modelling wings and reuirements for the write up

Modellingwings,understandingflightandreportingresults.OneofthegreatthingsaboutCFDisthatitallowsustomodelawiderangeofconditionsthatitwouldbeimpossibletocaptureinasingleexperiment.Inthisexerciseyouaregoingtomodelanaerofoilusingapremademesh,thenflyitinflowsofchangingincidencetodeterminetheliftanddragcoefficientsoftheaerofoilasafunctionoftheoncomingangleofattack.Bylookingcarefullyattheflowfieldyoushouldalsobeabletodeterminewhenthewingstalls,andwhattheeffectisontheliftanddrag.Thenyoucantrytomodeltheflowathighervelocityinthesupersonicregime.Youhavealreadymeasuredthepressuredistributionaroundasupersonicaerofoilinthe9x3windtunnel.InthismodellingexercisethesameRAE104aerofoilprofileisused.ToenableyoutodothisanRAE104aerofoilina3DthinsliceatzeroincidencehasalreadybeenmeshedandisuploadedintheWingModellingsectionoftheFluidsCourseworkModulesectionofDavidGillespiespageontheOsneyThermofluidsWebsite.

Torunthisopenworkbench(start_ansysthenrunwb2inthenewwindow),youdragaFluentsessionontotheWorkbench.NextopenFluentin3D.Youneedtoimportthemesh,thisisdonethroughFileImportMesh.Onceitisinthesystemyoucanviewitasnormal.Checkwheretheinletandoutletfromthesystemlie.Youcanscaletheunitstoanythatyouneed.ThesystemwillsolvemorequicklyifyouMeshReorderDomain,thistriestoreducethebandwidthofthesystemofequations.Themeshesareabitroughandreadybutshouldprovideareasonableindication

ofthepressure,liftanddragoftheaerofoilatlowspeeds,andyoucantrytorefinethemesh.Foreasysolutionrunatspeedsofunder100m/s.Wesuggestyoumakethemagnitudeoftheincomingflow20m/sfortheangleofattacktests.Whenyouopentheboundaryconditionsboxyouwillseethereare2itemsnamedinletfluidair**.Thesemustbothbesetasvelocityinletboundaryconditions,andforthelowspeedflowsyoucansettheoutlettobeanoutflow.AswewouldliketoreportliftanddragcoefficientsyouneedtosetthelengthtermintheReferencevaluestobethechordofthemodelwhichis80mminthiscase,thevelocitytothefreestreamvelocityandthedensitytothefreestreamdensity.FirstclickonComputefromandchooseeitheroftheinlets,andthenchangethelengthtermto80mm.

1MeshRAE104_0_revised3.msh

The flow is turbulentnot laminarchooseeither theSpalartAlmaras turbulencemodelor thekepsilon model these should be quick to run and is likely to be nearly as accurate as othertechniques.Thekepsilonalthoughalittleslower,seemsmorestableforthisproblem.Rememberthatthesolutionhasnotconvergeduntiltheresidualsarelowandflat.Youmayneedtoswitchofftheconvergencecriteria in theMonitorsResidualssectionandmanuallystop thesolutionwhenthecriteriaforsolutionareachieved.

Tosettheboundaryconditionwemustsetthevelocity,magnitudeanddirectionoftheflow.Ifrunningat20m/sthisstaysfixed.Torunat5degreesincidencetheXcomponentiscos(5)butmaketheYcomponentsin(5)asthesystemisnotsymmetricandisdesignedtoallowtheflowtodevelopfurtheronthedownstreamsideoftheflowdomain.Initialisethevelocityaszeroineachcasethisseemstobemorereliableinproducingaresult.Increasetheangleofattackineachrunyouconductandlookfortheonsetofstall.Itisalsointerestingtofindthepressuredistributionaroundtheaerofoil,notonlydoesthisletyoucalculatethelift,itwillallowyoutocomparethedistributionaspressurecoefficientCp=(ppinf)/0.5U2betweenthesubsonicandthesupersonicaerofoils.YoucanusetheResultsReportsForcestofindtheliftanddragdirectlyrememberthatyouwantthelift

anddragnormalandparalleltotheoncomingflowsosetyourDirectionVectorappropriately.Ifyouduplicatethesystemforeachsetofboundaryconditionsthenyouwillbeabletopullinmultipleresultsintothepostprocessortoallowyoutocomparetheflowfeaturesmoreeasily.Youshould:

Covertherange5toapproximately20degreesangleofattackthehighestanglesmaybedifficulttoconverge.IfnecessarygointotheSolutionControlsandreducethepressureandmomentumterms,sothatlessofthenewsolutionisusedineachiteration.Use~4degreeintervalsuntilyouareclosetostallandthenreduceto1degree.

Foreachangleofattackcapturethepressureandvelocitydistributions,thepressuredistributionoverthetopandbottomoftheaerofoil,thelift,dragandliftanddragcoefficients.

Plotacombinedplotofangleofattackvsliftcoefficientanddragcoefficient.Findtheslopeoftheangleofattackvsliftcoefficientgraphinthelinearportionandcompareyourresulttothetheoreticalvalueof2.

Plotliftcoefficientagainstdragcoefficient,andcommentontheeffectoftheonsetofstall.Itisuptoyoutodecidewhattoreport.CFDisjustlikeanyotherexperiment,weneedtodescribewhatwehavedoneandthenprovidetheresultstobackitup.Ofcourseyouhavelimitlesspicturesavailabletohelpyouexplainwhatphysicallyishappeningwhenairflowsoverawing.Wewouldbe

2BoundaryconditionsScreen

surprisednottofindacleardescriptionofwhyaerofoilsgenerateliftinthissectionofthewriteupalongwiththeresultsnotedabove.

SupersonicAerofoilthisisquitetrickyandmightrequireseveralattempts.Themodellinginthissectionistobackupyourexperimentalresults.Foryourwriteupwewanttoseeagoodwriteupoftheexperiment,resultinginaclearexplanationofwhyexpansionandshockwavesoccuronthesurfaceofthesupersonicwing.Ialsoexpectagoodexplanationofschlierenphotographyandwhatthedifferentlycolouredregionsonthewingrepresent.Oneofthekeythingstogetintoyourreportisacomparisonoftheshapeofthepressuredistributionovertheuppersideoftheaerofoilforthecaseofthesubsonicwingandthesupersonicwingandyoushouldspeculateabouttheimplicationsofthisforanaircrafttryingtoemploythissortofwingforlowsupersonicspeedflow.ModellingForthispartofthemodellingyouneedtosetpressureandnotvelocityboundaryconditions.Useyourmeasurementsfromtheexperimenttodeterminetheseandsettheboundaryconditionattheexitofthesystemtoapressureoutlet.Attheinletyoucanseteitherapressureboundaryconditionoravelocityboundarycondition.Notethatitisusuallybettertoinitialisefromzerovelocityandthenrunwiththesolutioncontrolparametersturneddownatthebeginningofthesimulation.Youalsoneedtochangethemodeloftheairfromaconstantdensitygastobeanidealgas.Youdothisinthematerialssectionofthemenu.Thesolutionislikelytomuchmoreunstableundertheseconditions.Youmaybeabletocoarsenandrefinethemeshinordertocapturetheexpansionandshockfeaturesthatyouhaveseenduringyourexperiment.Someamazinglygoodresultswereachievedin2014.Whatyouarelookingtoachieveisaqualitativeresultshowingthedramaticdensitychangesduringthepassageofflowovertheaerofoil.Youcanreportthepressuredistributionsasabove,butthesystemwouldnottolerateanythingotherthanaverysmallangleofattackwithoutconsiderabledifficultyinformingasolution,sorunwiththeflowcomingontothewingatzeroangleofattack.Yourfinalreportshouldbenomorethan10pages,submittedasgroupsof4.PleaseaddacoversheettothereportsothatIknowexactlywhoeachofyouare!DavidGillespie16/05/2014