GAMBIT 4 Volume Meshing and the Sizing Function

Introductory GAMBIT NotesGAMBIT v2.0 Jan 2002

Fluent User Services Center

www.fluentusers.com

Volume MeshingVolume Meshingand the

Sizing FunctionSizing Function

© Fluent Inc. 6/3/20124-1



www.fluentusers.com

Approach

To potentially reduce discretization errors, and to reduce cell count, a ‘high’ quality hex mesh is preferred.

For a hex mesh complicated geometries (volumes) typically need toFor a hex mesh, complicated geometries (volumes) typically need to be decomposed into simpler ones so that one of the hex meshing schemes can be used.In some instances, some geometries may be too complex and , g y pdecomposition for hex meshing is impractical or impossible. In these instances use a tet/hybrid mesh.

© Fluent Inc. 6/3/20124-2



www.fluentusers.com

Volume MeshingVolume Meshing Form:

Upon picking a VolumeGAMBIT will automatically choose a Type based on the solver selected and the combination of the face Types of the volume.In ambiguous cases GAMBIT chooses the Tet/Hybrid: TGrid combinationIn ambiguous cases, GAMBIT chooses the Tet/Hybrid: TGrid combination

Available element/scheme type combinationsHex

MapMapSubmapTet-PrimitiveCooperStairstep

Hex/WedgeCooper

© Fluent Inc. 6/3/20124-3

Tet/HybridTGrid



www.fluentusers.com

Volume Meshes - Hex ExamplesHex: Map Hex: Cooper

Hex: Submap Hex: Stairstep

Hex: Tet-Primitive

© Fluent Inc. 6/3/20124-4



www.fluentusers.com

Hex/Wedge and Tet/Hybrid Examples

Hex/Wedge: CooperHex/Wedge: Cooper

Tet/Hybrid: TGridy

© Fluent Inc. 6/3/20124-5



www.fluentusers.com

Hex Meshing - Map

Volumes that are mappable by default:Map Scheme

Volumes that are mappable by default: A logical cube All faces map-able (or Submap-able) and mesh is matching

mesh

mesh

© Fluent Inc. 6/3/20124-6



www.fluentusers.com

H M hi S bHex Meshing - SubmapVolumes that are Submap-able by default:

Submap Scheme

All faces map-able or submap-able Topological matching of opposite faces

mesh

mesh

© Fluent Inc. 6/3/20124-7



www.fluentusers.com

Hex Meshing - Tet-PrimitiveTet-Primitive scheme

All hex elements in a four-sided (tet) volumeVolumes directly meshable using Tet-Primitive scheme

Tet Primitive

Mesh

How the Tet Primitive Scheme worksConnect center points on edges, faces and the volume

© Fluent Inc. 6/3/20124-8

Map the four sub-volumes



www.fluentusers.com

Hex Meshing - CooperThe Cooper Scheme, in essence, projects or extrudes a face mesh (or a set of face meshes) from one end of a volume to the other and then divides upof face meshes) from one end of a volume to the other and then divides up the extruded mesh to form the volume mesh.

The projection direction is referred to as the Cooper direction.Faces topologically perpendicular to this direction are called Source facesFaces topologically perpendicular to this direction are called Source faces.

Source faces do not have to be premeshed.In practice, at least one source face must not be meshed and must span across the entire cross section.

Faces that intersect the source faces are referred to as Side faces.Side faces must be Mappable or Submappable.

Source Faces Side FacesS de aces

© Fluent Inc. 6/3/20124-9

Cooper direction



www.fluentusers.com

Permissible Cooper GeometriesCooper Scheme - permissible geometries

A volume with multiple source faces on multiple sidesHoles or “wells” are allowed

source facessource faces

source faces

source faces

© Fluent Inc. 6/3/20124-10



www.fluentusers.com

I i ibl C G iImpermissible Cooper Geometries

Cooper Scheme -pimpermissible geometries

(a) Cannot construct logical cylinder, the side faces of which is mappable(b) All source faces are meshed( ) C C

(a)

(c) Cannot use Cooper (readily) with multiple source faces on opposing ends.

A

B

C

© Fluent Inc. 6/3/20124-11

(c)(b)



www.fluentusers.com

C A li tiCooper ApplicationCooper scheme - Application

When the Cooper scheme is selected, a source face list box appears in the panel.GAMBIT will automatically select all source faces for direct Cooper-able volumes (scheme comes up as default).If GAMBIT chooses the sources faces you should check the source face list andIf GAMBIT chooses the sources faces, you should check the source face list and visually check for an intelligent selection. Change, if necessary.If GAMBIT fails to pick a set of source faces, you can either manually change the vertex types on the side faces or manually select the source faces.yp y

© Fluent Inc. 6/3/20124-12



www.fluentusers.com

Hex Meshing - Stairstep

The Stairstep scheme creates a single-block structured mesh.The Stairstep scheme creates and meshes a faceted volume the shape of which approximates the volume to be meshed.

The original volume is not meshed.Faceted volume:

is generated as a result of the meshing processis not connected to original volume.

Assignment of continuum and boundary zonetypes must be applied to faceted volume.

The Stairstep scheme can be used for quickmesh when boundary mesh is less important.

‘Body-fitted’ boundary mesh willb i l t d i f t

© Fluent Inc. 6/3/20124-13

be implemented in future.



www.fluentusers.com

Tetrahedral/Hybrid MeshingTetrahedral/Hybrid Mesh Scheme - TGridy

Automatic - most volumes can be meshed without decomposition.Use boundary layers to create hybrid grids (prism layers on boundaries to capture important viscous effects).Using on volumes that are adjacent to volumes that have been meshed with hex elements will automatically result in a transitional layer of pyramids.

Tet mesh second

© Fluent Inc. 6/3/20124-14

Hex mesh first



www.fluentusers.com

Tet/Hybrid Meshing: TroubleshootingQuality of the tetrahedral mesh is highly dependent on the quality ofQuality of the tetrahedral mesh is highly dependent on the quality of the triangular mesh on the boundaries.

Initialization process may fail or highly skewed tetrahedral cells may result if there exists:

highly skewed triangles on the boundaries.large cell size variation between adjacent boundary triangles.small gaps that are not properly resolved with appropriate sized triangular mesh.

Difficulties may arise in generation of hybrid mesh.Cannot grow pyramids from high aspect-ratio faces.Prism and pyramid generation may not work properly between surfaces forming very acute angles.

prism layer

© Fluent Inc. 6/3/20124-15

low quality pyramid

prism layer

acute angle



www.fluentusers.com

Si i F iSizing FunctionsSizing Function controls mesh distribution in a region of space (Edges, Faces, and Volumes) in a manner analogous to the way(Edges, Faces, and Volumes) in a manner analogous to the way grading controls mesh distribution on edges.Sizing Function accessed through Toolbar:

© Fluent Inc. 6/3/20124-16

Without a Size Function With a Size Function



www.fluentusers.com

Sizing Function TypesSizing Function requires the specification g q pof Type, Entities, and Parameters.Sizing Function ‘Type’ controls method by which scope of sizing function is y p gobeyed.

FixedScope is defined as a fixed region about a source.

CurvatureScope is defined as a region near highly

d fcurved surfaces.Proximity

Scope is defined as a region within a specified distance from objects

© Fluent Inc. 6/3/20124-17

specified distance from objects.



www.fluentusers.com

Sizing Function DefinitionEach Sizing Function Type requires the specification of:specification of:

EntitiesSource entity defines shape and location of the ‘origin’ of affected regionorigin of affected region.Attachment entities host the mesh that will be affected.

ParametersDefines growth rate of cells in affected region for Fixed and Curvature Type.Defines number of cells in gaps for Proximity TType.Defines extent of affected region in Attachment entities.Defines maximum cell size allowed in affected

© Fluent Inc. 6/3/20124-18

Defines maximum cell size allowed in affected region.



www.fluentusers.com

Fixed Sizing Function - SourceSource

Can be vertices edges faces or volumesCan be vertices, edges, faces, or volumesCan be internal or external to attachment entitiesSource entity defines shape of scope

© Fluent Inc. 6/3/20124-19



www.fluentusers.com

Sizing Function - AttachmentsThe attached entities host mesh to be affected.

© Fluent Inc. 6/3/20124-20



www.fluentusers.com

i d i i iFixed Sizing Function - Parameters

S iStart size Size adjacent to the source

Growth rate Ratio of two adjacent mesh-element edge size

Distance Determine boundary of size function

Size limitMaximum allowable size for attachment entity

© Fluent Inc. 6/3/20124-21



www.fluentusers.com

C Si i F iCurvature Sizing FunctionModifies size according to geometric curvaturecurvatureSources can only be face entitiesParameters

Angle - Maximum allowable angle between outward pointing normals for any two adjacent mesh elements located immediately adjacent to the surface of a sourceadjacent to the surface of a sourceOthers are as with Fixed

© Fluent Inc. 6/3/20124-22



www.fluentusers.com

Proximity Size FunctionSpecifies number of cells in face gap (3D) and edge gap (2D)Parameters

Cells per gap - number of mesh layers in the gap.p g p y g pDistance - maximum distance from the source at which size function applies.Size limit

LimitationsBecomes slow on large modelsImproper use may result in abrupt change in sizep p y p gSolutions

Use multiple size functionsSpecify large value for distance

© Fluent Inc. 6/3/20124-23

Increase resolution by changing the defaults for background grids

GAMBIT 4 Volume Meshing and the Sizing Function

Documents

Transcript of GAMBIT 4 Volume Meshing and the Sizing Function