Green Energy and Technology A. P. Schaffarczyk Introduction to Wind Turbine Aerodynamics Green Energy and Technology For furthervolumes: http://www.springer.com/series/8059 A. P. Schaffarczyk Introduction to Wind Turbine Aerodynamics 123 A.P.Schaffarczyk Mechanical EngineeringDepartment Universityof AppliedSciences Kiel Germany ISSN 1865-3529 ISSN 1865-3537 (electronic) ISBN 978-3-642-36408-2 ISBN 978-3-642-36409-9 (eBook) DOI 10.1007/978-3-642-36409-9 Springer Heidelberg NewYork Dordrecht London LibraryofCongressControlNumber:2014941105 (cid:2)Springer-VerlagBerlinHeidelberg2014 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionor informationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodology now known or hereafter developed. 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While the advice and information in this book are believed to be true and accurate at the date of publication,neithertheauthorsnortheeditorsnorthepublishercanacceptanylegalresponsibilityfor anyerrorsoromissionsthatmaybemade.Thepublishermakesnowarranty,expressorimplied,with respecttothematerialcontainedherein. Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) This book is dedicated to my wife Ulrike and our children Claudia, Carola and Kai Foreword The modern development of wind power is a remarkable story of the combined effort of enthusiastic entrepreneurs and skilled engineers and scientists. Today, wind power forms the most rapid advancing renewable energy resource with an annual growth rate of about 30 %. Within the last 20 years the size of wind turbines have increased from a rotor diameter of about 30 m to 150 m, corre- spondingtoanincreaseinpowerbyafactorofmorethan25.Inthesameperiodof time, the knowledge and scientific level of the aerodynamic research tools to develop optimally loaded rotor blades have increased dramatically. Today, wind turbine aerodynamicformsoneoftheresearchfrontiersinmodern aerodynamics. The aerodynamics of wind turbines concerns modeling and prediction of the aerodynamic forces on the solid structures of a wind turbine, and in particular on the rotor blades of the turbine. Aerodynamics is the most central discipline for predicting performance and loadings on wind turbines, and it is a prerequisite for design,development, andoptimizationofwindturbines.From anoutsider’spoint of view, aerodynamics of wind turbines may seem simple as compared to aero- dynamics of, e.g., fixed-wing aircraft or helicopters. However, there are several added complexities. Most prominently, aerodynamic stall is always avoided for aircraft, whereas it is an intrinsic part of the wind turbines operational envelope. Furthermore, wind turbines are subjected to atmospheric turbulence, wind shear fromtheatmosphericboundarylayer,winddirectionsthatchangebothintimeand in space, and effects from the wake of neighboring wind turbines. Introduction to Wind Turbine Aerodynamics, written by an experienced teacher and researcher in the field, provides a comprehensive introduction to the subject of wind turbine aerodynamics. It is divided into ten chapters, each giving a self-contained intro- duction to specific subjects within the field of wind turbine aerodynamics. The book begins with an overview of different types of wind turbines, followed by a comprehensive introduction to fluid mechanics, including different forms of the basic fluid mechanical equations and topics such as potential flow theory, boundary layer theory, and turbulence. The momentum theory, which still forms the backbone in the rotor design of wind turbines, is presented in a separate chapter, which, in a compressed form, describes the basic momentum theory as well as the important blade-element momentum theory. State-of-the-art advanced aerodynamic models are also presented. This includes vortex models as well as vii viii Foreword ComputationalFluidDynamics(CFD)techniquesandanintroductiontothemost common turbulence models. The last part of the book gives an overview of experiments in wind turbine aerodynamics and the impact of aerodynamics on bladedesign.Eachchaptercontainssomehomeworkproblemsandconcludeswith abibliography.IntroductiontoWindTurbineAerodynamicsisahighlytopicaland timely textbook of great value for undergraduate students as well as for trained engineerswhowishtogetafastintroductionintothemanyaspectsoftheamazing world of wind turbine aerodynamics. Jens Nørkær Sørensen, Technical University of Denmark, Kongens Lyngby, Denmark. Copenhagen, April 2014 J. N. Sørensen Preface Wind Energy has flourished over the last 20 years. At the time of this writing, approximately40 GWofwindpowerproductioncapacity,equivalentto80 billion US $ in investment, is added to the worldwide energy portfolio annually. About 2,40,000 (IEA Wind 2012 annual report) engineers are working in this field, and many of them are confronted with challenges related to aerodynamics. Even special academic programs (one of them being the M.Sc. Program in Wind Engineering at the consortium of universities in Schleswig-Holstein, the north- ernmostprovinceofGermany)nowareeducatingyoungpeopleinthisfield.From personal experience, the author knows how difficult it is to navigate through the jungle of scientific literature without being injured before having learned all the basics.Thistextbookintendstohelpyoungandoldinterestedpeopletogaineasier access to the basics of the fluid mechanics of wind turbines. The scope of this book is to take the reader on an interesting journey from the basics offluid mechanics to the design of wind turbine blades. The reader’s basic knowledge of mathematics (Vector calculus) seems to be necessary for under- standing, whereas fluid mechanics will be explained in what amounts to a crash course.Butthereshouldbenomistake:fluidmechanicsbeingnon-linear,evenas anappliedscience,isacomplicatedaffairandcanonlybelearnedandunderstood byone’s ownproblemsolving.Thisisespeciallytruewheninvestigatingturbulent flowwhichisofutmostimportancewheninvestigatingmechanical loadsonwind turbine parts. Therefore, the estimated amount of effort necessary for solving our selected problems at the end of each chapter ranges from 10 min to 10 months. The author will try to give the beginning reader a glimpse of established knowledgeabouttheaerodynamicsofwindturbines.Forthemorefamiliarreader, thisbookwillfacilitatethediscoveryof(andhopefullyunderstandingof)relevant advancedpapers.Inanyevent,allreaderswillgleanadeeperunderstandingwhich will allow them to perform analyses either by hand or by using computer codes. The structure is as follows: an introduction isgiven to inform the reader about the specific relevance of wind turbine aerodynamics, while touching on some interesting historical examples. Then a chapter on installed windmills and wind turbines is added to describe the basics of mainstream and unconventional tech- nology.Afterthatwe arereadytotackleourgeneralframework: thelaws offluid mechanics.Iwilltrytolimitthecontentstothosetopicsthatarecloselyrelatedto wind, understood as turbulent flow in the lower (boundary layer) part of the ix x Preface atmosphere. Because the rotor swept area now easily reaches several hectares ð¼104m2Þ something must be said about real-world inflow conditions, which are notconstantfromaspatialortemporalpointofview. Chapter 5isthecoreofthe book and explains the various versions of Momentum Theory as well as its limi- tations,followedbyadescriptionofapplicationsofitscounterpart:VortexTheory. After exhausting these classical methods, we then have to present the modern approach of solving the differential equations called Computational Fluid Dynamicsinsomedetail.Many effortshave beenexpendedoverthelast15years to bridge the gap from older, sometimes called simpler, theories to actual mea- surementsusingthesecomputationalmethods.Thisleadsdirectlytoatransitionin thenextchaptertoadiscussionoffree-fieldandwindtunnelmeasurements.After thatwewilltry toget evenclosertopractical,real things. Thismeansthatwe try to give examples of actual blade shapes from industry. We conclude with sum- marizing remarks and an overview of possible future developments. As this text grew out of my lectures on wind turbine aerodynamics in the first year of our international Master of Science Program, it is targeted toward this student audience. Nevertheless, I have tried as much as possible to make it valuable and informative for a much broader group of readers. Kiel, Germany, April 2014 A. P. Schaffarczyk Acknowledgments I want to thank several people for introducing me to wind turbine aerodynamics: first of all Sönke Siegfriedsen from aerodyn Energiesysteme GmbH, Rendsburg and Gerard Schepers, ECN, the energy research center of the Netherlands, who gave me a first chance in 1997. My very first contact with wind energy was actually somewhat earlier (1993) during supervising a Diploma-Thesis which is nowsummarizedasthestrangefarmerproblem(seeProblem5.1).Participationin several IEA wind tasks (11, 20 and 29) gave me the opportunity to interact with leading international experts. State of Schleswig-Holstein, Federal German and European funding helped contribute to important projects and finally an Interna- tional M.Sc. program inWindEngineering wasestablishedin2008inwhich now more than 160 young people from all around the world (hopefully) learned about wind energy technology. Mr. Lippert prepared mostof the graphs and Mr. John Thayer brought the text intomorecorrectEnglish.Anyerrors,however,aretheresponsibilityoftheauthor only. xi