Engineering models in wind energy aerodynamics

353 Pages·2012·16.42 MB·English

Engineering models in
wind energy aerodynamics
Development, implementation and analysis
using dedicated aerodynamic measurements
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. ir. K.C.A.M. Luyben
voorzitter van het College voor Promoties,
in het openbaar te verdedigen
op dinsdag 27 november 2012 om 15 uur
door
Jan Gerhard SCHEPERS
Ingenieur Luchtvaart en Ruimtevaarttechniek
geboren te Winterswijk.
2
Ditproefschriftisgoedgekeurddoordepromotors:
Prof.dr.G.J.WvanBussel
Prof.dr.Ir.G.A.M.vanKuik
Samenstellingpromotiecommissie:
RectorMagnificus Voorzitter
Prof.dr.G.J.WvanBussel TechnischeUniversiteitDelft,promotor
Prof.dr.ir.G.A.M.vanKuik TechnischeUniversiteitDelft,promotor
Prof.dr.ir.T.vanHolten TechnischeUniversiteitDelft
Dr.ir.L.M.MVeldhuis TechnischeUniversiteitDelft
Prof.dr.J.N.Sørensen TechnicalUniversityofDenmark
Dr.S.Schreck National Renewable Energy Laboratory
NREL
Ir.H.Snel Energie Onderzoek Centrum Nederland,
ECN
Prof.dr.ir.drsH.Bijl TechnischeUniversiteitDelft,reserve
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ISBN:978-94-6191-507-8
Allrightsreserved. Nopartofthematerialprotectedbythiscopyrightnotice
maybereproducedorutilizedinanyformorbyanymeans,electronicor
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Authoremail:[email protected]
TothegenerationofErikandSander: Thatthisworkmaybeamodest
contributiontoamorecleanandprosperousworld

Acknowledgements
Writing this thesis was a very pleasant task. It sometimes felt a bit like a
journey through time because some of the described results go back to the
1980’sand1990’swhenwindenergyresearchprojectswerecarriedoutwith
differentpeopleinaverydifferentsetting.
Most of the described work comes from cooperation projects. That means
thatthisworkwouldn’thavebeenpossiblewithoutthehelp,inspiration,ideas
andresultsofverymanycolleaguesin,butalsooutsideECN.
IverymuchwouldliketothankallmycolleaguesatECN-Wind. Althoughthis
thesis has largely been written in own time the findings come from projects
whicharecarriedoutwithintheECNorganizationtowhichmanycolleagues
contributed. ItisimpossibletolistallcolleaguesbutHermanSnelneedstobe
mentioned. Heinitiatedandcontributedtosomuchofthedescribedresearch
andhehasbeenatremendoussourceofinspirationforme. EdwinBothelped
mewiththecumbersometaskoffinetuningvariousfigures. Moreimportant,
he also worked on the fine tuning of the Farmflow modelling and the under-
standing of wind farm measurements, the results of which are described in
this thesis. I also would like to acknowledge my former group leader Peter
Eecenwhoalwayssupportedandstimulatedthewritingofthisthesisanddid
whateverhecouldtomakethispossible.
Making this thesis in Latex was a wise decision and I am still amazed by
the fact that it only took a split second to include large portions of text and
figures as produced in the 1980’s into the present report. And if something
went wrong due to my clumsiness there was always the help from a Latex
wizardlikeBernardBulder.
ii Acknowledgements
Another wizard is Koen Boorsma with whom I had the pleasure to work to-
getherintheMexnextproject. Severalfindingsfromthatprojectasdescribed
in this report are the result of very intense discussions behind a computer
screenwhenweprocessedthedataandtriedtounderstandthem.
Of course I also would like to thank Gijs van Kuik and Gerard van Bussel
for their support and their critical reviews. They gave me the opportunity to
spend some time at TUDelft to write this thesis. This gave me a better look
in their ”kitchen” by which I am even more impressed by the activities there.
EquallyimportantwasthesupportfromSylviaWillemswhomanagedtohelp
mewithreallyeverypracticalproblemwhichIhadtofaceindoingthisPhD.I
hope,alsofromtheperspectiveofNHL,foracontinuationofthecooperation
withDelft.
And then there is an almost infinite number of colleagues outside ECN with
whom I cooperated. It is impossible to mention them all but I always ex-
perienced this cooperation as very pleasant in particular during the various
project meetings at sometimes very exotic places. Through these coopera-
tionsIgotaccesstoawealthofuniqueresults, e.g. measurementsfromthe
world’s largest wind tunnel: NASA-Ames or from the largest off-shore wind
farms. Without these (and other) contributions this PhD wouldn’t have been
possibleatall.
Also, I donot want to forget the funding agencies. For example, the former
Senter NOVEM agency which sponsored many of the projects described in
this thesis. The same holds for the EU. Several results come from IEA pro-
jectswhichwerecarriedoutundertheauspicesoftheIEAExecutiveCommit-
tee,themembersofwhichwherealwaysveryhelpfulinestablishingmondial
research cooperations. These IEA projects formed and still form great op-
portunitiesforcooperationnotonlywithinEuropebutwithpartiesalloverthe
world,includingtheUS,Canada,Korea,JapanandChina.
Lastbutdefinitelynotleast. Irealizethatthereisn’taworseclichethanwrit-
ing that this thesis was only possible through the unconditional support from
my home front (Henrike, Erik, Sander). Still it needs to be written because it
is so very true! As a matter of fact this thesis has been my excuse for doing
completelynothingathomeformorethanayear. Iamreallylookingforward
toreturntonormallifeagain. Thankyou!
Summary
The subject of aerodynamics is of major importance for the successful de-
ploymentofwindenergy. Asamatteroffacttherearetwoaerodynamicareas
in the wind energy technology: Rotor aerodynamics and wind farm aerody-
namics. Thefirstsubjectconsiderstheflowaroundtherotorandthesecond
subject considers the (wake) flow within a wind farm. For both areas calcu-
lational models have been developed which are implemented in rotor design
and wind farm design codes respectively. Accurate rotor design codes en-
able a reliable design of wind turbines and an optimization towards a higher
energyproductionandlowerloads,i.e. towardsalowercostofenergy. They
arealsorequiredtoavoiddesignerrorsandhencetoreduceinvestmentrisks
ofwindturbinemanufacturers. Accuratewindfarmdesigncodesareneeded
to predict the production losses and the load increase on turbines in a farm
due to wake effects. They also support the optimization of wind farms (e.g.
throughfarmcontrol)bywhichtheenergylossesandtheloadincreasefrom
wakeeffects(andconsequentlythecosts/kWh)areminimized.
For both areas the complexity of models range from engineering methods
to very advanced Computational Fluid Dynamics (CFD) methods. The term
engineering method is meant to indicate a model which casts a complicated
flow phenomenon into a transparent form. This generally goes together with
aneconomiccomputerusage. Inthisrespectitisveryimportanttorealizethat
windenergydesigncalculationsareinherentlyverytimeconsumingbywhich
advancedCFDmodelsarestillbeyondtheroutinepossibilitiesofindustry. As
suchengineeringmethodsformtheonlyalternativeforthatpurpose.
Themainaimofthepresentthesisisthentodescribeseveraldevelopments
ofthelast25yearswhichhaveledtothepresentgenerationofaerodynamic
engineering models. It will be shown that much progress has been made
iv Summary
both on the field of rotor aerodynamics as well as on the field of wind farm
aerodynamics and that this progress was highly supported by the fact that
dedicatedaerodynamicmeasurementdatahavebecomeavailable. Thepro-
gress is illustrated by the engineering models which are developed and val-
idated by ECN in several large (inter)national cooperation projects in which
thesemeasurementsplayedanimportantrole. Theauthorofthisthesiswas
heavilyinvolvedintheseprojectsandoftenactedascoordinator. Sincethese
projectswereperformedinclosecooperationwithotherinstitutes(whichused
differenttypesofmodels),theactivitiesoftheauthorcanbeplacedinawider
context.
The first part of the thesis is devoted to rotor aerodynamics. Basically the
subject of rotor aerodynamics can be subdivided in two parts: The first part
deals with the global flow field around a wind turbine. This type of model-
ling is called induction aerodynamics, since its main goal is to determine the
induced velocities at the blade. The second part deals with the loads on a
wind turbine blade as a response to this flow situation and is called blade
aerodynamics. Current engineering models for rotor aerodynamics topic are
builtaroundtheBladeElementMomentum(BEM)theory. TheBladeElement
Momentum theory in itself is very basic, e.g. it is derived for 2-dimensional,
stationary, homogenous and non-yawed conditions. For this reason several
engineering models have been developed which overcome these simplifica-
tionsandwhichactasadd-on’stothebasicBEMtheory. Theseengineering
add-on’shavebeendevelopedforthefieldofbladeaerodynamicsandforthe
fieldofinductionaerodynamics.
Inthisthesisacomparisonismadebetweencurrentengineeringmodelsand
theengineeringmodelsfrom25yearsago. Theengineeringmethodsfrom25
yearsagowerenotmuchmorethantheverybasicBEMtheorywithaPrandtl
tiplosscorrectionandaturbulentwakecorrection. Moreoveratowershadow
modelbased onadipolemodel anda’geometric’ correctionforcone andtilt
anglewereincluded,whileyawwasmodelledwiththeadvancingandretreat-
ing blade effect only. Since then the models for airfoil aerodynamics have
beenimprovedbyaddingunsteadyandthree-dimensionaleffects. Theseun-
steadyeffectscanbedividedinviscousdynamicstalleffectsandnon-viscous
effectsatlowanglesofattack. Thethree-dimensionaleffectsoccuratthein-
nerpartofthebladewherestallisdelayedandattheouterpartwherethetip
decreasestheloads. Intermsofinductionaerodynamics,modelshavebeen
added for dynamic inflow, the azimuthal variation of the induced velocity at
yawandamodelforrootlosses.
v
TheprogressintherotoraerodynamicengineeringmodelsfromECNismainly
described along results of four subsequent IEA Tasks: IEA Task 14 and 18,
IEA Task 20 and IEA Task 29(Mexnext). An IEA Task (sometimes called an
IEA Annex) is a cooperative project carried out under auspices of the Inter-
national Energy Agency IEA. The goal of IEA Tasks 14 and 18 was to cre-
ate a database of detailed aerodynamic measurements which all have been
taken on turbines under atmospheric conditions. The goal of IEA Task 20
was to analyze the measurements which have been taken by the National
Renewable Energy Laboratory NREL on a 10 meter diameter wind turbine
whichwasplacedintheverylargeNASA-Ameswindtunnel. FinallyIEATask
29(Mexnext) analyzed the measurements which have been taken in the EU
ProjectMexicoonawindturbinerotorwithadiameterof4.5metersplacedin
theLargeLowSpeedFacility(LLF)oftheGermanDutchWindTunnel(DNW).
In all of these experimental programs pressure distributions were measured
atdifferentlocationsalongtherotorblades. MoreovertheMexicoexperiment
mappedtheflowfieldupstream,inanddownstreamoftherotorplane.
The detailed aerodynamic measurements from the IEA Tasks were found
to be very useful in the development, improvement and validation of these
engineering models because they made it possible to extract aerodynamic
phenomena which are hidden in the very global information from conven-
tionalmeasurementprograms. Itisconcludedthatonlydetailedaerodynamic
measurementsmaybeusedforvalidationofaerodynamicdesignmodels: A
validationonbasisofglobalturbine(blade)loadsdoesnotgiveadecisivean-
swer on the accuracy of aerodynamic models due to the fact that ’compens-
ating errors’ may occur. Moreover it will be shown that the measurements
revealed several shortcomings in aerodynamic engineering methods which
partlycouldbe’repaired’,sometimeswiththehelpofmorerefinedmodels.
Several recommendations are made on rotor aerodynamics. This includes
some specific further improvements which are still possible to the current
state of engineering models. Amongst other things, models for the annulus
averaged induction at yaw, tip loss effects and time constants at dynamic
inflow can be improved further. These improvements can be established
by calibrating engineering methods to results from more advanced aerody-
namic models (e.g. CFD or free vortex wake methods). The background
for this recommendation lies in thefact that the validationof these advanced
aerodynamic models with the detailed aerodynamic measurements from the
IEA Tasks showed a clear added value from such methods on these fields.
vi Summary
Moreover it is concluded that three-dimensional and unsteady effects on the
dragdeservemoreattention.
Howeverthemostimportantrecommendationisrelatedtotheobservationof
an unbalance in the aerodynamic wind energy society: Much effort is spent
onthedevelopmentofaerodynamicmodels(oftenoflittlemutualdifferences)
but the amount of experimental validation material is (too) limited. Therefore
it is recommended to intensify the activities on rotor aerodynamic measure-
mentsinboththewindtunnelandthefield. Specialattentionshouldbepaid
to the measurement of those phenomena which, until now, are still largely
concealed (e.g. boundary layer phenomena) or unclear (e.g. the relation
between blade loads and underlying flow field which is found puzzling in the
Mexicoexperiment).
The present thesis also describes the progress which has been made on
the field of wind farm aerodynamics. Opposite to the situation for rotor aero-
dynamics, where the BEM model can be appointed as the main model, the
variety of models for wind farm aerodynamics is much larger. This is partly
duetothefactthatawindfarmaerodynamicmodelshouldcovermuchmore
aspects: It should model both the aerodynamic behavior of the rotor (which
generates the wake) as well as the turbulent wake downstream of this rotor.
The fact that calculational time is such an extreme constraint adds to the di-
versity: As a consequence CFD modelling of wind farm aerodynamics often
onlyreferstothemodellingofthewakeandnottothemodellingoftherotor. It
alsomakesthatwindfarmandrotoraerodynamicsaresometimesconsidered
tobefullyseparatesubjects. Thisisseenasanundesireddevelopmentsince
the aerodynamics of the wake is largely determined by the aerodynamics of
therotorstandinginfrontofthewake.
In this thesis the main characteristics of the wake flow behind a wind tur-
binearedescribedtogetherwithasurveyofwindfarmaerodynamicmodels.
Most of the attention is focussed on an intermediate between the very basic
modelsandtheCFDcodes,i.e. theparabolizedwakemodels. Thesemodels
arerelativelyeconomicincomputerusage(bywhichtheyarestillconsidered
tobeengineeringmodels)wheretheymodeltheso-calledfarwakeinaphys-
icallyaccurateway. Thedisadvantageliesinthefactthattheygenerallyneed
anempiricaltreatmentofthenearwake. Thisagaingoestogetherwithavery
simplemodellingoftherotor. Theprogressinwindfarmaerodynamicmodels
isthenillustratedwithECN’swindfarmdesigncodeFarmflow(basedonthe
formerWakefarmwakemodel)whichcombinesaparabolizedk-(cid:15)turbulence