1

DESIGNANDCONSTRUCTIONOFALOWSPEEDWINDTUNNEL

By

JonathanD.Jaramillo

Athesissubmittedinpartialfulfillmentoftherequirementsforthedegreeof

BachelorofScience

HoughtonCollege

May2017

SignatureofAuthor…………………………………………….………………………………….DepartmentofPhysics

May13,2017……………………………………………………………………………………………………………...

Dr.KurtAikensAssistantProfessorofPhysics

ResearchSupervisor……………………………………………………………………………………………………………...

Dr.BrandonHoffmanAssociateProfessorofPhysics

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DESIGNANDCONSTRUCTIONOFALOW

SPEEDWINDTUNNEL

By

JonathanJaramillo

SubmittedtotheDepartmentofPhysicsonMay13th,2017inpartialfulfillmentofthe

requirementforthedegreeofBachelorofScience

Abstract

Despiteadvancesincomputationalaerodynamics,windtunnelsareandwillcontinuetobe

acornerstone in thedesignprocess forawiderangeofvehicles.Thismostlystems from

thedifficultiesofaccuratelyandefficientlypredictingturbulentflowfieldscomputationally.

Toexpandin-houseaerodynamicscapabilities,ageneral-purposelow-speedwindtunnelis

beingdesignedandbuiltintheHoughtonCollegePhysicsDepartment.Thiswindtunnelis

designed to reach test section speeds of up to 44.7m/s (100mph). To aid in the initial

design, semi-empirical formulas are used to estimate aerodynamic efficiencies and the

required fan-blower power as a function of various design choices. Tunnel geometry is

selected to optimize test section air flowquality, test section size, anddiffuser angle (to

avoidboundarylayerseparation),whiletheoveralltunnelsizeisconstrainedtofit inthe

allottedlaboratoryspace.Theproposedclosed-circuitwindtunnelisverticallyorientedto

reducefootprint,andis4.72m(15.5ft)longby1.67m(5.5ft)by0.762m(2.5ft)wide.The

overall design is presented and current design and construction progress is highlighted.

Additionally,futureresearchstudiesthatcouldutilizethewindtunnelarediscussed.

ThesisSupervisor:Dr.KurtAikensTitle:AssistantProfessorofPhysics

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TABLEOFCONTENTS

Chapter1BackgroundandMotivation...................................................................................51.1. ApproachestoFluidMechanics.................................................................................51.2. HistoryofExperimentalAerodynamics..................................................................91.3. Objective.........................................................................................................................11

Chapter2Theory........................................................................................................................132.1. FluidMechanics............................................................................................................132.1.1. TheoreticalDevelopment.................................................................................................132.1.2. Quasi-One-DimensionalFlow.........................................................................................142.1.3. Bernoulli’sPrinciple...........................................................................................................17

2.2. WindTunnels................................................................................................................182.2.1. ImportantParameters.......................................................................................................182.2.2. BasicDesignDecisions......................................................................................................212.2.3. SectionLossandPowerConsiderations....................................................................242.2.4. FansandDriveSystems....................................................................................................312.2.5. ConstructionRequirements............................................................................................31

2.3. MeasurementTechniques.........................................................................................322.3.1. PressureMeasurement.....................................................................................................322.3.2. VelocityMeasurement.......................................................................................................34

Chapter3DesignConsiderationsandMethodology.......................................................373.1. BasicDesignDecisions...............................................................................................373.2. Constraints,Methodology,andDesign.................................................................383.3. CurrentProgress..........................................................................................................40

Chapter4ConclusionsandFutureWork............................................................................464.1. Conclusions....................................................................................................................464.2. FutureWork..................................................................................................................46

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TABLEOFFIGURES

Figure1.AnexampleofagridmeshfortheNACA0012airfoil...................................................6Figure2.AdiagramoftherelativeaccuracyandcostofvariousCFDmethods....................8Figure3.AsketchofBenjaminRobin’swhirlingarm....................................................................10Figure4.AsketchoftheWrightbrothers’windtunne.................................................................11Figure5.Quasi-one-dimensionalflowthroughaduct...................................................................16Figure6.AnaerialviewoftheNationalFull-scaleAerodynamicsComplex........................22Figure7.TheNACA6-inchopenthroatwindtunnel......................................................................23Figure8.AschematicoftheUCDavisAeronauticalWindTunnelFacility...........................24Figure9.Adepictionoftheconicalangleforacone-shapeddiffuser.....................................27Figure10.Anexampleofcamberedairfoilturningvanesinacorner....................................28Figure11.Simplifieddiagramofau-tubemanometer..................................................................33Figure12.Asimplifieddiagramofapitot-statictube....................................................................35Figure13.Schematicoftheoverallwindtunneldesign................................................................38Figure14.Asketchoftheinsidedimensionsofthewindtunnel..............................................41Figure15.Thesteelframetosupportthewindtunnelwhencomplete................................42Figure16.AphotoofthefanandaCADdrawingofthefan........................................................43Figure17.Custom-madesheetmetalcomponentandsheetmetalturningvanes............43Figure18.Apreliminarydiagramoftheproposedwindtunnel...............................................47Figure19.ResearchersatNASAGlennproduce3-dscansoficeaccretiononairfoils....48Figure20.Ademonstrationofaplasmaactuatoractivelycontrollingtheairflow..........49

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Chapter1

BACKGROUNDANDMOTIVATION

1.1. ApproachestoFluidMechanics

Since the timeof theWrightbrothers, theneed for reliableaerodynamicpredictionshas

only increased. Demand from both military and private aeronautics sectors has driven

rapid increases in flight technology. Similarly, the automotive, nautical, civil engineering,

andmanyotherindustrieshaveincreasedthedemandforacomprehensiveunderstanding

of fluid flowaroundobjectssuchascars,shipsandbuildings.Therearethreetechniques

frequentlyusedforsolvingaerodynamicdesignproblems–theoretical,computational,and

experimental–eachhavingdistinctadvantagesanddisadvantages[1][2].

Theoreticalmethodsrefer toanalyticalapproaches to solving thegoverningequationsof

fluiddynamics.Thedistinctadvantagetoanalyticalmethods isthattheyresult inclosed-

formsolutions.Theformulasobtainedfromananalyticalapproachcangiveagreatdealof

insight as to how the system will behave under different conditions. Unfortunately,

analytical solutions are often impossible to obtain, except in caseswhere the governing

equationsare linearorcanbeapproximatedassuch.Onewaytoobtain lineargoverning

equations is to consider only simple geometries, such as flow through an infinitely long

pipe. That said, many problems of interest involve complex geometries such as cars or

planes.However,theoreticalapproachesareusefulforgainingintuitionforhowgivenflow

fieldswillbehaveandunderstandingbasic flowphenomena.Theoreticalapproachesalso

continue to play a vital role in validating computational methodologies. Because

computational solutions are approximations, it is helpful to test the accuracy of

computationalalgorithmsonproblemsforwhichthereareanalyticalsolutions.

Thesecondapproachtotheaeronauticdesignprocessiscomputational,oftenreferredto

as computational fluid dynamics (CFD). These methods use computer algorithms to

approximatethesolutionstothegoverningequationsoffluidmechanics.Foragivenflow

problemthegeometrymustbemodeledusingCAD.Initialandboundaryconditionsmust

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thenbedefinedandtheflowdomainmustbediscretizedintoagrid.Inmanyapproaches,

the solution is assumed tobe constant in each cell and thegoverningequations are also

discretizedtomodelhoweachcell'ssolutionchanges.Anexampleofagridforanairfoilis

showninFigure1.Onceanalgorithmischosen,thesimulationcanbeperformed.

Figure1.Anexampleofagridmesh for theNACA0012airfoil.Gridpointdensity increasesnear the leading and trailing edge of the airfoil becausethe solution near these points varies greaterwith space and time. FiguretakenfromRef.[3].

ThefieldofCFDhasseentremendousgrowthinthepastfiftyyearsduetothecontinuous

increase in computational power and memory of computers. The rate of computer

evolution during the 1960’s and 70’s led some researchers to predict that computer

simulationswouldeventuallybeabletomoreaccuratelyanalyzefreeflightscenariosthan

windtunnelsandwouldevenreplacethemcompletely[4].Unlikeanalyticalmethods,CFD

has the capacity tomodel complex geometries and does not need approximations to be

made to thegoverningequationsof fluidmechanics toobtain solutions.Thismeans that

fewphysical assumptions need to bemade to find a solution. CFD can also obtain time-

resolved flow solutions for any problem. Depending on the methodologies chosen, it is

feasibletoperformviscoussimulationsonafullaircraftinlessthanadayusingapersonal

computer[5].Furthermore,thereareproblemsforwhichexperimentaldataisdifficultor

impossibletoobtain.Forexample,takingmeasurementsinsideofajetengineisnotoften

practical. For this reason,CFD is an indispensableengineering tool that candramatically

reducetheamountofexperimentaltestingneededinanaerodynamicdesignprocess.

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DespitethemanyadvantagesofCFD,therearesomedisadvantagesthatinhibitCFDfrom

completelyreplacingwindtunneltesting.Likeanyfieldthatreliesonnumericalmethods,

CFD is subject to truncation errors in floating point calculations. Round-off error is

introduced in CFD because computers hold a finite number of digits in arithmetic

operations. This error is proportional to the number of grid points. CFD also introduces

discretization error. Discretization error is defined as the difference between the partial

differential equation solution and the discretized approximation to the differential

equation solution when round-off error is excluded [2]. Additionally, it might seem as

though solutions to increasingly complicated problems can be found more easily with

advancesincomputationalpower.However,thisisnotnecessarilythecase.Thisisdueto

difficultiesinsoftwareparallelization.ItisalsoimportanttopointoutthatwhileCFDcan

predictaerodynamicperformancewithincreasingaccuracy,engineersareofteninterested

in other features such as reliability, operability, and maintainability. For example, CFD

alonecannotpredict ifanenginecanrestartatcruisingspeed,orhowmany flighthours

canbeachievedbeforeserviceisneeded.

Another difficulty associated with CFD is that of turbulence modeling. One approach to

modelingfluidflowiscalledDirectNumericalSimulation(DNS)simulations.Thismethod

does not make approximations to the governing equations but often requires orders of

magnitudemoremeshpointsthanacomputationallycheaperalternativesuchasReynolds-

AveragedNavier-Stokes (RANS) [4] [6].Becauseof its computational cost,DNS is almost

exclusivelyusedfortheoreticalanalyses.WhileDNScanbeusedtostudymoreproblemsas

computational power increases, its application to most practical engineering problems

remainsinthedistantfuture.Alternatively,RANSisoneofthemostcommonCFDmethods

usedinengineering.ItreliesonstatisticallyaveragingtheNavier-Stokesequationsintime.

WhileRANSmethodscanbeusedforcasesofmildseparation,theyoftenarenotascapable

in cases ofmassively separated flows [5]. Another approach to CFD is called large-eddy

simulation (LES). LES is like RANS in that it makes assumptions that result in

approximationstotheNavier-Stokesequations.However,insteadofaveragingtheNavier-

Stokes equations, it filters the Navier-Stokes equations, removing high wave-number

components[7].ThecomputationalcostandaccuracyofthismethodfallsbetweenRANS

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andDNS.Figure2showstherelationshipbetweencomputationalcostandthenumberof

assumptions associated with each model. It should be stated that despite these

shortcomingsandthosediscussedpreviously,CFDhasproventobean invaluabletool in

the aerodynamic design process, dramatically reducing the number of design cycle

iterationsneededinthedesignprocess.

MoreAssumptions FewerAssumptions

LeastExpensive MostExpensive

RANS LES DNS

Figure 2. A diagram of the relative accuracy and cost of various CFDmethods.

The third approach to obtaining aerodynamic predictions is experimental. Historically,

windtunneltestinghasbeenusedasthebasisforthedesignprocessofallcontemporary

flyingvehicles,startingat thetimeof theWrightbrothers.Regardlessof the fact that the

numberofmilitaryaircraftprogramshasshownadownwardtrendsince the1950’s, the

numberofwindtunneltesthoursperprogramhasbeenonasteadyrise[8].Thiscanbe

attributedtothefactthatasaircraftbecomemoreadvanced,theamountoftestingneeded

to perfect a given design increases. Historical trends show that the amount of

experimentallyacquireddataforanaircraftprogramhavebeenandwillcontinuetogrow.

Windtunnels,however,arenotwithouttheirlimitations.Forexample,thecosttobuildand

maintainawindtunnelvariesdependingon itssizeand features,but it israrelyacheap

endeavor.Thisisbecausewindtunnelscantakeyearsandcountlessman-hourstodesign

andbuild.Makingaccuratemodelswithproperlycalibratedequipmentisadifficultprocess

andonlyincreasestheoperatingcostsofawindtunnel.Themainchallengeassociatedwith

windtunneltestingisproperlyreplicatingflowparameters.Forlowspeeds,this involves

building amodel of the object of interest and replicating the desired Reynolds number.

AchievinghighReynoldsnumbersintestscanoftenprovetobedifficultwithoutexpensive

infrastructure,andinsomecasesthedesiredReynoldsnumbercannotbeexperimentally

achieved at all. Furthermore, care must be taken to avoid measurement inaccuracies

9

related to the proximity of the tunnelwalls to the test object [9]. The fact remains that

despite growths in computational methodologies, CFD has not, and will not completely

replace experimental methodologies for the foreseeable future in solving aerodynamic

designproblems[1].

1.2. HistoryofExperimentalAerodynamics

Themodernconceptofawindtunnelhasonlybeenaround for150years.Prior towind

tunnelsthemostsystematicapproachtoexperimentalfluiddynamicsinvolvedattachinga

model to the end of a whirling arm. Benjamin Robins (1707 – 1751) is attributed with

employingthefirstwhirlingarmmechanismandheusedittomakelargeadvancementsin

our earlyunderstandingof fluidmechanics [10]. Figure3 is a sketchofRobins’whirling

armmechanism.ItwasnotuntilSirGeorgeCayley(1773-1857),whoalsousedawhirling

arm mechanism, began his work that experimental aerodynamics would contribute the

vital testdataneededtodesignwhat isbelievedtobethe firstrecordedheavier-than-air

vehicle inhistory [10].Cayley’s first successfulunmannedgliderhadawingspanof200

squarefeet.By1852hemadeatriplanethatintegratedmanyconceptsofmodernplanes:

fixedwings,fuselageandtail.Cayleywasthefirsttoconceptuallyseparatethemechanisms

behindliftandthrust.PriortoCayley,mostdesignsforflyingmachinesincorporatedone

mechanism forboth lift andpropulsion:ornithopters that flappedmechanicalwings [10,

11]. With this flying mechanism paradigm shift came the need for a better way to

experimentally measure forces such as lift and drag. Frank H. Wenham (1824-1908) is

frequently credited with designing and building the first wind tunnel [10]. As a council

member of the Aeronautical Society of Great Britain, he received funding to design an

apparatusthatpulledairthrougha3.66m(12ft)tubepastamodelusingasteam-engine-

poweredfanblower.

While the years after the creation of the first wind tunnel (1871) were full of ground

breakingexperimentsperformedbyWenhamandhis colleagues, thequestion remained:

didexperimentalresultsobtainedwithascalemodelbearanyresemblance to thoseofa

full-sizedaircraft[10]?Itwasnotuntil1885thatOsborneReynoldsshowedthattheflow

patternsoverascalemodelwouldbesameforafull-scalemodelifacertainparameteris

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thesame inbothexperiments[12].Thisparameter,whichwillbediscussed later, isnow

known as the Reynolds number. The significance of this discovery is that wind tunnels

could be used to generate meaningful experimental results for full-scale models by

completingtestingonscalemodelsiftheReynoldsnumberisproperlyreplicated.Another

fifteen yearswould pass beforeWilbur andOrvilleWright, frustratedwith the accepted

aerodynamicdesigntablesof theirday,woulduseresults fromtheirownwindtunnel to

buildtheworld’sfirstpowered,manned,heavier-than-airflyingvehicle[11,10].Asketchof

theWrightbrothers’windtunnelisshowninFigure4.

Figure3.A sketchofBenjaminRobin’swhirling arm.A string iswrappedarounda cylinderandamass is tied to theotherend (bottomright).Thestring ispassedoverapulleyandthecylinderandattachedarmrotateasthemassfalls.Themodelofinterestisattachedtotheendofthearm.ImagetakenfromRef.[13].

Sincetheearlydevelopmentofthemodernwindtunnel,avarietyofdifferentwindtunnels

havebeendevelopedformanyspecialpurposes.TheseincludetheNACAvariabledensity

tunnel,whichwascapableofperformingexperimentsathighReynoldsnumbers[14]and

thewindtunnelat theNASAAmesResearchCenter,witha40by80-foottestsectionfor

full-scale testing [15]. Further examples of specialized wind tunnel applications include

verticaland/orshortlandingandtakeoffvehicle(V/STOL)testingandacoustictesting.For

moreexamplesofmorerecentspecializedtunnelsandapplications,seeBarlow[1].

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Figure4.A sketchof theWrightbrothers’wind tunnelused todesign thefirst heavier-than-air powered flight vehicle. The fanwas belt driven andpowered by a small gas engine the Wright brothers used for poweringmachineryintheirbikeshop.Theairwaspushedthroughthetunnelbythefan and, as a result, the Wright brothers struggled to find a way tostraightentheswirlingmotionoftheairasitpassedthemodel.ImagetakenfromRef.[10].

1.3. Objective

As mentioned before, analytical and computational approaches to aerodynamic design

problemsarenot always sufficient.Asa result,Houghtonhas startedadesignproject to

buildanewgeneral-purpose low-speedwind tunnel toexpand its in-houseaerodynamic

testingcapabilities.ThiswindtunnelwillgiveHoughtontheabilitytoperformawiderange

of experiments. Specific examples include testsonplasmaactuators for flowcontrol and

testsonaircraftmodelswithicebuildup.Plansforfutureresearchwillbediscussedlater.

Furthermore, a general-purposewind tunnelwill giveHoughton the ability toperforma

varietyoflaboratoryactivitiesaspartofafluiddynamicscourse.

Thisthesisdiscussestheoveralldesignofthewindtunnelandthedesignprocessused.The

flow velocity throughout the tunnel was estimated using a quasi-one-dimensional

12

assumption.Giventhevelocity,semi-empiricalequationswereusedtopredict thetunnel

efficiency based on the air properties (e.g., test section speed and viscosity) and sizing

choices.AnOctavescriptwaswrittentoautomatethecalculations foravarietyofdesign

choices and estimate fan power requirements for each. Once construction of the wind

tunneliscomplete,thiswillprovideanotheropportunityforresearch:testingtheutilized

designprocess.Aretheestimatedsectionlossesandoverallpowerestimatescomparable

tothosemeasuredinthewindtunnel?Theseresultscouldbevaluabletothoseseekingto

designandbuildsimilarwindtunnelsinthefuture.

Theremainderofthisworkisasfollows.Chapter2describesthetheoryoffluidmechanics

andconceptsbehindwindtunneldesign.ThisincludestheNavier-Stokesequations,quasi-

one-dimensional flow, semi-empirical component efficiency equations, andmeasurement

techniques.Chapter3describesthespecificdesignprocessusedalongwithexplanationsof

designchoicesmade.Furthermore,Chapter3coversthedesignandconstructionprogress

made thus far. Lastly, Chapter 4 covers futurework and possible experiments inmore

depth.

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Chapter2

THEORY

2.1. FluidMechanics

2.1.1. TheoreticalDevelopment

There are three fundamental principles which govern fluid mechanics. These are

conservationofmass,Newton’ssecond lawofmotion,andconservationofenergy.These

threeprinciplesresultinequationswhichrelatevariousquantities(e.g.pressure,density,

velocity)astheyvaryintimeandspace. Onlyabriefoverviewoftheseequationswillbe

presented.Foramorein-depthdevelopmentseeAnderson[11]orCurrie[16].

First,considertheequationwhichfollowsfromtheprincipleofconservationofmass.Itcan

bewrittenas

!"∗

!$∗+! "∗&'

∗

!('∗ = 0 (1)

where"is the fluid density,+is the vector fluid velocity,$is the time,-is the vector of

spatial coordinates, anda superscriptasterisk indicatesdimensionalquantities.Equation

(1)isoftenreferredtoasthecontinuityequation[2].

The second governing equation of fluid mechanics comes from the principle of

conservationofmomentum,whichisanapplicationofNewton’ssecondlawofmotiontoan

elementof the fluid.The two typesof forces that actona fluidelementarebody forces,

suchasgravitationalandelectromagneticforces,andsurfaceforces,suchaspressureand

viscous forces. In many aerodynamic problems body forces are insignificant. Therefore,

theyareneglected.Theresultingequationsaregivenby

!("∗&/∗)

!$∗+! "∗&/

∗&1∗

!(1∗ =

!!('

∗ −3∗4'/ + 5∗!&'

∗

!(/∗ +

!&/∗

!('∗ − 4'/

23!&1

∗

!(1∗ (2)

14

where3isthepressureand5istheviscosity.Equation(2)isavectorequationindicatedby

thefreeindex,8.

The lastequationof fluidmechanicscomes fromconservationofenergy.Conservationof

energystatesthatthechangeintotalenergy(internalpluskinetic)isequaltothesumof

thetotalworkdoneonthesystemandanyenergytransferredbythermalconduction.This

equationisgivenby

!!$∗

"∗9∗ +12"∗&/

∗&/∗ +

!!(1

∗ "∗9∗ +12"∗&/

∗&/∗ &1

∗

=!!('

∗ &/∗ −3∗4'/ + 5∗

!&'∗

!(/∗ +

!&/∗

!('∗ − 4'/

23!&1

∗

!(1∗ + 4'/;∗

!<∗

!(/∗

(3)

where;isthethermalconductivity,9istheinternalenergyand<isthetemperature.Itis

importanttonotethatonlyfiveequationshavebeengiven,buttherearesevenunknowns:

density, pressure, temperature, internal energy, and threevelocity components.To solve

theseequationstwomorealgebraicexpressionsforpressureandinternalenergyshouldbe

used.Theyaregivenby

3∗ = "∗=∗<∗ (4)

and

9∗ = >?∗<∗ (5)

whereEquation(4)istheidealgaslawandEquation(5)isanexpressionfortheinternal

energy in termsof temperatureandspecificheatatconstantvolume,>? . In thiscase,=is

thespecificgasconstant,whichhasavalueof287Jkg-1K-1forair.Thesetwoequationsare

often substituted into the energy equation to give five equations with five unknowns.

Together,thesefiveequationsareknownastheNavier-Stokesequations.

2.1.2. Quasi-One-DimensionalFlow

A thorough treatment of quasi-one-dimensional flow is given by Anderson [11] and a

similarapproachtohisisusedhere.Whenconsideringtheairflowthroughawindtunnel,

15

oneapproachistousetheNavier-Stokesequationstomodeltheflowinthreedimensions.

However, no analytical solution exists and CFD would take time and computational

resources.Amoreeconomicapproachexists. Inwindtunnels, thecross-sectionalarea,@,

variesonlyas a functionof the lengthalong the loopof the tunnel,(,meaning@ = @(().

Forfluidmechanicsproblemsinvolvingducts,suchasestimatingwindtunnelefficiency,it

maybereasonable toassumethat the flow fieldpropertiesareuniformacrossanycross

section,andhenceonlyvaryasafunctionof(.Thisassumptionisvalidincaseswherethe

flowisassumedtobesteady,i.e.,notime-dependence.Furthermore,itisassumedthatthe

boundarylayersalongthesurfaceareverythin,andthatchangesinareawithrespectto(

aresmall.Ifchangesinareawithrespectto(aresmall,thentheAandBcomponentsofthe

velocity are small, allowing the fluid motion to be treated as though it is only in the(

direction.A flow that approximatelymeets these requirements is referred to as a quasi-

one-dimensional flow. Although quasi-one-dimensional flow is only an approximation of

thetime-dependentthree-dimensionalflowthatisreallyoccurringinthewindtunnel,the

resultsaresufficientlyaccurateforuseinestimatingtheefficiencyofthewindtunnel.

Themainresultneededforthepresentworkistheincompressiblequasi-one-dimensional

flow version of the continuity equation which comes from Equation (1). Using the

divergence theorem and assuming a time-independent flow, the integral form of the

continuityequationis

whereCis the the surface that bounds a given volume of fluid. This equation can be

appliedtoaductliketheoneshowninFigure5.Thefluidintheductisboundedbycross-

sectional area@Don the left,@Eon the right, and thewallsof theduct itself. In this case,

Equation(6)canbewrittenas

"& ⋅ GHI

= 0 (6)

"& ⋅ GHJK

+ "& ⋅ GHJL

+ "& ⋅ GHMNOO

= 0. (7)

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The last term describes the fluid flow that passes through the wall, which is zero.

BecauseGHpoints out of the volume, and x-velocity,u, and density are assumed to only

varywith(,theremainingtermscanbewrittenas

and

Therefore,fromEquation(7),

Equation (10) is the quasi-one-dimensional continuity equation. It is a statement of

conservation of mass and that mass flow through a duct is constant. In the case of an

incompressibleflowwhere"isconstant,

ForairflowatMachnumberslessthan0.3,thedensityvariesbylessthan5%.Therefore,

theincompressibleassumptionismadeinthedesignofthislow-speedwindtunnel.

Figure 5. Quasi-one-dimensional flow through a duct. The dashed linesboundthe fluidvolumeof interest. In thecaseof incompressible flow, theareaandvelocityarerelatedbyEquation(11).

"& ⋅ GH = −"D@D&DJK

(8)

"& ⋅ GH = "E@E&E.JL

(9)

"D@D&D = "E@E&E. (10)

@D&D = @E&E. (11)

@D

@E

&D &E

(

17

2.1.3. Bernoulli’sPrinciple

Bernoulli’sprincipleisastatementofconservationofenergy.Itmathematicallyexpresses

the idea that the sum of all forms of energy along a streamlinemust be the same at all

points.Thatis,

whereQis the acceleration due to gravity,Ais the elevation of the fluid,3is the static

pressure,&isthefluidvelocity,and"isthefluiddensity.Thefirstterm,DE"&E,isreferredto

as thedynamicpressure, andexpresses thekinetic energyof the fluid.The second term,

"QA,describesthegravitationalpotentialenergyofthefluid.Thelastterm,3, isthestatic

pressureofthesystemandcanbethoughtofasameasureofinternalenergyofthesystem.

Thisexpressionofconservationofenergyisonlyvalidunderacertainsetofassumptions.

Thefirstassumptionisthatthefluidisincompressible–thedensityisconstant.Secondly,

theflowissteadyanddoesnotchangewithtime.Thelastassumptionisthatviscousforces

arenegligible.

Whenconsideringwindtunneldesign, thechange ingravitationalpotentialenergyof the

fluidissmallcomparedtothatofthedynamicandstaticpressures.Asaresult,thistermis

oftenneglected.Thestagnationpressure,3R,isdefinedasthesumofthestaticanddynamic

pressures

The stagnation pressure of a system is constant under the same set of assumptions as

Bernoulli’sprinciple.Energy,however,isnotconservedinanon-idealsystem.Forexample,

viscous forces are not negligible throughout the wind tunnel. Therefore, inefficiencies

throughout the tunnel can be expressed as a drop in stagnation pressure. This idea is

fundamentaltothewindtunneldesignprocessandisdiscussedingreaterdetailinSection

2.2.3.

12"&E + "QA + 3 = constant (12)

3R =12"&E + 3. (13)

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2.2. WindTunnels

In this section, the Navier-Stokes equations are presented in non-dimensional form to

highlighttheimportantparametersforwindtunnelapplications.Section2.2.2coverssome

basicdesignfeaturesofwindtunnelsandgivessomeexamplesofoperationalwindtunnels

with these features. Next, Section 2.2.3 discusses the process and equations by which

powerestimatesaremadeforwindtunneldesign.Lastly,Sections2.2.4and2.2.5discuss

optionsforfanselectionandotherconsiderationsforconstructionofawindtunnel.

2.2.1. ImportantParameters

Wind tunnel testing is most often completed on scale models in an effort to better

understandflowfieldsaroundafull-scaleobject.Howdoesoneknowifflowfieldsaround

ascalemodelaresimilartoflowfieldsaroundafull-scaleobject?Anderson[2]definestwo

flowstobedynamicallysimilarif:

1) Thestreamlinepatternsaregeometricallysimilar.

2) The distributions ofY∗

YZ∗ ,\∗

\Z∗ ,]∗

]Z∗, etc., where subscript r denotes a reference

quantity,arethesamewhenplottedagainstcoordinatesnon-dimensionalizedby

acommonreferencelength.

3) Theresultingnon-dimensionalizedforces(e.g.lift,drag)arethesame.

In general, flow solutions are a function of all of the parameters included in the

dimensional Navier-Stokes equations. However, the Buckingham Pi theorem [17] states

thatthenumberoffreeparametersintheNavier-Stokesequationscanbereducedtothree.

Todothis,theNavier-Stokesequationsmustbenon-dimensionalized.Thisisachievedby

usingthesubstitutions

3 =3∗

3^∗&^∗E (14)

& =&∗

&^∗

19

$ =$∗&^∗

_^∗

" ="∗

"^∗

( =(∗

_^∗

where,asbefore,theasteriskrepresentsadimensionalquantity,thesubscript`denotesa

referencevalue,and_islength.Itiscommontochoosereferencevaluesthathavephysical

significance.Examplesincludethechordofthewingofanaircraftfor_^∗ ,andthefreestream

velocityfor&^∗ .

Using the variable substitutions given in Equations (14), the non-dimensional Navier-

Stokesequationsare

!"!$+! "&'!('

= 0, (15)

! "&/!$

+! "&/&1!(1

=!!('

−4'/3 +5=9

!&'!(/

+!&/!('

− 4'/23!&1!(1

,

(16)

!!$

3a − 1

+12"&/&/ +

!!(1

3a − 1

+12"&/&/ &1

=!!('

&/ −34'/ +5=9

!&'!(/

+!&/!('

− 4'/23!&1!(1

+5

a − 1 b`cE=94'/

!<!(/

(17)

20

whereais the specific heat at constant pressure dividedby the specific heat at constant

volume.Toclosetheseequations,theidealgaslaw(Equation(4))isnon-dimensionalized

usingthesamereferenceparameters,producing

3 ="<acE. (18)

There are three dimensionless coefficients that appear in these equations. The first

dimensionlessparameteriscalledtheMachnumber.Thisisgivenby

c =&^∗

d^∗ (19)

whered^ isthereferencespeedofsound.NextisthePrandtlnumber.Itdescribestheratio

oftheviscousdiffusionratetothethermaldiffusionrateandisgivenby

b` =>\∗5∗

;∗. (20)

In this expression,>\∗ is the specific heat of air at constant pressure and;is the thermal

conductivity of air. The last dimensionless parameter is the Reynolds number which

describestheratioofinertialforcestoviscousforcesinthefluid.Thisparameterisgiven

by

=9 ="^∗&^∗_^∗

5^∗. (21)

The Reynolds number is the similarity parameter of most interest for low speed wind

tunnels(i.e.M<0.3)[11].

Dimensionalanalysisofthegoverningequationsof fluidmechanicsshowsthattwoflows

willbesimilarif:

1) Thebodiesaregeometricallysimilar(i.e.,oneisascalemodeloftheother)

2) Thesimilarityparametersarethesameforbothflows

21

This is importantbecauseconductingexperimentsonscalemodels isoneof theprimary

activitiesofwind tunnels.Thismeans that full-scalebehavior canbepredictedbasedon

experimentalresultsifthenon-dimensionalflowparameterscanbeproperlyreplicatedin

thewindtunnel. However,itisdifficulttomatchboththeMachandReynoldsnumberin

mostcases,andmanytimesneithercanbematchedperfectly.Decisionsbasedonintuition

and knowledge of the system being testedmust bemade regardingwhich parameter to

morecloselyreplicateandwhetherreplicatingthemisnecessary[11].

2.2.2. BasicDesignDecisions

Thissectionwillhighlightsomeoftheimportantdecisionsinvolvedwithdesigningalow-

speedwindtunnelandgiveexamplesoftunnelsthatexhibitcommonfeatures.

It iscommonwhendesigningawindtunneltofirstconsiderthedesiredtestsectionsize.

The required size is determined by its intended application, laboratory size constraints,

andbudgetaryconstraints.Awiderangeofwind tunnelshavebeenbuiltbasedon these

constraints.Onthelargeend,theNationalFull-ScaleAerodynamicsComplexatNASAAmes

(seeFigure6)hastwotestsections.Thesmallertestsectionis12.19m(40ft)by24.38m

(80ft)andcanreachspeedsupto154.2m/s(345mph)andthelargertestsectionis24.38

m(80ft)by36.58m(120ft)andcanreachspeedsof51.41m/s(115mph)[18].Thelarger

testsectionisbigenoughtofitafull-scaleBoeing737init.Incontrast,itisnotuncommon

for educational wind tunnels to have test section diameters as small as 10 cm and be

limitedtospeedsoflessthan15m/s[1].Fordesignpurposes,agoodruleofthumbisthat

model airplanes and airfoils should take up at most 80% the width of the test section.

Furthermore,fortestarticleslikeairplanes,awidthtoheightratioof1.5iscommon.That

said,itisadvantageousformodelstobesmallcomparedtothesizeofthetestsection,as

this lowers the impact of the test section walls on the results [1]. For the same flow

velocity,largertestsectionsallowlargerobjectstobetestedandthereforealargerrangeof

Reynolds numbers can be achieved. The required power and cost of tunnel building

materials tend to vary with the test section hydraulic diameter squared [1], while the

power consumptionof the tunnel tends to increasewith the test section airspeed cubed

[19].

22

Figure6.Anaerialviewof theNationalFull-scaleAerodynamicsComplex.ImagetakenfromRef.[20].

Theseconddesignconsiderationtobemadeisifthetunnelistobeareturn(closedcircuit)

or non-return (open circuit) type. Closed circuit wind tunnels are designed to pull air

throughaclosedloop.Figure7isanexampleofaclosedcircuittunnelastheairpassesina

clockwisedirection.Conversely,opencircuittunnelspulltheairfromtheroomthroughthe

test sectionandexhaust itback into the room.Aschematicof theUCDavisAeronautical

WindTunnelFacility (AWT) is shown inFigure8.This is anexampleof a standardopen

returnwindtunnel. Mostsmallresearchtunnelswithtestsectionslessthan0.61m(2ft)in

diameter are open return [19], in part because of their lower construction costs. Open

circuitwind tunnels are often less energy efficient, but energy operating costs for small

windtunnelstendtoberelativelysmalltobeginwith[1].Therefore,largertunnelstendto

beclosedcircuitwitheitheroneortworeturnpaths.However,closedcircuittunnelsare

moreexpensivetoconstructandcanbemoredifficulttodesignthanopencircuittunnels.

Becauseopencircuittunnelsareopentotheatmosphereatthesettlingchamber,thestatic

pressureinthetestsection,wheretheairspeedisthehighest,islowerthanatmospheric

23

pressure.Thismeans that leaks in the test sectionare inevitablebecauseholesareoften

drilled into the test section to install models or probes. Open circuit tunnels are also

sensitive to obstructions in the room, such as furniture, other equipment or the walls.

Placinganopenreturntunneltooclosetoawallcanhaveadverseeffectsontestsection

flow.Therefore,openreturntunnelsrequirealargeroomastominimizethiseffect[19].

Figure 7. The NACA 6-inch tunnel is an example of an early open throatwind tunnel. Air is blown in a clockwise direction past the test section.ImagetakenfromRef.[21].

Another significantdesigndecision iswhether the test section itself isanopenor closed

type.Anopentestsection isa testsection that isnotenclosedonall sides.Conversely,a

closedtestsection isonewhich isclosedonallsides.Therearealsohybridandpartially

opentestsections.Figure7isanexampleofanearlywindtunnelwithanopentestsection.

Therearesomedisadvantagesforopentestsections.First,opentestsection(openthroat)

tunnelsdonotworkwithopencircuit tunnels thathaveablower in thediffuserbecause

theroomcanbecometooturbulent.Thismeansthateitherthetestsectionmustbeplaced

insideofaseparateair-tightroomorthetunnelmustbeclosedreturn.Openthroattunnels

24

can also suffer from pulsations, the sameway a pipe organ vibrates. If the object being

tested creates awake that is too big, the collector can interferewith the aerodynamics.

However,openthroattunnelsmakeiteasiertochangeandworkontestmodels,andmake

iteasiertoinsertmeasurementprobes.Ingeneral,closedthroatwindtunnelshavemore

advantages. However, several hybrid, partially open, convertible, or slotted wall wind

tunnelshavebeenmadeforuseintheautomotiveindustryandV/STOLdevelopment.

Figure 8. A schematic of the UCDavis AeronauticalWind Tunnel Facility.Thiswindtunnelisanopen-circuitdesign.Airentersthetunnelontheleft,is contracted through a nozzle, passes through the test section, expandsthrough a diffuser, passes the fan that powers the process, and exits thetunnelandreturnstotheroomontherightsideofthefigure.

2.2.3. SectionLossandPowerConsiderations

ToaidinthedesignoftheHoughtonCollegewindtunnel,ascriptwaswritteninOctave(an

open-sourceMATLAB alternative). This script has two primary purposes. The first is to

calculate the dimensions of various components of thewind tunnel given constraints on

selected components. The second purpose of this script is to estimate the required fan

specificationsneededtoachieveatestsectionspeedof44.7m/s(100mph).Thisisdone

by calculating the stagnation pressure drop across each component as a function of its

geometry,and theReynoldsnumberandMachnumber foreachcomponent.Thissection

will reviewthesemi-empiricalequationsused toestimate these losses. It is important to

notethattheseequationsarequasi-one-dimensional.Thecross-sectionalareaisallowedto

change throughout the wind tunnel and the air speed is only a function of the cross-

sectionalarea.

As mentioned in Section 2.1.3, inefficiencies can be thought of as a drop in stagnation

pressure throughout thewind tunnel.Wattendorf [22] considers losses in a return-type

windtunnelbyconsideringthe lossesofeachcomponent insuccession.Eckert,Mortand

25

Jope [23] take the same approach and compare the estimations given byWattendorf to

severalexistingwindtunnels.Asimilarapproachistakenhere.Thetunnelcanbebroken

into constant area sections, corners, diffusers, contracting sections, etc. In each of these

sections,apartfromthefan,thereareflowlossesduetofrictionbetweentheflowinggas

and solid boundaries. The successive drops in total pressure for each component are

balancedoutbythepressureriseacrossthefan.

The power loss, or drop in stagnation pressure, of each section is modeled as being

proportionaltothedynamicpressureattheentranceofthatsection.Forexample,thedrop

instagnationpressurefortheefgcomponentofthewindtunnelisgivenby

h3R' = i'12"&'

E = i'j' (22)

wherej' is the dynamic pressure at the entrance of theefgcomponent.i' is called the

pressure loss coefficient and can be estimated as a function of geometry of theefg

component and viscosity of the air. Then, the total efficiency of the wind tunnel is

calculated as the ratio of the flow’s kinetic energy in the test section to the energy

dissipatedinthetunnelduetofriction.Anexpressionforthisenergyratiois

kl =

12"&f

E

h3R''=

jfi' ' j'

(23)

wherethesubscripttreferstothequantityasmeasuredinthetestsection.Becauseboth

the numerator and denominator are effectively energy terms, both could also be

consideredasenergyperunittime.Thismeansthatthepowerdeliveredbythefanmustbe

at leastjf/kl . It is important tonotethat thisdefinition for therequired fanpowerdoes

not account for the electro-mechanical efficiency of the fan, but only the aerodynamic

efficiencyofthetunnelitself.

Giventhisdefinitionoftheenergyratio,itfollowsthatestimatingthelosscoefficient,i' ,for

eachcomponentofthetunneliskeytoestimatingtheaerodynamicefficiencyofthetunnel.

Wattendorf [22] provides semi-empirical formulas for the loss coefficient of a variety of

26

windtunnelcomponents.Thesearesummarizednext,startingwiththelosscoefficientfor

aconstantareasection.Thisisgivenby

iO = nopg (24)

wherenis the friction coefficient,pgis the hydraulic diameter, andois the length of the

component.Thehydraulicdiameter is a commonlyused termwhendealingwith flow in

rectangularducts.Itisdefinedas

pg =2dqd + q

(25)

whereaandbarethewidthandheightoftheduct.Thefrictioncoefficient,f,isgivenbythe

Prandtluniversallawoffriction[22]

1

n= 2 logDR =9 n − 0.8. (26)

This loss coefficient is used for the test section and any other sectionwhere the cross-

sectionalareaisconstant.

Whenconsideringthelosscoefficientforadiffuser,itiswrittenintermsoftheequivalent

conicalangle.Thisangle,asshownforaconeinFigure9, istheangleofexpansion.More

generally,itcanalsobeusedfordiffusersthatdonothaveacircularcrosssection.Inthis

case,theequivalentconicalangleistheangleofexpansionthatwouldoccuriftheentrance

andexitofthediffuserwerecircularandmaintainedthesamecross-sectionalareaoneach

endastheactualdiffuser.Mathematicallythiscanbeexpressedas

uv = tanwD12 @l − 1

o/pD (27)

where@listheratiooftheareaofthelargersidetotheareaofthesmallerside,pDisthe

hydraulicdiameterattheentrance,andoisthelengthofthediffuser.

27

Figure 9. A depiction of the conical angle for a cone-shaped diffuser. Theequivalent conical angle is the angle of expansion thatwould occur if theentrance and exit of the diffuser were circular and maintained the samecross-sectionalareaoneachendastheactualdiffuser.

Oneofassumptionsmadeaboutdiffuserlossesisthatthetotallosscanbebrokenintothe

sumoffrictionlossesandexpansionlosses,asindicatedby

ix = iy + ivz (28)

where

iy = 1 −1

@lE

n8 sin uv

(29)

and

ivz = iv |@l − 1@l

. (30)

iv(u)isgivenbyEckertetal.[23]fordiffuserswithasquarecrosssection

iv(u) =

0.09623 − 0.004152ufor0 < u < 1.5°

0.1222 − 0.04590u + 0.02203uE + 0.003269uÖ

−0.0006145uÜ − 0.00002800uá + 0.00002337uâfor1.5° ≤ u ≤ 5°

−0.01322 + 0.05866ufor5° < u.

(31)

28

Recommendedpracticeisfordiffuserstomaintainanequivalentconicalangleof3°orless

withatotalarearatioof lessthan3[1]. Ingeneral,theriskofboundarylayerseparation

increaseswithalargerequivalentconicalangleandarearatio[24].Insomecases,diffusers

with a more aggressive expansion are used. So-called wide-angle diffusers are used to

increase the nozzle contraction ratio for awind tunnel, which can improve overall flow

qualityinthetestsection.Barlow[1]statesthatthesediffusersoftenhavearearatiosof2-

4 and22.5°equivalent conical angles.While they canbeused to increase test sectionair

flowquality,theyintroduceadditionallossesandrequirescreenstoavoidboundarylayer

separation.Awide-anglediffuserisnotusedinthepresentdesignsofurtherdetailsarenot

includedhere.SeeMehta[25]formoredetailsonwide-anglediffuserdesign.

Forclosedcircuitwindtunnels,cornersareequippedwithturningvanestoreducelosses

andstraightentheairflowafterthecorner.Figure10diagramsanexampleofacornerthat

usescamberedairfoil turningvanes.Thesevanescanvary ingeometryandchord-to-gap

ratio.Barlow[1]givesanestimationforthelosscoefficientofacorneras

iã = 0.10 +

4.55logDR =9ã E.áå (32)

wheretheReynoldsnumberisbasedontheturningvanechordlength.Equation(32)isa

conservative estimate and losses can often be reducedwith careful design or increased

withpoordesign.Consequently,theuseofCFDforpressurelossestimatescanbeusefulin

choosingtheairfoilshape,thechordlength,andthegap-to-chordratio.Equation(32)only

includeschordlength,limitingitspredictivepower.

Figure 10. An example of cambered airfoil turning vanes in a corner. Theflowdirectionisspecifiedbythearrow.DiagramtakenfromRef.[26].

29

The nozzle primarily serves two purposes. The first is to increase themean air velocity

beforeitentersthetestsection.Thisallowsfortheairspeedintherestofthetunneltobe

lower relative to the test section. This is beneficial because the loss in each component

tends to vary with the local flow speed squared. The second is that the total pressure

remains close to constant across the contraction and, therefore, variations in velocity

becomesmallerfractionsoftheaveragevelocityatagivencrosssection.Thismeansthat

lessscreens(discussedfurtherbelow)areneededtoreduceturbulence,thereforereducing

pressurelossesevenmore[27].Thelosscoefficientforanozzleisgivenby

içf = 0.32nN?éoçpfè

(33)

whereoçisthelengthofthenozzleandpfèisthehydraulicdiameterofthetestsection.In

this case,nN?écan be found using Equation (26) based on the average of the Reynolds

numbersattheentranceandexitofthenozzle.Whilethelosscoefficientwillalsodepend

onthecontractionshape,theeffectoftheexactshapeofthecontractionisrelativelysmall

becausethe losses fornozzlesaretypicallyontheorderof3%thetotalwindtunnel loss

[1]. To properly design a nozzle, it is common to use computer simulations to check for

boundarylayerseparationandoptimizetheflowuniformityatitsexit–thestartofthetest

section. See Mehta [27] for a more detailed approach to contraction geometry and

separationpredictions.

Screens and honeycombs are used to straighten the flow and increase flow uniformity

immediatlybeforethenozzle.Eckertetal. [23]givesrelationsforscreenlosscoefficients

which are based on the porosity. The porosity of awiremesh is dependent on thewire

diameterandweavedensity.IfGêisthewiredensityandëêistheweavedensity,thenthe

porosityisgivenby

íè = 1 −GMëê

E

(34)

30

andthescreensolidityis

ìè = 1 −íè. (35)

Typicalporosityvaluesforwindtunnelscreensarebetween0.5-0.8[1].Theexpressionfor

thelosscoefficientofascreenisgivenby

iê = iîïñóilçìè +ìèE

íèE. (36)

Additionally, Idel’chik [28] gives a loss coefficient curve for screens as a function of the

Reynoldsnumberbasedonthewirediameter,=9M .Barlow[1]providesabest-fitcurveto

thisdata

ilç = 0.785=9M241

+ 1.0wÜ

+ 1.01. (37)

Idel’chik [28]gives themesh factor, iîïñó, tobe1.0 fornewmetalwire,1.3 foraverage

circularmetalwireand2.1forsilkthread.

Eckertetal.[23]givesanexpressionforthelosscoefficientforahoneycombas

ig = ògogpg

+ 31íg

E

+1íg− 1

E

(38)

where

òg =0.375

hpg

R.Ü

=vôwR.Dfor=9ô ≤ 275

0.214hpg

R.Ü

for=9ô > 275

. (39)

In these expressions,pg is the hydraulic diameter of the honeycomb cell,íg is the

honeycomb porosity,=9ôis the Reynolds number based on the honeycomb material

roughness and incoming flow speed,ogis the honeycomb length in the flow direction,

andhisthematerialroughness.MehtaandBradshaw[29]suggestthatabout25,000total

31

cells inahoneycombare sufficient for straightening theair flow. It shouldbenoted that

whiletheliteraturecitedherediscussesthelosscoefficientsforhoneycombs,itisnotclear

howtheroughnesslength,h,isdefined.Asaresult,aconstantguessofig = 0.5wasused

forthepressurelosscoefficientofthehoneycombinthisstudy,assuggestedbyBarlow[1].

2.2.4. FansandDriveSystems

The thrust of a fan and the drag in the various tunnel components varies with the fan

angularvelocitysquared[1].Whilesmallerwindtunnelstendtoonlyhaveangularvelocity

control, it iscommonin largerwindtunnels tocontrol the fanbladepitch, too.However,

havingbothisnotessential.Forcontrollingthefanangularvelocity,mostnewfacilitiesuse

variable-frequency drives for AC motors but some use solid-state controllers for DC

motors, instead. In some cases, themotor isplacedoutside thewind tunnelwith a shaft

leadingtothepropeller,butthisdesignisnotessential.Itisusefultomountmotorsonto

anti-vibration mounts and connect them to the tunnel with flexible coupling to reduce

vibrations [29]. It should also be noted that many wind tunnels have fan straighteners

placed downwind of the fan to reduce swirl. BothMehta [29] andBarlow [1] givemore

detailsonthepowersectionofthewindtunnel.

2.2.5. ConstructionRequirements

Whenbuildingwindtunnels,Barlow[1]suggeststhatcomponentsshouldbedesignedto

withstand the maximum stagnation pressure with a safety factor of around 4. The

structuralstrengthofthewindtunnelisprimarilytoavoidvibrationandtomakesurethat

themotorwillstayinplacewerethefantolosehalfofitsblades.Vibrationcontributesto

noiseandcan,overtime,reducethestructuralintegrityofthewindtunnel.Forthisreason,

thesupportforvariouscomponentsshouldbestiffeneduntiltheirnaturalfrequencyiswell

abovethefrequencyofthefan[1].

Windtunnelsaremadefromavarietyofmaterialsincludingwood,plywood,thinorheavy

metal, concrete,orplastic [1].For small researchand instructional tunnels,plywood isa

goodchoicefortheconstructionmaterial.Itallowsforholesandchipstobeeasilypatched

andforthewholecomponenttobereplacedifneedbe.Plywoodcanbeboltedtogetherand

32

supportedbywoodorsteelsupports.Theinteriorofthetunnelshouldbesmoothedwith

cornersandseamstreatedwithcaulkingorarubbergasket.

2.3. MeasurementTechniques

The primary purpose of aerodynamic testing is to understand the flow field around an

object. The flow field refers to the velocity vector, temperature, pressure, viscosity, and

densityasfunctionsofpositionandtime.Thismeansthataccurateresultsnotonlydepend

on thedesignof the tunnel itself,butalso the techniquesused tomeasure the flow field.

One of the challenges associatedwithmeasuring the flow field around an object is that

insertinganinstrumentintotheflowfieldwillalmostinevitablydisturbit.Designingand

implementing minimally invasive measuring techniques can prove to be a challenge.

Furthermore, obtaining accurate and precise flow field measurements with high time

resolution requires careful design and planning. This section overviews some of the

techniquesusedtoobtainmeasurements.

2.3.1. PressureMeasurement

Measuringpressureisavitalcomponentofwindtunneltesting.Pressureisdefinedasthe

normalforceperunitarea.Inthecontextofaerodynamics,thisforceisexertedonanarea

element due to a time rate of change ofmomentumof the gasmolecules impacting that

surface[11].Pressureisafunctionofbothtimeandposition.

Onetechniqueformeasuringpressureisthemanometer.Themanometerisonetheoldest

devicesformeasuringdifferentialpressure,orthepressuredifferencebetweentwopoints,

andisalsooneoftheeasiesttobuild.Therearemanydifferenttypesofmanometersbuta

simpleonecanbemadebymeasuringthedifferencesinheightofaliquidsittinginaglass

u-shapedtubewithitstwoendsconnectedtothereferenceandtestedpoints.Thisiscalled

au-tubemanometer.SeeFigure11foranexample.Thedifferenceinpressureisrelatedto

theheightdifferenceintheliquidby[1]

h3 = 3D − 3E = hℎ sin í Q "y − "N (40)

33

wherehℎis theheightdifferenceof the liquid in thecolumns,íis theanglebetween the

horizontalandtheplanemadebythetwocolumns,Qistheaccelerationduetogravity,"y

isthedensityoftheliquidinthetubes,and"Nisthedensityofthefluidinthewindtunnel.

Figure 11. Simplified diagram of a u-tube manometer. The pressuredifferencebetweenp1andp2canbefoundfromtheheightdifferenceofthefluidinthetwotubesanddensityofbothfluids.

Another way to measure pressure is using a pressure transducer. The term pressure

transducerreferstodevicesthatprovideanelectricpotentialorcurrent inresponsetoa

pressureorchangeinpressure[1].Themostcommonpressuretransducersaremadefrom

asmalldiaphragmthatdeformswhenapressuredifferential is introducedacross it.The

most common and inexpensive ways of measuring the deformation of the diaphragm

includecircuits thatmeasureachange incapacitanceor inductancedue to thechange in

geometry.Thesetypesofpressuregaugescanbemadeinexpensivelyandrelativelysmall,

but they do require periodic calibration. This means that a manometer or some other

reliablepressuremeasuringtechniqueisstillrequiredforcalibratingpressuretransducers

inawindtunnel.

34

It is important tonote that there are a varietyof othermethods formeasuringpressure

that are not discussed in depth here. For example, some wind tunnels use pressure-

sensitivepaintwhichchangescolorwhenthepressureappliedtoitchanges.SeeGregoryet

al. [30] for a more comprehensive review of pressure-sensitive paints. Similarly,

piezoelectrictransducersproduceanelectricfieldinresponsetoanappliedpressure.See

Barlow[1]foramoreindepthlookatpressuremeasurementmethods.

2.3.2. VelocityMeasurement

Oneofthemostcommonwaystomeasurefluidvelocityisusingapitot-statictube.Pitot-

static tubesarean instrumentwhichyield thedifferencebetween the totalpressureand

the static pressure of a moving fluid. The density of the fluid can be found using the

equation of state based on ameasured temperature and static pressure. This allows the

velocitytobecalculated.Theprincipleofapitot-statictubeliesinBernoulli’sequation(see

Section2.1.3),whichisastatementofconservationofenergy.SolvingEquation(12)forthe

velocity,gravitationaleffectscanbeneglected,giving

& =2 3R − 3

". (41)

This instrument is made of two tubes, one inside the other, with one orifice facing the

airflowandthesecondperpendiculartotheairflow.Anexamplepitot-statictubeisgiven

inFigure12.Asdepicted,theorificeatAmeasuresthetotalpressure,3R,asitbringstheair

to a stop, and the orifice atB senses the static pressure,p, as the airmovespast it. The

pressuresfromthetwoorificesareconnectedtoamanometerorpressuretransducerand

the pressure difference is approximatelyDE"&E. From this the velocity can be calculated

usingEquation(41).

The tipofapitot-static tubecanbeeitherroundedorsquare.Whilesquare tipped tubes

aremore accurate at higher angles relative to the incoming flow [1], hemispherical tips

offer less flow field disturbances further downstream near the second set of holes.

However,tip-effecterrorsbecomenegligentifthesecondsetofholesisplacedmorethan5

or6tubediametersdownstreamfromthebase[31].

35

Figure12.Asimplifieddiagramofapitot-statictube.ThetotalpressureismeasuredatAastheairisbroughttoastopgoingaroundthebendontheleft. The static pressure is measured at B as the air moves past it. Thevelocity can be calculated based on the difference between these twopressures.

Anotherwaytomeasureflowspeedisthermalanemometry.Thismethodtakesadvantage

ofthethermalandelectricalpropertiesofathinwireorfilm.Thefasterairmovespastthis

wire or film, the faster thermal energywill be transferred from it. This property can be

usedtomeasurethespeedoftheair.

To accomplish this, an electric current is passed through the wire or film to raise its

temperatureabovetheadiabaticrecoverytemperatureofthegas[1].Becausethewireor

filmisnotinthermalequilibriumwiththefluid,energyistransferredtothefluid.Therate

of energy transfer to the fluid is therefore a function of the air velocity. An electronic

controlsystemmonitorsthetemperatureofthewire,whichisafunctionofitsresistance,

and adjusts the power in order to maintain a constant temperature in the wire. The

airspeed, therefore, is a function of the power provided by the control system. When

properly calibrated, accurate velocity measurements can be obtained with frequency

responses of up to 50 kHz [32]. This makes thermal anemometry ideal for measuring

velocitiesinturbulentflowfields.

Hot-wiresensorsuseathinmetalwire,withatypicaldiameterof0.5–55mandatypical

lengthof0.1-1mm.Thewireisoftencomposedofplatinumortungsten,or,lesscommonly,

platinum-rhodiumoriridium.Hot-filmsensorsaremadeofathinplatinumfilmdeposited

A

B

36

onto a quartz support with another layer of quartz on top to reduce electromechanical

effects.Hot-filmsaremorecommonlyusedinliquidswhilehot-wiresaremorecommonly

used ingases [32]. Furthermore, forhot-wires, singleprobes can containmultiplewires,

whichcanbeusedforresolvingflowdirectionaswellasvelocity.

Duetothedevelopmentofthedigitalcamera,particleimagevelocimetry(PIV)hasbecome

a viableway tomeasure the vector velocity of a flow field [33].This is accomplishedby

introducingtracingparticlestotheairandmeasuringtheirvelocityastheyfollowtheflow.

Thetracingparticlesneedtobesmallenoughtoproperlyfollowtheair.Theparticlesare

thenilluminatedandtwoimagesoftheparticlesarecaptured.Thevelocityoftheparticles,

and therefore the fluid, canbe calculated from the averagedisplacementof theparticles

within an interrogationwindow and the time between the two frames. This results in a

vectorfieldofthefluidflowattheresolutionofthechoseninterrogationwindowsize.PIV

systems are commonly used for two-dimensional flow field resolutions, but three-

dimensionalflowfieldscanberesolvedwiththeuseoftwocameras[34].LiuandKatz[35]

used a four-exposure PIV system tomeasure acceleration of a flow field. From this they

couldcalculatethepressuredistributionofthetestsection.PIVsoftwareisavailableboth

commercially and though open-source options. OpenPIV is one such open-source option

andthereareversionsforMatlab,PythonandC++[36].

37

Chapter3

DESIGNCONSIDERATIONSANDMETHODOLOGY

3.1. BasicDesignDecisions

Now that the theory and background information on wind tunnel design have been

presented,adiscussiononthedesignmethodologyusedforthisprojectwillbegiven.First,

general wind tunnel layout decisions will be discussed. Then the procedure used to

determine the dimensions of the wind tunnel is given followed by the loss coefficient

estimatescalculatedbasedonthisgeometry.Lastly,areviewofthecurrentprogressmade

onthedesign,construction,andpurchasingofneededmaterialsisgiven.

AsmentionedinSection2.2.2,mostwindtunnelswithatestsectionhydraulicdiameterof

lessthan0.61m(2ft)areoftheopenreturntype.Whiletheexactsizeofthetestsection

waslatercalculatedfromgeometricandpowerefficiencyconstraints,itwasapparentfrom

the beginning of the design process that the test section for theHoughton Collegewind

tunnel would be less than 0.61 m in diameter. However, due to the small size of the

laboratory,anopenreturntunnelwouldbeimpractical.Effectsfromtheendsofthetunnel

being too close to the walls of the roomwould sharply reduce efficiency, lowering test

section speeds. Another consideration is that open-return tunnels are louder than

similarly-sized closed-return tunnels. Sound levels are made even more important here

because of the small laboratory space and its proximity to classrooms and offices. As a

result,aclosedreturndesignwasselected.Figure13 isaschematic forageneralclosed-

returntunnel.

After deciding upon a closed type return, it was decided that including a wide-angle

diffuser (discussed in Section 2.2.3)was not beneficial to the overall design of thewind

tunnel. Wide-angle diffusers are placed upstream of the settling chamber. They are

designedtoslowtheairdownbeforepassingitthroughthehoneycombsandscreens.They

alsoallowforalargernozzlearearatiowhich,inturn,createsamoreuniformflowfieldin

thetestsection.

38

Theaforementionedcodewasusedtocalculatethesizeofvariouscomponentsofthewind

tunnel given area ratio constraints on the wide-angle area ratio, nozzle area ratio, first

diffuserarearatio,diffuserangleandatotallengthof4.72m(15.5ft).Aftertestingseveral

different combinations of these parameters, it became apparent that the desired test

sectionsizeandspeedswouldnotbepossibleifawide-anglediffuserwasincluded.

Figure13. Schematicof theoverallwind tunneldesign thathighlights themostimportantcomponents.

3.2. Constraints,Methodology,andDesign

Inthissection,specificconstraintsimposedonthewindtunneldesignarediscussed.Then

theoveralldesignmethodologyisdescribedasarethedetailsofthechosendesign.

Thefirstconstraintthatwasimposedonthedesignprocesswasatestsectionvelocityof

44.7m/s(100mph).Smallerspeedswouldnotbeidealforaccomplishingthetypesoftests

the physics department at Houghton College hopes to do with this tunnel. Specifically,

lowerspeedswould further limit therangeofReynoldsnumbers thatcouldbeachieved.

The second constraint placed on the design of the wind tunnel is the area ratio of the

nozzle.Itisidealtomaximizethisvaluebecauseitwillcausetheairintherestofthetunnel

tomoveslowerrelativetotheairinthetestsection.Becausethelossesofeachcomponent

varywiththelocalizedairspeedsquared,maximizingthisvaluewillincreasetheefficiency

ofthetunnel.Thismeansthatanozzleratioofascloseto6aspossiblewasdesired(based

onasuggestionfromMehta[29]).However,thespecificdesignofthecontractionrequires

39

computationalanalysistooptimizetheflowuniformityandpressureloss.SeeBrassardet

al[37],Fang[38],Fangetal.[39],Leifsson[40],Mikhail[41],andMehtaandBell[27]for

workonspecificcontractiondesigns.

Ultimately a nozzle area ratio of 5.45 and an area ratio of 3 for the first diffuser were

selected.Thiswasdoneusingcodethatiteratedthoughvariousnozzleratiosfrom5to6by

0.05andvariousdiffuserangles2°to3°by0.05°.Alengthbydiameterratioof2.5forthe

test section, 0.8 for the nozzle, and 0.5 for the setting chamberwere chosen based on a

suggestion given by Barlow [1]. The resulting test section size and required fan

specificationsneededtoachievethedesiredtestsectionspeedwereprintedtoatextfile.

Priortothisrefiningprocess,afanhadbeenpurchasedandonlynozzleanddiffuserarea

ratiosthatallowedthefantofitwereconsidered.Moredetailsonthepurchasedfanwillbe

giveninChapter4.TheequationsgiveninSection2.2.3wereusedtocalculatethepressure

loss coefficients foreach section.Asmentionedbefore, these coefficientshavebeennon-

dimensionalizedbydividingthepressureloss ineachsectionbythedynamicpressureof

thetestsection.

Basedonthechosennozzlearearatio,allremaininggeometricdimensionsarecalculated

by the code and presented in Table 1. Figure 14 illustrates a sketch of the inside

dimensionsoftheproposedwindtunnel.Airwillmovecounterclockwisethroughtheloop.

The script also computes the loss coefficient, pressure loss, and air speed for each

component,giveninTable2.Itshouldbenotedthatthefirstdiffuseristhesectionwiththe

highest losses.Thismakessense,notonlybecause it is the longestcomponent,but ithas

the second highest air speed, behind the test section. Note that corners, although not

estimatedtomakealargecontributiontotheoveralllosses,havemuchlargerlossesifthey

arenotdesignedandbuiltproperly.Becausethenozzlemakesuponlytwopercentofthe

totallosses,theprimarygoalofitssubsequentdetaileddesignshouldbemaximizingflow

uniformity. Maximizing the efficiency of the nozzle is not a concernwhen its losses are

smalltobeginwith.

For the purpose of estimating required fan characteristics, the energy ratio (k^) is

calculatedusingEquation(23).Itsvalueis2.60whichmeansthatthewindtunnelrequires

40

1.73kWtobeaddedbythefantoreachtestsectionspeedsof44.7m/s(100mph).Thisis

equivalentto2.32hp.Giventhesizeofthetestsection,thefanwouldneedtodisplace3.84

m3s-1(8,130ft3min-1)ofairandhaveatotalpressureriseof452Pa(1.82in.ofwater).The

acquired fan (described in the next section) is capable of meeting these operational

requirements.

3.3. CurrentProgress

Manyoftherequiredmaterialsandcomponentsofthewindtunnelhavebeenpurchased.

See Table 3 for a list of all purchased items along with their cost, distributor and

manufacturer.Thisincludes19.1mm(¾in.)mediumdensityoverlay(MDO)plywoodand

13mm (½ in.) acrylic needed to build thewind tunnel. MDO plywood’s primary use is

buildingconcretemolds.Thismeansthatithasexcellencestructuralstabilityandoneside

thatissmoothed.Thesecharacteristicsareidealforbuildingourwindtunnelbecausethe

airpressure insidethetunnelwillbeaboveatmosphericandthesmoothnesswillreduce

friction.Furthermore,onesideof the tunnelwillbeconstructedoutofacrylicso thatair

flow throughout the tunnel can be visualized and important tunnel components can be

seen. Inaddition,asteel framehasbeenconstructedoutofUnistrut tosupport thewind

tunnelonceitiscomplete.(Figure15).Unistrutwasselectedasabuildingmaterialbecause

of its adjustability. The bolts which hold the joints together can be loosened and the

horizontalbarscanbemoved,allowingthetunneltobeplacedastomatchtheheightofthe

fan.

Forthefan,avaneaxialfanmanufacturedbyNewYorkBlower(Figure16)wasselectedto

meettherequirementscoveredinSection3.2.Thisfanis53.3cm(21in.)indiameterwith

ninebladespitchedat33.0°.Ithasa5.6kW(7.5hp)motorandcanspinatamaximumof

3500rpm.Becauseithasacircularcrosssection,twocustom-madetransitions(Figure17)

havebeen fabricatedbyNordfab tobeused to convert fromacircular cross section toa

square cross section. The fan was selected to match cross sectional area with the

components before and after it as closely as possible. The fan is driven by a variable

frequency drive (VFD) that has been purchased and installed in the laboratory. A C10

CustomizableVectorACDriveVFDmadebySaftronicswasselectedtodrivethefan.

41

Table1.Alistofthedimensionsofeachcomponentofthetunnel.Thisliststartsat the testsectionandworksdownstreamfromthere.Note that forthepurposeofestimatinglosses,themotorandtransitionsbetweensquareandcircularcrosssectionweremodeledasaconstantareasectionwithalosscoefficientcalculatedusingEquation(24).

Figure14.Asketchoftheinsidedimensionsofthewindtunnel.Theairwillpasscounterclockwisethroughtheloop.Notethatadetaileddesignforthenozzlehasnotbeencompleted.Thenozzleshownhereusesstraightwallstoconnectthesettlingchamberupstreamandthetestsectiondownstream.

Component UpstreamWidth(cm)

UpstreamHeight(cm)

DownstreamWidth(cm)

DownstreamHeight(cm)

Length(cm)

TestSection 35.9 23.9 35.9 23.9 73.3Diffuser1 35.9 23.9 49.5 49.5 192.4Corner1 49.5 49.5 49.5 49.5 49.5Edge1 49.5 49.5 49.5 49.5 45.6Corner2 49.5 49.5 49.5 49.5 49.5Fan 49.5 49.5 49.5 49.5 173.9Diffuser2 49.5 49.5 68.4 68.4 180.7Corner3 68.4 68.4 68.4 68.4 68.4Edge2 68.4 68.4 68.4 68.4 30.5Corner4 68.4 68.4 68.4 68.4 68.4SettlingChamber 68.4 68.4 68.4 68.4 34.2Nozzle 68.4 68.4 35.9 23.9 54.7

42

Figure15.ThesteelframeconstructedoutofUnistruttosupportthewindtunnelwhencomplete.TheacquiredvaneaxialfanfromNewYorkBlowerisalsovisible.

Table2.A tableof theestimated losscoefficient, stagnationpressure loss,percentage of total losses and velocity for each component of the windtunnel.Theliststartswiththetestsectionandprecedescounter-clockwiseasshowninFigure13.

Component LossCoefficient StagnationpressureLoss(Pa)

LossPercentage Velocity(m/s)

TestSection 3.07e-2 36.2 7.8 44.70Diffuser1 1.23e-1 144 31.3 44.70SafetyScreen 6.51e-2 76.6 16.6 15.69Corner1 2.13e-2 25.0 5.4 15.69Edge1 1.66e-3 1.95 0.4 15.69Corner2 2.13e-2 25.0 5.4 15.69Fan 5.82e-3 6.84 1.5 15.69Diffuser2 9.30e-3 10.9 2.4 15.69Corner3 6.22e-3 7.26 1.6 8.20Edge2 2.14e-4 0.252 <0.1 8.20Corner4 6.22e-3 7.32 1.6 8.20SettlingChamber 2.40e-4 0.283 <0.1 8.20HoneyComb 1.68e-2 19.8 4.3 8.20Screen 7.63e-2 89.8 19.5 8.20

Nozzle 7.80e-3 9.17 2.0 8.20

43

Figure16.Aphotoofthefan(left)andaCADdrawingofthefan(right).

Figure17.Custom-madesheetmetalcomponentfortransitioningbetweensquareandcircularcrosssections(left)andsheetmetal turningvanes forcorners.(right).

44

A variety of other products have been acquired for thewind tunnel also. Turning vanes

(Figure17)wereorderedfromAeroDynetobeinstalledinthecorners.Thesewillreduce

lossesastheairpassesaroundthecorners.AhoneycombsheetwasorderedfromPlascore,

Inc.aswellasscreensfromW.S.Tyler.AsafetyscreenwaspurchasedfromMetalsDepotto

beplacedafterthefirstdiffuser.Thisscreenwillhelptostopanythingthatcomesloosein

thetestsectionfromenteringthefan.

45

Table3.Alistofthematerialspurchasedsofaralongwithapriceandproductdetails.

TUNNELCOMPONENTS Price Shipping PurchasedFrom Manufacturer(ifdifferent) Item

$1168.31 $149.95 Professionalplastics ½”thickclearcastacrylicpaper-masked

sheet@+/-0.125”cuttolerance

$588.87 VFDs.com Saftronics C102007-1,ACVectorDriveVFD

$2,409.00 $172.00 AdvancedAir,Inc. NewYorkBlower Vaneaxial,21"Diameter,Arrangement

4M,33degreebladepitch.Comeswitha

7.5HPmotorwithamaximumspeedof

3600RPM.

$498.53 $199.80 LordandSons,Inc. AtkoreInternational,Inc. Unistrutandvariousconnections

$508.00 $100.00 AeroDyne H-E-PVane,200feet

$62.50 $13.46 Plascore,Inc. PC2-250B2.000-13,27"x27",2"thick,

1/4"cell,honeycomb.

$407.79 $- BelmontHardware 8Single-Sided,3/4"sheetsofMedium

DensityOverlay(MDO),2x4's

$117.92 $15.00 W.S.Tyler 4-27"x27"0.009"diameterwire,316SS

finemesh.Theseare16-countmesh

screenswith73.3%openarea.

$49.84 $35.13 MetalsDepot 20"x20"WeldedWireSteelMesh,1"x1"

cellswith0.118"diameterwire

$630.00 $203.63 ABFlow-tek NordfabDucting Customductsquaretocircular

transition:19.5"x19.5"to21.25"

diameter,length=18.25"

INSTRUMENTATION

$70.50 $16.96 Dwyer,Inc. 2-MARKIIM-700PAU-Tube

Manometers

$270.00 $28.68 UnitedSensor UnitedSensor PCC-18-KLandPCC-8-KLpitot-static

tubes

TotalProducts TotalShipping TOTAL $6,781.26 $934.61 $7,715.87

46

Chapter4

CONCLUSIONSANDFUTUREWORK

4.1. Conclusions

Thepurposeofthisprojectwastodesignageneral-purposelow-speedwindtunnel.Code

waswritteninOctavetocalculatethegeometryofeachcomponentofthetunnelgivenaset

of constraints and geometry choices. The area ratio of thenozzle and first diffuserwere

chosentomaximizetheratioofthetestsectionairspeedtotheairspeedintherestofthe

tunnel.Becausetheenergylossesineachcomponenttendtovarywiththelocalairspeed

squared,itisidealtomaximizethenozzlearearatiotoincreasetheefficiencyofthetunnel.

The equivalent conical angle is constrained to reduce the chance of separation in the

diffusers,whichwouldgreatly increasethepressure loss.Giventhemaximumsizeof the

windtunnel,thecodeiteratesthroughnozzleanddiffuserarearatios,calculatingrequired

sizesforeachcomponentandensuringthereisenoughroomfortheselectedfan.

For each component, the code calculates the loss coefficient using a semi-empirical

equation.The losscoefficient isbasedstrictlyonthegeometryof thecomponentandthe

viscosityoftheair.Fromthis,thetotalefficiencyofthewindtunnelisestimated.Basedon

preliminaryanalysesusingthecode,afanwaspurchasedprovidingsufficientcapabilities

forthepresentapplication.Thewindtunnelandfanhavebeendesignedtoachieveatest

sectionspeedof44.7m/s(100mph)anddetailedsizinginformationhasbeengiven.The

proposedwindtunnelwillbe4.72m(15.5ft)longand1.67m(5.49ft)tall.Thetestsection

willbe0.239m(9.42in.)tall,0.359m(14.1in.)wideand0.733m(28.8in.)long.Todate,

manyofthenecessarycomponentsandmaterialsforthetunnelhavebeenpurchasedand

detaileddesignworkisinprogress.

4.2. FutureWork

In this section, immediate and more distant future work is discussed. In the immediate category,

a detailed design and drawing of the wind tunnel is under way, based on the basic design

outlined in this work. This will ensure that the plywood and acrylic will be cut to the correct

47

dimensions to ensure that each component has the correct interior dimensions of the air. Figure

18 is a preliminary version of this drawing, detailing the configuration of plywood and acrylic.

After this drawing is complete, construction of the tunnel can commence, with the exception of

the nozzle and corners. Currently, work is being done to simulate airflow through the nozzle to

select a contraction design which maximizes flow field uniformity in the test section. Similarly,

work is being done to simulate airflow through the corners of the wind tunnel. The goal of these

simulations is to determine the optimal spacing of the turning vanes to reduce losses and

maximize flow uniformity in the corners. After simulations on the corners and nozzle have

yielded optimal designs, they can be built.

Figure18.Apreliminarydiagramoftheproposedwindtunnel.

In the more distant future, and once the construction of the wind tunnel is complete,

instrumentation will need to be installed. This includes drilling holes to insert pitot-static tubes in

the settling chamber and test section. Also, a balance for supporting and measuring forces on a

48

model (e.g. lift, drag) needs to be installed in the test section. After the instruments have been

installed and calibrated, the tunnel can be used for experiments.A few options for experiments

are listed next.

As mentioned in section 1.3, there are many examples of future research that could be conducted.

One example is studying the effects of ice accretion on airplane wings. As ice builds up on

airplane wings it can adversely affect drag and lift and, consequently, the flight of the plane. Our

proposed wind tunnel is not capable of operating at freezing temperatures, but results from

NASA Glenn could be used to help us study this problem. The Icing Research Tunnel at NASA

Glenn can operate with air temperatures below freezing [42]. Water vapor is then injected into

the tunnel and ice begins to build up on the model they are testing. Researchers at NASA Glenn

scan the ice buildup to make a 3-d digital model. Figure19 is a picture of a model with ice

buildup and its corresponding 3-d model. These models can be replicated with a 3-d printer and

tested in the Houghton College wind tunnel.

Figure 19. Researchers atNASAGlenn can produce 3-d scans (left) of iceaccretiononairfoilsthathavebeenproducedintheIcingResearchTunnel(right).ImagestakefromRef.[42].

A second example of possible future research is plasma actuation. Plasma actuation is a

techniqueforactivelycontrollingairflow,e.g.,aroundanairfoil.Airfoilsatahighangleof

attackareatriskforseparationtooccur.Separationiswhentheboundarylayerbecomes

detachedfromthesurfaceofanobject.Foraircraft,separationonthewingsisreferredto

asstall.Whenstalloccurs,thereisarapidlossinliftandincreaseindrag.Thiscanbeseen

49

in the top image of Figure 20. Plasma actuators use a pair of electrodes with a high

potentialdifferencebetweenthem(3-12kV)atthefrontandmiddleofanairfoiltoionize

the air as it passes over the front edge of the airfoil. This attracts the ionized air to the

surface of the airfoil to reduce the chance of separation [43]. The bottom of Figure 20

demonstratesactivecontrolofairflowaroundanairfoil.

Figure20.Ademonstrationofaplasmaactuatoractivelycontrollingtheairflow around an airfoil. Smoke is being used to visualize the flowfield andclearly shows separation when the actuators are off (top). The flowsmoothlyfollowstheairfoilsurfacewhentheyareon(bottom).ImagetakefromPostandCorke[43].

Lastly,theHoughtonCollegelow-speedwindtunnelwillbeusedforteachingapplications

inthefuture.Specifically,itwillbeusedinthecontextofanundergraduatefluidmechanics

course. This canbeuseful in helping students understand fundamental fluidphenomena

and measurement methodologies. For example, Reynolds [44] suggests a lab for

undergraduates that focuses on plotting the lift of an airfoil as a function of air speed.

Undergraduatewind tunnel labsareuseful forbothphysicsandengineeringstudents,as

wellasnon-technicalstudentsinterestedinaviation.

50

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[29]R.D.MehtaandP.Bradshaw,TheAeronauticalJournalofTheRoyalAeronauticalSociety83(826),443(1979).[30]J.W.Gregory,K.Asai,M.Kameda,T.LiuandJ.P.Sullivan,ProceedingsofTheInstitutionOfMechanicalEngineers,PartG:JournalOfAerospaceEngineering222(2),249(2008).[31]K.G.MerriamandE.R.Spaulding,NACATN546(1935). [32]G.Comte-Bellot,AnnualReviewOfFluidMechanics8(1),209(1976).[33]A.Melling,Meas.Sci.Technol.8(12),1406(1997). [34]M.S.Iliescu,G.D.Ciocan,andF.Avellan,ProceedingsoftheASME2002JointU.S.-EuropeanFluidsEngineeringDivisionConference,(2002),pp.311-316. [35]X.LiuandJ.Katz,ExperimentsinFluids41(2),227(2006).[36]Z.J.Taylor,R.Gurka,G.A.Kopp,A.Liberzon,IEEETransactionsonIntrumentationandMeasurement,59(12),3262(2010). See also http://www.openpiv.net [37]D.BrassardandM.Ferchichi,JournalofFluidsEngineering127(1),183-185(2005).[38]F.M.Fang,JournalofFluidsEngineering119(2),454(1997). [39]F.M.Fang,J.C.ChenandY.T.Hong,JournalofWindEngineeringandIndustrialAerodynamics89(3),247(2001).[40]L.LeifssonandS.Koziel,JournalofComputationalScience7,1(2015).[41]M.N.Mikhail,AIAAJournal17(5),471(1979).[42]S.Lee,A.Broeren,H.Addy,R.SillsandE.Pifer,NASATM217702(2012). [43]M.L.PostandT.C.Corke,AIAAJournal42(11),2177(2004).[44]R.S.Reynolds,JournalofAviation/AerospaceEducation&Research14(2),9(2005).