MOVING MODEL RIG FOR HIGH SPEED TRAIN AERODYNAMICS.docx

MOVING MODEL RIG FOR HIGH SPEED TRAIN AERODYNAMICS.docx

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MOVINGMODELRIGFORHIGHSPEEDTRAINAERODYNAMICS

MOVINGMODELRIGFOR

HIGHSPEEDTRAINAERODYNAMICS

TJohnson

AEATechnologyRail

JubileeHouse

4,StChristopher’sWay

PridePark

DerbyDE248L

Yterry.johnson@aeat.co.uk

ABSTRACT

TheAEATechnologyRailMovingModelRigisauniquefacilityforinvestigatingandevaluatingaerodynamiceffectscausedbyhighspeedtrainsintheopenairandintunnels.Modeltrains,usuallyat1/25thscale,canbefiredalongthe150mlongtesttrackatfullscalespeedsupto305km/h.Therearetwoparalleltrackswhichpermitsimultaneousfiringsinoppositedirectionsandallowpassingtraineffectstobemodelled.TheRigissuitedformeasurementsofpassingpressuresintheopenair,pressurewavesintunnels,aswellasslipstreamairspeedsatthetracksideandonplatforms.ThispaperbrieflydescribesthereasonstheRigwasconstructedandthetechnicalrequirementsneededtoensuremodelscalemeasurementsarerepresentativeoffullscale;itdescribeshowtheRigworksandprovidessomecasestudiesillustratingworkthathasbeenundertakenusingtheRiginthepast.TherelevanceoftheRigtonewaerodynamicrequirementsbeingintroducedintorailwayTSIsisoutlined.Finally,futuredevelopmentsplannedfortheRig,whichenhanceitscapabilityinthefieldofhighspeedtrainaerodynamics,aredescribed.

KEYWORDS:

Aerodynamics,modelling,testing,highspeedtrains,pressures,airspeed,tunnels

INTRODUCTION

Intheearly1980’saneedforamovingmodeltestfacilityforrailwaytunnelaerodynamicswasrecognisedbyBritishRailResearch.Fullscaletestswereexpensive（andstillare!

）,requiredcomplexplanningandlongleadtimesandwerelabourintensive.Inaddition,theambientconditionswereuncontrollableandthepresenceofadverseweatherconditions,forinstance,wouldofteninvalidateaday’stesting,orattheveryleast,addanuncertaintyintotheanalysisoftheresults.Finally,testingwaslimitedtotrainsthatwerealreadybuiltandinfrastructurethatwasalreadyinplace,limitingthepotentialforinvestigating‘what-if’designs.DespitetherapidgainsinknowledgeofrailwayaerodynamicsmadetothatdatebyBritishRailResearch,theoreticalandcomputationalmethodswerestillinadequatetoinvestigateaerodynamicproblemssufficientlywelltoobviatetheneedfortesting.

Thetechnicalrequirementsformodelscaletestingofrailwayaerodynamicswereeasilyestablished:

Reynolds’numberandtrainMachnumberhadtobenearenoughtofullscalevaluestoensurethatthemodelscaleresultswouldberepresentativeoffullscale.TheReynolds’numberensuresthatscaleeffectsarenotimportantandthetrainMachnumberensuresthatpressurewaves,generatedwhenatrainentersatunnel,behaveinthesamephaseastheirfull-scaleequivalents.FromtheextensivewindtunneltestexperienceoftheBritishRailResearchstaff,itwasknownthatReynolds’numbereffectswouldbesmallifascaleofgreaterthanabout1/30thwasused.ThetrainMachnumber,（ietrainspeeddividedbyspeedofsoundinair）,couldonlybematchedtofullscaleiffullscaletrainspeedswereused,（ignoringexoticwaystochangethespeedofsound）.Finallytherigwasbuilttoaccommodate1/25thscaletrainmodels（orlargerifrequired）travellingatspeedsupto200km/h.Thefirstversion,consistingofasinglefiringtrackwascompletedin1988.DevelopmentoftheMovingModelRig（MMR）hascontinuedsince1991,drivenbytheincreaseintrainspeedsprimarilyinEurope,butalsointheUK.AmajorextensionoftheMMR’scapabilitywastheadditionofasecondfiringtrack,permittingstudiesofpassingtrainsattwodifferenttrackseparations.Thiswascompletedin1992.FurtherdetailsoftheMMRdevelopmentarereportedbyDalleyandJohnson（1999）.

AlthoughthereareothermovingmodelfacilitiesforthestudyofrailwaytunnelaerodynamicsinJapanandtheNetherlands（DalleyandJohnson,1999）,theMMRisuniqueinseveralaspects.Theotherfacilitiesuseaxi-symmetrictrainmodelswhichcorrectlyrepresentthecross-sectionalareaevolutionofthetrainsbeingstudied,andthesearefiredalongwiresthroughthecentreofpipesrepresentingthetunnelsofinterest.Thesefacilitiesareusedtostudypressurewavephenomenainrailwaytunnels.Bycontrast,theMMRhasrealisticallydetailedtrainmodelsthatrunalongtracksfittedalongthebottomofthetunnel,muchasinreallife.Thisallowstheeffectoftraindesigndetailstobeaccuratelycapturedaswellasmodellingthecorrectrelationshipbetweenthetrainandtheground.Furthermore,theMMRisafacilitywhichisnotjustlimitedtothestudyofrailwaytunnelaerodynamics,butcanalsobeusedtostudytheaerodynamicphenomenageneratedbytrainsrunningintheopenair.

MECHANISMOFTHEMMR

Overview

TheMMRconsistsoftwoparallelfiringtrackswithoveralllengthof150m.Athirdtrack,atawidertrackspacingisalsoinplaceandcanbeconfiguredtoallowmodelstofiredalongit.Thereisacentraltestsectionof50minlength.Eitherendofthetestsectionthereisanaccelerationsectionandabrakingsection,eachabout50mlong.Theparalleltracksallowmodelstobefiredfromoppositeendsiftraincrossingeffectsareofinterest.

Thecentraltestsectionhasabuilt-upflooronwhichmodeltunnelscanbelocatedfortunnelaerodynamictesting.At1/25thscale,thisallowstunnelsuptoafull-scalelengthof1250mtobemodelled.Suchtunnelsneedtobecarefullypressure-sealedtothetestsectionbasetoensurethatthereisnoleakagetoatmosphereofthetrain-generatedpressuresfromthetunnelsduringtesting.

Oncethemodelhasbeenlaunched,itwilltravelatapproximatelyconstantspeedoverthetestsection,unlessthereisamodeltunnelinplace.Inthiscase,thetrainlosesabout1-2m/sinspeedduetotheincreasedaerodynamicresistanceinthetunnel.

Realistictrainmodels,usuallybuiltto1/25thscale,aretestedontheMMR.Theyareusuallyoffourvehiclesupto4minlength,whichnormallyisadequatefortheinvestigationoftrainaerodynamics,inordertolimitmodelweight.Themodelsthemselvesareconstructednowadaysusinglightweightexpandedfoam,shapedusinganaccuratelymanufacturedmould.Theyarebuiltaroundanaluminiumchassisattachedtowheelsonwhichthemodelactuallyruns.Thetrainbogiesandwheelsareconstructedfrombalsawoodandareattachedtothemodeltrains.Othermaterialswereusedinthepastformodelconstruction,butfoamoffersagoodcompromisebetweenstrength,weightandeaseofreplacement.Typicalmodelweightsarebetweenthreeandeightkilogrammes.AtrainmodelisshownontheMMRinFigure1.

Figure1TrainmodelontheMovingModelRigatDerby

Duetotherealismofthetrainmodels,aerodynamicmeasurementsontheMMRhavebeenfoundtoreplicatefullscaleveryaccurately,aswillbeshowninthebriefcasestudiespresentedlaterinthispaper.Onlyintheunderbodyregionofthetrain,betweenthetracks,doestheflowaroundthemodeltrainnotreplicatefullscale.Thisisduetothemodelstracks,whichdonotmodelrealrails,astheyarerequiredtorestrainthemodelfromliftingduringacceleration,andthechassiswheels.

AccelerationSection

Thetrainmodelsarepoweredby,whatisineffect,abungeerubbercatapult.Severallargeelasticbungeecordsaretensionedtogetherusinganelectricwinchpriortomodellaunch.Typically,thebungeesareloadedbetween7-11kN,dependingontheweightofthemodelandthespeedrequiredforthemodelrun.Whenreleased,theenergystoredinthebungeesistransmittedviaafiringcordpassingthroughacomplexpulleygearingsystemtothemodel,whichisthenaccelerateduptospeedintheaccelerationsection.Asthetrainmodelreachesacertainpointalongtheaccelerationsectionthecord,whichisattachedtoahookunderthemodel,dropsoffandthemodelproceedstocoastalongthetrackstothetestsection.

Thepulleygearingsystemisneededtocontroltheratethattheenergyistransmittedtothemodel,whichwouldotherwisebedamagedbytheacceleration.Thetopspeedachievedusingafullmodelis305km/halongthetestsection.

TheschematicoftheMMR,showninFigure2,showstheaccelerationsectionandmechanisminmoredetail.

MovingModelRig

Figure2SchematicoftheMovingModelRig,showingtheaccelerationandbrakingsystems

BrakingSection

Oncethemodelhaspassedthroughthetestsection,itentersthebrakingsection.Here,ahookunderthetrainmodelpicksupacordattachedtoapistonviaasecondpulleysystem.Themovingmodeldrawsthepistonintoadeformabletube,anditisthedeformationenergywhichcausesthemodeltoslowdownandstop.Again,apulleygearingsystemisnecessarytoreducethebrakingrateandtominimisedamagetothetrainmodel.Figure2showsmoredetailsofthebrakingsectionandbrakingmechanism.

Instrumentation

MeasurementsareusuallymadeontheMMRusingground-basedinstrumentation,mountedeitherattracksideorinthemodeltunnels.Fourteenchannelsofmeasurementdatacanbeloggedsimultaneously.Pressuretransducersareusedforpressuremeasurements,andeightchannelsofhotwireanemometryarealsoavailableformeasuringairspeeds.Trainpositionandspeedismeasuredusinglasereventdevicesatthestartofthetestsection.Highdatasamplingratesarerequiredaseventsoccurat25timesfasterthanreallifeforascaleof1/25th.

Bespokesoftwarecontrolstensioningthebungees,modelfiring,safetyanddataacquisition.Allsignalsaredigitisedandrecordedusingthesamesystem,whichallowsthemeasurementdatatobeviewedstraightaftereachrunforcheckingpurposes.

Thereisal