Turbulent Pipe Flow.docx
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Turbulent Pipe Flow.docx
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TurbulentPipeFlow
TurbulentPipeFlow
ProblemSpecification
Let'srevisitthepipeflowexampleconsideredinthepreviousexercise.Asbefore,theinletvelocityis1m/s,thefluidexhaustsintotheambientatmosphereanddensityis1kg/m3.Forµ=2x10-5kg/(ms),theReynoldsno.basedonthepipediameterandaveragevelocityattheinletis
AtthisReynoldsnumber,theflowisusuallycompletelyturbulent.
Aturbulentflowexhibitssmall-scalefluctuationsintime.ItisusuallynotpossibletoresolvethesefluctuationsinaCFDcalculation.Sotheflowvariablessuchasvelocity,pressure,etc.aretime-averaged.Unfortunately,thetime-averagedgoverningequationsarenotclosedi.e.theycontainfluctuatingquantitieswhichneedtobemodeledusingaturbulencemodel.Noturbulencemodeliscurrentlyavailablethatisvalidforalltypesofflowsandsoitisnecessarytochooseandfine-tuneamodelforparticularclassesofflows.Inthisexercise,you'llbeturnedlooseonvariantsofthek-εmodel.Butintherealworld,treadwithgreatcaution:
youshouldevaluatethevalidityofyourcalculationsusingaturbulencemodelverycarefully(which,ahem,meansthatthereisnogettingawayfromstudyingfluiddynamicsconceptsandnumericalmethodsverycarefully).FLUENTshouldnotbeusedasablackbox.Thek-εmodelsconsistoftwodifferentialequations:
oneeachfortheturbulentkineticenergykandturbulentdissipationε.Thesetwoequationshavetobesolvedalongwiththetime-averagedcontinuity,momentumandenergyequations.Soturbulentflowcalculationsaremuchmoredifficultandtime-consumingthanlaminarflowcalculations.Thisisanexercisetowhetyourappetiteforturbulentflowcalculations.
Step1:
CreateGeometryinGAMBIT
Ifyouwouldprefertoskipthemeshcreationsteps,youcandownloadthemeshhere(rightclickandselectSaveAs...)andgostraighttostep4.
Sincetheflowisaxisymmetric,thegeometryisarectangleasintheLaminarPipeFlowtutorial.Wewillfirstusea100x30mesh(i.e.100divisionsintheaxialdirectionand30divisionsintheradialdirection).
Wecouldcreatethismeshfromscratch,asintheLaminarPipeFlowtutorial,butinstead,wewillmodifytheprevious100x5togetthe100x30mesh.ThiswillintroduceyoutotheartofmodifyingmeshesinGAMBIT.
Step2:
MeshGeometryinGAMBIT
LaunchGAMBIT
Createafoldercalledpipe2ataconvenientlocationtouseasyourworkingfolder.Copyyourpipe.dbsfilecontainingthe100x5meshfromtheLaminarPipeFlowtutorialtothisfolder.Ifyoudon'thavethisfile,here'sacopy(right-clickandselectSaveAs...).Renamethisfileaspipe100x30.dbs.We'llmodifythisfiletoobtainthemeshfortheturbulentpipeflowsimulation.
StartGAMBITandloadpipe100x30.(Refertostep1oftheLaminarPipeFlowtutorialifyou'veforgottenhowtodothis.)RecallthatGAMBITwillusetheidpipe100x30asthedefaultprefixforallfilescreatedduringthissession.
Tomakebestuseofscreenrealestate,resizetheGAMBITandbrowserwindowssothatyouapproximatethisscreenarrangement.ThiswayyoucanreadinstructionsinthebrowserwindowandimplementtheminGAMBIT.
Themeshfromtheprevioustutorialshouldbedisplayed.Tofitthemeshtothesizeofthewindow,select:
GlobalControl>FittoWindow
DeletePreviousFaceMesh
Thefirststepwehavetodoisremovetheoldfacemesh.Recallthatthefacemeshisbuiltontopofmeshededges,therebyformingthegrid.Inthiscase,wedon'twanttoremovetheunderlyingedgemeshes.Sotodeleteonlythefacemesh,select:
OperationToolpad>MeshCommandButton
>FaceCommandButton
>DeleteFaceMeshes
Sinceweonlyhaveoneface,shift-clickanyedgeoftheboundingrectangletoselectthefacemeshwewanttodelete.ThefaceyouhaveselectedshouldbecomeredandthenameofthefacelistedintheDeleteFaceMesheswindowinthedropdownbox.
Now,becausewedon'twanttodeletetheedgemeshes,unchecktheRemoveunusedlowermeshbox.
ClickApply.
CheckthatthefacemeshhasbeenremovedintheGAMBITGraphicsWindow.
RemeshEdges
Sincewearestillgoingtouse100divisionsforthehorizontaledges,weonlyneedtoremeshtheverticaledges.
Toresolvethemuchhighergradientnearthewallforaturbulentflow,wewillusesmallergridspacingnearthewallbyemployinggridstretching.
Foreachverticaledge,wewillspecifythedivisionlengthnexttothewalltobe0.001andthetotalnumberofdivisionstobe30.InGAMBIT,eachedgehasadirectionassociatedwithitasshownbyanarrow.Wewillsetthisarrowtopointawayfromthewall.Thenthedivisionnexttothewallbecomesthe"FirstLength"andthedivisionnexttotheaxisbecomesthe"LastLength".We'llspecifythe"FirstLength"tobe0.001andthetotalnumberofdivisionstobe30fortheedge;GAMBITwillautomaticallycalculatetheappropriatevalueforthe"LastLength".
OperationToolpad>MeshCommandButton
>EdgeCommandButton
Selecttheverticaledgesbyshift-clickingoneachofthem.Noticetheredarrowthatappearsontheedgewhenitisselected.Makesurethesearrowsarepointingdown(towardstheaxisandawayfromthewall).Ifbothofthesearrowsarepointinginthewrongdirection,youcanreversethembyclickingReversenexttoPickwithlinks.However,ifonlyoneoftheedgesneedstobereversed,youcandothatbyshift-middleclickingonthatedge.You'llhavetozoomintobeabletodothis.(RecallthatyoucanzoominbyholdingdowntheCtrlkeyandthendraggingaboxwithyourleftmousebutton.Double-clickwiththemiddlemousebuttontogobacktothelastview.)
ForTypeintheMeshEdgesmenu,selectFirstLengthfromthedropdownbox.NexttoLength,typein0.001.
Wewant30divisionsoneachoftheverticaledges;soselectIntervalCountfromthedropdownboxunderSpacingandenter30inthetextboxtoitsleft.
ClickApply.
Ifyouzoominontherightedge,youshouldseethefollowing:
Notethatthemeshspacingissmallernearthewallasindicatedbythebluecirclesontheedge.
RecreateFaceMesh
Thenextstepistorecreatethefacemeshontopoftheseedgemeshes.Thisisthesameprocedureasintheprevioustutorial:
OperationToolpad>MeshCommandButton
>FaceCommandButton
>MeshFaces
Shiftleft-clickonthefaceandclickApply.Themeshedareashouldlooklikethisafterzoomingin:
Step3:
SpecifyBoundaryTypesinGAMBIT
Recallthatwecreatedthefollowingboundarytypesforthe100x5meshintheLaminarPipeFlowtutorial:
EdgePosition
Name
Type
Left
inlet
VELOCITY_INLET
Right
outlet
PRESSURE_OUTLET
Top
wall
WALL
Bottom
centerline
AXIS
Theseboundarytypesarestillretainedeveniftheedgesareremeshedsincetheedgesthemselveswerenotdeleted.Toverifythis:
OperationToolpad>ZonesCommandButton
>SpecifyBoundaryTypes
CheckthatthefollowingisintheName/Typelist:
Additionally,clickonshowlabels.YoushouldnowbeabletoseeeachoftheboundarynamesontherespectiveedgesintheGraphicsWindow.Verifythattheboundarytypesspecificationiscorrect.
SaveandExport
Asintheprevioustutorial,wewillnowsaveandexportthemesh.
MainMenu>File>Save
MainMenu>File>Export>Mesh...
Typeinpipe100x30.mshfortheFileName:
.SelectExport2dMeshsincethisisatwo-dimensionalmesh.ClickAccept.
Checkthatpipe100x30.mshhasbeencreatedinyourworkingdirectory.
ExitGAMBIT:
MainMenu>File>Exitandsavethesession.
Step4:
SetUpProbleminFLUENT
Ifyouhaveskippedthepreviousmeshgenerationsteps1-3,youcandownloadthemeshbyright-clickingonthislink.Savethefileaspipe100x30.msh.Youcanthenproceedwiththeflowsolutionstepsbelow.
LaunchFLUENT
LabApps>FLUENT6.3.26
Select2ddp(2D,double-precisionversion)fromthelistofoptionsandclickRun.
ImportFile
MainMenu>File>Read>Case...
Navigatetoyourworkingdirectoryandselectthepipe100x30.mshfile.ClickOK.
ThefollowingshouldappearintheFLUENTwindow:
Checkthenumberofnodes,faces(ofdifferenttypes)andcells.Thereare3000quadrilateralcellsinthiscase.Thisiswhatwe'dexpectsinceweused30divisionsintheradialdirectionand100divisionsintheaxialdirectionwhilegeneratingthegrid.Sothetotalnumberofcellsis30*100=3000.
Also,takealookunderzones.Wecanseethefourzonesinlet,outlet,wall,andcenterlinethatwedefinedinGAMBIT.
Grid
First,wecheckthegridtomakesurethattherearenoerrors.
MainMenu>Grid>Check
Anyerrorsinthegridwouldbereportedatthistime.Checktheoutputandmakesurethattherearenoerrorsreported.Thenselect:
MainMenu>Grid>Info>Size
Thefollowingsummaryaboutthegridshouldappear:
Let'slookatthegrid:
MainMenu>Display>Grid...
Makesureall5itemsunderSurfacesareselected.ThenclickDisplay.Rememberthatwecanzoominusingthemiddlemousebutton.Zoominandadmirethegrid.Howmanydivisionsarethereintheradialdirection?
RecallthatyoucanlookatspecificcomponentsofthegridbychoosingtheentitiesyouwishtoviewunderSurfaces(clicktoselectandclickagaintodeselectaspecificboundary).ClickDisplayagainwhenyouhaveselectedyourboundaries.Usethisfeatureandmakesurethattheboundarylabelscorrespondtothecorrectgeometricentities.
ClosetheGridDisplayWindowwhenyouaredone.
DefineSolverProperties
MainMenu>Define>Models>Solver
ChooseAxisymmetricunderSpace.Asinthelaminarpipeflowtutorial,we'llusethedefaultsofsegregatedsolver,implicitformulation,steadyflowandabsolutevelocityformulation.ClickOK.
MainM
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