MESHING AND GEOMETRY.docx

MESHING AND GEOMETRY.docx

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MESHINGANDGEOMETRY

5MeshingandGeometry

TableofContents

5.1ConstructingGeometry247

5.1.1GeometryMethodsinFLOW-3D247

GeneratePrimitivesUsingtheFLOW-3DSolidsModeler248

ImportingI-DEASUniversalFiles248

ImportingSTLFiles249

ImportingANSYSTetrahedralorSurfaceTriangleFiles251

ImportingTopographicalDataFiles251

OperationsonGeometry（LimitersandTransformations）252

LimitationsAssociatedwithGeometryGeneration252

5.1.2ConstructingGeometryUsingtheGUI252

CreatingGeometry253

TransformationOptions262

ModifyingGeometryintheTreeStructure263

CreatingandModifyingBaffles265

DeletingGeometry266

5.1.3ConstructingGeometryUsingtheTextEditor266

ApplyingLimiters266

ApplyingTransformations267

CopyingComponents267

VirtualComponents268

ZeroBlockageObstacles268

ComponentProperties269

5.2MeshBuilding270

5.2.1MeshingBasics270

Definitions271

5.2.2MeshBuildingMechanics272

General272

MeshBuildingintheWorkspace273

MeshBuildingintheTreeStructure274

MeshBuildingintheTextEditor278

MeshTipsandRulesofThumbforMeshGeneration278

5.3ReviewingtheFAVORizedGeometryandMesh281

5.1ConstructingGeometry

GeometryisconstructedinFLOW-3Dbyassemblingsolidgeometric"objects"orcomponentswhich,whencombined,definetheflowregionforasimulation.Theflowgeometryisthenembeddedinthecomputationalgridbythepreprocessorwhichcomputesthefractionalareasofthegridcellfacesopentoflowandthecorrespondingfractionalvolumesofthecellsthatareopen.Thistechnique,referredtoasFAVOR™,shortforFractionalArea/VolumeObstacleRepresentation,offersasimple,accuratewaytoavoid"stair-step"boundariesandgivesagoodgeometricdescriptioneveninrelativelycoarsegrids.

TheFAVOR™methodallowscomplexgeometriestobeembeddedinthecomputationalmeshbycomputingtheareafractions（AFT,AFR,AFB）onthecellfacesalongwiththevolumefraction（VF）opentoflow.

Definitions

Therearetwokeydefinitionsforgeometryconstruction:

Components（knowninearlierversionsofFLOW-3DasObstacles）andSubcomponents（formerlyknownasRegions）.

Components—representtheportionsofagivensimulationthatwillsharecommonpropertiesandinitialconditions.Componentshavespecificproperties（e.g.,thermalconductivity,porosityandroughness）thatdistinguishthemfromeachother.Acomponentmaynotnecessarilybeacontinuousentity.Forexample,twounconnectedspherescanbedefinedasonecomponentconsistingoftwosphericalsubcomponents.

Subcomponents—representsolidobjects,holesandcomplementswithinagivencomponent,andaremadeupofimportedSTLorotherfiles,orFLOW-3Dprimitives.

Asimulationcanhavemultiplecomponents,andeachcomponentmayconsistofmultiplesubcomponents.Componentscanbeconstructedbydefiningasinglesubcomponentor,formorecomplexflowregions,byaddingandsubtractingaseriesofsubcomponents.Asinglecomponentisshownbelow.Thecomponentiscomprisedofonesolidsphericalsubcomponentandonecylindricalsubcomponentcreatedasahole.TheleftimageshowsthegeometryinBooleanview（View/Boolean）andtherightimageisshownafterapplicationoftheFavorizertool.

BooleanViewFAVORizedview

5.1.1GeometryMethodsinFLOW-3D

Specificationofflowregiongeometrycanbeaccomplishedusingtwomethods.OneapproachistoconstructthegeometrygraphicallyintheWorkspaceusingtheGUI.TheotherapproachistoentertheproperinformationinnamelistOBSintheinputfileusingtheTextEditor（Utilities>TextEditorintheGUI’smainmenu）.

TherearefivebasicmethodsfordefiningsubcomponentsinaFLOW-3Dcomputationalgrid:

primitivesgeneratedbytheFLOW-3Dsolidsmodeler,I-DEASuniversalfiles,Ansystetrahedralfiles,stereolithography（STL）files,andtopographicaldatafiles.Intheinputfile（prepin.*）,theselectionisgovernedbythevariableIGEN（L）.Combinationsofthefivemethodscanbeusedtodefineacomponent.Thefollowingdiscussiondescribesthesefivemethodsinmoredetail.

Note:

InFLOW-3D,geometryisalwaysinterpretedinCartesiancoordinatesevenifthegridhasbeendefinedincylindricalcoordinates.

Geometryfileswithspacesinthefilename（suchas“partone.stl”）cannotbeusedbytheFLOW-3Dsolver.Changethefilenamebeforeimportingthefile,toeliminatethespace.

GeneratePrimitivesUsingtheFLOW-3DSolidsModeler

TheFLOW-3Dsolidmodelerisbasedthedefinitionofregionsintermsofquadraticfunctions.Itcanbeusedtoquicklyandeasilyconstructgeometry.Sincequadraticfunctionsincludesuchprimitiveshapesasspheres,cylinders,cones,ellipsoidsandtoroids,thismodelercanbedescribedasonebasedonprimitiveshapes;butquadraticfunctionsactuallyhavemoregenerality.Theessentialideaistodefineasubcomponentbyspecifyingthecoefficientsofthegeneralquadraticfunction:

F（x,y,z）=CX2•x²+CY2•y²+CZ2•z²+CXY•x•y+CXZ•x•z+CYZ•y•z+CX•x

+CY•y+CZ•z+CRXY•

+CC

wherex,yandzareCartesiancoordinateswiththesameoriginandunitsusedtodefinethegrid.

Byconvention,thesubcomponentconsistsoftheregionwherethefunctionF（x,y,z）evaluatestoanegativevalue.Changingthesignofallcoefficientswill,therefore,definethecomplementarysubcomponent.

SubcomponentsaredefinedwherethefunctionF（x,y,z）islessthanzero.

Additionally,bysettingIOH（L）=0,thesubcomponentbecomesaholewhereF<0.

Thedefaultvalueforallcoefficientsiszero,exceptfortheconstanttermCC,whichdefaultstoaverysmallnegativenumber.Thissettingallowsonetosimplyusethecoordinatelimiterstodefinesimpleshapes（e.g.,blocks,cylindersandspheres）withouthavingtosetanyfunctioncoefficients.Whencombinedwithscaling,translationsandrotations,thisfeaturebecomesquitepowerful.

ImportingI-DEASUniversalFiles

FLOW-3DuserscanimportCADdatageneratedintheformoftetrahedralelements.TheonlydataneededbyFLOW-3Darethenodecoordinatesandtheconnectivity（seethefigurebelow）.ThisdataisassumedtobeinSDRC'sUniversalFileformatfromitsI-DEASprogram,butotherformatscouldalsobeaccommodated.Thepre-processorreadsthefilespecifiedbyFIDEAS（L）='filename'.Thedefaultfilenameis"cadfnn.inp"wherennisatwo-digitrepresentationofthesubcomponentnumberL.

Alldefinedsubcomponentscanhavecoordinatelimitersaswellastranslationsandrotationsappliedtothem.

TypicalTetrahedralElement

ImportingSTLFiles

MostCADvendorshavedevelopedinterfacestoconvertsolidmodelsintoaformatusedforstereolithography.Thisformatisknownasthestereolithography,orSTL,format.Fileswritteninthisformatusuallyhaveextension.STL.

IntheSTLformat,thesolidobjectsurfaceisapproximatedbytriangles.VertexcoordinatesandthefacetnormalforeachtrianglearestoredintheSTLfile.Thenormalisusedtodistinguishtheinteriorfromtheexterioroftheobject.Specifically,theunitnormalalwayspointsawayfromtheobject.AllcoordinatesaredefinedinaCartesiancoordinatesystem.

Theobjecttobeapproximatedmustberepresentedbyclosedsurfaces,eachunambiguouslydefininganenclosedvolume.Thatis,themodeldatamustspecifytheinside,outsideandboundaryoftheobject.

AnSTLfilecanbeeitherACSIIorbinary.TheASCIIformatfileisusuallysubstantiallylargerthanthebinaryfilebutitallowstheusertoseethedataandmodifyitifneedbe.ThegeneraloutlineoftheASCIIformatisshownbelow:

solid

facetnormal-1.00.00.0

outerloop

vertex1.01.01.0

vertex1.01.02.0

vertex1.02.01.0

endloop

endfacet

facetnormal0.0-1.00.0

outerloop

vertex

endloop

endfacet

endsolid

Eachfilesectionstartingwithfacetnormal...andendingwithendfacetdescribeonetriangle.Thefirstandlastlinesofthefilecontainanoptionaltitlefortheobject.Theorderinwhichthetrianglesarelistedisunimportant.

Triangleisdefinedbyvertexcoordinatesandthenormaln.

FLOW-3DcanimportboththeASCIIandbinarySTLformat.Theformattypeisdetectedbythepre-processorautomatically.STLinputdataisrequestedinPREPIN.INPfileinnamelistOBSinasimilarwayasitisdonefortheI-DEASCADdata.Namely,ifsubcomponentnumberListobedefinedbySTLdata,thenthefollowinginputhastobespecified

IGEN（L）=3

ThecorrespondinginputfileisspecifiedusingFSTL（L）='filename'.Thedefaultfilenameis"stlfnn.inp",wherenncorrespondstothesubcomponentnumber,e.g.,forL=1nn=01andforL=12nn=12.

Themainrequirementforthesurfacedefinedineachfileisthatitbeclosed.Eachfilecancontainmorethanoneclosedsurface.ThetotalnumberoftrianglesineachSTLfileisunlimited.

OncetheSTLinputhasbeendefined,thespecifiedsurfacecanbemanipulatedinthesamewayasanyotherregion,thatis:

∙Spatiallimitscanbeimposed;

∙Thesurfacecanbescaled,rotatedandtranslated;scalingisusefulforunitconversion;

∙Thesurfacecanbecombinedwithothergeometricalinputtocreateacompletecomponent,e.g.,holescanbecreatedintheobjectdefinedbythesurfaceortheobjectitselfcandefineaholeinapreviouslydefinedobject;

∙Physicalpropertiescanbeassignedtoeachcomponent（ingeneralconsistingofseveralsubcomponents）individually.

EachtriangleisprocessedonlyonceintheordertheyappearintheSTLfile,sotheCPUtimeislinearlyproportionaltothenumberoftriangles.Theprocessingtimeisalsolinearlyproportionaltothenumberofcellsinthegrid.Calculationsforeachtriangleonlyinvolvethosemeshcellsthatlieinthevicinityofthe