Wake Development of Airfoils with Serrated Trailing Edges

22 Pages·2016·3.46 MB·English

Liu, X., Kamliya Jawahar, H., & Azarpeyvand, M. (2016). Wake

Development of Airfoils with Serrated Trailing Edges. In 22nd AIAA
CEAS Aeroacoustics Conference [AIAA 2016-2817] American
Institute of Aeronautics and Astronautics Inc. (AIAA).
https://doi.org/10.2514/6.2016-2817
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10.2514/6.2016-2817
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Wake Development of Airfoils with Serrated Trailing Edges
XiaoLiu∗, Hasan KamliyaJawahar† and MahdiAzarpeyvand‡
UniversityofBristol,Bristol,United Kingdom,BS81TR
Wakedevelopmentandaero-acousticperformanceofairfoilsfittedwithtrailing-edgeserrationshave
beenstudiedexperimentally. Asymmetric(NACA0012)andanasymmetric(NACA65(12)-10)air-
foilwithdifferenttypesof trailing-edgeserrationshave been testedover awiderange of anglesof
attack and Reynolds number. The steady aerodynamic force measurements have shown that the
useoftrailingedgeserrationsleadstosignificantreductionofliftcoefficientatlowanglesofattack.
ParticleImageVelocimetry(PIV),two-dimensionalLaserDopplerAnemometry(LDA)andhot-wire
measurementswerecarriedoutinordertoimproveourunderstandingofthewakedevelopmentand
theenergy content of the turbulentcoherent structures. Significant reduction in the airfoils wake
turbulencelevelhasbeenobserved,causemayincludetheinteractionbetweentheflowfieldoverthe
tipand root planesandthreedimensionalhorse-shoeandstreamwisestructuresinthenearwake.
Resultshavealsoshownthattheuseofslottedsawtoothreductioncanleadtosignificantreduction
ofturbulentkineticenergyandReynold’sshearstressinthenearwakeregion.Thereductionoftur-
bulencewithinthewakecansignificantlyreducetheturbulenceinteractionnoiseincontra-rotating
propellers,rotor-statorconfigurations,etc.
Nomenclature
2h = amplitudeofserration
AoA = angleofattack
c = chordlength
C = dragcoefficient
D
C = liftcoefficient
L
C = maximumliftcoefficient
L,max
d = slotwidth
H = slotdepth
Re = chordbasedReynoldsnumber
c
TE = trailingedge
TKE = turbulencekineticenergy
U = flowspeed
U = freestreamwindspeed
0
u′u′ = streamwiseReynoldsnormalstress
v′v′ = spanwiseReynoldsnormalstress
u′v′ = Reynoldsshearstress
α = angleofattack
α = serrationangle
s
λ = serrationwavelength
∗PhDStudent,DepartmentofMechanicalEngineering,AIAAStudentMember,[email protected]
†PhDStudent,DepartmentofAerospaceEngineering,AIAAStudentMember,[email protected]
‡Senior Lecturer and Royal Academy of Engineering research fellow, Department of Mechanical Engineering,
[email protected]
I. Introduction
THE increasing popularity of air travel and rapid growth in the number of airports close to the city limits have
increasednoisepollutionleadingtonegativephysiologicalandpsychologicaleffectsonthesurroundingresidents.
Therefore,stringentstandardshavebeensetbyICAOforaircraftnoisecontrol,attractingresearchersinteresttoaero-
acoustics. Noisegeneratedfromaircraftcanbebroadlyclassifiedintheairframenoiseandenginenoise. Theengine
noisehasbeenreducedconsiderablyoverthelasthalfcenturybytheintroductionofhighbypassratioturbofanengines
and introduction of chevrons in the back of the nacelle and the engine exhaust nozzle. However, the reduction of
broadbandnoisecomponentfromthe compressor,dueto theinteractionof laminarandturbulentflowswith blades,
hasremainedachallengingtask. Itisalsobelievedtoplayasignificantroleintheoverallenginenoise[1]andalsoin
noisecreatedfromcontrarotatingpropellers.
Oneofthedominantcausesofairfoilselfnoiseistheinteractionoftheairfoilsurfacewiththeturbulenceproduced
byitsownboundarylayer,thustheturbulenceintensityoftheinflowandtheinteractionofturbulenceattheboundary
layerwiththeairfoiltrailing-edgeplaysamajorroleinairfoilselfnoise[2]. Thetrailingedgenoiseissignificantat
lowMachnumbersduetotheefficientscatteringoftheturbulentfluctuationsoverasolidtrailingedge[3]. Inorder
toreducethisdominanttrailingedgenoiseseveralpassivemethodssuchasserratedtrailingedge[4–23],poroussur-
face[15,24]andbrushes[25,26]havebeeninvestigatedoverthepasttwodecades.Inparticular,serratedtrailingedges
havebeenofsignificantinterestamongsttheresearchcarriedoutonpassivetrailingedgetreatments. Initiallyanalyt-
icalinvestigationsbyHowe[4]haveshowedthatsimplemodificationssuchassawtoothandsinusoidalserrationsto
thetrailingedgecouldreducetheintensityofthetrailingedgeradiationandthemagnitudeofreductiondependsupon
thefrequencyandthelengthandspanwisespacingoftheteeth. Furtherexperimentalinvestigation[11,12,14,15,27]
showedthattheserratedtrailingedgeinsertscanachievea self-noisereductionofupto2 and6 dBwithsignificant
reductionat high frequencieswhilst maintainingan acceptable aerodynamicperformance. The more recentexperi-
mentsshowedthatfurthernoisereductioncanbeachievedbytheuseofmorecomplextrailingedgessuchas: slitted
edge,sawtoothwithhole,slitted-sawtoothedgeandrandomlyserratedtrailingedges,etc. Eventhoughtrailingedge
serrationshaveproventobevalidmeansofairfoilnoisereduction,themechanisminwhichitoccursisnotcompletely
understood.Therefore,detailedstudyoftheaerodynamicperformance,wakedevelopment,turbulencelevelandnoise
reductionsourceoftreatedairfoilshavebeencarriedoutinordertobetterunderstandthenoisereductionmechanism.
Thepresentedpaperisadetailedstudyofthewakedevelopmentandaero-acousticbehaviourofairfoilwithand
without trailing edge serrations. Aerodynamic force measurements were carried out for a wide range of angles of
attackandReynoldsnumberhavebeenperformed.Theflowmeasurementshavebeencarriedoutusingmethodssuch
asPIV,LDAandhot-wire.Thewindtunnelandexperimentalsetupalongwiththemeasurementtechniquesemployed
in the current study are discussed in Sec .II. The results for force measurements, wake developmentand the wake
energycontenthasbeendiscussedinSec.III.
II. Experimental andComputational Setup
A. WindTunnelandExperimentalSetup
AcomprehensivestudyoftheaerodynamicperformanceforasymmetricalNACA0012andacamberedasymmetrical
NACA 65(12)-10 airfoil with and without trailing edge serrations were performed in three different wind tunnels
at University of Bristol. The wind tunnels were: (i) large low speed closed-circuit wind tunnel with an octagonal
working section of 2.1 m × 1.5 m × 2 m, contraction ratio of 3:1 and a stable velocity range of 10 m/s to 60 m/s;
(ii)lowturbulenceclosed-circuitwindtunnelwithanoctagonalworkingsectionof0.8m×0.6m×1m,contraction
ratioof12:1,maximumvelocityof100m/sandwithturbulencelevelaslowas0.05%;(iii)openjetwindtunnelwith
diameterof1.1m,maximumreliablespeedof30m/sandminimumturbulencelevelof0.05%.
1. ForceMeasurementsSetup
Theaerodynamicliftanddragforcesforboththeairfoilsweretestedintheclosed-circuitlargelowspeedwindtunnel.
To reducethe probablethree-dimensionalityeffectassociated with vortexsheddingand drag, clear circularPerspex
end-plateswithadiameter0.34mwereusedontheairfoiltoinducetwo-dimensionality. Theedgesofthesecircular
end-plateswere chamferedto reduce flow distortion. The spacing between the sides of the end-platesto the airfoil
hasaratioofmorethanseven,assuggestedbyBoutilier[28],inordertoobtainaccuratemeasurementsofliftforces
andvortexsheddingfrequencies. TheforcemeasurementsweretakenusingAMTIOR6-7-2000forcebalance. This
force balance measures the aerodynamic forces by passing a voltage induced by the force balance through AMTI
MSA-6 strain gauge amplifier, which is then converted into force (N) using the builtin control system provided in
theLabVIEWsystemdesignsoftware. Athoroughdataindependencytestusingdifferentsamplingfrequencieswas
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carried out before choosing a sampling frequency of 37 Hz which provided the best data independency and least
uncertainty in the results [29]. The final data sets were collected for a time period of 30 s and the post-processed
resultsarepresentedinSectionIII.
2. ParticleImageVelocimetrySetup
Thewakedevelopmentandtheenergycontentoftheturbulencestructurewithinthewakeoftheairfoilswerestudied
using two-dimensional two-componentParticle Image Velocimetry. The laser sheet with a thickness of 1 mm was
createdusingaDantecDualPower200mJNd:YAGlaserwithawavelengthof532nm,thetimeintervalbetweeneach
snapshotswere15µswitha repetitionrateof25Hz. A mixtureofPolyethyleneglycol80withameandiameterof
1µmwereusedtoseedtheairinsidethelowturbulencewindtunnel. Atotalnumberof1600imageswerecaptured
usingaFlowSense4MPCCDcamerawitharesolutionof2078×2078pixelsand14bitcorrespondingtofieldview
of11.3cm×11.3cm. TheimageswereanalysedwiththeDynamicStudiosoftwarefromDantec. Theiterativeprocess
yeildgridcorrelationwindowof16×16pixelswithanoverlapof50%resultinginafacialvectorspacingof0.43mm.
3. Hot-WireAnemometrySetup
Flow steady and unsteady velocity field measurements were carried out using single and cross-wire constant tem-
perature hot-wire anemometry. Flow measurements were taken using Dantec 55P16 single sensor hot-wire probes
with a 5 µm diameter and 1.25 mm long platinum-plated tungsten wire sensor for streamwise component. Dantec
55P51gold-platedcross-wireprobewithtwo 3mmlongplatinum-platedtungstenwire sensorswith5 µm diameter
placed 90◦ to each other were used for simultaneousmeasurementof normaland streamwise velocity components.
The hot-wire probeswere driven by Dantec StreamlinePro CTA 91C10 modulesand controlled througha National
InstrumentNI9215 four channel module. Both hot-wire probes were calibrated using the Dantec 54H10 two point
modehot-wirecalibrator. Ayawcalibrationforthecross-wirewasperformedbetweentheangleof45◦ and−45◦ in
ordertodeterminetherelationshipbetweentheeffectivecoolingvelocityforeachwireandthevelocitycomponents
uandv. Theflowfieldmeasurementswereloggedatafrequencyof20kHz.Thehot-wireprobeswereplacedon1m
longsteelrodthatwasconnectedtothetraversesystemthatwasplacedawayfromtheflowfieldandtraversesystem
wascontrolledusingMatlab. Thetraversesystemwasplacedawayfromthewindtunnelastonotinfluencetheflow
fieldandblockageofthetunnelflow.
4. LaserDopplerAnemometrySetup
TwocomponentLDAmeasurementswith6-Wattargon-ionlaserwereperformedwithinthewakeoftheairfoils. The
systemwasoperatedinbackscattermodewithafocallengthof600mm. Velocitycomponentsweremeasuredatthe
orthogonalintersectionof the dual-green514.5nm and dual-blue488.0nm laser beams. Themeasurementvolume
atthebeamintersectionwasaprolongedspheroidshapewithdiameterof0.16mmandlength2.9mm. Thesystem
ismountedonafullyautomatedtraversesystemandcontrolledbyinbuiltsoftwarepackagebyDantec(BurstWave)
(a) NACA65(12)10 (b) NACA0012
usedfordatacollectionandpostprocessing.
Baseline
Baseline
Blunt Airfoil
Blunt Airfoil
10 mm 10 mm
Serrated Airfoil
Serrated Airfoil
2h 2h
140 mm 140 mm
a) NACA65(12)-10 b) NACA0012
Fig.1. Airfoilconfigurationsemployedinthecurrentstudy
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B. SerrationSetup
TheairfoilsweremanufacturedusingRAKU-TOOLrWB-1222polyurethaneboardandmachinedusingacomputer
numericalcontrol(CNC)machine. Boththeairfoilsweredesignedwitha1.5mmblunttrailing-edgewithaslotthat
hadadepthof15mmandthicknessof0.8mmalongthespan-wisedirection,asshowninFig.1forthepurposeof
installingflatplateserrationsatthetrailing-edge. Thereisnogeneralconsensusintheliteratureastohowtodefine
thetrailing-edgeserrations. Therearetwomethodspreviouslyusedbyresearchers:(i)tousetheserrationasasimple
flaptotheexistingairfoilwhichincreasestheairfoilsurfaceareaand(ii)tokeeptheairfoilsurfaceareaconstantas
suggestedbyGruber[5,11–13].Inordertounderstandtheeffectsofbothtreatmentstwodifferentbaselineshavebeen
consideredinthisstudy. TheBluntairfoilusestheserrationsasanadditionalflapconfiguration(thusincreasingthe
airfoilsurfacearea),whereastheBaselineairfoilisdesignedtohavethesamesurfaceareaasthatoftheserratedairfoil
asshowninFig.1.
TheairfoilgeometriesshowninFig.1aand1bhaveaspanlengthofL = 0.45mbutwithvaryingchordlengths.
TheBluntairfoilhasachordlengthofc=0.14mwithnotrailingedgeflatplateinserts. Baselineairfoilhasaslightly
differentchordlengththatwasachievedbyaflatplateinsertswithoutserrationsatthetrailing-edgeoftheBluntairfoil.
Thisincreasestheeffectivechordlengthtoc=0.155mandthusincreasesthesurfaceareaoftheairfoil.
Twotypesofserrationsbasedontheirgoodnoisereductionperformancefrompreviousanalyticalandexperimental
studies[1,4,5,11–13]wereselectedforthepresentstudy. Thetwotypesofserrationstestedandpresentedhereare
thesawtoothserration(Fig.2a)andslotted-sawtoothserration(Fig.2b). Inordertocoverawiderangeofserration
anglesgeometricalparametersoftheserrationssuchastheamplitude2h,periodicitywavelengthλ,angleofserration
edge α , slot width d and slot depth H were varied. The selected parameterspresented in Table. 1 are due to their
s
excellent noise reduction performance as demonstrated in previous experiments [5,11–13]. Even though a variety
ofserrationgeometricalparameterswereusedforexperimentation,onlya fewserrationsarepresentedinthispaper
showninTable.1.
a) Sawtooth b) Slotted-sawtooth
Fig.2. Serratedtrailing-edgetreatmentstestedinthecurrentstudy
Table1. Geometricalparametersoftrailing-edgeserrations.
Configuration SerrationTypes 2h λ λ/h α d H
s
(mm) (mm) - (deg.) (mm) (mm)
Case1 Baseline1 0 - - - - -
Case2 Baseline2 15 - - - - -
Case3 sawtooth 30 3 0.2 2.86 - -
Case4 sawtooth 30 9 0.6 8.53 - -
Case5 sawtooth 30 22.5 1.5 45 - -
Case6 slotted-sawtooth 30 9 0.6 8.53 0.5 15
Case7 slotted-sawtooth 30 9 0.6 8.53 0.5 5
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C. SimulationSetup
PreliminaryReynolds-averagedNavier-Stokes(RANS)numericalsimulationshavebeencarriedouttofurtherinves-
tigateandvisualisetheflowbehaviouraroundserratedtrailingedgeofthetestedairfoil.Airfoilsimulationshavebeen
carriedoutforNACA65(12)-10atanglesofattack0◦,5◦and10◦forachordbasedReynoldsnumberofRe =3×105.
c
Thecomputationaldomaininthespanwisedirectionis27mmrepresenting20%ofchordlengthandisequaltothree
wavelength of the serration enabling the use of periodic boundary conditions in the spanwise direction. The mesh
approximatelyhad2.3millionelementsandwascreatedusingsnappyHexMeshsuppliedwithOpenFOAM.Thesim-
ulations were carried using OpenFOAM opensource code and SpalartAllmaras turbulence model. The airfoil wall
wassetto havea y+ valueof30usedalongwithwall functions. Thecloseto wallregionoftheairfoilwasdensely
populatedforthefirst10mmin-ordertobettercapturetheboundarylayer.
a) TipFlow b) RootFlow
Fig.3. PreliminaryRANSresultsdepictingtheflowfieldovertheserrationsforα=10◦forchordbasedReynoldsnumberofRec=3×105.
III. Results
A. AerodynamicForceMeasurements
The lift and drag measurementsof NACA 0012 and NACA 65(12)-10airfoils for the Baseline, Blunt and Serrated
configurationswerecarriedoutforawiderangeofanglesofattackandchord-basedReynoldsnumbersRe =2×105
c
to6×105, correspondingtoflowvelocitiesofU = 20m/sto U = 60m/s. However,forthepurposeofbrevitythe
resultsforRe =3×105and5×105,correspondingtoflowvelocitiesU =30m/sandU =50m/swillbepresented
c
here. Thetestswerecarriedoutforanglesofattack(α)rangingfrom0◦ to20◦ and−5◦ to20◦ forNACA 0012and
NACA65(12)-10,respectively.Thetestedsawtoothserrations(Fig.2a)hadvaryingwavelengthsofλ=3mm,9mm
and22.5mmandamplitude2h = 30mm. Theslotted-sawtoothserrations(Fig.2b)hadawavelengthandamplitude
ofλ=9mmand2h=30mmalongwithslotwidthofd =0.5mmandtwodepthsofH =5mmand15mm.
1. NACA65(12)-10
ThemeasuredliftanddragcoefficientsforthethreeconfigurationsofNACA65(12)-10airfoilarepresentedinFig.4.
TheuppertwoplotsinFig.4 showstheforcecoefficientsfortwoflowconditionswith Re = 3×105 (Fig.4a)and
c
5×105(Fig.4b)andforairfoilwithsawtoothserrations. ThebottomtwoplotsinFig.4showstheforcecoefficients
for similar flow conditions for airfoil with slotted-sawtooth serrations. The force measurement results for NACA
65(12)-10airfoilwithsawtoothserrationsforthepresentedflowconditionsshowareductioninliftcoefficientofup
to15%foranglesofattackrangingfrom−5◦ to10◦ comparedtotheBaselineairfoil. NACA65(12)-10airfoilwith
slotted-sawtooth serrations shows an even greater reduction in lift coefficient of up to 30% for the entire range of
anglesofattackwiththemaximumreductionobservedatloweranglesofattack. Airfoilswithtrailing-edgeserrations
showincreasedliftcoefficientinthepre-stallregionbutdoesnotparticularlychangethestallbehaviouroftheairfoil.
Theresultsalso showa prominentlossinliftcoefficientforBluntairfoilcomparedtoBaseline airfoilwhichcanbe
attributedtothedecreaseinsurfacearea. Theliftcoefficientoftheserratedairfoilsshowsasignificantincreaseinlift
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1.6 0.8 1.6 0.8
(a) U=30 m/s (b) U=50 m/s
1.4 1.4 Baseline
Baseline
1.2 0.6 1.2 0.6
1 1
CL0.8 0.4CDCL0.8 0.4CD
Blunt airfoil
0.6 0.6 Blunt airfoil
0.4 0.2 0.4 0.2
0.2 0.2
0 0 0 0
1.6 0.8 1.6 0.8
(c) U=30 m/s (d) U=50 m/s Baseline
1.4 1.4
Baseline
1.2 0.6 1.2 0.6
1 1
CL0.8 0.4CDCL0.8 0.4CD
Blunt airfoil
0.6 0.6 Blunt airfoil
0.4 0.2 0.4 0.2
0.2 0.2
0 0 0 0
-5 0 5 10 15 20 -5 0 5 10 15 20
α(deg) α(deg)
Fig.4. LiftanddragcoefficientresultsforNACA65(12)-10fittedwithdifferentserrations
(a)sawtoothserrations.Triangles:Red:λ=22.5mm;Blue:λ=9mm;Green:λ=3mm.(b)slotted-sawtoothserrations
(λ=9mm,2h=30mm,d=0.5mm).Square: Blue: H=5mm;Red: H=15mm.
overthewholerangeofanglesofattackincomparisonwithBluntairfoil.Thedragcoefficientfortheserratedairfoils
arehigherthanthatoftheBaselineandBluntairfoil,foranglesofattacklargerthan10◦,itisalsoevidentthatthedrag
increaseswithincreaseinserrationwavelength.
2. NACA0012
TheNACA0012airfoilforcemeasurementsforallthetestedconfigurationsareshowninFig.5. Theliftcoefficient
resultsshownoticeabledifferenceintheoverallbehaviourofsymmetricNACA0012airfoiltothatoftheasymmetric
NACA65(12)-10.NACA0012airfoilfittedwithserrationsshowsalmostnoreductioninliftcoefficient,especiallyfor
smallanglesofattack,rangingfrom0◦ to9◦ showingthattheeffectofserrationsonliftcoefficientislesssignificant
onNACA0012comparedtothatofNACA65(12)-10,particularlyatlowerspeedswithRe =3×105. Theserrations
c
significantlyaffectthemaximumliftcoefficientoverthecriticalanglesofattackrangesfrom9◦ to15◦,butitappears
to show improvementin the lift performance at deep stall angles from 17◦ to 20◦ and also increase the stall angle
from13◦ to14◦ forRe = 5×105. TheliftperfomancesforNACA0012airfoilwithserrationissimilartoorbetter
c
thantheBluntairfoilespeciallyathighanglesofattack. Asmentionedearlier,basedonthepreviousanalyticaland
experimentalresults in [1,4,5,11–13], sharp sawtooth (small α ) and slotted-sawtooth serrationsprovide large and
s
robustnoisereductionoverawidefrequencyrange. Theaerodynamicresultspresentedhere,however,haverevealed
thatsucheffectivetrailing-edgetreatment(intermofnoise)arepronetosignificantaerodynamiclosses.
B. WakeDevelopement
Detailed flow measurements at several downstream wake locations have been performed using both LDA and PIV
measurement techniques for NACA 65(12)-10 airfoil in order to better understand the effects of serration on the
aerodynamicperformanceoftheairfoil. Thewakemeasurementswerecarriedoutatanglesofattack,α=0◦,5◦,10◦
and15◦forchordbasedReynoldsnumberRe =3×105correspondingtoaflowvelocityofU =30m/s. ForLDAthe
c
wakemeasurementwereloggedatdownstreamlocations(Fig.6)0.2c,0.3c,0.5c,1.0c,1.5cand2.0crelativeto the
trailingedgeoftheBaselineairfoil. Eachmeasurementlinewaspopulatedwith60measurementpointstoaccurately
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1.2 0.6 1.2 0.6
(a) U=30 m/s (b) U=50 m/s
Baseline Baseline
1 1
0.8 0.4 0.8 0.4
CL0.6 CDCL0.6 CD
Blunt airfoil
0.4 0.2 0.4 Blunt airfoil 0.2
0.2 0.2
0 0 0 0
1.2 0.6 1.2 0.6
(c) U=30 m/s (d) U=50 m/s
Baseline Baseline
1 1
0.8 0.4 0.8 0.4
CL0.6 CDCL0.6 CD
Blunt airfoil Blunt airfoil
0.4 0.2 0.4 0.2
0.2 0.2
0 0 0 0
0 5 10 15 20 0 5 10 15 20
α(deg) α(deg)
Fig.5. LiftanddragcoefficientsforaNACA0012airfoilfittedwithdifferentserrations
Triangles:λ=9mm;squares: sawtoothserrationwithλ=3mmwavelength;pluses:slotted-sawtoothserrationwithλ=9mm,
d=0.5mmandH=5mm;stars:slotted-sawtoothserrationwithλ=9mm,d=0.5mmandH=15mm.
Fig.6. Chord-wiselocationsusedalongthetipandrootoftheserrationforLDAmeasurements.
capture the flow behaviour at the wake region. Results presented from Figs. 7 to 18 compare the flow behaviour
betweenBaseline,tipandrootlocationsofsawtoothserration(λ=9mm)andslotted-sawtoothserration(λ=9mm,
2h=30mm,d =0.5mmandH =15mm)atdifferentanglesofattack.
Thewake velocityprofilesforbaseline andserratedNACA 65(12)-10airfoilsatα = 0◦ are presentedin Fig. 7.
Inthe near-wakeregion, x = 0.2c,0.3cand0.5c, itcan beobservedthattheflow atthe tip positionof thesawtooth
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follows similar wake profile to that of the Baseline but with a slightly higher velocity deficit. The higher velocity
deficitofthetipflowisduetothelargereffectivechordlengthofthetipflow. Thediplocationofthetipwakeisthe
sameasthatoftheBaselineflow. Unlikethetipflow,theflowfromtherootexhibitsaverydifferentbehaviour. The
rootflowhasasmallervelocitydeficit,withthediplocationmovedupwardduetheflowthroughtheserrationvalleys.
The wake velocity deficit and the dip location at far-wake locations, x = 1.0c,1.5c and 2.0c, gradually disappear
betweensawtoothandBaselinebutitisevidentfromtheresultsthattherootflowcansignificantlychangethenature
ofthewakedevelopment,evenatsmallanglesofattack. InthecaseofaNACA65(12)-10airfoilfittedwithaslotted-
sawtooth,thetipvelocitydeficitisnotablylargerthanthatoftheBaselineandthestandardsawtoothserration. The
flowattherootpositionhasalsomovedslightlyupwardcomparedtothesawtoothandBaseline. Resultsalso show
thatthe use of serrationsin the case ofa NACA 65(12)-10atsmallanglesofattack, lead to smaller flow deflection
angle.
AsshowninFigure7theflowbehaviourinthecaseofanairfoilwithstraightTEispredominantlytwodimensional,
oftenwithastrongω vortexappearingneartheTEduetoflowseparationontheairfoiluppersurface.However,inthe
z
caseofserratedairfoils,thepresenceofinclinedlines(serrations)leadtosignificantchangeoftheflow,introductionof
threedimensionalvorticesduetothepressuredifferenceovertheserration,resultingintheappearanceofeffectively
x-ylayersofflow,whichretaintheirpropertiesforalargeaxialdistancefromtheairfoilofparticularinteresthereis
toinvestigate.
Theturbulentkineticenergy(TKE)profilesforNACA65(12)-10airfoilwithtreatedanduntreatedtrailingedges
forα = 0◦ arepresentedinFig.8. Inthevicinityofthetrailingedge, x = 0.2c,0.3cand0.5c,itcanbeobservedthat
theTKEmagnitudeofthesawtoothissimilartothatoftheBaseline. Inthenear-wakeregion,theTKEofthebaseline
airfoilshowsaweakdoublepeakbehaviourduetotheupperandlowerboundarylayersoftheairfoil.Thedouble-peak
behaviouris even more evidentfor the tip-flow, due to the increase in the effectivechord-lengthof the airfoil. The
TKE of the root flow, however, peaks only in an area in the upper side of the airfoil, indicating the presence of an
upward flow throughthe serration valleys. In the case of slotted-sawtooth, the peak due to the airfoilpressure-side
boundarylayerismuchlargerthanthatofthesawtoothserration.Resultshavealsoshownthatinthefar-wakeregion,
x = 1.0c,1.5cand2.0c,theTKEfortheairfoilfittedwithsawtoothandslotted-sawtoothserrationsisslightlywider,
butgenerallymatchwiththeBaselineairfoil.
0.2 Baseline
U=10m/s
U = 30m/s Root position
0
0.1 Tip position
c
0
0 2.
c Airfoil =
y/ α=0(deg) x
-0.1
-0.2 c c c c c
2 3 5 0 5
0 0. 0. 0. 1. 1.
= = = = = =
-0.3 x x x x x x

0.2
U = 30m/s U=10m/s
0 c
0.1 2.0
=
x
0
c Airfoil
y/ α=0(deg)
-0.1
-0.2 c c c c c
2 3 5 0 5
0 0. 0. 0. 1. 1.
= = = = = =
-0.3 x x x x x x
Fig.7. WakemeanvelocityforNACA65(12)-10airfoilatα=0◦.
TheReynoldsstresstensorsprofilesforNACA65(12)-10airfoilatα=0◦ispresentedinFig.9. Theresultsshow
thattheslotted-sawtoothhaslargershearstresscomparedtothesawtoothatnearwakelocations,x=0.2cand0.3c.
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0.2 TKE/U2 Baseline
U = 30m/s 0 Root position
0
0.1 0.01 Tip position
0
c Airfoil
y/ α=0(deg)
-0.1
-0.2 c c c c c c
2 3 5 0 5 0
0 0. 0. 0. 1. 1. 2.
= = = = = = =
-0.3 x x x x x x x

0.2 TKE/U2
0
U = 30m/s
0 0.01
0.1
0
c Airfoil
y/ α=0(deg)
-0.1
-0.2 c c c c c c
2 3 5 0 5 0
0 0. 0. 0. 1. 1. 2.
= = = = = = =
-0.3 x x x x x x x
Fig.8. WaketurbulentkineticenergyforNACA65(12)-10airfoilatα=0◦.
0.1
(a) x = 0.2c (b) x = 0.5c (c) x = 1.0c (d) x = 2.0c

0
h
ot
wto y/c-0.1
Sa Baseline
-0.2 Root

Tip
-00..31
(e) x = 0.2c (f) x = 0.5c (g) x = 1.0c (h) x = 2.0c

h
ot 0
o
wt
d-sa y/c-0.1
e
ott -0.2
Sl

-0.3
-4 -2 0 2 4 -4 -2 0 2 4 -4 -2 0 2 4 -4 -2 0 2 4
-u′v′/U02 x 10-3 -u′v′/U02 x 10-3 -u′v′/U02 x 10-3 -u′v′/U02 x 10-3
Fig.9. ReynoldsstresstensorforNACA65(12)-10airfoilatα=0◦.
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