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21stAIAAAppliedAerodynamicsConference,Orlando,FL,23-26June2003.
1
AmericanInstituteofAeronauticsandAstronautics
IMPROVEMENTSTOWALLCORRECTIONS
ATTHENASALANGLEY14X22-FTSUBSONICTUNNEL
VenkitIyer,DavidD.Kuhl,LockheedMartin
and
EricL.Walker,NASALangleyResearchCenterHampton,VA
SeniorMember,AIAA;AerodynamicsSectionSupervisor,LangleyProgramOffice.Member,AIAA;AeronauticalEngineer,LangleyProgramOffice.StudentMember,AIAA;ResearchEngineer,ResearchFacilitiesBranch,AAAC.
ABSTRACT
The new wall pressure measurement system and the
TWICS wall correction system for the 14x22-Ft
subsonic tunnel are described. Results froma recent
semispan test and a full-span test are presented.
Comparison with existing classical methods of
correction is shown. A modification of the TWICS
codetotreattheeffectduetoadeflectedwakefromahigh-lift wing is also discussed. The current
implementation ofTWICS for the 14x22-Ft tunnel is
showntobeanimprovementoverexistingmethods.
1. INTRODUCTION
Windtunnelfacilitiesallovertheworldareconstantly
seekingimprovementsintestingcapabilitiesandinthe
qualityofdataprovidedto thecustomers.Themetrics
ofperformanceisnolongerthenumberofdatapoints
taken,butrathertheefficientmannerinwhichthetest
objectivesareachievedbyacquiringqualitydata.Two
items of importance in the area of data quality are:statistical analysis of repeatability of data based on
periodic check-standard testing1, and correction for
interferenceduetothetunnelwalls.Inparticular,wall
interference appears as a bias error on the measured
data,whichneedstobequantifiedinordertoarriveat
theequivalentfree-airvalues.Itisimportanttodothis
accuratelybecausesubtlephenomenasuchasReynolds
numbereffectscanbemaskedotherwise.
Inthisdrivetoimproveperformanceanddataquality,
wind tunnels are retro-fitted with new hardware, data
acquisition systems and analysis software. The
14x22-Ft facility at the NASA Langley Research
Center, thesubjectof this paper,underwent anumberof modifications during 2001-2002, including a new
drive motor and a new wall pressure measurement
systemamongvariousotherupgrades.Specifically,the
purposeofthenewwallpressuremeasurementsystem
istomeasuremoreaccuratelythepressuresignatureat
the walls with or without the test article. This
measurementinturnisusedinspecifyingtheboundary
condition for a wall interference correction code.
Correctionsto obtaintheequivalent free-airconditions
are therefore computed based on boundary
measurements and forcemeasurements. The general
experience is that this leads to a more accurate
estimation comparedto theclassicalcorrectionsbasedonliftanddragonly.Ofcourse,thisclaimneedstobe
substantiated with extensive cross-tunnel validation
testsusingdifferentsizemodels.
Thispaperisadescriptionoftheimprovementsinwall
correction estimation using the new wall pressure
systematthe14x22-Ftfacility.Section2describesthe
newwallpressuremeasurementsysteminmoredetail;
detailed analysis of the quality of these wall
measurementsisgiveninReference2.Thisisfollowed
bySection 3 which presents wall interference basics,
terminologyanddefinitionofcommontermsused.The
classical methods used previously at the facility arediscussed inSection4. Thewall signaturemethod is
thenintroducedinSection5withadetaileddescription
oftheTWICSwallsignaturemethodnowimplemented
in the tunnel. Sections 6 and 7 discuss the two
importantpartsoftheTWICSimplementation,viz.,the
pre-computedperturbationvelocitydatabase(PVD)and
theemptytunnelcalibrationdatabaseusedasatarein
estimatingtheincrementalwallsignatureforaspecific
testpoint.Sections8and9presentsomeresultsfroma
recentsemispanandafull-spantestatthefacility.As
anexampleofimprovementsmadeonthebasicTWICS
method,theissueofdeflectedwakeanditseffectonthe
wall correction ispresented inSection10. Finally,a
brief summary and concluding remarks are given inSection11.
21st Applied Aerodynamics Conference23-26 June 2003, Orlando, Florida
AIAA 2003-395
Copyright 2003 by the American Institute of Aeronautics and Astronautics, Inc.The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes.All other rights are reserved by the copyright owner.
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2. THENEW14x22-FTTUNNELWALL
SYSTEM
The14x22-Fttunnelunderwentextensivemodifications
inthe Fall 2001 toSpring 2002 time period. A new
main drive motor and a new boundary-layer suction
grating were installed during this time in addition to
varioustunnelcontrolanddataacquisitionupgrades.A
newwallpressuremeasurementsystemwasalsoputin
place. Prior to the upgrade, wall pressures were
measuredinthecenterofthetwosidewalls(calledthe
NorthandSouthwalls)andinthecenteroftheceiling
alongarowof30wallportlocationsdistributedalong
the 40-fttestsection length. However, the quality of
wall pressure measurements was poor due to inferior
measurement instrument accuracy, poor surface and
orifice quality and possibly due to various leakage
flowsintothetestsection3.
Thenewwallpressuremeasurementlayoutisshownin
Figure1.Itconsistsof12rowsof31wallpressuretaps
(4rowsin each ofthe south, ceiling and northwalls;
seeFigure1 forwallandrowdesignationconvention).
Thewallportdistributiondensityin theodd-numbered
rows is selected such that a higher resolution is
obtainedatthefrontmodelcartcenterlocationof17.75
ft.Theportdistributionintheeven-numberedrowsis
weightedsuchthatahigherresolutionisobtainedatan
alternate downstream model centerlocation. Thewall
orificesaredrilledintoplateswhichareaffixedtothe
wallin30 ftsegments.Variousgapsinthewallswere
pluggedorsealedtoreduceleakage.
The pressure lines are connected to electronically
scanned pressure(ESP) moduleswhich aresupplieda
reference pressure from a pressure calibrator unit
(PCU).TheESPmoduleshaveafullscalerangeof10
inchesofwater(0.361psi).Sincethetunneldynamic
pressure(Q)canbeashighas0.95psi,theESPmodule
referencepressureissetdependingonthetunnelQso
thatthewallESPpressuredifferentialiswellwithinits
stated range. Wall measurements are thus made in a
smaller range tuned to the tunnel Q resulting in
improvementsinmeasurementaccuracy.
3. OVERVIEWOFTUNNELWALL
CORRECTIONS
Wallinterferencereferstothechangesinthemeasured
tunnel-stream reference conditions and model
parametersduetotheconstrainingeffectsofthetunnel
walls. Inthe case ofsolid walls, the natural outward
expansionof thestreamlinesisprevented, causingthe
flowaboutthemodeltoaccelerateoverthatinfreeair.
Open jet boundaries allow the flow to over-expand,
causing the flow atthe boundary toslowand balloon
outward. The flow at a ventilated wall is somewhere
between these two extremes. These changes in the
boundaryconditions canmakethe tunnel flow around
themodelsubstantiallydifferentfromthefree-airflow.
Thetunnel measuredvaluesreflect thischangedflow,
and corrections must be applied to the measured and
derivedvaluestogettheequivalentfree-airvalues(i.e.,
whenthewallsareremoved).References4and5givea
comprehensivereviewonthetopicofwallinterference.
To illustrate the idea further consider a three-
dimensional point source in free-stream simulating a
Rankineforebody.Thefree-airsolutionisgivenbythe
sum of the free-stream potential and the source
potential. The perturbation velocities in free-air are
simply the derivatives of thesource potential. When
this singularity is enclosed by four solid walls, flow
tangency is imposed at the boundaries which isobtainedbyplacinganinfinitenumberofsingularities
atreflectionlocationsoffthewalls,followingthewell-
known method of images. Therefore, the additional
perturbation potential introduced by the walls is the
sum of all the image potentials. Correction for wall
interferencethus corresponds to the sumof reflection
potentials, which can be evaluated once the original
singularitystrengthisknown.
Wallinterferencecorrectionisthusaspatiallyvarying
function. The traditional assumption is that the
perturbation velocity field can be approximated by a
change inthe angleof attack and the tunnel velocity.Any left-over differences are usually second-order
effects. However, when they become significantly
largesuchasinthecaseofalargemodel,themeasured
datamaybecomeuncorrectablebytraditionalmethods.
ACFDsolutionofthein-tunnelflowisthenrequired.
Thisishowevercostlyforroutineuseatawindtunnel
facility.
The perturbation velocity field is usually computed
using a wall signature method in which a simplified
representation of the model is made using potential
singularityelementssuchas pointsinks,pointsources,
and point or line doublets. Once the perturbation
velocity solution based on the measured boundary
condition imposed is known, wall corrections can be
quantified by averaging the interference flow field
alongmodelortunnelreferencelines.Thecorrections,
termed primaryor mean wall interferencecorrections,
are given in terms of a blockage correction and an
angleofattackcorrection.
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Theblockagecorrectionisobtainedastheaverageofstreamwise perturbation velocities (normalized by
tunnelreferencevelocity)alongthemodelaxis.Thisis
proportionaltotheratioofthemaximummodelfrontal
crosssectionareatothetunnelcrosssectionarea.This
correction is applied to the measured values of MachnumberManddynamicpressureQ.Thecorrectionsfor
MandQareobtainedas
211
2
MM
M
= +
(1)
( )22Q
MQ
= (2)
Theangleofattackcorrectionisobtainedasaweighted
average of the perturbation velocity in the lifting
direction.Thewingchordlineisthereferenceline
customarily used for this averaging, although someclassicalmethodspreferthechordline.Weightingis
done based on the spanwise load distribution. These
primary correctionsleadto correspondingcorrections
onforcecoefficientsCLandCD.Abuoyancycorrection
onCDduetowallinterferenceisalsocomputedfrom
the perturbation velocity gradient averaged along the
model axis. Pitching moment coefficient corrections
are computed based on changes in flow curvature
calculatedfromspatialvariationsintheangleofattack
corrections.
Theinterferenceflowfieldinthevicinityofthe model
isalsoofinterestinordertoassesstheextentofchangein mean corrections with change inreference lines or
pointswheretheyareaveraged.Theselocalvaluesof
corrections are presented as contour plots at different
tunnelsections.
To summarize, wall corrections for a test point are
reportedasfollows:
1. A blockage parameter, and an angle of attack
correction, ; these are the primary correctionsobtainedbyaveragingtheperturbationflowfield.
2. Corrections ontunnelMachnumber and dynamic
pressure, which are functions of the blockageparameter,.3. Corrections on lift, drag and moment coefficients
which result from the changed tunnel reference
velocity,angleofattackandflowgradients. Theseare
derivedcorrectionsobtainedfromtheperturbationflow
field.
4. Contour plotsof perturbation velocityfield inthe
modelvicinity;theseshowtheextentofthedeparture
of local corrections from the averaged correction
values.
Thefree-airorcorrectedvaluesareobtainedbyadding
the corrections to the measured values. For a lifting
modelinasolid-walledtunnel,thefree-airorcorrectedvalues ofM, Q, , CD are larger than the measured
values;free-airCLisusuallydecreased.Dependingon
the model and the test, corrections on roll moment,
sideforceandtailincidencecanalsobecalculated.
4. EXISTINGMETHODSATTHE14x22-FT
TUNNEL
Theexistingwallcorrectionsusedinthefacilityrelyon
standardformulaebasedonmodelandtunnelgeometry,
measuredliftanddragvalues.Thisclassicalblockage
correction is documented in References 4, 6, 7 and
various other references dating back to 1960. It
consists of a fuselage volume blockage term, a wing
blockage term and an attached wake blockage term
based on the apparent drag coefficient. The jet
boundary correction (equivalent to an angle of attack
correction)isalsoappliedbasedontheliftandinduced
dragcoefficients. While this method is of acceptable
accuracyforasmallmodelinattachedflowconditions,
large departures are possible when dealing with large
modelsathigh-liftconditions.Formodelswithalarge
deflectedwake,amethodbasedonHeysonswork8is
applied to calculate the change in the angle of attack
anddynamicpressureinthemodelregion.
Unfortunately, many questions exist regarding thevalidity of the assumptions required for derivation of
theseclassicalmethods.Inaddition,noassessmentof
correction uncertainty is available. Improving the
accuracy of wall corrections involves the use of the
measured wall-pressure boundary condition, which
incorporatesa morerealisticcharacterizationofthe in-
tunnel flow into the correction method and hence a
tightercontrolontheaccuracyofcorrections.
5. THETWICSWALLSIGNATUREMETHOD
TheTransonicWallInterferenceCorrectionSystem(or
TWICS) is a wall correction code based on the wallsignature method developedfor transonic tunnelswith
ventilatedwalls,originallyfortheAmes11-FtTunnel9.
Ithasbeenimplementedforthe14x22-Fttunnelin the
solid-wallorwallsdownconfiguration.TWICSand
its predecessor code WICS10
were developed by
Ulbrichbyusingastrategyofgloballyfittingthewall
signature. A brief summary of the method is given
below.
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TWICS uses the pressure signature at the walls
(actuallytheincrementalvaluerelativetotunnelempty
signature)asthebasisforcomputingwallinterference
corrections.Themodelisrepresentedbyanumberof
pointdoublets(tosimulatevolumeandwakeblockage)
andlinedoublets(tosimulateeffectsduetolift).Thefar-field effect due to the assumed singularity
distributionismatchedwiththewallsignature.Thisis
done in a global fitting procedure, which yields the
strengths of the singularities as the solution. The
perturbation velocities are then computed based on
superposition of standard solutions of point and line
doublets which are contained in pre-computed
databases of perturbation velocity solutions.
Corrections for each test point are obtained by
interpolationfromthedatabaseatnear-realtimespeeds.
Compressibility is modeled using Prandtl-Glauert
scaling.Asaresult,thereisanupperlimittotheMach
numberintheapplicationofthismethod.
TheTWICScodeisdesignedtoworkinunisonwitha
panel-method-generated database of wall interference
solutionsbasedonpointdoubletsforblockageandline
doubletsfor lift interference. Byappropriatelysetting
theboundaryconditionsinthepanelmethod,ventilated
wallscanalsobemodeled.Sincethe14x22tunnelis
operatedasasolid-wallfacility,thisfeatureofTWICS
isnotused.Asimplerprocedurebasedonthemethod
ofimagesisusedtogeneratethedatabase.
ThestringofpointdoubletsusedintheTWICSmethod
tomodelvolumeandwakeblockagescanbeshownto
beequivalenttothesource-sinkpairusedin theWICSmethod. The advantage is that the individual point
doubletstrengthscannowbeweightedinproportionto
the cross section area of the test article thereby
increasingthemodelfidelity.InTWICS,theeffectof
the sting is also modeled using a chain of weighted
point doublets, in an analogous fashion to volume
blockage. This shortcut implies the use of a
considerablysmallertunnelcalibrationtestmatrix.
TheinputsforTWICSaredescribedbelow:
1. Tunnel empty signature: The wall signature is
defined in terms of the 12 rows with 30 orifices ineach. This calibration data is required for a specified
range of Mach numbers (or equivalently, dynamic
pressures).Forfullspanmodels,thesignaturewiththe
modelsupport(whichistheverticalpostatX=40ft)at
severalpitchanglesisalsorequiredatvariousoperating
conditions. Several such calibration sets are required
depending on the support system used and the floor
boundarylayersuctionsystem(BLRS)state(onoroff).
SeeSection7fordetailsofthetunnelcalibration.
2. Wallsignatureforagiventestpoint.
3. Testpointvaluesofuncorrectedforceandmoment
values, Mach number, reference velocity at model
centerofrotationandanumberofothertestandmodel
attitudeparameters.
4. Perturbation velocity database (PVD): This is alargetableofpre-computedperturbationvelocitiesused
in signature matching and wall interference
computation.Thedatabasedependsonthewallorifices
layout, tunnel section, Mach number and lift vector
direction. SeeSection6fordetailsofgeneratingthese
databases.
5. Modelsingularitydistributionandgeometrydata.
6. Referencelines alongwhichweightedaveragesof
interferencearetobecomputedandplanesalongwhich
localvaluesofwallinterferencearetobecomputed.
7. Port flags used to de-select specific wall orifices
thatarenottobeusedinthecalculationforagiventest.
In addition to this, the code rejects additional wall
orificesbasedonstatisticsofthefit(seeReference10
for the original rejection criteria). Here an improved
rejection criteria is used based on wall data quality
analysis (see Reference 2 for details). Hence the
original statistical rejection logic used in the code is
turnedoff.
The calculation steps used in TWICS are described
below:
1. Processing of input test data:Foreach test point,
the wall signature is read in. Subsequently, the
corresponding tunnel empty signature is interpolated
fromthecalibrationdatabaseandsubtractedtogettheincrementalortaredwallsignature.
2. Computationoftheequivalentlinedoubletstrength
frommeasuredliftandmodelgeometry:Thestrengthis
thendistributedalongthespanasperinputorcomputed
weights. These weights are based on the estimated
wingloadingdistribution.
3. Interpolation from the perturbation velocity
database (PVD): This is done to estimate the lifting
effectpartofthesignatureateachport.Theliftingpart
of the signature is then subtracted from the tared
signaturetogettheblockageeffectateachport.
4. LeastsquaresfittingandinterpolationfromPVDto
calculatethestrengthsofthepointdoublets:Thetwounknowns computed here are the volume blockage
strength and the wake blockage strength. This step
representsthecoreofthecalculationprocedure.
5. Interpolation from PVD to compute wall
interference at any point in the test section (within
reference grid limits) by superposition of all
singularities: Mean corrections are then calculated
using weighted averaging. Force and moment
coefficient corrections are then computed. The
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streamwisedistributionofblockageis usedtoestimate
thebuoyancycorrection.
6. Iteration using corrected tunnel parameters, if
necessary.
Corrections are computed for each point in a polarindependently. The primary mean correction due to
blockageisappliedascorrectionsonMachnumber,M
and dynamic pressure, Q (added to corresponding
measured values). Upwash correction is applied as
correctionontheangleofattack.CorrectionsonCL,CD
and pitching moment coefficient CM are computed
basedontheprimarymeancorrectionsofblockageand
angle ofattack. Inaddition,model-induced buoyancy
correction is also calculated and added to CD. The
method also computes local variations of interference
corrections, which are useful in determining if the
averaging assumption is truly representative of the
interferencefieldinthemodelregion.
6. PRE-COMPUTEDPERTURBATION
VELOCITYDATABASES
The databases for TWICS are large tables of pre-
computed solutions of perturbation velocities induced
by unit singularities (point doublets or line doublets)
enclosedby the tunnel walls. Inthese databaseruns,
thesingularitiesareplacedatdifferentlocationsinthe
tunnel (termed the singularity grid) and solutions are
calculated at different locations inside the tunnel
(termedthereference grid)as wellas atthe wall port
locations.
The database table is thus a function of a number of
variablesasgivenbelow.
Typeofsingularity:Thepointdoubletsingularityisthe
basicbuildingblockforsolidorwakeblockagedueto
amodelinthetunnel.Thisisthesameasapointsource
plus a pointsinkin closeproximity. The line doublet
singularityis thebuildingblock forliftinterference in
the tunnel. This is equivalent to a chain of point
doubletsstartingatthelinedoubletlocationstretching
toinfinity.
Orientationofsingularity:Pointdoubletdirectionused
hereis always pointingin theupstream direction (i.e.,point sink to point source direction is X ). Line
doubletdirectionisoppositetotheliftvectordirection
(i.e.,chainofpointdoubletspointingawayfromthelift
direction). In order to handle a case with model roll
whereliftdirectionchanges,calculationsaredoneat45
degree intervals ofthe line doublet angle inorderto
facilitateinterpolations.
Singularity location: The locationis determined by a
specifiedsingularitygrid;themodelandsupportsystem
shouldbecontainedwithinthisgrid.
Solution location: The location is specified by a
reference grid. Reference lines along which wall
corrections are averaged (such as fuselage centerline,wing chord line) should be contained within this
grid. Solutions are also computed at all pressure
measurement locations on the tunnel walls. The
solutionatatunnelinteriorpointconsistsofthethree
perturbationvelocitycomponents( u,v,w);thesolution
atthewallconsistsofonlythestreamwisecomponent
u.
Model configuration: The database depends on
whether the model configuration is semispan or full-
span. The location of the singularity and reference
grids are dependent onthe model configuration. The
solutionalsodependsonthemodelconfigurationsince
thesemispan model-mounting wall becomesan image
planewiththeeffectivetunnelsectiondoublinginsize.
Mach number: With the use of the Prandtl-Glauert
scalingfactor2
1 M = ,theeffectivetunnelcross
section is reduced by this factor. The singularity
strengths also scale with this factor (2
for point
doublet,3
forlinedoublet).
Note also that the solutions calculated at the wall
include the direct effect of the singularity. This can
thenbecomparedtothewallsignature.Thesolutions
calculatedattheinteriorofthetunneldonotincludethe
direct effect of thesingularity(itis thusequivalent tothesummationofsolutionsfromonlytheimagesofthe
singularity which is the true definition of the
interferencesolution).
InTWICS,oncetheactuallocationofasingularityin
the tunnel is established, multi-parameter linear
interpolation is used to find the corresponding
perturbation velocitysolution. For walllocations, the
actual coordinates of the wall ports are used in the
database. A master databaseis first generated for all
thewallports;acustomizeddatabaseforagiventestis
then derived from it depending on the ports actually
selectedforuseinaparticularTWICSrun.
7. TUNNELCALIBRATIONDATA-BASES
Measuring the tunnel-empty wall signatures
periodicallyisarecommendedqualitycontrolmeasure
for the wall system. It is also required in TWICS to
subtractoutsystematicvariationsfromorificetoorifice
and the effect due tothe tunnel boundarylayer. The
assumptionmadehereis thattheseeffectsareconstant
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withorwithoutthemodelinthetunnel.Thecalibration
isdoneforanumberof Qvalues,sothatinterpolation
canbedoneforthetest-pointvalueof Q.Forfull-span
models, the model sting is also considered in the
TWICS simulation, so there is no need to obtain a
calibrationwith thestingor othersupportcomponentswhicharealignedwiththewake.However,otherparts
ofthesupportsystem(suchastheverticalpostforthe
14x22tunnel)shouldbe includedinthecalibration. If
thegeometryofthesecomponentschangewithangleof
attack,forexample,thenthecalibrationshouldbedone
in a range of such parameters. Interpolation for the
tunnel empty signaturewill thenbe a multi-parameter
oneinvolving forexample,Qandtheangleofattack.
Separate databases are required with floor boundary
layerremovalsystemonandoff.
8. EXAMPLERESULTS:SEMISPANMODEL
Wall corrections for a large semispan wing tested
recently at the facility will now be discussed. The
model is a trapezoidal planform high-lift wing with
fixed-setting flaps and slats. A photo of the model
installed inthetunnelisshown inFigure2. Some of
the important model and tunnel geometry parameters
andflowconditionsaregiveninthefollowingtable.
Wingarea 22.028sqft.
Wingchord,
chordsweeps30,26.1
Root,tip,aero.and
geom.chords
4.6,1.7,3.3,
3.1ft
Halfspan,aspectratio 7.088ft,4.561Tunnelcrosssection 14.75by21.75
ft
Modelcenterof
rotation
X=17.75,
Y=7.25ft
TunnelQ 57.8psf
Machnumber,
velocity
0.2,230fps
Angleofattack 4to36
ThetestwasrunwiththeBLRSon.Alltestrunswere
made ata fixedunitReynolds number of1.4 million.
The CLvs. CD curveforthis wingmodel isshown in
Figure 3, which is typicalof a high-lift configuration.
Stallangleis34to35withamaximumCLof3.0anda
CD,stallof0.7.
Thesingularitydistributionisspecifiedaprioriwitha
number of X direction point doublets along the
fuselageaxisandwakeseparationlines.Linedoublets
originatingfromthechordlineareusedtomodelthe
lifting effect with an elliptic loading distribution.
Figures 4a and 4b show the distribution used in
TWICS. The specified locations correspond to zero
angle of attack; for a particular data point, these
locations are displaced as a function of the model
kinematicsandattitude(,inthepresentcase).
Wallcorrectionresultsforarepresentativerun(apolar
of28pointsfrom4to35)isgivenhere.Figure5
showsthemeancorrectionsforthisrun.Thecorrection
on theblockageparameter is shownas well as thecorrectionsonQ.Theblockagestaysbelowthe0.25%
valueforanglesofattackbelow25 .Between25and
34,blockageincreasestodoublethislevel(0.5%)due
toalargercontributionfromthewake.Above34,the
wing begins to stall and the large separated wake
increasestheblockageevenfurther.Alsoshowninthe
plotof Qarethecorrectionvaluescalculatedbytheexisting classical procedure as well as using the
Maskellprocedure11
.Thetwo-stepformoftheMaskellmethod estimates the separated flow effects on
blockageusing a formulation based on flat plate flow
measurements.TheblockagepredictedbyTWICSfalls
inbetweentheclassicalandMaskellresults.
The angle of attack correction shown in Figure 5 is
largelyafunctionofliftandfollowstheCLvs.shape.
Amaximum correctionof1.6isobtainedat =32which means theequivalent free-airangle of attack is
33.6. In TWICS, the correction is obtained byaveraging at the chord line. The classical values
obtainedbythejetboundarycorrectionofReference6,
alsoshowninFigure5,seemtobemoreinagreementwith the TWICScorrections obtained by averaging at
thechordline.
Corrections on the force and moment coefficients are
showninFigure6.TheD
C andL
C valuesfromthe
classical approach are also shown to be in close
agreement, with a slight departure only near stall
conditions.ThebuoyancycorrectiononD
C isshown
to be quite small, varying linearly with the angle of
attack to a maximum value of 25 drag counts. The
changed aerodynamic characteristic due the wall
interferenceintheformofacorrected L DC C curveisalsoshowninFigure6.Overall,asseenfromFigures
5-6,thewallcorrectionsaresubstantialandneedtobe
consideredwhenevaluatingthefree-airperformanceof
thewing.
AgoodwaytoverifyhowtheTWICSmodelsimulates
the real flow is tocomparethe measured incremental
wall signature with the TWICSprediction. Shown in
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Figure7aretheincrementalwallsignaturesalongthe
12rowscomparedwiththeTWICSvaluesfor3angles
of attack. Theoverallagreement isremarkably good,
exceptatRows1and12athighanglesofattack.Some
of this difference is due to a deflected wake which
tracks closer the north wall and is not simulated inTWICS.Section10providesmoredetailsoncapturing
thiseffectwhichleadstoabetteragreement.Itisalso
possible that the simple lift model used in TWICS is
also responsible for part of this difference in the
signatures.
9. EXAMPLERESULTS:FULL-SPANMODEL
Wall corrections for a relatively small full span wing
(reference area only 2% of tunnel cross section,
comparedto7%forthetrapezoidalwing)willnowbe
presented.Thewingisanellipticalplanformwingwith
symmetric NACA 0012 sections. This is a more
limitingtestcaseforthewallpressuresysteminthatthe
wall signatures are much weaker (only 10% of the
trapezoidal wing signatures for the same angle of
attack).Someoftheimportantmodelparametersand
flowconditionsaregiveninthefollowingtable.
Wingarea 6.25sqft.
Wingsweep 0
Rootchord 1.2ft
Span,aspectratio 6.75ft,5.625
Modellength 2.8ft
Modelcenterof
rotation
X=17.75,Y=0
ft
TunnelQ 2137psf
Machnumber,
velocity0.050.32,
6350fps
Angleofattack 3to12
Aphotoofthemodelinstalledinthetunnelalongwith
thesupportsystemisshowninFigure8.Thesupport
systemconsistsofasting,arollcoupling,alongsting
(cannon), and a vertical post. The support system
moves in the pitch plane with the model fixed at the
X=17.75, Y=Z=0 position. The aerodynamic
characteristicofthemodelisshowninFigure9.
A tunnel-empty calibration with the support system
aloneinstalledinthetunnelwithrepeatrunsatseveral
Q and values is used here. Generally, an emptytunnelcalibrationcanalsobeusedwithoutanyofthe
wake-oriented support system components (these can
be modeled in TWICS). However, the vertical post
shouldbepartoftheemptytunnelcalibration.Forthe
present case, the only good calibration set that is
availableistheonewiththecompletesupportsystemin
thetunnel. Thisessentiallymeansthat thesingularity
distributionusedherewillcorrespondtoonlythemodel
andwakeandnotanyofthestingcomponents.
Thesingularitydistributionusedfortheellipticalwing
model is shown in Figure 10. Wall correctioncalculationsweredoneforanumberof Qvaluesinthe
range3to12.Resultsindicatethattheblockageisnominallyconstantacrossthisrangeat0.001.Itwas
alsofoundthatforQvalueslessthan10psf,thescatter
in the computed blockage values is large, due to the
wall systemnotableto distinguishthe weaksignature
from other random variations. The corrections onM
and Qarealsoconstantintherangeandtheyscalewith M and Q as given by Equation (1) and (2)
( /M M = ; / 2Q Q = , approximately). The
angle of attack correction scales with with
/ =0.011, approximately. The classical value
calculatedfromchartsinReference7isalso0.011.
Thecorrectionsontheforcecoefficientsareshownin
Figure11.Thecorrectionondragreachesamaximum
of20dragcountsat12 angleofattack;correctionson
liftanddragduetobuoyancyarenegligiblysmall.
Comparison of measured incremental wall signatures
withTWICS-fitvaluesat=3,2and12isshowninFigure12.Agoodglobalfithasbeenobtainedevenat
these low wall signature values. Note that the wall
signaturescaleis0.01comparedtothe0.1scaleused
forthetrapezoidalwingcaseinFigure7.
10. DEFLECTEDWAKEANALYSIS
Thestandardapproachusedinclassicalwallcorrection
methodsas well asin wall signature methodssuchas
TWICSistomodelwall-inducedliftinterferenceusing
a number of line doublets distributed along the wing
span with starting locations along the chord line.
Eachline doublet is alsoequivalent to a semi-infinite
streamwise string of point doublets, pointing opposite
to the lift vector direction, and originating from the
same chord locations of the wing. For a three-
dimensionalwingtherewillbeanumberofsuchpoint
doubletstringsatdifferentspanlocations.
For high-lift wings as well as rotorcraft in forward
flight,thevorticityinthewakeisorientedatanangleto
the freestream in an average sense. In addition, the
correspondingpointdoubletvectorsareinclinedto the
free-streamatanangleprimarilydependentonthelift-
drag ratio, as modeled by Heyson8. Heysons wake
deflectionmodelthusassumesastraight-linetrajectory
for thisvorticity track. This model is equivalent toa
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8
semi-infinite string of point doublets placed along a
deflected wakeline,withthe pointdoubletdirectionat
an angle to the lift vector direction as shown in the
sketchinFigure13a.
The Heyson model calculates thedeflected wake wallinterferencevelocitiesintwosteps.Firsttheeffective
wakedeflectionangleiscalculated.Thisisafunction
of the wing span, area, lift, induced drag, tunnel
geometryandfree-streamvelocity.Thesecondstepin
the Heyson model is to calculate the singularity
strengthsandcorrespondinginterferencevelocities.As
shown Figure 13b for the trapezoidal wing case, the
pointdoubletsaresplitintotwotypes,onepointingto
YproducingthelifteffectandanotherpointingtoX
producingtheinduceddrageffect.
Since the results from the first step of the Heyson
analysisrevealedthatthevorticitymaybedeflectedby
asmuchas10ontheaveragefromthechordplaneat
maximum lift condition, a modified version of the
TWICScodewas created toinclude these effects. In
order to implement the deflected wake, the line
doubletsusedinTWICSwerechangedtoYdirection
pointdoubletstringsnotingthatasummationofachain
ofthesepointdoubletsisequivalenttothelinedoublet
witha90orientation.Itisthenpossibletodeflectthe
vorticity track by specifying the point doublets to be
located along the deflected line. This requires the
generation of a new PVD database with interference
solutionsduetoYdirectionpointdoublets.Thefirst
partoftheHeysonmodelwasaddedtoTWICStofirst
computetheeffectivewakedeflectionangle. Changeswere made in the interpolation scheme in TWICS to
locate the deflected lifting point doublets in the
databasegridfollowedbyinterpolationandsummation.
Changeswerealsomadetodeflectthewakeblockage
pointdoubletsbyacorrespondingangle;thiseffectwas
howeverfoundtobeminor.
Figure 14 shows the wall signature fit from the
deflectedwakeversionofTWICSfor=32(forrows1and12,mostaffectedbythedeflectedwake),which
canbecompareddirectlywithFigure7fromtheregular
TWICScalculation.Itisclearthatthedeflectedwake
modificationhasimprovedthefitinthesetworowsofthe South and North walls. Figure 15 further
demonstratesthatthisistrueforalltherows,showing
thatthestandarddeviationofthefithasdecreasedasa
result of this modification. Note that the global
standard deviation drops by a factor of almost two.
Finally, Figure 16 shows the change in the mean
corrections as a result of the deflected wake
modification. A closer agreement with the classical
valuescanbeobserved.
11. CONCLUSION
Awallsignature-basedcorrectionmethodforthewalls-
down configuration of the 14x22-Ft tunnel has been
implemented based on a new wall pressure
measurement system at this tunnel. Example resultsfromrecentsemispanandfull-spantestsatthefacility
have been presented. Comparison with classical,
closed-formmethodsofwallcorrectionindicatethatthe
wall signature method provides comparable values.
Further, the model-provided wall signatures compare
very well with measured values in a range of model
sizes, angle of attack and Q values. Although true
validation of results require cross-tunnel tests using
scaledmodels,itcanbestatedthatthenewmethodis
an improvement over the existing classical methods
since boundary measurements are used in addition to
forceandmomentinputsandsincemodelshowsclose
agreementwiththemeasuredwallpressures.
REFERENCES1. Hemsch,M,Grubb, J., Krieger,W., andCler, D.,
Langley Wind Tunnel Data Quality Assurance
CheckStandardResults,AIAA2000-2201,June2000.
2. Kuhl,D.D.,andEverhart,J.L.,Measurementand
Control of the Uncertainty of Scanning Pressure
Transducer Measurements, AIAA 2003-3816, June
2003.
3. Iyer, V., and Everhart, J.L., Application of
Pressure-Based Wall Correction Methods to Two
NASA Langley Wind Tunnels, AIAA 2001-2472,
June2001.
4. Garner,H.C.,Rogers,E.W.E.,Acum,W.E. A.,and Maskell, E. C., Subsonic Wind Tunnel Wall
Corrections,AGARDograph109,October1966.
5. Ewald, B. F. R. (Editor), Wind Tunnel Wall
Correction,AGARDograph336,October1998.
6. Quinto,P.F.,andOrie,N.M.,Langley14-by22-
FootSubsonicTunnelTestEngineersDataAcquisition
andReductionManual,NASATM4563,June1994.
7. Barlow,J.B.,Rae,Jr., W.H., Pope, A,Low-Speed
Wind Tunnel Testing, ThirdEd., John Wiley & Sons,
1999,pp.367-427.
8. Heyson,H.H.,LinearizedTheoryofWindTunnel
Jet-Boundary Corrections and Ground Effect for
VTOL-STOLAircraft,NASATRR-124,1962.9. Ulbrich, N. and Boone, A.R., Determination of
theWallBoundaryConditionoftheNASAAmes11ft
TransonicWindTunnel,AIAA2001-1112.
10. Ulbrich, N., The Real-time Wall Interference
CorrectionSystemoftheNASAAmes12-footPressure
Tunnel,NASA/CR-1998-208537,July1998.
11. Maskell,E.C., ATheoryof theBlockageEffects
onBluff Bodiesand StalledWings ina Closed Wind
Tunnel,R&M3400,November1963.
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9
Y
Z
-15-10-5051015
-5
0
5
Cross section view looking downstream
Southwall
Northwall
R
ows
912
Rows 58
Rows14 Z
Y
X
Z
0 10 20 30 40-10
0
10
INFLOW
PLANE
Port distribution on the South Wall
Row 1
Row 2
Row 3
Row 4
X
Y
0 10 20 30 40
-10
0
10
Top view and port distribution on the ceiling
INFLOW
PLANE
Rear modelmounting cart
Front modelmounting cart
Row 5
Row 6
Row 7
Row 8
Figure1.Newwallpressureportsforthe14x22-Fttunnel(ellipticalwingmodeloutlinesshown).
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10
Figure2.Photoofthetrapezoidalwingmountedinthe
14x22-Ft. tunnel. Photo courtesy of NASA Langley
PhotoLab.
Figure3.Aerodynamicscharacteristicsofthe
trapezoidalwing.
X
Y
0 10 20 30
-10
0
10
South wall
North wall
Turn table
Modelcenterof
rotation,
X=17.7
5
Figure4a.Singularitydistributionfortrapezoidalwing,
topview.
X
Z
15 20 25
-5
0
5
Point doublets for body blockage
Line dblt.start locn.s
Pt. doublets forsep. wake
Ceiling
Modelcenterof
rotation,X=17.7
5
Figure 4b. Singularity distribution for trapezoidal
wing,sideview.
CL
CD
0 0.5 1 1.5 2 2.5 3 3.50
0.2
0.4
0.6
0.8
1
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11
ANGLE OF ATTACK (Uncorrected)0 10 20 30 40
0
0.005
0.01
0.015
0.02
BLOCKAGEFAC
OR,
TWICS
ANGLE OF ATTACK (Uncorrected)
0 10 20 30 400
0.5
1
1.5
2
Q,psf
TWICSclassical
Maskell
ANGLE OF ATTACK (Uncorrected)0 10 20 30 40
0
0.5
1
1.5
2
3/4 chord line avg.1/4 chord line avg.classical method
Figure5. Meancorrectionsforthetrapezoidalwingin
the14x22-Fttunnel.
ANGLE OF ATTACK (Uncorrected)0 10 20 30 40
-0.02
0
0.02
0.04
0.06
0.08
0.1
COEFFT.
CORREC
TIONS
CL
CD
CD,Buoy
CL-clas.
CD
-clas.
ANGLE OF ATTACK (Uncorrected)
0 10 20 30 400
0.002
0.004
0.006
0.008
0.01
PITCH.
MOMENTCORRECTION
CL
0 1 2 30
0.2
0.4
0.6
0.8
1
CD
CorrectedUncorrected
Figure 6. Coefficient corrections for the trapezoidal
winginthe14x22-Fttunnel.
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12
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 1 (South Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 9 (North Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 5 (Ceiling)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 2 (South Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 6 (Ceiling)
X=17.7
5ref.l
ine
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 10 (North Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 3 (South Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 7 (Ceiling)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 11 (North Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 4 (South Wall)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 8 (Ceiling)
0 10 20 30 40-0.1
-0.05
0
0.05
0.1 ROW 12 (North Wall)
ROW 1 (South Wall) ROW 5 (Ceiling) ROW 9 (North Wall)
ROW 2 (South Wall) ROW 6 (Ceiling) ROW 10 (North Wall)
ROW 3 (South Wall) ROW 7 (Ceiling) ROW 11 (North Wall)
ROW 4 (South Wall) ROW 8 (Ceiling) ROW 12 (North Wall)
Figure 7. Measured and TWICS-fit wall signatures for the trapezoidal wing test. Symbols are incremental
measurements; lines are TWICS-fit values. Three angles of attack 4 (squares, dash-dot line), 10 (triangles,
dashedline),32(circles,solidline)areshown.X-axisisthestreamwisedistanceX;Y-axisistheincremental
normalizedwallvelocity.
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Figure 8. Photoof the ellipticalwingmountedin the
14x22-Ft. tunnel. Photo courtesy of NASA Langley
PhotoLab.
CL
-0.25 0 0.25 0.5 0.75 10
0.02
0.04
0.06
CD
CorrectedUncorrected
Figure9.Aerodynamiccharacteristicsoftheelliptical
winginthe14x22-Fttunnel.
X
Y
16 18 20 22 24
-4
-2
0
2
4
Tunnel center line
X=17.75 ref. line
Pt. doubletsfor wake
Line dblt.sfor lift
Pt. dblt.s
for vol. blockage
Figure10.Singularitydistributionfortheellipticwing
modelinthe14x22-Fttunnel.
ANGLE OF ATTACK (Uncorrected)0 5 10
0
0.001
0.002
COEFFT.
CORRECTIONS
CD
CL
CD,Buoy
Figure 11. Coefficient corrections for the elliptical
winginthe14x22-Fttunnel.
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14
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 1 (South Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 5 (Ceiling)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 9 (North Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 2 (South Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 10 (North Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 3 (South Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 7 (Ceiling)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 11 (North Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 4 (South Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 8 (Ceiling)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 12 (North Wall)
0 10 20 30 40-0.01
-0.0075
-0.005
-0.0025
0
0.0025
0.005
0.0075
0.01 ROW 6 (Ceiling)
X=17.7
5ref.line
ROW 1 (South Wall) ROW 5 (Ceiling) ROW 9 (North Wall)
ROW 2 (South Wall) ROW 6 (Ceiling) ROW 10 (North Wall)
ROW 3 (South Wall) ROW 7 (Ceiling) ROW 11 (North Wall)
ROW 4 (South Wall) ROW 8 (Ceiling) ROW 12 (North Wall)
Figure 12. Measured and TWICS-fit wall signatures for the elliptical wing test. Symbols are incremental
measurements;linesareTWICS-fitvalues.Threeanglesofattack3(squares,dash-dotline),2(triangles,dashed
line),12(circles,solidline)areshown.X-axisisthestreamwisedistanceX;Y-axisistheincrementalnormalized
wallvelocity.
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X
Y
) wake deflection
=90-
X
Y
) wake deflection
-X dir. pt. dblts for induced drag effect
-Y dir. pt. dblts for lift effect
Figure13.Part(a)(top),Heysonwakedeflectionmodel.Part(b)(bottom),Superpositionusedinthe
Heysonmodel.
X0 10 20 30 40
-0.1
-0.05
0
0.05
0.1 ROW 12 (North Wall)
X0 10 20 30 40
-0.1
-0.05
0
0.05
0.1 ROW 1 (South Wall)
Figure 14. Comparison of measuredincrementalwall
velocities (symbols) with computed values from a
modifieddeflectedwakeTWICScode,=32.
ROW NUMBERS
STD.
DEVIATION
1 2 3 4 5 6 7 8 9 10 11 120
0.005
0.01
0.015
0.02
Average value for all rows,TWICS modified for deflected wake
Average value for all rows,TWICS original version
TWICS originalTWICS modifiedfor deflected wake
Figure 15. Improvement in the standard deviation ofthe TWICS fit to the measured wall pressures along
each wall row with the deflected wake modification,
exampleshownaboveisfor=32.
ANGLE OF ATTACK (Uncorrected)0 10 20 30 40
0
0.5
1
1.5
2
Q,psf
TWICS
Classical
TWICS-Defl. Wake
ANGLE OF ATTACK (Uncorrected)0 10 20 30 40
0
0.5
1
1.5
2
1/4 chord (Defl. Wake)classical
Figure16.Improvementinwallcorrectionswiththe
TWICSdeflectedwakemodification.