參考文稿2021-06學(xué)習(xí)

1、Lecture 4 Unstructured Volume MeshingIntroduction to ANSYSICEM CFDGeneral ProcedureLoad/create surface meshAs in Shell Meshing chapterFor Delauney, Advancing Front, ANSYS TGrid, Hex-DominantEither of these types run from geometry will automatically create surface mesh using global and local Shell Me

2、sh settings without any user input/editingIf in doubt, run Shell Mesh first, then from “existing mesh”Compute MeshMesh Compute Mesh Volume MeshDefine density regions (optional)Applying mesh size within volume where geometry doesnt existFirst decide global volume mesh parametersGlobal Mesh Setup Volu

3、me Meshing ParametersSelect Mesh TypeSelect Mesh Method for chosen TypeSet options for specific MethodsSet mesh sizesGloballyLocallyPart/Surface/Curve Mesh SetupFor From geometry:OctreeCartesianDefine volumetric regionMaterial pointsTypically for octree on complex modelsMultiple volumes possibleComp

4、ute Prism (optional)As separate processAlso option to run automatically with tetra creationMesh TypesTetra/mixedMost used typePure tetraTetra with prism layersPrism is discussed in a later presentationPrisms from tri surface meshHexas from quad surface meshPyramids to cap off any quad faces from pri

5、sm sides or hex prism layersTetra with hex coreCan also have prismsHexa filling majority of volumeTetra (from Delaunay algorithm) used to fill between surface mesh or top of prism layers and hex corePyramids to make conformal between tetra and hexa quad facesA Hybrid mesh can be created by merging a

6、 tetra mesh with a structured hex meshPure tetraTetra/PrismTetra/Prism/HexcoreMesh Types - ContinuedHexa-DominantUses existing quad meshGood quality hex near surfaceTypically poor interior qualityTypically not suited for CFD, but good enough for static structural analysisNot covered in detail hereCa

7、rtesianMethods available in CartesianBody fittedStaircaseHexa-CoreAutomatic pure HexaRectilinear meshFastest method for creating volume meshNot covered in detail herePoor quality interiorMesh Input SourcesInput sources for tetra/mixed meshing modules1. Start from just geometryOctree tetraRobustCan “

8、walk” over small features and gaps/holesCartesianStair stepBody Fitted Cartesian Mesh sizes set on geometry or globally2. Start from a closed, watertight shell meshDelaunay and T-GridQuickAdvancing FrontSlow mesh size growth for several layers (6 or so) from surface, then fast growth after that into

9、 volumeHexa CoreHex Dominant3. Start from both geometry and shell meshOctree tetra onlyPortions of model already shell meshedShell meshed section may not be 100% respected (may adjust shell mesh some)Set mesh sizes on geometry of unmesh portionsVolume filling methodsCreate BodyMaterial point and bod

10、yMaterial point used by tetra octree to instruct which volume regions to keepVolume elements will be in the same part as the material pointUsed in hexa blocking as a part for placing blocksDelaunay, Adv Front, and Tgrid also use this if Flood fill after completion is on in global settingsMaterial po

11、int method is most robustBy Topology method automatically creates a material point in every closed volumeRequires build diagnostic topology first to determine connectivityCan save you the work of creating a lot of material points for each regionAny regions not completely closed (yellow curves indica

12、ting gaps/holes) will not get a material point so this is less robustMesh Methods - Octree Type - Tetra/MixedMethod - Robust (Octree)Same as Shell Meshing Patch IndependentExcept volumetric tetras are keptAdvantagesGood choice for complex and/or dirty geometryLittle or no geometry cleanup necessaryN

13、o time wasted with detailed shell meshingDont want to spend time defeaturing geometryDisadvantageIntensive on CPU usage and memoryJust set appropriate mesh sizes on geometryGlobal sizes (max size, curvature/proximity based)By parts (Part mesh setup spreadsheet)SurfacesCurvesReview Shell Meshing chap

14、terPart/Surface/Curve Mesh SetupOctree Tetra Process The Octree process creates an initial mesh of “Maximum size” elements which fills a bounding region completely encapsulating the geometry.Initial conditionsGeometry including surfaces, curves and points (from Build Topology) Mesh size set globally

15、 and/or on surfaces/curves/densitiesOptional material point could also be createdAll saved in the tetin fileOCTREE SubdivisionTetra Process, ContdMesh then subdivided to meet the entity size parametersFactor of 2 in 3-dimensions, hence the name OctreeNodes are projected to model surfaces, curves, an

16、d pointsAutomatic “flood fill” process finds volume boundaries Initial element assigned to part name of material pointAdjacent layers added to same part until boundary surfaces are reachedElements outside the domain are marked into a reserve part name called ORFN, then deletedFlood fill User defined

17、 volumes kept ORFN region is discardedMaterial point2:1 growth ratio because edges get split in halfTetra Octree Process, ContdSmoothOctree mesh is initially composed of regular right angle tetrasSince nodes are simply moved to nearest geometry, smoothing is necessarySmoother can be set to run autom

18、atically to improve quality in Global Mesh SetupOr run afterwards: Edit Mesh - Smooth Mesh GloballyOctree Method CharacteristicsGeometryMesh detailMeshResulting mesh is independent of the underlying arrangement of surfacesNot all surface edges need to be captured!Surfaces edges only captured if curv

19、e exists thereDelete curves to ignore hard edgesOr “filter” points/curves under Build Diagnostic TopologySliver ignoredVolume first generated independent of surface model and nodes projected onto surfacesSurface mesh is created when outside tetras are cut awayGeometry Requirements for Octree TetraTe

20、tra requires a reasonably enclosed surface modelRun Build Diagnostic Topology to find gaps/holesOctree can tolerate gaps smaller then the local element size (approximately 1/10th the element size or less)Keep points and curves at key features and hard edgesFilter curves and points by angle with Buil

21、d Diagnostic TopologyCreate Material points to define volumesWill material point(s) if none exists (named CREATED_MATERIAL#)Set Global, Part, Surface, Curve Size ParametersSimilar to Shell Meshing sectionMissing inlet surfaceGeometry Repair tools quickly locate and fix these problems.Hole highlighte

22、d by yellow single edge curveRunning tetra octree shows leakage pathUsing Points and Curves with Tetra OctreeCoarse mesh walks over detail in surface modelMesh captures detail Curves and points not included Mesh size specified only on surfaces Curves and points included Mesh size specified on curves

23、 and surfaces Curves and points affect which features are captured by the mesh! Build Topology easily creates the necessary points and curves easily with filter by angleCompute Mesh Tetra OctreeRun options: Compute Mesh Volume Meshing ParametersCreate Prism LayersWill create prisms marked under Part

24、 Mesh SetupImmediately after tetra calculationPrism layers grown into existing tetra meshCreate Hexa-CoreWill retain tri surface mesh (or tri and prisms), throw away tetra mesh and regenerate volumeFill volume interior with Cartesian hexasCap off hexas with pyramidsMap tetra to tri or top prism face

25、 with Delaunay filling algorithmInputSelect GeometryAll, VisiblePart by PartMeshes each part separatelyMesh not conformal between partsFrom fileSelect tetin file (save memory by not loading it)Use Existing Mesh PartsSelect Parts that are already surface meshedUses Make Consistent to match octree vol

26、ume mesh to existing surface meshCurvature/Proximity Based RefinementCurvature/Proximity Based RefinementOctree and Patch Independent Surface MesherAutomatically subdivides to create elements that are smaller than the prescribed entity size in order to capture finer features Min size limit value ent

27、ered is multiplied by the global Scale Factor and is the minimum size allowed for the automatic subdivisionUsed primarily to avoid setting up meshing parameters specifically for individual entities thus allowing the geometry to determine the mesh sizeConvenient for geometry with many fillets of vary

28、ing curvaturePrescribed size is adequate herePrescribed element size: Surface/Curve Max. Element Size times Scale FactorAuto subdivision at tighter radius of curvatureMin Size Limit: multiplied by Scale Factor = global minimumCurvature Based RefinementRefinementApproximate number of elements along c

29、urvature if extrapolated to 360oTo avoid subdivision always to global minimum which would otherwise result in too many elementsSubdivision will stop once number of elements along curvature is reachedApprox max number of edges along 360oWont exceed global minimum set by min size limit valueExampleSpe

30、cified refinement achieved with larger elementsGlobal minimum (min size limit) not realized, not necessary to capture curvaturePrescribed sizeMin size limitRefinement = 12Proximity Based RefinementElements in GapNumber of cells desired in narrow gapsThe default is one element in the gap to just mode

31、l the featureCan specify more than one element in gapsWill not make elements smaller than the Min size limitExampleOnly one element in gapCant go smaller than Min size limitHave to set smaller Min size limit to allow the 5 elements in gapPrescribed size Min size limitCells in Gap = 5Prescribed size

32、Min size limit (1/5th smaller)Cells in Gap = 5Workshop 4.1Workshop 4.1 Engine Block ModelBuild diagnostic topologyOctree meshMesh with surface sizesSmooth meshView cut planeCurvature/proximity refinementDelaunay mesh9/16/05Inventory #002281C1-19Thin CutsBAcIf the face of a tetra element has a surfac

33、e/line node on part “A” then it may not have a surface/line node on part “B”Note: If the surfaces of the two parts, A and B, meet, then the contact curve must be in a third part, C, or the thin cut will fail.Only works with Tetra OctreeTo avoid holes in thin solids/narrow gaps when mesh size is much

34、 larger than gap (can handle up to 10 times gap distance)Define thin cuts by selecting two parts and then Add them to the list of defined Thin cutsThe two sides of the thin cut must be in different parts2 types:1. Offset surfaces with a gap between2. Small angle ( 30o)Mesh Methods - DelaunayType - T

35、etra/MixedMethod - Quick (Delauney)Start from a good quality, closed surface meshCan be quad and tri elementsFrom Shell MeshFrom OctreeFrom imported surface meshSetup Options:Delaunay SchemeStandard: Delaunay scheme with a skewness-based refinementTGlib: TGrid Delaunay volume grid generation algorit

36、hm that utilizes a more gradual transition rate near the surface and faster towards the interiorUse AF: TGrid Advancing Front Delaunay algorithm which has smoother transitions than the pure Delaunay algorithm.Memory Scaling Factor: To allocate more memory than originallySpacing Scaling Factor: Growt

37、h ratio from surface (1 1.5 typically)Fill holes in volume mesh: Use to fill holes/voids in existing volume mesh . E.g. if bad quality region is deletedMesh internal domains: For multiple sets of closed volumes in one modelFlood fill after completion: For multiple volumes Will assign tetras within c

38、losed volume to Part designated by Body or Material PointVerbose output: For troubleshootingInitially distributes nodes so as the centroid of any tetra is outside the circumsphere of any neighboring tetraMesh Methods Advancing FrontType - Tetra/MixedMethod - Smooth (Advancing Front)Same as Quick (De

39、launey) butUses advancing front method that marches tetras from surface into interiorAlgorithm from GE/CFXResults in more gradual change in element size“Better” but finer mesh, more elements than DelaunayElements grow slowly for first few layers from surface, then growth rate increases into volume m

40、oreInput surface mesh has to be of fairly high qualitySetup Options: Do Proximity CheckingCheck to properly fill small gapsLonger run timeCan create pyramids from quadsQuads need to be a 10 aspect ratio or lessDelaunay can handle much higher quad aspect ratiosRespects densitiesMesh Methods ANSYS TGr

41、idType - Tetra/MixedANSYS TGridRuns Tgrid through an extension module Good mesh qualityFast mesh generationSetup Options: Flood fill after completion: Same as octree Flood fillVerbose output: This option writes more messages to help in debugging any potential problemWill not respect densitiesWill no

42、t mesh to quads. It converts them to trianglesSimilar to Advancing Front, but does not group elements as close near surfaceComparisonOctreeDelauneyAdv.frontANSYS TGridNote the 2:1 growth ratio and square volume elements of the octree methodThe 3 filling methods (Delaunay, Adv Front, Tgrid) allow a smoother growth (less than 2:1) than octreeDensity RegionCreate Mesh Density Define volumetric region with smaller mesh size where no geometry exists, e.g. wake region behind a wing Not actual geometry!Mesh nodes not constrained to density ob