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Introductory GAMBIT Notes

GAMBIT v2.1 Jul 2003

Fluent User Services Center

www.fluentusers.com

Volume Meshing

and the

Size Function

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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 to

be decomposed into simpler ones so that one of the hex meshingschemes can be used.

In some instances, some geometries may be too complex and

decomposition for hex meshing is impractical or impossible. In these

instances use a tet/hybrid mesh.

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Volume Meshing

Volume Meshing Form:

Upon picking a Volume

GAMBIT will automatically choose a Type based on the solver selected andthe combination of the face Types of the volume.

In ambiguous cases, GAMBIT chooses the Tet/Hybrid: TGrid combination

Available element/scheme type combinations Hex

Map

Submap

Tet-Primitive

Cooper

Stairstep Hex/Wedge

Cooper

Tet/Hybrid

Tgrid

Hex-Core

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Volume Meshes - Hex Examples Hex: Map

Hex: Submap

Hex: Tet-Primitive

Hex: Cooper

Hex: Stairstep

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Hex/Wedge and Tet/Hybrid Examples Hex/Wedge: Cooper

Tet/Hybrid: Tgrid

Tet/Hybrid: Hex-Core

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Hex Meshing - Map

Volumes that are mappable by default:

A logical cube

All faces map-able (or Submap-able) and mesh is matching

Map Scheme

mesh

mesh

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Hex Meshing - Submap

Volumes that are Submap-able by default:

All faces map-able or submap-able

Topological matching of opposite faces

Submap Scheme

mesh

mesh

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Hex Meshing - Tet-Primitive

All hex elements in a four-sided (tet) volume

Volumes directly meshable using Tet-Primitive scheme

How the Tet Primitive Scheme works

Connect center points on edges, faces and the volume

Map the four sub-volumes

Tet-Primitive scheme

Tet Primitive

Mesh

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Hex Meshing - Cooper

The Cooper Scheme, in essence, projects or extrudes a face mesh (or a setof 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 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.

Cooper direction

Source Faces Side Faces

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Permissible Cooper Geometries

source faces

source faces

Volume containing

multiple holes

Multiple source faces and

multiple interior loops

Source faces are not

parallel to each other

source faces

source faces

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Steps to Use the Cooper Tool

When the Cooper scheme is selected, a source

face list box appears in the panel.

IfGAMBIT chooses the sources faces

Check the source face list and visually check for

an intelligent selection

If necessary, change the source faces selected by

GAMBIT.

IfGAMBIT fails to pick a set of source faces

Manually select the source faces

If necessary, manually change the vertex types

(discussed in lecture 6) on some of the side faces

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Getting the Cooper Tool to Work (1)

Problem: Mesh on Source

Faces A and B can not be

projected onto mesh on Source

Face C

Work around: Remove Mesh on

Face C.

As a general rule, do not

premesh all of the source faces.

A

B

C

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Problem: "Close" interior loops onopposing source Faces A and B

The Cooper tool fails if the interiorloops (when projected onto asingle face) intersect or are "close".

Work around: Split Face A.Neither of the faces A1 and A2have interior loops.

A

B

Interior

loops

A1 A2

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Problem: No logical cylinderexists: If Faces A and B aresource faces, then Face C mustbe either mappable orsubmapple. Face C has a void

and can only be paved.

Work around: Split the Volume with

a Face. Use Face A1 as one source

face for Volume 1 and use Face C2 as

one source face for Volume 2.

A

B

C

A1

C2

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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 process

is not connected to original volume.

Assignment of continuum and boundary zone

types must be applied to faceted volume.

The Stairstep scheme can be used for quick

mesh when boundary mesh is less important.

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Tetrahedral/Hybrid Meshing

Tetrahedral/Hybrid Mesh Scheme - TGrid

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.

Hex mesh first

Tet mesh second

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Tet/Hybrid Meshing: Troubleshooting

Quality 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.

low quality pyramid

prism layer

acute angle

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Hex - Core Meshing

Tetrahedral/Hybrid Mesh Scheme

Hex - Core

Combines Tet/Hybrid mesh with core

Cartesian mesh

Fewer cells with full automation and

geometric flexibility

Non Conformal Meshes Created:

With Size Functions

With Defaults (split quads into tri

elements)

Quad on boundary of

hex core

Split into 2

Triangles

Hanging edge

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Hex - Core: No Size Function

1 (default)

Split boundary quad into

2 triangles

hanging edges created

(NOT allowed in FIDAP)

Smooth boundary hexes 0

Boundary quads are NOT

split

Pyramid (transition)

elements created

Boundary hexes notsmoothed

Geometry: Cylinder

Edit Default: Hexcore_Quad_Surface_Split = 1 (default) or 0

Hex Core Tets Pyramids

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Hex - Core: With Size Function

Hanging nodes created


Geometry: Elliptical Cylinder

Edit Default: Hexcore_Quad_Surface_Split = 0

Hex Core Tets Pyramids

Boundary cells split

at transition only


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Size Functions Size Function controls mesh distribution in a region of space

(Edges, Faces, and Volumes) in a manner analogous to the way

grading controls mesh distribution on edges.

Size Function accessed through Toolbar:

Size Functions are designed to improve mesh with Tets even

though they can be used with a hex mesh

Multiple Size Functions: Curvature and Proximity

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Size Function Types

Size Function requires the specification ofType, Entities, and Parameters.

Size Function "Type" controls method bywhich scope of sizing function is obeyed.

Fixed

Scope is defined as a fixed region abouta source.

Curvature

Scope is defined as a region near highlycurved surfaces.

Proximity

Scope is defined as a region within aspecified distance from objects.

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Sizing Function Definition

Each Size Function Type requires the specificationof:

Entities

Source entity defines shape and location of the

"origin" of affected region.

Attachment entities host the mesh that will beaffected.

Parameters

Three Parameters define the characteristics of the

size function

The two parameters common to all three size

function types are the Growth rate and Size limit.

The third parameter is different for each of the

three size function types.

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Fixed Size Function - Source

Source Can be vertices, edges, faces, or volumes

Can be internal or external to attachment entities

Source entity defines shape of scope

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Size Function - Attachments

The attached entities host mesh to be affected.

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Fixed Size Function

Parameters

Start size: Size adjacent to the source

Growth rate: Ratio of two adjacent mesh-element edge size

Size limit: Maximum allowable size forattachment entity

Small growth rate Large growth rate

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Curvature Size Function

Source Entities can only be Faces

Parameters

Angle: Specifies the maximum allowableangle () between outward pointingnormals for any two adjacent mesh

elements

Growth rate and Size limit: same as forFixed

LargeSmall

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Proximity Size Function

Specifies number of cells in face gap (3D) and

edge gap (2D)

Parameters

Cells per gap : number of mesh layers in the gap

Growth rate and Size limit: same as for fixed Limitations

Becomes slow on large models

Improper use may result in abrupt change in size

Solutions

Use multiple size functions Increase resolution by changing the defaults for

background grids

Cells/gap = 2

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