A century of wind tunnels since Eiffel

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Bruno Chanetz. A century of wind tunnels since Eiffel. Comptes Rendus Mécanique, Elsevier, 2017,

345 (8), pp.581-594. �10.1016/j.crme.2017.05.012�. �hal-01570740�

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A century of wind tunnels since Eiffel

Bruno Chanetz

ONERA,ChemindelaHunière,BP80100,91123Palaiseaucedex,France

a r t i c l e i n f o a b s t r a c t

Articlehistory:

Received15November2016 Accepted8March2017 Availableonline13July2017

Keywords:

Windtunnels Laminar Turbulent Subsonic Supersonic Hypersonic

Flyhigher,faster,preservethelifeoftestpilotsandpassengers,manychallengesfacedby mansincethedawnofthetwentiethcentury,withaviationpioneers.Contemporaryofthe firstaerialexploits,windtunnels,artificiallyrecreatingconditionsencounteredduringthe flight,havepowerfullycontributedtotheprogressofaeronautics.

Buttheuseofwindtunnelsisnotlimitedtoaviation.Theresearchforbetterperformance, coupledwithconcernforenergysaving,encouragesmanufacturers ofgroundvehiclesto performaerodynamictests.Buildingsandbridgestructuresarealsoconcerned.

ThisarticledealsprincipallywiththewindtunnelsbuiltatONERAduringthelastcentury.

Somme windtunnelsoutsideONERA,even outsideFrance,are alsoevocatedwhentheir characteristicsdonotexistatONERA.

©²⁰¹⁷Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Frombirthtoﬁrstevolutions

1.1. Windtunneldeﬁnition

Wind tunnels are facilities (circular, elliptical or rectangular tunnels) in which the wind is produced by fans or by compressedairto studyandmeasure theactionofthe airflowaround asolid. Thetest sectionis thepartofthecircuit where thesolid is studied. Invented inthe late nineteenth century,these aerodynamic laboratoriestook off inthe early twentieth century. The method is based on the principle of relativity enunciated by Isaac Newton in 1687: the forces exertedonasolidimmersedinafluidandthefluidarethesameeitherthesolidmoveswithacertainspeedthroughthe fluidatrest,orthefluidmoves,withthesamerelativevelocitytothesolidthatitisimmobile.

InFrench,windtunnelisstilldesignatedbytheterm“soufflerie”,whichwascorrectforthefirstfacilities,sinceafanwas blowingairupstream(relativetothedirectionofflow)ofthetest-section. Thefirstevolutionwastosuckairdownstream ofthetestsection.Theprecursorsoftheothercountriesinthescienceofflightusetermsthatdonotprejudgewhetherthe directionoftheairis movingin thecircuit:windtunnelinEnglish, WindkanalinGerman,galleriaaerodinamicain Italian, andaerodinamicheskayatrubainRussian[1].Anyway,windtunnelshavelargelycontributedtothedevelopmentofaviation, reducingthenumberofaccidents,thussavingthelivesofpilotsandmaintainingtheequipment.Theyalsoallowedreplacing the“flair”ofthepioneersbythe“art”oftheengineers,accordingtothebeautifulsentencebyGustaveEiffel.

E-mailaddress:chanetz@onera.fr.

http://dx.doi.org/10.1016/j.crme.2017.05.012

1631-0721/©²⁰¹⁷Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccessârticleûnder^the^CC^BY-NC-ND^license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Fig. 1.Aérodromeof Captain Ferdinand Ferber in Chalais–Meudon (© ONERA).

1.2. Competitorsforgroundinvestigation

The wind tunnel is avery convenient experimental investigation mean,which hassurpassed the alternative methods based onthe directmovement of thesolid inthe air.However, atthe beginning ofthetwentieth century,there was no consensus.IndeedArmanddeGramont,DukeofGuiche,whowasmeasuringthedistributionofpressureonawingfixedon amovingcar,wasadetractorofEiffel,whowasusingawindtunnel.HeevenchallengestheworkofEiffelanddisapproves the transposition ofthe resultsobtained inhis wind tunnel tothe reality of airplanes inflight. Todefinitively closethe controversyabouttheequivalencebetweenrealandrelativemovements–althoughthequestionhadbeenalreadysolved inthemiddleofthe19thcenturythroughtheexperiencesofDuchemin,solvingtheparadoxofduBuat[1]–Eiffelrequests theintervention ofthegreatmathematicianHenriPoincaré.Shortlybeforethedeath ofPoincaréin1912, Eiffelreceiveda noteinlinewithhisexpectations:“Thereisnoreasonthattheforcesexertedontheplatebyauniformairflowdifferfrom thosethatwouldoccurifthisplateismovinginacalmair”.Poincaréadds:“itisclearthatonlytherelativemovementis significant”[2].

Belowaredescribedthesealternativeprocessesexistingatthebeginningofthe20thcentury:

– Horizontal rectilinear motion

•ⁱⁿ^1901,â^flight^testing^was^performed^by^the^German^company^Siemensônâ^train^travelingât¹⁶⁰^km/h,

•ⁱⁿ^1909,^thanks^to^a^1.4-km^private^railway^track,^theAeroTechnicalengineeringInstitute(IAT)ofSaint-Cyr-l’Écoletested wings;

•^during^the^first^decadeôf^the^twentieth^century,Ârmand^de^Gramont,^dukeôf^Guiche,^tested^wingsⁱⁿ^his^car^[2].

– Vertical rectilinear motion

•ⁱⁿ ^1908, ^before ^the^building ôfîts ^first ^wind^tunnel, ^GustaveÊiffel ^conceived â^very îngenious^drop ^test ^machine ^to study the drag of solids [2,3]. He installed the device on the eponymous tower’s second floor, takingadvantage of its 115 m in height. His first studies, awarded by the French Academy of Sciences in 1908, allowed him to found the fundamental lawsof airresistance.In thisperiod,with thegrowth ofaviation, Eiffelbrought newideas andhis knowledgeofaerodynamicsledhimtounderstandtheeffortsoftheaironasolid.

– Combination of the horizontal and vertical motions

•În^1904,^Ferdinand^Ferberⁱⁿ^Meudon^performedâ^device^by^takingâdvantageôf^theêffectsôf^gravity^to^moveâ^plane alongacable.Hisplane,suspendedfromaslidingcarriagealongatensionedcablebetweenpylons(seeFig. 1).Gustave Eiffel hadalsoconsidered suchadevice,calledaérodrome,fromthefirst floorofthetower, priortotherealizationof itswindtunnel.

– Circular motion:

In 1906, the AeroTechnical engineering Institute (IAT)of Saint-Cyr-l’École testedsolids attachedto the endsof rotary arms.Thiswayallowsaccesstotangentialspeedsallthemoreimportantthatthearmislongandthespeedgreat.

1.3. Theblowingwindtunnelsandtheintroductionofthecollector

EveniftheﬁrstwindtunnelknowninGreatBritainoperatedbycompressedairejector(VenhamFrancis,1871,thenthat Horatio Phillips, 1884),thefollowing wind tunnelsused theairmoved by a fan disposedupstream ofthe windtunnel’s testsection(seeFig. 2a):CharlesRenard(France,1896),HiramMaxim(GreatBritain,1896),KonstantinTsiolkovsky(Russia,

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Fig. 2.a: Fan upstream of the model. b: Fan downstream of the model.

1897),OrvilleandWilburWright(USA,1901),andAugusteRateau(France,1909).Weshouldalsomentionthewindtunnel, smoke machinebuilt byÉtienne-Jules Marey(France, 1899),which permitted therealizationofthe first visualizations of fluidflows[4].

Afirstdevelopmentconsistsintheintroductionofthecollector.Disposedupstreamofthetestsection, theconvergent part increasesthe velocity in the test section under the mass conservationlaw for the flow, which is expressed for an incompressibleflow,by:

V·^S=^constant

whereV representsthevelocityoftheﬂowandS thesurfaceintheconsideredsection.

1.4. Theintroductionofsuction

Thepassagetosuctionisalsoanimportantstepintheartofwindtunnels.Thefanisnowdisposeddownstreamofthe testsection (seeFig. 2b)andtherebynolongerinterfereswiththemodel,whichisagreatimprovementtoguaranteethe quality ofﬂow. Accordingto thisprinciplethe followingwind tunnels wereconceived: NikolaiZhukovsky (Russia,1902), Thomas Stanton(Britain, 1903), DimitriRiabouchinsky (Russia,1905), GustaveEiffel(France, 1909).Suction wind tunnels havebeendevelopedveryquickly.Therearecurrentlyseveralhundredmajorfacilitiesofthiskindintheworld,whileonly lessthanadozenwereactivebefore1914.

1.5. Eiffel’swindtunnelsandtheintroductionofthediffuser

In1909,EiffelbuiltitsfirstwindtunnelatthefootoftheEiffelTowerintheChampdeMars.Thisfacilitywasoperational till 1911to studyaerodynamics.The airwas moved by a fan driventhanks to a 50-horsepower electricmotor.The first testswere tocheckthattheprincipleofwindtunnelsimulation–wheretheaircraftoritsmodellyinginairinmotion– isconsistentwiththereality –wheretheaircraftismovingintheairconsidered asnon-movingasafirstapproximation.

To this aim, Eiffel compares the wind tunnel results with those previously obtained with the drop test machine. This comparison being successful, Eiffel’s wind tunnel was then available for the pioneers in their conquest of air: Farman, Bleriot,Voisin,Bréguet[5].

In1911, themunicipalityofPariswantingto recovertheareaonwhichthewind tunnelislocated,Eiffelhadtoleave the Champde Mars. In thebeginning of 1912, heinstalled in the district ofAuteuil a newwind tunnel withincreased performance, this new laboratory being inaugurated on 19 March. The originality of the wind tunnel at Auteuil is the presenceofthefollowingsuccessiveelements,fromupstreamtodownstream:

– a collectororconvergent,whereair isacceleratedfroman entrancesection, fourmeters indiameter,to anoutletof twometers;

– aclosed experimentalchamber wherethefree streamﬂow isproduced (i.e.withoutwalls); whereasatthe timethe otherwindtunnelshadaguidedtestsection;

– a recoveryattheexitof thetest chamber, madeofa smallcollectorwhosesection is largerthan thesection ofthe convergentatthetestchamberentrance;

– adiffusermadeofdivergingtubeinordertorecoverpressureinfrontofthefan(seeFig. 3).Thesame50-horsepower engineasthatalreadyusedintheChampdeMars,wasusedagain.ThewindtunneloftheChampdeMarsreacheda speedof18m/sina1.5-m-in-diametertestsection.Withthesameengine,thenewwindtunnel,thankstothediffuser, reached32m/s(anincreaseof78%speed)inasectionof2metersindiameter(anincreaseof78%also),henceamass ﬂowgainofmorethan200%.

– Ahelicalfanwithadiameterof3.80mandaweightof8.500kgwasspeciallypurchasedforthewindtunnelofAuteuil, thefan oftheChampde Marswindtunnelbeingused forasecond smallwindwitha collectoroutletone meterin diameter.

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Fig. 3.Introduction of the diffuser between the test section and the fan.

Fig. 4.Watercolor drawing of the Eiffel wind tunnel located in the rue Boileau (© Aérodynamique Eiffel).

The diffuser,which was aninnovation ofEiffel (see Fig. 4), was thesubjectofa patentdated 28November 281911:

“Theadditionofadiffusertoimprovetheperformanceofamachinetoproduceartificialairflow”.Thisinventionwasrich in consequences,asitallowed Eiffel todrasticallyreduce theelectricpower requiredforsucha facility.The efficiencyof thedevicecomesfromtheBernoullilaw,statingthatpressureandvelocityvaryinversely.Indeed,byreducingvelocity,the diffuser increasedthepressureoftheairexitingthetestsection. Thepressuredifferenceoneithersideofthefan isthen much lessthantheone thatwouldexistifthefan werelocated directlyjustdownstreamofthetest-section. Thisdevice leadstoareductionintheelectricpowerrequiredtoextracttheair.“Thusthediffusersavesmoneysincepowerisreduced byafactorof3.Theadvantageofthisrecoverydeviceisclear,whichallowedustorealizethepresentlargefacility”(Eiffel [5]).From thatdate allwindtunnels havebeenequippedwithadiffuser.GustaveEiffel(see EiffelinFig. 5) willaskthat theofficialdesignation,“Aerodynamicdevice systemEiffelParis”shouldbeengravedona1-m-by-0.5-mmarbleplaqueon allfacilitiesusinghistechnique,withoutaskingforroyalties.

1.6. ThePrandtlwindtunnelinGöttingen

Another major innovation consisted in making air circulate in a closed circuit. The ﬁrst installation of this kindwas built atthe University ofGöttingen,in Germany, by LudwigPrandtl in1909. Afterbeing suckeddownstream of thetest section, theairisguidedthroughfoursuccessivebends,andthenreadmittedupstreamofthecollector.Thistype ofwind tunnel, calledPrandtlwind tunnel–orGöttingen windtunnel– leads toa betterfuel eﬃciencyandprovidescontrolled test conditions(pressure, temperature, humidity).Modernwind tunnelscombinePrandtl’s innovation(closed circuit) and Eiffel’sone(diffuser).

1.7. ThecontroversybetweenEiffelandPrandtlconcerningthedragofspheresandtheirruptionoftheReynoldsnumberin aerodynamics

At the Auteuillaboratory, Eiffel continuedthe activities undertaken atthe Champ de MarsLaboratory concerning the aerodynamics ofthe drag ofa sphere. The measurements carriedout at theChamp de Mars’ windtunnel ata speed of 15 m/srevealed values ofthe drag coeﬃcient less thanhalf those found forlower speeds by Professor AugustFöppl in theGöttingenLaboratory,headedbythefamousLudwigPrandtl.Föppldidnothesitatetowritethat theFrenchcolleague hadcommittedanerror,butaforgivablemistakewithrespecttoits age.Therefore,Eiffel decidedtoundertake newtests

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withspheres ofdifferent diameters.He discovered that,for eachsphere, there aretwo flow regimes: oneat low speeds correspondingtothecoefficientfoundinGöttingen(withboundarylayerseparationinlaminarflow)andtheother,athigher speeds,correspondingtothecoefficientfoundintheChampsdeMarsfacility(withboundarylayerseparationinturbulent flow).Thetransitionbetweenthetworegimesalwaysoccursforthesamevalueoftheproductvelocity× ^diameter.

Iftheviscosityisconstant,whichisthecaseatlowvelocities,thisproductvelocity×^diameter^is^similar^to^the^Reynolds number:

Re=ρ_·V·^L/μ

where ρisthedensityoftheactualﬂow, V itsvelocity, μtheairdynamic viscosity,andLthecharacteristiclengthofthe studiedsolid.

Thus,theexperimentalstudiescarriedoutinhislaboratoryofAuteuilhaveﬁrstdemonstratedtheimportantroleofthe Reynoldsnumberinaerodynamics.

Theexampleofthesphererevealsthecomplexityoftheboundary-layerbehavior,whichsometimesleadstoparadoxical laws.The laminarboundary layer generatesa lower friction drag than theturbulent one, butits earlierseparation leads to amore disturbed wake,whichis theorigin ofa form(pressure)drag largerthan inthe turbulent case. Therefore,by triggering transition, one reduces more the drag due to the wake than one increases the friction drag due to a more importantturbulentboundarylayeronthesphere.Theturbulentregimeismorefavorabletotheglobaldragbalance,which isthe sumofthefriction andformdrags.This phenomenonisillustrated by an oldexperimentcarried outatONERA in a watertunnelby Werlé[6].Fig. 6ais relativeto thecaseobtainedfora Reynoldsnumberbasedon thediameterequal to200,000,andFig. 6bisrelativetothecaseofa Reynoldsnumberequalto300,000.Onesees clearlythelessperturbing wakeinthesecondcase(withaturbulentseparation)thanintheﬁrstcase(withanearlierlaminarseparation)[7,8].

Anemblematicapplicationofthephenomenonisgivenbythegolfballwithdimplesthattriggertransitionandpostpone more downstream theseparation of theboundary layer. The reducedwake resulting fromthis turbulent boundarylayer promotes thepenetration intheairby reductionofthe globaldrag,which leadsto an increaseinthe golfballrange.At theorigin, theinventorsofgolfmanufacturedleatherballs, takingcare toplacetheseamson thebackto havea smooth envelope favorable to penetration into the air. Theylater discovered that the best golfballs were damaged balls whose roughenvelopetriggersatransitionoftheboundarylayer[8,9].

ThecontroversybetweenPrandtlandEiffelaboutthedragofthespherehighlightstheessentialroleplayedbyexchanges betweenresearchersfortheadvancementofknowledge.Thankstotheseconfrontations,scienceprogressesandthatwasthe goalpursuedbyGustaveEiffel:“ItismainlytheprogressinaviationthatIwantedbycreatingthislaboratorywherethetests

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Fig. 6.a: Laminar regime at the boundary layer separation. b: Turbulent regime at the boundary layer separation (© ONERA).

are absolutelyfree, butwheretheresultswouldbepublished, inthegeneralinterest,throughreportsorcommunications to scientiﬁc societies”.GustaveEiffel wasa greatresearcherwho hasnot playedthe roleofa suspiciousinventor against potential competitors.He offeredall manufacturers themeansto testtheir modelsfree ofcharge,iftheir resultswere to be published andaccessibletoall. Itwas truly thewind ofresearch thatwas blowingatthelaboratory ofAuteuilunder the leadershipof Eiffel.He publishesTheFrictionoftheAirandAviation, a booktranslated into both German andEnglish.

Furthermore,itscontributiontothisemergingscienceofaerodynamicswas recognizedintheUSA,wherehereceived the goldmedalofLangleyin1913,whichhadonlybeenawardedtoWilburandOrvilleWrightbefore.

2. Facilitiesfromthebeginningofthe20thcenturytomodernwindtunnels 2.1. Therulesofsimilarity

Duringthetwentiethcentury,thedevelopmentofwindtunnelshascontinued,drivenbytheurgentneedtorespectthe similarityrulestoensurethevalidityofthetests.Aﬁrstevidentrequirementwastohaveamodelwiththesamegeometric shapeastheoriginalobject.

Asecondconditionwasrequiredtomeetthecharacteristicsoftheﬂow:theMachnumberMhastoberespected:

M=V/c

where V representsthevelocityoftheﬂowandcthecelerityofsound.

Thethirdsimilarityruleistoperformtestsmaintaining–asfarasitmightbe–theReynoldsnumber(Re)oftheﬂow.

Iftheairinthewindtunnelhasthesamecharacteristicsastherealone(densityandviscosity)andthesamevelocity, theReynoldsnumberisautomaticallyrespectedinamodelattherealscale.Sinceatthebeginningoftheepicofaviation, the sizeofaircrafts wassmall, theﬁrst meantoreachthe convenientReynoldsnumberbeingtoincrease thesize ofthe windtunnels.ThelargeChalais–MeudonS1Chwindtunnelmeetsthesespeciﬁcations.

Duetotherapidincreaseinaircraftsize,itwasnotsuitabletoincreasetoomuchthesizeofthefacilities.Thus,nowadays the experiments areagain performedwithreducedmodels. Torespectthe convenient Reynoldsnumber,we play on the other parametersoftheformula:densitybypressurizingtheﬂuidandviscositybycoolingtheﬂuid.TheF1windtunnelis apressurizedwindfacilityandtheEuropeanWindTunnel(ETW)isacryogenicwindtunnel.

2.2. ThelargeChalais–MeudonS1Chwindtunnel(France)

Antonin Lapresle, collaborator and successorof Eiffel at the Auteuil wind tunnel, designed, at the requestof the Air Ministry, thelargeChalais–Meudonwind tunnel(Hauts-de-Seine)thathadbeenbuiltbetween1932and1934fortesting full-scale airplanes (see Figs. 7 and 8). It was equippedwith a collectorwith a reduction ratio of3.5, witha mouth of 350 m² thatcapturestheoutsideairandallowsittoreachavelocityof180km/hintheinlettestsectionof100m².The diffuser,agianttubeshapedlikeatruncatedcone(ellipticalsection,10mofverticalaxisandhorizontalaxisof18m),is madeofreinforcedconcretewithathicknessof70mmandalengthof38meters.Downstreamofthediffuser,thesuction chamber,withsixfansof1,000horsepowereach,allowstheairextraction.Theinstallation,whichhadbecomeobsolete,has beendisusedin1977. Classiﬁedonthelistofthehistoricalmonumentsin2000,thisfacilityhastested, inmanynational programs,aircraft,automobilesandbuildingsaerodynamics[10].

2.3. TheF1windtunnelofLeFauga–Mauzac(France)

ToreplacethelargeS1ChwindtunnelofChalais–Meudon,other facilitieswere emerging.Foraerospacetesting,apres- surized F1windtunnelwasbuiltnearToulouse,atLeFauga-Mauzac.Operationalsince1974, thistunnelclosedcircuithas

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arectangulartestsection(4.5 m×^{3.5 m)}^where^the^flow^speed^reaches⁴³⁰^km/h.Pressurizingthefacility(upto3.85bar) increasesthedensityoftheflow,hencemaintainingahighReynoldsnumbercompensatingthereducedsizeofthemodel imposedbytherelativelysmallwindtunnelsize.Ithasnotbeen,however,possibletosavemorethanafactorof4onthe Reynoldsnumberbypressurizing,sincethenverylargedynamicloadswillappearonthestructures(walls,floors).

2.4. TheEuropeanTransonicWindTunnel(ETW)inCologne(Germany)

ToobtainhighernumberofReynolds,closetotheonethatcharacterizesthecurrentlargecivilaircraft,wechosetoplay ontheair’sdynamicviscosity μ,whichisproportionaltothesquarerootoftemperature.Bycoolingthecircuit,acryogenic windtunnelisobtained. Insucha facility,theReynoldsnumberincreasesnotonlyduetothedecreaseinviscosity,since itisincludedinthe denominatorintheformulagivingtheReynoldsnumber,butcoolingalsoinduces an increaseinthe densityviatheidealgaslaw:

ρ=^p/r·^T

where ρ is thedensity inkg m⁻³, p the pressure inPa,r the universal gas constantequal to 287J kg⁻¹K⁻¹ and T the temperatureexpressedinK.