New vortex‐lift and tangential‐force models for HAWT aerodynamic load prediction

Horizontal axis wind turbines (HAWTs) experience three‐dimensional rotational and unsteady aerodynamic phenomena at the rotor blades sections. These highly unsteady three‐dimensional effects have a dramatic impact on the aerodynamic load distributions on the blades, in particular, when they occur at high angles of attack due to stall delay and dynamic stall. Unfortunately, there is no complete understanding of the flow physics yet at these unsteady 3D flow conditions, and hence, the existing published theoretical models are often incapable of modelling the impact on the turbine response realistically. The purpose of this paper is to provide an insight on the combined influence of the stall delay and dynamic stall on the blade load history of wind turbines in controlled and uncontrolled conditions. New dynamic stall vortex and nonlinear tangential force coefficient modules, which integrally take into account the three dimensional rotational effect, are also proposed in this paper. This module along with the unsteady influence of turbulent wind speed and tower shadow is implemented in a blade element momentum (BEM) model to estimate the aerodynamic loads on a rotating blade more accurately. This work presents an important step to help modelling the combined influence of the stall delay and dynamic stall on the load history of the rotating wind turbine blades which is vital to have lighter turbine blades and improved wind turbine design systems.     

Aerodynamic loads calculation and analysis for large scale wind turbine based on combining BEM modified theory with dynamic stall model

The aerodynamic loads for MW scale horizontal-axis wind turbines are calculated and analyzed in the established coordinate systems which are used to describe the wind turbine. In this paper, the blade element momentum (BEM) theory is employed and some corrections, such as Prandtl and Buhl models, are carried out. Based on the B-L semi-empirical dynamic stall (DS) model, a new modified DS model for NACA63-4xx airfoil is adopted. Then, by combing BEM modified theory with DS model, a set of calculation method of aerodynamic loads for large scale wind turbines is proposed, in which some influence factors such as wind shear, tower, tower and blade vibration are considered. The research results show that the presented dynamic stall model is good enough for engineering purpose; the aerodynamic loads are influenced by many factors such as tower shadow, wind shear, dynamic stall, tower and blade vibration, etc, with different degree; the single blade endures periodical changing loads but the variations of the rotor shaft power caused by the total aerodynamic torque in edgewise direction are very small. The presented study approach of aerodynamic loads calculation and analysis is of the university, and helpful for thorough research of loads reduction on large scale wind turbines.     

A new stall delay algorithm for predicting the aerodynamics loads on wind turbine blades for axial and yawed conditions

Stall delay is a complicated phenomenon that has gained for many years the attention of industry and academics in the fields of helicopter and wind turbine aerodynamics. Since most of the potential flow theories still rely on the use of 2D aerofoil data for simulating loads on a rotating blade, less degree of accuracy is expected because of 3D rotational effects. In this work, a new model for correcting the 2D steady aerodynamic data for 3D effects is presented. The model can reduce the uncertainty in the blade design process and, subsequently, make wind turbines more cost‐effective. This model combines the stall delay model of Corrigan and Schillings, a modified version of an inviscid stall delay model, a new modification factor to account for the effect of the angle of attack changes and a new tip loss factor. Furthermore, the model applies the use of the separation factor of Du and Selig to evaluate the area on the rotor disc where stall delay is most prominent. The new stall delay model was embedded in a free‐wake vortex model to estimate the aerodynamic loads on the National Renewable Energy Laboratory Phase VI rotor blades consisting of the S809 aerofoil sections. The results in this study confirm the validity of the 3D corrections by the proposed new model under both axial and yawed flow conditions. Copyright © 2017 John Wiley & Sons, Ltd.     

Load Mitigation with Bending/Twist‐coupled Blades on Rotors using Modern Control Strategies

The prospect of installing blades that twist as they bend and/or extend on horizontal axis wind turbines provides opportunities for enhanced energy capture and/or load mitigation. Although this coupling could be achieved in either an active or a passive manner, the passive approach is much more attractive owing to its simplicity and economy. As an example, a blade design might employ coupling between bending and twisting, so that as the blade bends owing to the action of the aerodynamic loads, it also twists, modifying the aerodynamic performance in some way. For reducing loads the blades are designed to twist towards feather as they bend. For variable‐speed pitch‐controlled rotors, dynamic computer simulations with turbulent inflow show that twist coupling substantially decreases fatigue damage over all wind speeds, without reducing average power. Maximum loads also decrease modestly. For constant‐speed stall‐controlled and variable‐speed stall‐controlled rotors, significant decreases in fatigue damage are observed at the lower wind speeds and smaller decreases at the higher wind speeds. Maximum loads also decrease slightly. As a general observation, whenever a rotor is operating in the linear aerodynamic range (lower wind speeds for stall control and all wind speeds for pitch control), substantial reductions in fatigue damage are realized. Copyright © 2002 John Wiley & Sons, Ltd.     

Rapid optimization of stall‐regulated wind turbine blades using a frequency‐domain method: Part 2, cost function selection and results

A fast and effective frequency‐domain optimization method was developed for stall‐regulated blades. It was found that when using linearized dynamics, typical cost functions employing damage‐equivalent root bending moments are not suitable for stall‐regulated wind turbines: when the cost function is minimized, the edgewise damping can be low, and the flapwise damping can approach zero during an extreme operating gust. A new cost function is proposed that leads to nicely balanced stall behavior and damping over the entire operating windspeed range. The method was used to design the blades of two multi‐MW, stall‐regulated, offshore wind turbines, comparable with the NREL 5 MW and NTNU 10 MW pitch‐regulated turbines. It is shown that the optimal stall‐regulated blade has a unique aerodynamic profile that gives high flapwise and edgewise damping and a uniform mean power output above the rated windspeed. The blades are described in sufficient detail that they can be used in further aeroelastic analyses, to compare large stall‐regulated and pitch‐regulated turbines. Copyright © 2014 John Wiley & Sons, Ltd.     

HAWT dynamic stall response asymmetries under yawed flow conditions

Horizontal axis wind turbines can experience significant time‐varying aerodynamic loads, potentially causing adverse effects on structures, mechanical components and power production. As designers attempt lighter and more flexible wind energy machines, greater accuracy and robustness will become even more critical in future aerodynamics models. Aerodynamics modelling advances, in turn, will rely on more thorough comprehension of the three‐dimensional, unsteady, vortical flows that dominate wind turbine blade aerodynamics under high‐load conditions. To experimentally characterize these flows, turbine blade surface pressures were acquired at multiple span locations via the NREL Phase IV Unsteady Aerodynamics Experiment. Surface pressures and associated normal force histories were used to characterize dynamic stall vortex kinematics and normal force amplification. Dynamic stall vortices and normal force amplification were confirmed to occur in response to angle‐of‐attack excursions above the static stall threshold. Stall vortices occupied approximately one‐half of the blade span and persisted for nearly one‐fourth of the blade rotation cycle. Stall vortex convection varied along the blade, resulting in dramatic deformation of the vortex. Presence and deformation of the dynamic stall vortex produced corresponding amplification of normal forces. Analyses revealed consistent alterations to vortex kinematics in response to changes in reduced frequency, span location and yaw error. Finally, vortex structures and kinematics not previously documented for wind turbine blades were isolated. Published in 2000 by John Wiley & Sons, Ltd.     

A solution‐based stall delay model for horizontal‐axis wind turbines

This work proposes a new solution‐based stall delay model to predict rotational effects on horizontal‐axis wind turbines. In contrast to conventional stall delay models that correct sectional airfoil data prior to the solution to account for three‐dimensional and rotational effects, a novel approach is proposed that corrects sectional airfoil data during a blade element momentum solution algorithm by investigating solution‐dependent parameters such as the spanwise circulation distribution and the local flow velocity acting at a section of blade. An iterative process is employed that successively modifies sectional lift and drag data until the blade circulation distribution is converged. Results obtained with the solution‐based stall delay model show consistent good agreement with measured data along the National Renewable Energy Laboratory Phase VI and Model Experiments in Controlled Conditions rotor blades at low and high wind speeds. Copyright © 2014 John Wiley & Sons, Ltd.     

Digital tuft analysis of stall on operational wind turbines

Operational wind turbines are exposed to dynamic inflow conditions because of, for instance, atmospheric turbulence and wind shear. In order to understand the resulting three‐dimensional and transient aerodynamics effects at a site, a 10m stall‐regulated upwind two‐bladed wind turbine was instrumented for a novel digital tuft flow visualization study. High definition video of a tufted blade was acquired during wind turbine operation in the field, and a novel digital image processing algorithm calculated the blade stall directly from the video. After processing O(10⁵) sequential images, the algorithm achieved a ?5% bias error compared with previous manual analysis methods. With increasing wind speed (5m/s to 20m/s) the fraction of tufts exhibiting stalled flow increased from 5% to 40% on the outboard 40% of the blade. The independently measured instantaneous turbine power production correlates highly with the stall fraction. Some azimuthal variation in the stall fraction associated with dynamic stall induced by vertical wind shear was seen with a maximum in the 45–90° azimuthal location. The high detail, quantitative image processing method demonstrated good agreement with the expected behaviour for a stall‐regulated wind turbine and revealed azimuthal variation because of shear‐induced dynamic stall. The amount of reliable blade stall data to be obtained from digital tuft visualization has hereby been vastly increased. Copyright © 2015 John Wiley & Sons, Ltd.     

百千瓦中型变速变桨风电机组叶片设计研究

百千瓦级叶片一般采用定桨方式运行,依靠叶片失速进行功率控制,机组运行过程中无法维持较高的效率。基于100 kW变速变桨机组的运行特征,提出了一种100 kW级中型叶片的设计方法。气动设计采用了BEM方法,利用Harp_opt中的优化算法获得较高的气动性能;结构及载荷设计参考IEC标准进行,采用Focus进行铺层设计及结构特性分析。所设计叶片的长度为10.029 m左右,极限及疲劳载荷特性满足GL IIA类风场的运行要求。     

Assessment of a comprehensive aeroelastic tool for horizontal‐axis wind turbine rotor analysis

This paper presents the development of a computational aeroelastic tool for the analysis of performance, response and stability of horizontal‐axis wind turbines. A nonlinear beam model for blades structural dynamics is coupled with a state‐space model for unsteady sectional aerodynamic loads, including dynamic stall effects. Several computational fluid dynamics structural dynamics coupling approaches are investigated to take into account rotor wake inflow influence on downwash, all based on a Boundary Element Method for the solution of incompressible, potential, attached flows. Sectional steady aerodynamic coefficients are extended to high angles of attack in order to characterize wind turbine operations in deep stall regimes. The Galerkin method is applied to the resulting aeroelastic differential system. In this context, a novel approach for the spatial integration of additional aerodynamic states, related to wake vorticity and dynamic stall, is introduced and assessed. Steady‐periodic blade responses are evaluated by a harmonic balance approach, whilst a standard eigenproblem is solved for aeroelastic stability analyses. Drawbacks and potentialities of the proposed model are investigated through numerical and experimental comparisons, with particular attention to rotor blades unsteady aerodynamic modelling issues. Copyright © 2016 John Wiley & Sons, Ltd.     

A new correlation on the MEXICO experiment using a 3D enhanced blade element momentum technique

The blade element momentum (BEM) theory is based on the actuator disc (AD) model, which is probably the oldest analytical tool for analysing rotor performance. The BEM codes have very short processing times and high reliability. The problems of the analytical codes are well known to the researchers: the impossibility of describing inside the one-dimensional code the three-dimensional (3D) radial flows along the span-wise direction. In this work, the authors show how the 3D centrifugal pumping affects the BEM calculations of a wind turbine rotor. Actually to ascertain the accuracy of the analytical codes, the results are compared with rotor performance, blade loads and particle image velocimetry measurements of the model experiment in controlled conditions. A reliable agreement with the measurement is obtained. A good improvement is gained when the blade stall state modified aerofoil data instead of the original aerofoil data are used in the calculations.     

Modeling dynamic stall on wind turbine blades under rotationally augmented flow fields

This paper presents an investigation of two well‐known aerodynamic phenomena, rotational augmentation and dynamic stall, together in the inboard parts of wind turbine blades. This analysis is carried out using the following: (1) the National Renewable Energy Laboratory's Unsteady Aerodynamics Experiment Phase VI experimental data, including constant as well as continuously pitching blade conditions during axial operation; (2) data from unsteady delayed detached eddy simulations (DDES) carried out using the Technical University of Denmark's in‐house flow solver Ellipsys3D; and (3) data from a reduced order dynamic stall model that uses rotationally augmented steady‐state polars obtained from steady Phase VI experimental sequences, instead of the traditional two‐dimensional, non‐rotating data. The aim of this work is twofold. First, the blade loads estimated by the DDES simulations are compared with three select cases of the N‐sequence experimental data, which serves as a validation of the DDES method. Results show reasonable agreement between the two data in two out of three cases studied. Second, the dynamic time series of the lift and the moment polars obtained from the experiments are compared with those from the dynamic stall model. This allowed the differences between the stall phenomenon on the inboard parts of harmonically pitching blades on a rotating wind turbine and the classic dynamic stall representation in two‐dimensional flow to be investigated. Results indicated a good qualitative agreement between the model and the experimental data in many cases, which suggests that the current two‐dimensional dynamic stall model as used in blade element momentum‐based aeroelastic codes may provide a reasonably accurate representation of three‐dimensional rotor aerodynamics when used in combination with a robust rotational augmentation model. Copyright © 2015 John Wiley & Sons, Ltd.     

Aerodynamic effects of leading‐edge tape on aerofoils at low Reynolds numbers

A systematic wind tunnel study was conducted to gain an understanding of the aerodynamic effects of leading‐edge tape, which is typically used on small wind turbines as a protection from blade erosion. The wind tunnel tests included lift and drag measurements over the Reynolds number range from 150,000 to 500,000. In addition, flow visualization experiments were carried out. Various tape configurations were tested on five aerofoils, namely the BW‐3, FX 63‐137, S822, SG6042 and SG6051. Although the magnitude of the aerodynamic effects of the tape was aerofoil‐dependent, it was found that extending the tape beyond 5% chord and staggering multiple tape layers were most beneficial in minimizing the loss in aerofoil performance. The practical significance of the results on wind turbine performance is discussed. In particular, the data for the SG6042 aerofoil were used to quantify the effects of the tape on the power coefficient of small variable‐speed wind turbines. Overall, the different tape configurations tested reduced the power coefficient by no more than 2·1%. From the trends shown, however, larger reductions in power coefficient should be expected for larger wind turbines than those considered, particularly if two layers of tape are used. In light of this study, guidelines for optimum application are suggested. Copyright © 1999 John Wiley & Sons, Ltd.     

The application of passive air jet vortex‐generators to stall suppression on wind turbine blades

An experimental study was performed to assess the feasibility of passive air jet vortex‐generators to the performance enhancement of a domestic scale wind turbine. It has been demonstrated that these simple devices, properly designed and implemented, can provide worthwhile performance benefits for domestic wind turbines of the type investigated in this study. In particular, this study shows that they can increase the maximum output power coefficient, reduce the cut‐in wind speed and improve power output at lower wind speeds while reducing the sensitivity to wind speed unsteadiness. A theoretical performance analysis of a 500 kW stall‐regulated wind turbine, based on blade element momentum theory, indicates that passive air jet vortex‐generators would be capable of recovering some of the power loss because of blade stall, thereby allowing attainment of rated power output at slightly lower average wind speeds. Copyright © 2016 John Wiley & Sons, Ltd.     

海上浮式水平轴风力机气动特性研究

以NREL-5 MW风力机为研究对象,基于叶素动量理论,考虑动态失速、风剪切及塔影效应等气动修正模型,开发Matlab非定常气动载荷计算程序,研究浮式水平轴风力机气动特性。结果表明:为保证风力机气动载荷模拟的正确性,气动修正模型必不可少;基础运动对风力机气动性能有显著影响,基础运动使风力机输出功率增大,但同时存在较大的振荡幅度,导致功率输出不稳定;叶片变桨失效导致功率输出更加不稳定。     

Comparison of different numerical approaches to the study of the H-Darrieus turbines start-up

Self-start capability is an important feature of wind turbines. It allows to obtain simpler and cheaper turbines not actively controlled. Different approaches to describe the self-start of an H-blade Darrieus rotor are presented and compared in the present work. The Blade Element Momentum (BEM) approach was compared with two and three-dimensional CFD simulations. The tip–speed ratio versus power coefficient curves and the evolution of the trust forces over a blade revolution highlighted the limits and the strengths of each approach.The BEM model showed remarkable limits to describe to describe the self-start behaviour of the tested geometry. The principal limits of the BEM approach can be ascribed to the absence of well documented aerofoil databases for low Reynolds number and the inadequate modelling of dynamics effects. The 2D simulation allowed to highlight the unsteady features of the flow fields, and the presence of a complex vortices pattern which interact with the blade. Furthermore the comparison between 2D and 3D data demonstrated the importance of 3D effects such as secondary flows and tip effects. These effects were proved to have a positive effect on start-up, increasing the torque characteristic for tip–speed ratio of 1. The start-up capability of H-Darrieus appears to be influenced by many different factors, which include secondary flows, three-dimensional aerodynamic effects and the finite aspect-ratio of the blades.     

Development of an aeroelastic code based on three‐dimensional viscous–inviscid method for wind turbine computations

Aerodynamic and structural dynamic performance analysis of modern wind turbines are routinely estimated in the wind energy field using computational tools known as aeroelastic codes. Most aeroelastic codes use the blade element momentum (BEM) technique to model the rotor aerodynamics and a modal, multi‐body or the finite‐element approach to model the turbine structural dynamics. The present work describes the development of a novel aeroelastic code that combines a three‐dimensional viscous–inviscid interactive method, method for interactive rotor aerodynamic simulations (MIRAS), with the structural dynamics model used in the aeroelastic code FLEX5. The new code, called MIRAS‐FLEX, is an improvement on standard aeroelastic codes because it uses a more advanced aerodynamic model than BEM. With the new aeroelastic code, more physical aerodynamic predictions than BEM can be obtained as BEM uses empirical relations, such as tip loss corrections, to determine the flow around a rotor. Although more costly than BEM, a small cluster is sufficient to run MIRAS‐FLEX in a fast and easy way. MIRAS‐FLEX is compared against the widely used FLEX5 and FAST, as well as the participant codes from the Offshore Code Comparison Collaboration Project. Simulation tests consist of steady wind inflow conditions with different combinations of yaw error, wind shear, tower shadow and turbine‐elastic modeling. Turbulent inflow created by using a Mann box is also considered. MIRAS‐FLEX results, such as blade tip deflections and root‐bending moments, are generally in good agreement with the other codes. Copyright © 2017 John Wiley & Sons, Ltd.     

Self‐induced vibrations of a DU96‐W‐180 airfoil in stall

This work presents an analysis of two‐dimensional (2D) and three‐dimensional (3D) non‐moving, prescribed motion and elastically mounted airfoil computational fluid dynamics (CFD) computations. The elastically mounted airfoil computations were performed by means of a 2D structural model with two degrees of freedom. The computations aimed at investigating the mechanisms of both vortex‐induced and stall‐induced vibrations related to a wind turbine blade at standstill conditions. In this work, a DU96‐W‐180 airfoil was used in the angle‐of‐attack region potentially corresponding to stall‐induced vibrations. The analysis showed significant differences between the aerodynamic stability limits predicted by 2D and 3D CFD computations. A general agreement was reached between the prescribed motion and elastically mounted airfoil computations. 3D computations indicated that vortex‐induced vibrations are likely to occur at modern wind turbine blades at standstill. In contrast, the predicted cut‐in wind speed necessary for the onset of stall‐induced vibrations appeared high enough for such vibrations to be unlikely. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamic stall modelling of the S809 aerofoil and comparison with experiments

An analysis of dynamic stall for the S809 aerofoil has been performed in conjunction with the Leishman–Beddoes dynamic stall model that was modified for wind turbine applications. Numerical predictions of the lift, drag and pitching moment coefficients were compared with measurements obtained for an oscillating S809 aerofoil at various reduced frequencies, mean angles of attack and angle of attack amplitudes. It was found that the results using the modified model were in good agreement with the experimental data. Hysteresis in the aerodynamic coefficients was captured well, although the drag coefficient was slightly underpredicted in the deep stall flow regime. Validation against the experimental data showed overall good agreement. The mathematical structure of the model is such that it can be readily incorporated into a comprehensive analysis code for a wind turbine. Copyright © 2006 John Wiley & Sons, Ltd.