WIXLIB

Sizing : EdgesBehaviorSoft: Sizing will be influenced by global sizing functions such as those based onproximity and/or curvature as well as local mesh controlsHard: Size control is strictly adhered to Transition between hard edges (or any edge with bias) and adjacentedgeand face meshes may be abrupt Hard edges or edges with bias will override Max Face Size and Max SizepropertiesSoftHard35Influenced by globalProximity advancedsize function.No influence from otherglobal settings

Sizing : FacesElement SizeDefines the maximum element size on the faceFace meshed withconstant elementsizeEdge curvature isresolved36

Sizing : Body (volume)Element SizeDefines the maximum cell size on the BodyTetrahedron patchconforming meshCutCellmeshWithoutbody sizingWithoutbody sizingWith bodysizing37Body meshed with maxcell size definedWith bodysizing

Sizing : Sphere of InfluenceSphere of Influence : on Vertex– Available with or without Advanced Size Functions– Sets the average element size around the selected vertex– Inputs: Sphere radius and Element size Center of the sphere is defined by a model vertex Good resolution at the vertex The element size will be applied to all entitiesconnected to the selected vertex38

Sizing : Sphere of InfluenceSphere of Influence : on Bodies– Available with or without Advanced Size Functions– Constant element size is applied within the confinesof a sphere– Use coordinate system to define the center of theSphere39

Sizing : Bodies of InfluenceBodies of influence (BOI)– Lines, surfaces and solid bodies can be used to refinethe mesh– Accessible when ASF is OnLine BOIsSurface BOIWithout BOIsThe ‘Body of Influence’ itself will not be meshed40Solid BOI

Mapped Face Meshing Creates structured meshes on selected mappable surfaces– Mapped Face Meshing with advanced control is supportedfor Sweep, Patch Conforming, Hexa Dominant Quad Dominant and Triangles– Mapped Face Meshing with basic control is supportedfor MultiZone Uniform Quad/Tri and Uniform Quad– RMB on Mesh and Show/Mappable Faces to display allmappable facesIf Mapped Face Meshing fails, ( ) icon appears adjacent to corresponding objectin the Tree outline. The mesh will still be created but will ignore this control.41v

Mapped Face Meshing: Vertex Type ‘Side’, ‘Corner’ and ‘End’ controls for vertices, to define strategyforMapping42Vertex TypeIntersecting Grid LinesAngle BetweenEdgesEnd00 — 135 Side1136 — 224 Corner2225 — 314

Mapped Face Meshing: ExampleEE EEE ESEEEESEEEEEEVertex type is dependant to the attached face the mesh can be individually controlled on all faces43

Mapped Face Meshing: Radial No. of Divisions If face is defined by two loops, then the “Radial Number of Divisions”field is activated Specify the number of divisions across the annular region Useful for creating number of layers across an annulusMapped face is swept to createpure hex mesh44

Match Control Define periodicity on faces (3D) or edges (2D) The two faces or edges should be topologically and geometrically the sameA match control can only be assigned to one unique face/edge pairMatch controls are not supported with Post Inflation AlgorithmMatch Control with Patch Independent tetrahedrons not supported yet– Two types of match controls available: Cyclic and Arbitrary– Not available for CutCell meshingMatching facemeshIf ‘Match Control’ fails, ( ) icon appears adjacent to correspondingobject in the outline Tree, however the mesh is created ignoring it45

Match Control: Cyclic Define Rotational periodicFull ModelCut BoundariesPeriodic ModelModel is symmetrical at 90 so slice the body along dotted lines in DesignModelerSelected Facesfor Match controlMatching facemesh46

Match Control: Arbitrary Two faces or edges to be matched, can be arbitrarily locatedFull Model : Tube BanksSymmetric Periodic ModelFace Mesh before ‘Match Control’Model extracted through ‘Slice’operation in DMFace Mesh after ‘Match Control’Coordinate systems needto be suitably defined atthe faces to be matched47Matching face mesh

Pinch To improve quality Pinch controlremoves small features (edges ornarrow regions) at the mesh level The Pinch feature is supportedfor the following mesh methods: Patch Conforming TetrahedronsThin Solid SweepsHex Dominant meshingQuad Dominant Surface MeshingTriangles Surface meshing– Not supported for CutCellmeshing– More details in lecture 5 “MeshQuality”48

InflationUsed to generate prism layers (as explained in Global settings chapter)Inflation layer can be applied to faces or bodies using respectively edgesorfaces as the boundaryInflation layer grown on edge boundary (red)Inflation layer grown on face boundary (red)49

Workshop 4 – Local Mesh Controls50

AppendixContents 51Edge, Face & Body Sizing OptionsSizing: Sphere of InfluenceContact SizingRefinementInflationObject generator for copying mesh settings to like bodies

Edge, Face and Body Sizing OptionsCurvature Normal AngleMaximum allowable angle that one element edge is allowed tospanAvailable only when Use Advanced Size Function is set to either On: Proximity and Curvatureor On: CurvatureYou can specify a value from 0 to 180 degrees or accept the default. (A value of 0 resets theoption to its default.)The default is calculated based on the values of the Relevance and Span Angle Center optionsGrowth RateRepresents the increase in element edge length with each succeeding layer of elements. (Forexample, a growth rate of 1.2 results in a 20% increase in element edge length with eachsucceeding layer of elements.)Available when Use Advanced Size Function is onSpecify a value from 1 to 5 or accept the defaultThe default is calculated based on the values of the Relevance and Transition optionsMust be always lower or equal the global growthrate52

Sizing : Sphere of InfluenceSphere of Influence : on Edges– Available only if Advanced Size Function is OFF– Use coordinate system to define the center of the SphereMesh on the entity and otherproximity entities that lies withinthe sphere of influence is affected53

Sizing : Sphere of InfluenceSphere of Influence : on Faces– Available only if Advanced Size Function is OFF– Elements within the sphere will have given average elemen t size– Use coordinate system to define the center of the sphere54

Contact Sizing Generates similar-sized elements on contact faces between parts– Two options Element Size. The size of the elements on contact faces respects the valueof Element Size specified Relevance. The size of the elements on contact faces are determinedinternally by spheres of influence with automatic determination of radiusand size depending on the value of specified Relevance– Not available for CutCell MeshingMesh withoutContact Sizing55

Contact Sizing Note that the mesh is still non-conformal across the contact region To insert a Contact Sizing in the Mesh tree select the contact regionfrom Contacts list and drag it to Mesh object, or use RMB on theMesh56

Refinement Valid for only for faces or edges Not available for Patch Independent Tetrahedrons, CutCell, UniformUniform Quad meshing methodsQuad/Tri Refinement is applied after the creation of mesh with rest of thesettings Refinement level can vary from 1 (minimal) to 3 (maximum) A refinement level of “1” breaks up the edges of the elements into half– The Refinement control may be automatically suppressed when use inflationdepending on how the inflation/refinement is used. See the user’s Guide for detailsResultant mesh may be of poor qualityOnly the selected face is affected and rest is almost unchanged57

InflationTransition Ratio: Attempts to match the sizeof last prism layer with thatof next Tet cellGrowth Rate: It determines the relativethickness of adjacent inflationlayersDefaults:58Transition Ratio: 0.5Growth Rate: 1.2Transition Ratio: 0.8Growth Rate: 1.2Transition Ratio: 0.27Growth Rate: 1.1Transition Ratio: 0.27Growth Rate: 1.4Transition Ratio: 0.272Growth Rate: 1.2

Object Generator Object generatorallows user to copy tree objects attached toan entity to several entities. Entities may or may not be similar. However, this works best forcopying mesh settings on similar entities. With mesh controls, it provides an easy way to assign similarcontrols to a group of objects.In this example, multiple mesh controls areused to define mesh for a bolt body59There are 4 similar bolt bodies onwhich these mesh settings can becopied easily using Object Generator

Object Generator To use the Object Generator:– In the standard toolbar, click the View Object GeneratorGenerator window.button to view the Object– In the Tree Outline, select the mesh control to becopied.– In the Geometry window, select the geometry to which the mesh control should be copied in theObject Generator window.– In the Object Generator window, select any required options and press Generate.For mesh controls requiring multiple geometry,primary set of entities should be selected in“Generate From”. The entities for rest of theinputs can be grouped in Named Selections.For example, in this case input for “Source”comes from set of source faces grouped in“Source Faces” Named Selection. This can becreated based on instance information if it exists,or similar sized faces, etc. See slides on NamedSelection Worksheet. Similarly, Named Selectioncan be provided for “Target”.60

Object GeneratorFor copying inflation settings, two sets of entities will be required, first one for “Geometry” onwhich inflation needs to be applied and second one for “Boundary”. Here, set of faces for“Boundary” is grouped in “INF” Named Selection.61

Introduction to ANSYS MeshingModule 5:62Mesh Quality

Mesh QualityWhat you will learn from this presentation 63Impact of the Mesh Quality on the SolutionQuality criteriaMethods for checking the mesh qualityTools to improve quality in MeshingPinchVirtual topology

Preprocessing WorkflowGeometry CreationORGeometry ImportSketches andPlanes3D OperationsExtrude, Revolve,Sweep, etcGeometry ImportOptionsDirect CAD/BiDirectional CADGeometryOperations3D OperationsBoolean, BodyOperations, Split,etcMeshingMeshingMethodsHybrid Mesh: Tet,Prisms, PyramidsHexa Dominant,Sweep meshingGeometryCleanup andRepairAssemblyMeshingAutomaticCleanupGlobal MeshSettingsMerge, Connect,Projection, FlowVolumeExtraction, etcLocal MeshSettingsSizing,Body/Sphere ofInfluence, MatchControl, etc64SolverCheck MeshQuality

Meshing Process in ANSYS Meshing65

Impact of the Mesh QualityGood quality mesh means that Mesh quality criteria are within correct range– Orthogonal quality Mesh is valid for studied physics– Boundary layer Solution is grid independent Important geometric details are wellcapturedBad quality mesh can cause; Convergence difficulties Bad physic description Diffuse solutionUser must Check quality criteria and improve grid if needed Think about model and solver settings before generating thegrid Perform mesh parametric study, mesh adaption 66

Impact of the Mesh Quality on theSolution Example showingdifference between amesh with cellsfailing the qualitycriteria and a goodmesh Unphysical values invicinity of poorquality cells67

Impact of the Mesh Quality on the Solution Diffusion exampleMesh 1(max,avg)CSKEW (0.912,0.291)(max,avg)CAR (62.731,7.402)VzMIN -90ft/minVzMAX 600ft/minLarge cell sizechangeMesh 2(max,avg)CSKEW (0.801,0.287)(max,avg)CAR (8.153,1.298)VzMIN -100ft/minVzMAX 400ft/min

Grid DependencyNb Cells Solution run withDP 0DP 1DP 25003 00024 000DP 0DP 3190 000 x8DP 41.5 MDP 3multiple meshes Note : For all runs thecomputed Y is validfor wall function (firstcell not in laminarzone)2%69

Grid Dependency Hexa cells can be stretched instream direction to reducenumber of cells Bias defined on inlet andoutlet walls Bias defined on inlet edges– 16 000 cells ( DP2)– Delta P 310 Pa ( DP3)70

Hexa vs. Tetra Hexa: Concentration in one directionHexa– Angles unchanged Tetra: Concentration in one direction– Angles change Prism: Concentration in one directionTetra– Angles unchanged Solution for boundary layerresolutionPrism– Hybrid prism/tetra meshes– Prism in near-wall region, tetra in volume– Automated– Reduced CPU-time for goodTetra (in volume)boundary layer resolutionPrisms (near wall)71

Mesh Statistics and Mesh MetricsDisplays mesh information for Nodes and ElementsList of quality criteria for the Mesh Metric Select the required criteria to get details for quality It shows minimum, maximum, average and standard deviationDifferent physics and different solvers have different requirements formesh qualityMesh metrics available in ANSYS Meshing include:––––––––Element QualityAspect RatioJacobean RationWarping FactorParallel DeviationMaximum Corner AngleSkewnessOrthogonal QualityFor Multi-Body Parts, go to corresponding body in Tree Outlineto get its separate mesh statistics per part/body72

Mesh Quality MetricsOrthogonal Quality (OQ)Derived directly fromOn cellOn faceA1Fluent solver discretization For a cell it is the minimum of:Ai f i Ai f i Ai ci Ai ci computed for each face iA1c1f3c3e1c2f1f2A3e3A2e2A2A3Ai eiFor the face it is computed as the minimum of Ai ei computed for each edgeIWhere Ai is the face normal vector and fi is a vector from the centroid of the cell to the centroidof that face, and ci is a vector from the centroid of the cell to the centroid of the adjacent cell,where ei is the vector from the centroid of the face to the centroid of the edgeAt boundaries and internal wallsci is ignored in the computations of OQ730Worst1Perfect

Mesh Quality MetricsOptimal (equilateral) cellSkewnessTwo methods for determining skewness:1. Equilateral Volume deviation:Skewness optimal cell size cell sizeoptimal cell sizeApplies only for triangles and tetrahedronsActual cellCircumsphere2. Normalized Angle deviation: max e e min ,Skewness max 180 ee Where e is the equiangular face/cell (60 fortets and tris, and 90 for quads and hexas)– Applies to all cell and face shapes– Used for hexa, prisms and pyramids74 max min0Perfect1Worst

Mesh QualityMesh quality recommendationsLow Orthogonal Quality or high skewness values are not recommendedGenerally try to keep minimum orthogonal quality 0.1, or maximum skewness 0.95.However these values may be different depending on the physics and the locationof the cellFluent reports negative cell volumes if the mesh contains degenerate cellsSkewness mesh metrics spectrumExcellentVery 50-0.800.80-0.940.95-0.970.98-1.00Orthogonal Quality mesh metrics spectrum75UnacceptableBadAcceptableGoodVery 70-0.950.95-1.00

Aspect Ratio2-D: Length / height ratio: δx/δyδyδx3-D Area ratio Radius ratio of circumscribed / inscribedcircleLimitation for some iterative solvers A 10 100 (CFX: 1000)Large aspect ratio are accepted wherethere is no strong transversegradient (boundary layer .)76

Mesh quality: skewness Two methods for determiningskewness:1. Based on the equilateralvolume:optimal (equilateral) celloptimal cell size - cell size Skewness optimal cell sizecircumcircle Applies only to triangles andtetrahedra. Default method for tris and tets.actual cell2. Based on the deviation from anormalized equilateral angle: Skewness (for a quad) max Applies to all cell and faceshapes. Always used for prisms andpyramids.max90 90,90min maxmin24

Equiangle skewness Common measure of quality is based on equiangle skew. Definition of equiangle skew:maxmax180e, eeminewhere:––– largest angle in face or cell.min smallest angle in face or cell.e angle for equiangular face or cell.max e.g., 60 for triangle, 90 for square. Range of skewness:0best1worst maxmin

Mesh quality: smoothness and aspect ratio Change in size should be gradual (smooth).smooth changein cell sizelarge jump incell size Aspect ratio is ratio of longest edge length to shortest edgelength. Equal to 1 (ideal) for an equilateral triangle or a square.aspect ratio 1high-aspect-ratio quadaspect ratio 1high-aspect-ratio triangle26

Striving for quality A poor quality grid will cause inaccurate solutions and/or slowconvergence. Minimize equiangle skew:– Hex and quad cells: skewness should not exceed 0.85.– Tri’s: skewness should not exceed 0.85.– Tets: skewness should not exceed 0.9. Minimize local variations in cell size:– E.g. adjacent cells should not have ‘size ratio’ greater than 20%. If such violations exist: delete mesh, perform necessarydecomposition and/or pre-mesh edges and faces, and remesh.Value ofSkewnessCell able0.80-0.95 0.95-0.99poorsliver0.99-1.00degenerate

Grid design guidelines: resolution Pertinent flow features should be adequately resolved.flowinadequatebetter Cell aspect ratio (width/height) should be near one where flow ismulti-dimensional. Quad/hex cells can be stretched where flow is fully-developedand essentially one-dimensional.Flow DirectionOK!28

Grid design guidelines: smoothness Change in cell/element size should be gradual (smooth).smooth changein cell sizesudden changein cell size — AVOID! Ideally, the maximum change in grid spacing should be 20%: xi xi 1xi 1xi1.229

Grid design guidelines: total cell count More cells can give higher accuracy. The downside is increasedmemory and CPU time. To keep cell count down:– Use a non-uniform grid to cluster cells only where they are needed.– Use solution adaption to further refine only selected areas. Cell counts of the order:– 1E4 are relatively small problems.– 1E5 are intermediate size problems.– 1E6 are large. Such problems can be efficiently run using multipleCPUs, but mesh generation and post-processing may become slow.– 1E7

5 Global mesh controls are used to make global adjustment in the meshing strategy, which includ