Thick Airfoils and High Lift

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Thick Airfoils and High Lift
Gaunaa, Mac; Sørensen, Niels N.; Bak, Christian
Published in:
Research in Aeroelasticity EFP-2007-II
Publication date:
2009
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Gaunaa, M., Sørensen, N. N., & Bak, C. (2009). Thick Airfoils and High Lift. In T. Buhl (Ed.), Research in
Aeroelasticity EFP-2007-II (pp. 103-114). Danmarks Tekniske Universitet, Risø Nationallaboratoriet for
Bæredygtig Energi. Denmark. Forskningscenter Risoe. Risoe-R No. 1698(EN)

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Research in Aeroelasticity EFP-2007-II

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Edited by Thomas Buhl
Risø-R-1698(EN)
June 2009
Author: Thomas Buhl (ed.) Risø-R-1698(EN)
Title: Research in Aeroelasticity EFP-2007-II June 2009
Division: Wind Energy Division

Abstract (max. 2000 char.): ISSN 0106-2840
This report contains results from the EFP-2007-II project "Program ISBN 978-87-550-3759-5

for Research in Applied Aeroelasticity". The main results can be
summed up into the following bullets:
• 2D CFD was used to investigate tower shadow effects on both
upwind and downwind turbines, and was used to validate the
tower shadow models implemented in the aeroelastic code
HAWC2.
• Using a streamlined tower reduces the tower shadow by 50% Contract no.:
compared to a cylindrical tower. Similar reductions can be ENS 033001/33033-0266
achieved using a four legged lattice tower.
• The application of laminar/turbulent transition in CFD
Group's own reg. no.:
computations
1110065-01 to 1110065-08
for airfoils is demonstrated. For attached flow over
thin airfoils (18%) 2D computations provide good results
while a combination of Detached Eddy Simulation and laminar/ Sponsorship:
turbulent transition modeling improve the results in stalled Danish Energy Authorities

conditions for a thick airfoil.
• The unsteady flow in the nacelle region of a wind turbine is
Cover :
dominated by large flow gradients caused by unsteady shedding
Multi element airfoil
of vortices from the root sections of the blades.
• The averaged nacelle wind speed compares well to the
freestream wind speed, whereas the nacelle flow angle is highly
sensitive to vertical positioning and tilt in the inflow.
• The trailing edge noise model, TNO, was implemented and
validated.
The results showed that the noise was not predicted accurately,
but the model captured the trends and can be used in
airfoil design. The model was implemented in the airfoil design
tool AIRFOILOPT and existing airfoils can be adjusted
to maintain the aerodynamic characteristics, but with reduced
noise in the order of up to 3dB in total sound power level and
up to 1dB with A-weighting.
• 2D CFD simulations are performed to verify their capability
in predicting multi element airfoil configurations. The present
computations show good agreement with measured performance
from wind tunnel experiments.
• The stochastic fluctuations of the aerodynamic forces on blades
Pages: 147
in deep-stall have an insignificant effect on the risk of stallinduced
Tables: 19
vibrations predicted by quasi-steady aerodynamic References:
models, but more realistic models of deep-stall aerodynamics
must be developed to finally conclude on the real risk of Information Service Department
Risø National Laboratory for
stallinduced vibrations at standstill.
Sustainable Energy
• Finite element analysis shows that local blade cross section
Technical University of Denmark
deformations P.O.Box 49
caused by global blade deflection do not have significant DK-4000 Roskilde
Denmark
influence on the aerodynamic performance.
Telephone [phone]

[email protected]
Fax [phone]
www.risoe.dtu.dk
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1 Preface 6
2 Summary 7
2.1 Introduction 7
2.2 EvaluationofTowerShadowEffectsonVariousWindTurbineConcepts 7
2.3 CharacterisationoftheUnsteadyFlowintheNacelleRegionofaModernWind
Turbine 7
2.4 CorrelationBasedTransitionModelingofLaminartoTurbulentTransition
8
2.5 Aeroacoustics 8
2.6 DesignofHighLift,LowNoiseAirfoilsandHighAerodynamicPerformance
8
2.7 PredictionofMultiElementAirfoilsWiththeEllipSysCode 9
2.8 Stall-inducedvibrationsofabladesectionindeep-stall 9
2.9 GlobalBladeDeflectionsEffectonLocalAirfoilDeformationandPerformance
9
3 EvaluationofTowerShadowEffectsonVariousWindTurbineConcepts 11
3.1 Introduction 11
3.2 ComputationalSolver 12
3.3 ComputationalSetup 13
3.4 NRELPhaseVIResults 17
3.5 UPWINDTurbineResults 19
3.6 Conclusions 28
4 Characterisationofthe UnsteadyFlowin the NacelleRegionofa ModernWind
Turbine 30
4.1 Introduction 30
4.2 ComputationalSetup 31
4.3 ComputationalParameters 32
4.4 Results 33
4.5 Conclusions 39
5 CorrelationBasedTransitionModelingofLaminartoTurbulentTransition 42
5.1 Introduction 42
5.2 Codedescription 42
5.3 Computationalgrid 43
5.4 2DAirfoilResults 45
5.5 3DCylinderComputations 51
5.6 Flowoverathickairfoil 55
5.7 DiscussionofAirfoilResults 58
5.8 Acknowledgement 59
6 ValidationofandAirfoilOptimizationwithTrailingEdgeNoiseModel 61
6.1 Introduction 61
6.2 ValidationofaTrailingEdgeNoiseModel 61
6.3 AirfoilsAeroacousticOptimization 65
6.4 Conclusions 72
7 DesignofHighLiftAirfoilsWithLowNoiseandHighAerodynamicPerformance
76
7.1 Nomenclature 76
7.2 Introduction 76
7.3 MethodforAirfoilDesign 77
7.4 StrategyforAirfoilDesign 78
7.5 AirfoilDesignswithNoiseReduction 80
7.6 Conclusions 85
8 NoisepredictionofNACA64418airfoil 87
8.1 Introduction 87
8.2 Governingequations 87
8.3 High-orderdiscretizations 88
8.4 Results 91
8.5 Conclusions 92
9 PredictionofMultiElementAirfoilsWiththeEllipSysCode 95
9.1 Introduction 95
9.2 Codedescription 95
9.3 Computationalgrids 95
9.4 Results 96
9.5 Conclusion 99
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9.6 Acknowledgement 99
9.7 Appendix:Geometrydescription 101
10 Thickairfoils&Highlift 103
10.1 Introduction 103
10.2 HowAndWhyDoMultipleElementAirfoilsWork 105
10.3 MultiElementAerodynamicComputationalTool 106
10.4 ParametricStudyofTwo-ElementAirfoils 110
10.5 Conclusions&Furtherwork 112
11 Stall-inducedvibrationsofabladesectionindeep-stall 114
11.1 Models 115
11.2 Stall-inducedvibrations 117
11.3 Numericalexample 118
11.4 Conclusion 120
12 Global Blade Deflections Effect on Local Airfoil Deformation and Performance
122
12.1 TheWindTurbineBlade 122
12.2 AerodynamicForces 122
12.3 FiniteElementModel 122
12.4 DeformedAirfoilSection 125
12.5 Conclusion 127
13 GearDynamics 134
13.1 Doublecontacttheory 134
13.2 Interface 138
13.3 ValidationExample 140
13.4 Conclusion,Perspective,andFutureWork 142
14 Completelistofpublicationsfromtheproject 143
14.1 Journalpapers 143
14.2 Conferencepapers 143
14.3 Oralpresentations 144
14.4 Books 144
14.5 PhDTheses 144
14.6 MScTheses 145
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(cid:7) (cid:8)(cid:9)(cid:5)(cid:10)(cid:11)(cid:12)(cid:5)
TheEnergyResearchProject"ProgramforResearchinAppliedAeroelasticity,EFP-2007-II"
wascarriedoutincooperationbetweentwoinstitutesattheTechnicalUniversityofDenmark
(DTU),RisøNationalLaboratoryforSustainableEnergy(RisøDTU)andtheDepartmentof
MechanicalEngineering(DTUMEK),from1April2008to31March2009.Fromtheonsetof
theproject,sixmilestonesweredefinedwhichrepresentthemainpartoftheresearchactivity.
Apartfromtheworkfocusedonthemilestones,alsoanalysisofcurrentproblemsandfurther
developmentoftheexistingmodelswerecarriedout.
SeveralresearchersatDTUMEKandRisøDTUhavebeeninvolvedintheprojectworkand
havecontributedto the researchpresentedin this report.To enablereferenceto the different
partsofthereport,thenamesoftheauthorsareindicatedforeachchapter.Itshould,however,
beemphasizedthatthereportisnotadetailedreportofthecompleteactivitywithintheproject.
Thus,notallofthecontributorstotheprojectappearasauthorstothedifferentchapters.Fora
detaileddescriptionoftheresultsfromtheproject,pleaseseeChapter14inwhichacomplete
listofpublicationsintheprojectcanbefound.
AtDTUMEK,thefollowingresearchersfromtheFluidMechanicsSectionoftheDepartment
ofMechanicalEngineeringhavebeeninvolvedintheproject:
KurtS.Hansen
MartinO.L.Hansen
GabrielHernandez
RobertMikkelsen
WenZhongShen
JensN.Sørensen
StigØye
AtRisøDTU,primarilytheresearchersfromtheAeroelasticDesignGrouphavecontributed
totheproject:
PeterB.Andersen
ChristianBak
AndreasBechmann
FranckBertagnolio
ThomasBuhl
MadsDøssing
MacGaunaa
AndersM.Hansen
MortenH.Hansen
BjarneS.Kallesøe
GunnerC.Larsen
TorbenJ.Larsen
HelgeA.Madsen
HelenMarkou
FlemmingRasmussen
NielsN.Sørensen
NielsTroldborg
FrederikZahle
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(cid:13) (cid:14)(cid:15)(cid:16)(cid:16)(cid:11)(cid:9)(cid:17)
(cid:0)(cid:2)(cid:3) (cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:6)(cid:12)(cid:8)(cid:5)
This report contains the results from the EnergyResearch Project "Programfor Research in
AppliedAeroelasticity,EFP-2007-II"coveringthe periodfromApril 1st 2008to March 31st
2009. The partners in the project are the two institutes at Technical University of Denmark
(DTU),RisøNationalLaboratoryforSustainableEnergy(RisøDTU)andtheDepartmentof
MechanicalEngineering(DTUMEK).Theoverallobjectivesoftheprojectaretoensurethe
developmentofanaeroelasticdesigncomplex,whichcancreatethebasisforthenextgener-
ationofwindturbinesandmakenewdesignconceptspossible.Theprojectformsastrategic
cooperationbetween Risø DTU and DTU MEK and the wind turbine industry with empha-
sisonobtainingasuitablebalancebetweenlong-termstrategicresearch,appliedresearchand
technological development. To obtain synergy between the different subjects and to ensure
an optimal, dynamic cooperation with the industry, while maintaining the continuity of the
research, the project is organizedas a research program within applied aeroelasticity with a
combinationofresearchactivitieswithspecificshort-termtargetswithinoneyearandgeneral
continuouslong-termresearchactivities.Thisresearchprojecthasbeenthetenthinarowof
one-yearprojects,whichhasensuredacontinuousdevelopmentsince1997,wheretheactivity
inthisrowofprojectsisdescribedin[1–10].
(cid:0)(cid:2)(cid:0) (cid:13)(cid:14)(cid:15)(cid:16)(cid:10)(cid:15)(cid:6)(cid:12)(cid:8)(cid:5) (cid:8)(cid:17) (cid:18)(cid:8)(cid:19)(cid:20)(cid:7) (cid:21)(cid:22)(cid:15)(cid:9)(cid:8)(cid:19) (cid:13)(cid:23)(cid:20)(cid:11)(cid:6)(cid:24) (cid:8)(cid:5) (cid:25)(cid:15)(cid:7)(cid:12)(cid:8)(cid:10)(cid:24) (cid:26)(cid:12)(cid:5)(cid:9) (cid:18)(cid:10)(cid:7)(cid:27)(cid:12)(cid:5)(cid:20)
(cid:28)(cid:8)(cid:5)(cid:11)(cid:20)(cid:29)(cid:6)(cid:24)
A2DCFDstudyhasbeencarriedouttoevaluatetheeffectsoftowershadowonabladesection
for an upwind turbine and three downwind turbine concepts; one with a traditional tubular
tower,anotherwithastreamlinedtowerandlastly,onewithafourleggedconfiguration.Two
turbineswereusedasbasisforthestudy:FirstlytheNRELPhaseVIturbine,whichwasused
for validation against experimentalresults; secondly the UPWIND 5 MW reference turbine,
sinceitisrepresentativeinsizeofmodernturbines.
The2Dsimulationssucceededincapturingtheunsteadyinteractionbetweenthebladesection
and the tower wake, and the results for the Phase VI turbine were in good agreement with
the experimental data for low wind speed cases, whereas for higher wind speeds with flow
separation,theagreementwasnotasfavourable.Forthedownwindtubulartowerconfiguration
oftheUPWIND turbine,thetowerwakegaveriseto asignificanttowershadowwith a 20%
reductioninthebladenormalforce.Changingthetowerclearancedidnotgiverisetosignificant
reductionofthetowershadow.Usingastreamlinedtower,however,reducedthetowershadow
50% comparedto the cylindricaltower,andeliminatedthe unsteadyblade/vortexinteraction
seenontheconventionaltower.Simulationswithafourlegconfigurationshowedthatthetower
shadowcouldbereducedwithsuchaconfiguration.Thetowerwakewas,however,significantly
moreturbulent.
Comparisonof the Navier-Stokes simulations and the aeroelastic code HAWC2 showed that
theoverallagreementwasgood,andthatappropriateadjustmentofthetowerdragcoefficient
canbeusedtofittheHAWC2resultstotheCFDsimulations.
(cid:0)(cid:2)(cid:30) (cid:28)(cid:22)(cid:15)(cid:7)(cid:15)(cid:11)(cid:6)(cid:20)(cid:7)(cid:12)(cid:24)(cid:15)(cid:6)(cid:12)(cid:8)(cid:5) (cid:8)(cid:17) (cid:6)(cid:22)(cid:20) (cid:31)(cid:5)(cid:24)(cid:6)(cid:20)(cid:15)(cid:9) !(cid:16)(cid:8)(cid:19) (cid:12)(cid:5) (cid:6)(cid:22)(cid:20) "(cid:15)(cid:11)(cid:20)(cid:16)(cid:16)(cid:20) #(cid:20)$(cid:12)(cid:8)(cid:5)
(cid:8)(cid:17) (cid:15) %(cid:8)(cid:9)(cid:20)(cid:7)(cid:5) (cid:26)(cid:12)(cid:5)(cid:9) (cid:18)(cid:10)(cid:7)(cid:27)(cid:12)(cid:5)(cid:20)
A3DNavier-Stokessolverhasbeenusedtoinvestigatetheflowinthenacelleregionofawind
turbinewhereanemometersaretypicallyplacedtomeasuretheflowspeedandtheturbineyaw
angle. A 500 kW turbine was modelled with rotor and nacelle geometry in order to capture
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thecomplexseparatedflowinthebladerootregionoftherotor.Anumberofsteadystateand
unsteadysimulationswerecarriedoutforwindspeedsrangingfrom6m/sto16m/saswellas
twoyawandtiltangles.Theflowinthenacelleregionwasfoundtobehighlyunsteady,domi-
natedbyunsteadyvortexsheddingfromthecylindricalpartofthebladeswhichinteractedwith
the rootvortices fromeachblade,generatinghightangentialvelocities in the nacelleregion.
Forpureaxialinflowtheaveragednacellewindspeedvariedapproximatelylinearlywiththe
freestreamwindspeed,whereasthenacelleflowanglechangedsignificantlywithwindspeed.
Thenacelleanemometryshowedsignificantdependenceonbothyawandtiltangleswithyaw
◦
errorsofupto10 whenoperatinginatiltedinflow.
(cid:0)(cid:2)& (cid:28)(cid:8)(cid:7)(cid:7)(cid:20)(cid:16)(cid:15)(cid:6)(cid:12)(cid:8)(cid:5) ’(cid:15)(cid:24)(cid:20)(cid:9) (cid:18)(cid:7)(cid:15)(cid:5)(cid:24)(cid:12)(cid:6)(cid:12)(cid:8)(cid:5) %(cid:8)(cid:9)(cid:20)(cid:16)(cid:12)(cid:5)$ (cid:8)(cid:17) ((cid:15))(cid:12)(cid:5)(cid:15)(cid:7) (cid:6)(cid:8) (cid:18)(cid:10)(cid:7)(cid:27)(cid:10)*
(cid:16)(cid:20)(cid:5)(cid:6) (cid:18)(cid:7)(cid:15)(cid:5)(cid:24)(cid:12)(cid:6)(cid:12)(cid:8)(cid:5)
For thin airfoils a very good agreement is observed between measurements, Xfoil computa-
tionandpredictionsbythe2DEllipSyscodeforlowanglesofattack.Fora thickairfoiland
acylinderthecouplingoftheDESmethodologyandthelaminar/turbulenttransitionmodelis
demonstrated.Fortheairfoilthesecomputationsshowanimprovedagreementwiththemea-
sureddatainstall,comparedto2Dcomputations.Inthecylindercasethecomputationsshow
thatthecodeiscapableofpredictingtheflowfromthelaminarregionataReynoldsnumberof
10tothefullyturbulent/transitionalflowataReynoldsnumberaboveonemillion.Theuseof
thecombinedDES/transitionalmethodologyimprovesthepredictionofthedragcrisisonthe
cylinder compared to pure DES simulation. Additionally, the DES/transitional methodology
predictsthecorrectflowphenomenaincludinglaminarseparationandturbulentreattachment
followedbyturbulentseparation.
(cid:0)(cid:2)+ ,(cid:20)(cid:7)(cid:8)(cid:15)(cid:11)(cid:8)(cid:10)(cid:24)(cid:6)(cid:12)(cid:11)(cid:24)
Theaimofthiscontributionistwo-sided.Firstly,anexistingtrailingedgenoisemodelisvali-
datedbycomparisonwithexperimentaldata.Measuredairfoilsurfacepressurefluctuationsare
comparedwiththecomponentofthemodelthatrelatestheturbulentboundarylayercharacter-
isticstothesurfacepressure.Farfieldsoundpressurelevelsarealsoconsideredforcomparisons
betweenmodelresultsandexperimentaldata.Secondly,themodelisimplementedintoanair-
foildesigncodethatisnormallyusedforaerodynamicoptimization.Anexistingwindturbine
airfoilis optimizedin orderto reduceits noise emission,tryingat the same time to preserve
itsaerodynamicperformancesandgeometriccharacteristics.Themodificationsresultingfrom
thisnewdesignareanalyzed.
(cid:0)(cid:2)- .(cid:20)(cid:24)(cid:12)$(cid:5) (cid:8)(cid:17) /(cid:12)$(cid:22) ((cid:12)(cid:17)(cid:6)0 ((cid:8)(cid:19) "(cid:8)(cid:12)(cid:24)(cid:20) ,(cid:12)(cid:7)(cid:17)(cid:8)(cid:12)(cid:16)(cid:24) (cid:15)(cid:5)(cid:9) /(cid:12)$(cid:22) ,(cid:20)(cid:7)(cid:8)(cid:9) (cid:5)(cid:15))(cid:12)(cid:11)
1(cid:20)(cid:7)(cid:17)(cid:8)(cid:7))(cid:15)(cid:5)(cid:11)(cid:20)
Noisefromwindturbineshasseveralsourcessuchasgear,generatorandaerodynamics.This
workfocusesontheaerodynamicnoise,alsocalledaeroacoustics.Thegenerationofthisnoise
iscommonlydividedintodifferentsources,wherethenoisegeneratedfromthetrailingedge
is one of a few dominant sources. The objective of this work is to design airfoils which are
aerodynamicallyhighperformingandhavereducednoise.Thefirstofthreestepsinthisinves-
tigationwasimplementationandvalidationofanadvancedtrailingedgenoisemodel(TNO),
whichshowedthatthemodeldoesnotpredictthenoiselevelaccurately,butcapturedthecor-
rect trends. Thus, the model can be used qualitatively in a design process. The second step
involved the implementation of the model in the airfoil design tool AIRFOILOPT and test-
ingtheimplementationbyreducingthenoiseontheRisø-B1-18andtheNACA63418airfoils
showedthatthenoisecouldbereducedbetween1and2.5dB,whilestillmaintainingitsaero-
dynamiccharacteristics.Thethirdandlaststepwasthedesignofahighperformanceandhigh
liftairfoilwhichhadits basisintheRisø-C2-18.Whilemaintainingtheaerodynamicperfor-
(cid:10) (cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:2)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:12)(cid:13)(cid:14)