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Modelling wings, understanding flight and reporting results. One of the great things about CFD is that it allows us to model a wide range of conditions that it would be impossible to capture in a single experiment. In this exercise you are going to model an aerofoil using a pre‐made mesh, then ‘fly’ it in flows of changing incidence to determine the lift and drag coefficients of the aerofoil as a function of the oncoming angle of attack. By looking carefully at the flow field you should also be able to determine when the wing stalls, and what the effect is on the lift and drag. Then you can try to model the flow at higher velocity in the supersonic regime. You have already measured the pressure distribution around a supersonic aerofoil in the 9x3 wind tunnel. In this modelling exercise the same RAE104 aerofoil profile is used. To enable you to do this an RAE104 aerofoil in a 3‐D thin slice at zero incidence has already been meshed and is uploaded in the Wing Modelling section of the Fluids Coursework Module section of David Gillespie’s page on the Osney Thermofluids Website. To run this open workbench (start_ansys then runwb2 in the new window), you drag a Fluent session onto the Workbench. Next open Fluent in 3‐D. You need to import the mesh, this is done through File – Import – Mesh. Once it is in the system you can view it as normal. Check where the inlet and outlet from the system lie. You can scale the units to any that you need. The system will solve more quickly if you Mesh – Reorder – Domain, this tries to reduce the bandwidth of the system of equations. The meshes are a bit rough and ready but should provide a reasonable indication of the pressure, lift and drag of the aerofoil at low speeds, and you can try to refine the mesh. For easy solution run at speeds of under 100 m/s. We suggest you make the magnitude of the incoming flow 20 m/s for the angle of attack tests. When you open the boundary conditions box you will see there are 2 items named inlet‐fluid‐air**. These must both be set as velocity inlet boundary conditions, and for the low speed flows you can set the outlet to be an outflow. As we would like to report lift and drag coefficients you need to set the length term in the Reference values to be the chord of the model – which is 80 mm in this case, the velocity to the freestream velocity and the density to the freestream density. First click on Compute from and choose either of the inlets, and then change the length term to 80 mm. 1 “MeshRAE104_0_revised3.msh”

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Modelling Wings and Reuirements for the Write Up

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  • 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


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