘开孔可以降低翼涡干扰效应,但对翼型升力系数有一定的影响;宽度为2.5%弦长的直孔能在翼型升力系数损失较小的情况下有效地降低翼涡干扰效应,且适用范围较广.     

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

The computational ?uid dynamics (CFD) methods are applied to aerody-namic problems for large scale wind turbines. The progresses including the aerodynamic analyses of wind turbine pro?les, numerical ?ow simulation of wind turbine blades, evalu-ation of aerodynamic performance, and multi-objective blade optimization are discussed. Based on the CFD methods, signi?cant improvements are obtained to predict two/three-dimensional aerodynamic characteristics of wind turbine airfoils and blades, and the vorti-cal structure in their wake ?ows is accurately captured. Combining with a multi-objective genetic algorithm, a 1.5 MW NH-1500 optimized blade is designed with high e?ciency in wind energy conversion.     

A coupled blade element momentum – Computational fluid dynamics model for evaluating tidal stream turbine performance

A modelling approach based on blade element momentum theory is developed for the prediction of tidal stream turbine performance in the ocean environment. Through the coupling of the blade element momentum method with computational fluid dynamics, the influence of upstream hydrodynamics on rotor performance is accounted for. Incoming flow onto the rotor can vary in speed and direction compared to free-stream conditions due to the presence of obstructions to the flow in the upstream, due to other devices for example, or due to the complexity of natural bathymetries. The relative simplicity of the model leads to short run times and a lower demand on computational resources making it a useful tool for considering more complex engineering problems consisting of multiple tidal stream turbines. Results from the model compare well against both measured data from flume experiments and results obtained using the classical blade element momentum model. A discussion considering the advantages and disadvantages of these different approaches is included.