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2022 | Book

Handbook of Wind Energy Aerodynamics

Editors: Dr. Bernhard Stoevesandt, Dr. Gerard Schepers, Dr. Peter Fuglsang, Dr. Yuping Sun

Publisher: Springer International Publishing


About this book

This handbook provides both a comprehensive overview and deep insights on the state-of-the-art methods used in wind turbine aerodynamics, as well as their advantages and limits. The focus of this work is specifically on wind turbines, where the aerodynamics are different from that of other fields due to the turbulent wind fields they face and the resultant differences in structural requirements. It gives a complete picture of research in the field, taking into account the different approaches which are applied. This book would be useful to professionals, academics, researchers and students working in the field.

Table of Contents

1. The Issue of Aerodynamics in Wind Energy

Wind turbine aerodynamics remains to be a central field in the development of wind turbines. It comprises several aspects, and methods to be understood really make the best use of it in today’s wind turbine and wind farm development. This chapter gives an overview of all aspects in wind energy aerodynamics covered within this book and the reasons why these aspects are covered. This way it already gives an overview for developers on all the issues wind turbine might face, when dealing with topics related to wind turbine aerodynamics.

Bernhard Stoevesandt, Gerard Schepers, Yuping Sun, Peter Fuglsang

Aerodynamic Blade Design

2. Economic Aspects of Wind Turbine Aerodynamics

Wind turbines are aerodynamically driven machines. The energy produced is chiefly associated with the efficiency of their rotors to convert the kinetic energy of wind to mechanical power. Still, mechanical power must be transformed to electrical through a drivetrain, and the whole system should be kept in place through a support structure. The selection of the proper architecture and the design of these three main subsystems (rotor, drive train, support structure) allow for much freedom, but at the end of the day, it is driven by economical aspects. The optimal turbine design for a given site (onshore/offshore) with known external conditions is the one which can produce electricity in the lowest possible cost, usually expressed through a metric called levelized cost of electricity (LCoE). We acknowledge the fact that since a few years, wind farms are subjected to variable market price mechanisms, and the value of the produced electricity is depending on the market specifics, possibly leading to a different optimum than the one suggested by minimum LCoE. As, however, LCoE remains a pure metric for technology assessment, the goal of this chapter is to make the connection between LCoE and technological selections and design aspects, with focus on rotor aerodynamics. Mastering this connection allows for better understanding the critical areas where the emphasis should be placed for improving cost-efficiency of wind turbine designs.

Panagiotis Chaviaropoulos
3. The Actuator Disc Concept

Actuator disc theory is the simplest rotor theory possible: the rotor is replaced by a permeable disc carrying an axisymmetric force field. It is more than a century old, with a first analytical result obtained by Froude in 1889. In 1918 Joukowsky published the first rotor performance prediction for a helicopter rotor in hover; in 1920 Betz and Joukowsky published the maximum efficiency of wind turbine rotors. In modern rotor design codes, this momentum theory still forms the basis, be it with many adaptations and engineering add-ons. This chapter treats the actuator disc theory in two versions. Best known is the classical theory relating to an actuator disc with thrust acting against the flow but without torque, so without wake swirl. This theory gives the Betz-Joukowsky limit. The results deviate when applied to a flow annulus instead of the entire stream tube, due to the role of the pressure exerted by one annulus to the other. The momentum theory for discs with thrust and torque is relevant for rotors operating with high torque at low rotational speed. For increasing rotational speed, the performance increases from zero to the Betz-Joukowsky limit. In all flow cases, with or without torque, the velocity vector in the meridional plane appears to be constant at the disc. For the performance per annulus and the performance with torque, the deviation from the classical momentum theory is explained by classifying force fields as conservative or non-conservative and investigating their impact on energy and momentum balances.

G. A. M. van Kuik
4. Airfoil Design

This chapter describes how to carry out an aerodynamic design of airfoils. Performance measures to use when designing airfoils for wind turbines are described where the focus is on the design of airfoils for horizontal axis wind turbines. Mainly the characteristics to be considered in the airfoil design process is described, where the methods used for numerical optimization are only briefly described. The airfoil performance in relation to wind turbine rotor performance is described with the corresponding objective functions, constraints, and design variables in the design process. Also, the available prediction methods are described briefly. A general description of the setup of the design problem in terms of the objective function, the constraints, and the design variables are given, followed by a simple example of how to handle an airfoil design problem. In the end, an outlook is given.

Christian Bak
5. Rotor Blade Design, Number of Blades, Performance Characteristics

The design of Blade planform design of the blade planform relates to the choices about the main geometrical characteristics of the blade, namely, chord, twist and blade thickness. In addition to these main quantities, the blade sweep, prebend and cone angle can be considered part of the planform design, though from a purely aerodynamic point of view, their effect on the overall performance of the wind turbine is limited. For a full three-dimensional description of the blade geometry, we also need to define the pitching axis and the stacking axis (i.e. the relative positioning of sections with regard to the pitching axis). These parameters do not affect the 2D aerodynamics of the individual blade sections, but they can have a secondary effect on 3-D aerodynamics.

Giorgos Sieros
6. Blade Design with Passive Flow Control Technologies

This chapter focuses on the application of passive flow control technologies to wind turbine blades. The motivation of using these technologies is always an enhancement of the wind turbine performance (increase of power production, load reduction, noise reduction, etc.) in comparison to the standard blade.Passive flow control solutions can be limited to static add-ons or involve more significant modifications of the blade for dynamic approaches. Furthermore, these technologies can be included in the initial design of the blade or included later as add-ons to improve the performance of an existing blade design.A large number of passive technologies have been proposed for wind turbine applications, although the level of maturity is not the same for all of them ranging from conceptual studies in some cases to commercial products in others. Some representative examples of specific technologies are included in this chapter: vortex generators, static miniflaps, root spoilers, serrations, winglets, passive flaps, and aeroelastic coupling. For each technology, some aspects related to the state of the art, main concept, impact on the wind turbine performance, application, and design have been described.Finally, passive flow control technologies have to be integrated into the design process of wind turbines. To select and properly apply the most suitable technology for each specific problem, the chapter highlights the importance of modeling tools, design methodologies, objectives and restrictions, design parameters, and scale of impact of each passive flow control solution. In addition, from a general point of view, some design guidelines have been mentioned.

Álvaro González-Salcedo, Alessandro Croce, Carlos Arce León, Christian Navid Nayeri, Daniel Baldacchino, Kisorthman Vimalakanthan, Thanasis Barlas

Aerodynamic Modeling Techniques

7. History of Aerodynamic Modelling

Windmills are around us for more than 1000 years, no doubt that many technicians, engineers, and scientists have paid attention to design, build, and improve these fantastic machines that made life easier and enabled the technical development of society to a next level of well-being. The next section provides the major keywords identified in this contribution followed by an introduction section in which the research area of aerodynamic modelling of wind turbines is addressed shortly, from initial historic experimental research to the introduction of the work of some famous scientists in fluid mechanics.The next section “Aerodynamic Modelling of Wind Turbines, an Exciting Story of Its Development and Technology” is the body of the chapter. This section is subdivided into seven subsections which describe the development of wind turbine aerodynamics in a chronological sequence starting with the historic development of windmills and the first aerodynamic theories for design and performance analysis of wind turbines. A separate subsection is dedicated to the emergence of lift-driven vertical axis wind turbines, and then two more subsections are dedicated to typical design issues such as the choice of the number of blades and the design and application of dedicated aerfoils for wind turbine application. The next section is dedicated to operational conditions that occur quite often in wind turbine operation, such as operation under yawed inflow conditions and the operation of wind turbine in wind farms. Section “Aerodynamic Modelling of Wind Turbines, an Exciting Story of Its Development and Technology” is concluded with a subsection on wind turbine augmentation principles and devices.Section “Cross-References” contains the cross-references to other contribution of the handbook, and the final section, Section “References”, provides the references used throughout this contribution.

Gerard J. W. van Bussel
8. Interacting Boundary Layer Methods and Applications

In this chapter the derivation of the viscous integral boundary layer equations is presented in an unsteady, two-dimensional form. Closure sets for both laminar and turbulent flow conditions together with a laminar to turbulent transition method are given. The solution methods for the inviscid region and the viscous-inviscid interaction coupling scheme are briefly discussed. The numerical solution of the integral boundary layer equations are first presented assuming a prescribed solution for the inviscid flow region and then for the coupled viscous-inviscid interacting boundary layer method.

Hüseyin Özdemir
9. CFD Simulations for Airfoil Polars

In this chapter on CFD simulations for airfoil polars, we focus on studies that are relevant to wind turbine airfoils, which have been investigated at reasonably high Reynolds numbers (i.e., Re>˜1×106). We specifically focus on topics such as solution approaches, grid characteristics, effects of turbulence models, near/post-stall behavior predictions, transition modeling, and Reynolds number effects. We include a sample group of selected studies covering relevant airfoils in wind energy research. More research papers can be found using the references given in this paper as a starting point. The main objective is to provide some guidance to the reader regarding how to set up a good CFD simulation to obtain airfoil polars by giving relevant examples from the literature.

Nilay Sezer-Uzol, Oğuz Uzol, Ezgi Orbay-Akcengiz
10. Dynamic Stall

Dynamic stall is a complex fluid dynamics problem that occurs on an airfoil during rapid, transient motion in which the angle of attack goes beyond the static stall angle. Since the instantaneous sectional aerodynamic loads may surpass the static values, dynamic stall events often dictate the operational load range in several systems, including wind energy machines. Typically the phenomenon of dynamic stall is modelled using semi-empirical or the so-called engineering approaches, derived from 2D wind tunnel tests. However, extrapolation to wind energy machines’ behaviour must be done carefully as real conditions of DS occurrence arise as a combination of complex, interacting phenomena, including 3D aerodynamic features (e.g. due to yaw misalignment and rotational augmentation) and also blade structural vibrations.

Ricardo Santos Pereira
11. Thick Sections

A key aspect in blade design is the selection of the airfoils to be adopted. This means generally selecting or designing section families where the percentage thickness can vary between 12% of the chord at the tip and 100% of the chord at the very root, where the blade is connected to the hub.The present chapter provides an overview about the main aspects concerning thick sections and the major challenges around their development. It appears in fact on simulation side that the tools have some limitations in predicting thick airfoil performance, which introduces larger uncertainty on controlling their behavior. At the same time, a rich and reliable experimental database of wind tunnel tests is not available for the inner region airfoils. The wind tunnel testing itself of thick sections offers several challenges and makes more difficult to model this class of shapes.Finally, an overview of most popular features to improve the aerodynamics of these shapes is included in the chapter.

Francesco Grasso
12. The Effect of Add-Ons on Wind Turbine Blades

Add-ons improve power production and reduce noise on wind turbine blades. Appling vortex generators and Gurney flaps to inner part of blade increases lift force and increases power production with 0.3–3% more AEP. Design of vortex generators depends mainly on fin height, fin angle, chordwise location, shape of fin, and spacing between fin pairs. Design of Gurney flaps depends mainly on height. A winglet at the tip of the blade reduces tip-loss and increases power production with 0.5–1% more AEP. Design of winglet depends mainly on height of winglet and if it is an upwind or downwind pointing winglet. Serrated trailing edge applied at the outer part of the blade reduces trailing edge turbulence boundary layer noise with 2–3 dB. Design of winglet depends mainly on relative length of STE of airfoil chord, aspect ratio of tooth length, and the STE flap inclination to airfoil chord line.

Kristian Godsk
13. Pragmatic Models: BEM with Engineering Add-Ons

This chapter discusses several aspects related to engineering methods in wind turbine design codes. Current engineering models for rotor aerodynamics topic are built around the Blade Element Momentum (BEM) theory. The Blade Element Momentum theory in itself is very basic, e.g., it is derived for two-dimensional, stationary, homogenous, and non-yawed conditions. For this reason, several engineering models have been developed which overcome these simplifications and which act as add-ons to the basic BEM theory. This chapter describes the BEM theory, the most important engineering add-ons, and an assessment of BEM with engineering add-ons with results from higher fidelity models and measurements.

Gerard Schepers
14. CFD for Wind Turbine Simulations

The use of computational fluid dynamics (CFD) for three-dimensional wind turbine rotor simulations has become recently more and more popular in wind energy research. Also, in the industry, this tool has started to play a crucial role in the analysis of blade or rotor aerodynamics. In this chapter, the numerical methods to simulate blade or rotor performances are illustrated. In particular, an overview of the state of the art in terms of simulation setup, the corresponding grid requirements, and the proper turbulence models is provided. Moreover, a considerable number of verification and validation cases for both experimental and numerical reference wind turbine models are presented. Finally, the added value of the full rotor simulations as kernel for the development of reduced order methods for load simulations is illustrated by means of extensive literature sources.

Elia Daniele
15. Aeroelastic Simulations Based on High-Fidelity CFD and CSD Models

This chapter focuses on the challenges rising when modeling the aeroelasticity of modern wind turbines utilizing high-fidelity methods. A comprehensive review of the state of the art is presented at the beginning, including engineering models. Since the aeroelastic models consist of a flow and a structural solver, a detailed description of the modeling and simulation techniques is provided, including the basic requirement for coupling a computational fluid dynamics (CFD)-based solver with a computational structural dynamics (CSD)-based solver. The challenges related to the simulation of large rotating bodies, as well as moving grids, are described.In the numerical analysis of the aeroelasticity, the blades could be structurally modeled by mainly three different elements. These are beam, shell, and solid elements, by which the accuracy level of the results could be improved. Therefore, different fidelity levels of structural discretization of the wind turbine are discussed in terms of using these elements. To model the blade using beam elements, a cross-sectional analysis tool is needed to extract the beam structure properties out of the full three-dimensional (3D) geometry of the blade. Coupling CFD to CSD needs great attention at the coupling interface between both solvers.Since they have different grid resolution, a mapping grid technique is needed to translate the data at the interface between the nonmatching grids. Moreover, the coupling scheme should be carefully chosen based on the required accuracy level.The chapter ends by presenting high-fidelity results of a state-of-the-art wind turbine model. The effect of the geometrical nonlinearity of the wind turbine blades is discussed. Comparisons between the different structural elements are described based on these results. The effect of the aerodynamic model fidelity is introduced.

M. Sayed, P. Bucher, G. Guma, T. Lutz, R. Wüchner
16. Aeroelastic Stability Models

In this chapter aeroelastic stability for wind turbines is discussed. The complete wind turbine mode shapes, the harmonic modal components, and the main instabilities are explained, possible resonances addressed, and methods to analyze and improve the stability of a wind turbine design are discussed. The main instabilities that current size wind turbines could suffer from are stall-induced vibrations (edgewise and flapwise, idling instabilities, and vortex-induced vibrations) and classical flutter. The stability of a design can be evaluated using linearized stability tools or nonlinear time domain tools. It is also possible to evaluate damping of some modes on an actual wind turbine. Current size wind turbine has become more flexible, and due to the large deformations, it is required to use advanced blade models when analyzing the stability of the turbine.

Jessica G. Holierhoek

Experimental Approaches to Wind Turbine Aerodynamics

17. Wind Tunnel Wall Corrections for Two-Dimensional Testing up to Large Angles of Attack

An accurate representation of two-dimensional airfoil characteristics measured in a wind tunnel generally requires the inclusion of corrections for interference effects that exist due to the presence of the wind tunnel walls. This chapter discusses the most commonly used correction schemes both for streamlined and separated flow regimes. The classical correction method based on small velocity perturbations gives very good results up to angles of attack of about 20 degrees for chord-to-tunnel height ratios c/h up to 0.36. Even with separation of the boundary layer at a chord location of 30% the corrected pressure distribution matches that of a much smaller model with c/h =  0.15. In the deep-stall range of angles of attack, where the flow separates from the leading edge, the method based on the wake analysis by Maskell with a blockage factor of 0.96 seems to give good results for two-dimensional models up to c/h values of 0.27. A comparison with measurements corrected with the matrix version of the pressure signature method, which uses the pressure distribution on the tunnel walls, shows that the latter leads to slightly larger corrections. Maskell’s method, for which the blockage parameter of 0.96 apparently is based on a single measurement of a two-dimensional flat plate, seems to give better results when a value of 1.03 is used.

W. A. Timmer
18. Examples of Wind Tunnels for Testing Wind Turbine Airfoils

Wind tunnel testing of airfoils is an indispensable part of the wind turbine design process. Especially very large wind turbines with 100m+ blades demand robust airfoils with highly accurate aerodynamic data during the design phase which requires special attention for wind tunnel testing. This chapter provides an overview of wind tunnels that are suitable to support these demands in wind turbine airfoil testing. Starting with two historic wind tunnels, NASA Langley Low-Turbulence Pressure Tunnel and Velux wind tunnels which were supporting designers and researchers at some stage since the beginning of wind energy, a total of 13 wind tunnels are elaborated in terms of both their specifications and the measurement methods. Moreover, a summary of different tests performed in each of these wind tunnels is given. Although the challenges in wind turbine airfoil testing are still out there, it can be concluded that both more precise measurement techniques and modern wind tunnels with special features will serve to tackle these challenges in the near future.

Özlem Ceyhan Yilmaz
19. Wind Tunnel Rotor Measurements

To investigate the effects of rotation without the complicating effect of unknown stochastic inflow, wind tunnels have been used for 3D rotating measurements despite their size limitations. Several examples of past experiments and their lessons learned are given, illustrating the challenges with respect to scaling and measurement instrumentation. An outlook to the future is given anticipating on the further development of instrumentation to quantify aerodynamic loads and flow on smaller scales. Developments to reproduce more realistic inflow conditions in a controlled way are expected to further close the gap between wind tunnel and field measurements. To further drive down the costs of wind energy there is a large need to perform aero-elastic and high Reynolds number testing in a controlled rotating environment.

Koen Boorsma
20. 3D Wind Tunnel Experiments
Measurement Techniques

A lot of effort was recently spent by the wind energy community on developing wind tunnel experiments. These complement full-scale field tests as more measurements can be implemented with less effort, and the overall uncertainty about data is generally lower since environmental conditions are precisely controlled. Progress in wind tunnel experiments and the quality of resulting measurements is closely related to technology of wind turbine models. These are often highly-sensorized and capable of reproducing the aero-servo-elastic response of full-scale machines. This section of the handbook introduces the reader to measurement techniques available in 3D wind tunnel experiments. It provides a review of the turbine model tests that were carried out in the topics of turbine control, wake modeling, wind farm control and floating turbines. The state-of-the art of turbine models technology is presented with reference to some recent examples. Typical measurements of turbine and wind tunnel sensors are described, showing what it is possible to measure and how.

Alberto Zasso, Alessandro Fontanella, Marco Belloli
21. Corrections and Uncertainties

This section deals with a review of how to estimate accuracy of wind tunnel measurements with emphasis to those important for wind turbine aerodynamics.

Alois Peter Schaffarczyk
22. Doppler Lidar Inflow Measurements

During the last few decades, remote sensing devices have established themselves as reliable and effective tools for the measurement of the wind speed. The early applications focused mainly on radar (radio detecting and ranging), but afterward the developed knowledge and principles were transferred to Doppler lidar (light detection and ranging) as well. Lidar uses infrared laser to remotely retrieve a wind speed estimate along the line-of-sight of its emitted laser beam, by relying on the Doppler effect. Lidar has many different applications nowadays, ranging from simple commercial site assessment and power curve measurement to dedicated research on wind field reconstruction methodologies and the application to wind turbine control. This chapter on Doppler lidar inflow measurements attempts to both explain the basic Doppler lidar principles and, at the same time, shed a light on the currently common industrial applications as well as the current academic research.

Marijn Floris van Dooren
23. Load Measurements on Wind Turbines

The chapter load measurements on wind turbines of the Aerodynamic Handbook gives an overview of different direct and indirect methods of measuring physical quantities on wind turbines. It focuses on a practical realization in an in situ environment and points out some dos and don’ts.Especially scientist and students with rather office-based conditions of work should get an impression of the options and challenges that may present themselves when taking field measurements.

Malte Fredebohm, Nora Denecke
24. Surface Pressure Measurements

This Surface Pressure Measurements chapter provides information useful for conceptualizing, designing, and operating full-scale turbine blade pressure measurement systems. Accurate blade pressure measurements depend on thorough understanding of multiple elements within the measurement chain. These include tap disruption of the boundary layer, internal flow dynamics of the orifice, dispersive pressure measurement tube dynamics, compensation for inertial effects due to rotation, discrimination of faint signals in the presence of noise, and digital sampling compatible with system dynamics. Equally important to accurate measurement is foundational knowledge of blade aerodynamic interactions likely to be encountered. These blade flow fields tend to be dominated by rotational augmentation and dynamic stall, which are highly three-dimensional and strongly unsteady. Admittedly, blade pressure measurement is one of the most challenging undertakings in full-scale rotor testing. However, with thorough understanding and careful design, large-scale rotor pressure measurements will deliver measured data that cannot be acquired by alternate means.

Scott Schreck

Aerodynamics and Turbulence

25. Introduction to Turbulence

The turbulent wind is the resource for wind turbines as well as the operating condition for the conversion process from wind into electrical energy. In this chapter, the fundamental aspects of turbulence are considered. The characteristics of turbulence are discussed based on common practices within the wind energy industry with the aim of a standardized characterization as well as the approaches of the scientific turbulence community. These different methods are discussed with respect to the description and quantification of statistical aspects. The aim of this chapter is to provide substantial background information on turbulence, important for an advanced understanding of the operating conditions of wind turbines.

Joachim Peinke, Matthias Wächter, Raúl Bayoán Cal
26. Turbulent Inflow Models

This chapter gives a short overview of different methods used for turbulence generation in the field of wind energy. The wind fields can be used as an inflow for computational fluid dynamics or blade element momentum-based simulations. For all presented models, the mathematical background is given, and it is discussed which advantages and drawbacks they have. The main focus lies on statistical properties in terms of one- and two-point statistics. This includes variance, autocorrelations, cross correlations, and spectral properties. First different recycling methods are explained, namely, the weak and the strong recycling methods. In the following sections, synthetic coherent eddy methods are shown which approximate the turbulent properties well. Those are the digital filtering method and the random spots method. Also an inflow model based on continuous-time random walks is demonstrated which considers higher-order statistics, the increment statistics. In the last section, two spectral methods are in the focus which are used in a wide range in the field of wind energy, the Sandia method, and the Mann model.

Sebastian Ehrich
27. Wind Shear and Wind Veer Effects on Wind Turbines

This chapter highlights key contributions to the scientific literature on the sources of wind shear and wind veer in the atmospheric boundary layer, observations of shear and veer, and the effects of shear and veer on wind turbine power production, wind turbine wake evolution, and wind turbine loads. As wind turbines have grown larger, they encounter deeper and more complicated regions of the atmosphere. Over this height, profiles of wind speed shear and wind direction veer play a quantifiable role. Changes in the wind speed and wind direction across the vertical extent of a wind turbine rotor disk modify the inflow vector on the blades of the turbine, thereby affecting the magnitude and orientation of the lift and drag forces of the blade’s airfoil. These changes can affect the power production and loads on large modern turbines, as well as the evolution of the wake that could affect a downwind turbine.

Julie K. Lundquist
28. Turbulence of Wakes

The size of wind turbines has been increasing steadily over the past decades, and the majority of these turbines are built in wind farms. As downstream turbines will be operating in the turbulent wakes of the upstream turbines, it becomes more and more important to understand the turbulence evolution mechanisms within the wake of a wind turbine for improved load calculations, wind farm layout optimization, and wind farm control methods. In this chapter, the evolution of turbulence within the wake of a single turbine exposed to uniform and atmospheric boundary layer inflow is therefore discussed in detail with views on velocity components, turbulence intensity, length scales, Reynolds stress, energy spectra, and intermittency. Approaches to include turbulence in wake models are explained. Further, turbulence in wakes of yawed turbines will briefly be commented on, and a comparison of turbulence generated by an actuator disk and a wind turbine will be given since the actuator disk concept is an established concept to simplify simulations and experiments.

Ingrid Neunaber

Wind Farm Aerodynamics

29. Wake Structures

This chapter will concentrate on the near wake that also can be divided into very near and near wake. Over the past years, the length of the near wake has become a very important parameter with the appearance of increasingly larger wind farms. The focus of this chapter will be on the physics and possible ways of modeling or estimating the near wake length.The chapter is outlined with a background introducing the subject, a section with basic features and theorems introducing basic concepts, a section describing the wake structure followed by a description of influence of turbulence. The chapter will end with a “rough and ready model” as one way of estimating the length of the near wake followed by an end note with final comments and recommendations.

Stefan Ivanell
30. Industrial Wake Models

This chapter deals with the description of wind turbine wakes by means of reduced-complexity flow models. These models offer the appeal to conduct a vast number of simulations of the wake flow for different atmospheric boundary conditions in short time, thus, they are usually the models of choice in the wind energy industry for assessing and optimizing long-term energy production. The intent of the chapter is to provide an overview over a subset of the most prominent wake models, their physical approximations, and the resulting equations that the models use to describe the flow. At the end, two algorithms are presented to derive a converged wind farm flow by superimposing the wake deficits derived from the presented single wake models. This offers the readers the opportunity to implement a wind farm flow model with their wake model of choice themselves.

Jonas Schmidt, Lukas Vollmer
31. Wake Meandering

The present chapter deals with wake meandering – its physics, its modeling, and its consequences for production and loading of wind turbines erected in wind farms.Wake meandering is the phenomenon describing the dynamics of wind turbine wakes. Nowadays there is almost unanimous agreement in the wind energy community that wake meandering is caused by large turbulent eddies in the atmospheric boundary layer. In the introductory part of this chapter, an accounting of the development leading to this conclusion will be given. This includes both full-scale experiments using advanced lidar technology, scaled wind tunnel experiments using both boundary layer wind tunnels and conventional wind tunnels, and last, but not least, detailed unsteady computational fluid dynamics large eddy simulations with wind turbines modeled as actuator lines.Recognizing the fundamental physics behind the wake meandering phenomenon, both high-fidelity and medium-fidelity modeling approaches are described. Being related to large-scale turbulence structures in the atmospheric boundary layer, impact from atmospheric boundary layer stability should be expected, and this important aspect is therefore also included in the modeling part.The chapter is concluded with various example applications ranging from wind turbine production prediction over wind turbine load prediction to optimal wind farm layout, for which both accurate production and load prediction are needed.

Gunner Chr. Larsen
32. CFD-Type Wake Models

CFD-type wake models are widely used to investigate the physics of the flow through wind turbines and wind farms, in particular with regard to the interaction between the atmospheric boundary layer and wind turbine wakes. This distinguishes them from engineering-type wake models that are preferred in industrial applications which need simple and fast results. The first part of this review focuses on the theory of CFD-type wake modeling. The three most widely used CFD-type wake models are described in detail: The standard uniformly loaded actuator disk model, an advanced actuator disk model including distributed forces and rotational effects and the actuator line model. A few other actuator models will be covered briefly. Furthermore, methods to model the effect of tower and nacelle are reviewed as well as the implementation of control mechanisms as speed, yaw and pitch control. Sections on tip-loss corrections, the inclusion of blade generated turbulence and the process of smearing the forces on the computational grid complete the first part. The second part of this review focuses on applications of CFD-type wake models, starting with an overview over verification and validation studies. Next, various application fields are reviewed, grouped by topic, with emphasis placed on the impact of atmospheric stability and terrain.

Björn Witha
33. Wind Farm Cluster Wakes

The last couple of decades have seen developments of large wind farms partly in close distance forming “wind farm clusters.” Those wind farm clusters often consists of several hundreds of wind turbines which interact with the atmospheric boundary layer. Measurements of wind farm cluster wakes have been carried out in recent years showing wake length of up to 100 km downstream of wind farm clusters. The advancement of remote sensing tools is promising as typical scales of the wakes are approached. As several orders of magnitude in scales have to be covered from the flow around the single turbine to the far distant wake, there are many challenges in deriving models and wind farm parametrizations for flow models. Single situations have quite extensively been studied based on remote sensing and also aircraft data. However, the impact on wind resources and annual energy production (AEP) is so far based on model results only and needs further insights using wind farm production data that will be available in future years due to the current expansion of wind farms.

Martin Dörenkämper, Gerald Steinfeld
34. Wind Tunnel Testing of Wind Turbines and Farms

This chapter reviews the wind tunnel testing of scaled wind turbines and farms, which in recent years is finding an increased interest by the scientific community for aerodynamic, aeroelastic and control applications. The chapter starts by reviewing the fundamental scaling laws, which reveal the quantities that need to be matched between full-scale system and scaled model for similarity to hold. The scaling requirements are then combined with the additional constraints due to physics, manufacturing, actuation and instrumentation, leading to a best compromise design of the scaled model for a given application. The analysis highlights that a scaled model cannot in general be an exact and complete replica of its full-scale counterpart. Nonetheless, sophisticated scaled experimental setups characterized by a significant realism can be developed for a plethora of different complex applications, in support of physical understanding, model validation and calibration, and the low-cost low-risk testing of new ideas and concepts. The chapter is concluded with an overview of turbine and farm experiments developed by the authors.

Carlo L. Bottasso, Filippo Campagnolo
35. Wake Measurements with Lidar

Since its emergence in the mid-2000s, Doppler wind lidars specifically developed for the wind industry were used for a number of different types of applications including field experiments that were focused on studying wind turbine wakes. Wake flows are to be considered as complex flows and represent for this reason a specific challenge to a remote-sensing lidar instrument. There are various approaches to overcome this challenge including the combination of certain lidar scan strategies and model assumptions as well as the application of more than one lidar device with overlapping planes or intersecting beams, respectively. The flexibility lidar technology offers is directly linked to the need for validating a specific measurement strategy in a particular situation. A respective framework is proposed with the definition of lidar use cases.Beside a comprehensive introduction to lidar measurement principles and strategies, that are relevant for the measurement of wake characteristics, and the use case concept, we give an overview about published wake measurement campaigns. The listed field experiments are categorized with respect to the applied types of lidar instruments and measurement strategies as well as the studied wake features. When considering the limitations the technology may have in particular situations carefully and assessing the measurement uncertainties in a sufficient way, lidars can be very powerful tools for studying wake flows as well as for quantifying specific wake characteristics.

Julia Gottschall
36. SAR Observations of Offshore Windfarm Wakes

Satellite synthetic aperture radar (SAR) is the only operational instrument providing information on near-surface ocean wind fields with coverage of up to several hundred kilometres and spatial resolutions below 100 m. SAR is independent of daylight and cloud conditions and therefore an interesting information source for the analysis of wakes behind offshore windfarms. In this chapter, an overview is given on the research that was done on this subject so far. In a first step, the basic measurement principle of SAR is explained, and typical imaging configurations and operation modes are presented. A summary is given of important past, present and future satellite SAR missions launched by various nations and organisations. In this context, the growing amount of data, which are available for analysis, and the existing efforts to ensure a continuity of SAR data acquisitions are pointed out. Both potentials and limitations of the system are discussed with a focus on offshore windfarm-related issues. Of particular importance is the fact that the basic quantity measured by SAR is the sea surface roughness. It is clear that the relationship between the roughness and wind speeds at higher levels is not straightforward, in particular in complex environments like the surroundings of offshore wind farms, and this will be discussed in some detail. Other complications can be caused by image features, which are actually related to oceanic processes like ocean current divergence. Finally, a basic limitation of SAR wind measurements is the fact that information about wind direction can only be obtained in a very indirect way and even that is not guaranteed. In practice, many users of SAR data therefore use additional data from numerical models or in situ stations. Approaches to estimate the wake length from SAR data are presented, and some derived results concerning the relationship between wake length and atmospheric stability are discussed. The discussion also includes some image features, which appear counterintuitive at first sight, like an apparent increase of surface roughness within about 10 km downstream offshore windparks observed on some SAR scenes. Additional applications of SAR data in the context of offshore windfarming, like the assessment of wind energy potential on larger spatial scales, are briefly addressed as well.As one major conclusion drawn in this chapter, it is strongly recommended to use SAR data in combination with other sources of information, like in situ data or numerical model simulations. Different options and challenges associated with data merging of this kind are discussed.

Johannes Schulz-Stellenfleth, Bughsin Djath
37. Met Mast Measurements of Wind Turbine Wakes

This chapter addresses met mast measurements for wind turbine wake research, where we tackle the matter by posing and answering three questions. By asking ourselves what the value is of met mast measurements for wake/aerodynamic research, we describe how met masts can be and have been used in aerodynamic research, accompanied by a number of illustrative examples. Second, we trigger the question how to obtain the highest quality in met mast measurements. In this respect, we have identified standardization in the measurement chain itself, the definition of success criteria and a good understanding between what is required on model side, and what is practically possible on the experiment side. Third, we wonder what have we learned from wind turbine wake measurements, where we describe in more detail some experiments, returning to the illustrative examples. These reflect upon quantifying power deficits of waked turbines, quantifying offshore wind farm wake effects and quantifying wake deflection of misaligned wind turbines.

J. W. Wagenaar
38. Aerodynamics of Wake Steering

This chapter discusses the mechanisms that enable wake steering within a wind farm with a focus on wake steering performed using yaw misaligned turbines as this is the most popular approach to wake steering, although there are others. Wake steering is a type of wind farm control in which wind turbines in a wind farm operate with an intentional yaw misalignment to mitigate the effects of its wake on downstream turbines in order to increase overall combined wind farm energy production. This chapter goes into detail regarding the dominant aerodynamic characteristics that are present when a turbine operates in yaw misaligned conditions and suggests analytical models that can capture these effects. A detailed analysis of large-scale flow structures generated in wind farm control through yaw misalignment is presented. A collection of counter-rotating vortices, produced from a misaligned turbine, deforms the shape of the wake and produces asymmetric effects with oppositely signed yaw angles. These vortices generated by an upstream misaligned turbine can also deflect wakes of downstream non-misaligned turbines. This chapter also addresses the importance of modeling these counter-rotating vortices in analytical models for wind farm control design and for accurately quantifying the impacts of wake steering on gains in power production in larger wind farms.

Jennifer King, Paul Fleming, Luis Martinez, Chris Bay, Matt Churchfield
39. Optimizing Wind Farm Layouts

Optimizing wind farm layouts is to optimize the designable variables in a wind farm, during the farm planning before the wind farm construction. Based on the best knowledge of the planners on the wind farm, i.e. the wind farm modelling, the wind farm daily operation is simulated. Then the designable variables are adjusted, to obtain the possible best targets such as the maximum power generation, the minimum cost of energy, etc. The task of optimizing wind farm layout is implemented by establishing the optimization problem and solving it with proper methods. This chapter gives a short overview of the wind farm layout optimization in the field of wind energy. After briefly introducing the basic concepts and guidelines, the problem formulation of the wind farm layout optimization is presented, consisting of wind farm modelling, objective function, constraints and computational complexity. Different methods used for automated wind farm optimization are also presented, as the reference solutions to the task, including the calculus based methods, the heuristic optimization algorithms and the hybrid approaches. Additionally, the research needs and trends in wind farm optimizations, and the commercial software for optimizing the wind farm layout are provided in this chapter.

Xiao-Yu Tang

Alternative Concepts

40. Kites for Wind Energy

Airborne wind energy systems (AWES) represent an emerging industry which is built on the premise that energy can be extracted from the wind at a cheaper overall cost than with conventional approaches. In an AWES, energy is produced by a flying apparatus, either by onboard generators or by a generator located on the ground. The flying apparatus is attached to a tether, which allows the flying apparatus to fly across the predominant wind direction, which increases the relative flow velocity experienced by the energy producing surfaces. This technique, often referred to as crosswind energy generation or crosswind flying, is fundamental to maximizing the energy production of an AWES.

Paul Williams, Evgeniy Pechenik
41. Vertical-Axis Wind Turbine Aerodynamics

Horizontal-axis wind turbines (HAWTs) are widely studied and have proven their technological capabilities. However, wind turbines are moving into new environments, such as floating far-offshore or urban applications, where the operational conditions are significantly different. Vertical-axis wind turbines (VAWTs) could be more suitable and compatible in these environments, hence, the interest in VAWTs is rekindling. Although vertical-axis wind turbines have a long history, the behavior of these turbines and their complex flow field is still not fully understood. The lack of understanding the complex unsteady aerodynamics of VAWTs and the challenge to predict the loads and performance of this kind of turbines accurately, has led to systematic failures and as such variable interest in VAWTs throughout history. Advancing the understanding and modeling of VAWT’s aerodynamics will be crucial to advance the technology further.This chapter highlights the main aerodynamic phenomena and challenges of vertical-axis wind turbines. First, an introduction is provided on the VAWT history and (dis-)advantages. The basics of VAWT aerodynamics and the various rotor simplifications/representations are presented. Further, the state-of-the art aerodynamic modeling techniques, specifically for VAWTs, are discussed. Since VAWTs are inherently unsteady, the main unsteady phenomena that play a crucial role in VAWT aerodynamics are summarized. Finally, wake aerodynamics and the importance of airfoil design for VAWTs are highlighted.

Delphine De Tavernier, Carlos Ferreira, Anders Goude


42. Wind Turbine Aerodynamic Noise Sources

In this chapter, the basic phenomena and mechanisms responsible for wind turbine noise are investigated. Current scientific knowledge from theoretical and experimental points of view and existing studies on the subject are reviewed. Here, the focus is on aerodynamic noise sources as these are in usual conditions the main contributors to wind turbine noise, although some other noise mechanisms are also shortly discussed. Individual aerodynamic noise sources are investigated first. Then, the focus is set on noise from a wind turbine, and finally from a wind farm, as a whole.

Franck Bertagnolio, Andreas Fischer
43. Wind Turbine Noise Propagation

The noise emitted by operating wind turbines will be influenced along its propagation to distant receivers by a large number of factors, including wind direction, wind and temperature gradients, atmospheric turbulence, and the ground effects arising both from the local acoustical properties of the ground itself and from screening or enhancement due to terrain. This chapter presents an overview of different environmental sound propagation calculation methodologies, of varying complexity, as applied to wind turbines. Available methods can broadly be separated into two categories: empirical or engineering methods and more complex numerical methods. The latter possess the ability to provide highly accurate representations of propagation for specific parameters in certain conditions, but are generally complex and computationally intensive. Their accuracy in practical use will be limited by the imperfect knowledge of the full range of atmospheric and ground conditions along the propagation path. However, they are successfully used to assess the parametric sensitivity of received noise levels and to isolate and better understand individual propagation effects. For most practical applications, engineering methods provide reasonably accurate predictions, provided they are applied correctly within their appropriate scope. Accuracy in this context should be interpreted within the context of the natural variability of environmental sound fields.

Matthew Cand, Andrew Bullmore, Timothy Van Renterghem
44. Measuring and Analyzing Wind TurbineNoise

Noise from wind turbines is often a decisive parameter when introducing a wind turbine project and noise data must be reliable. The IEC 61400-11 measurement methods for wind turbine noise emission are the most recognized methods and provide data for siting as well as for comparison between makes and models. The measurement method has been used since 1999 and makes it possible to follow the development in noise from the early versions in the 100 kW range to today’s MW wind turbines. For development purposes more advanced measurement methods like microphone arrays are used to give detailed information on the sources of noise on a wind turbine.

Bo Søndergaard, Tomas R. Hansen, Stefan Oerlemans
45. Wind Turbine Noise Mitigation

Well-designed modern wind turbines are relatively quiet compared to other large industrial machines. Despite increasing rotor diameters, noise levels from onshore wind turbines have stabilized or even reduced. Nevertheless, the noise of wind turbines still constitutes an important hindrance for the widespread application of onshore wind energy. Many onshore wind turbines need to run at reduced power to meet neighbor noise limits. This chapter provides an overview of noise reduction technologies applied in the industry. Newly developed low-noise technologies are continuously implemented in new and existing wind turbines. In this way, the cost of clean wind power can be reduced while addressing societal concerns.

Stefan Oerlemans
46. Direct Prediction of Flow Noise Around Airfoils Using an Adaptive Lattice Boltzmann Method


Mikaël Grondeau, Ralf Deiterding
Handbook of Wind Energy Aerodynamics
Dr. Bernhard Stoevesandt
Dr. Gerard Schepers
Dr. Peter Fuglsang
Dr. Yuping Sun
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