© Fluent Inc. 10/14/20154-1 Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002 Fluent User Services...

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Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002

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Volume Meshingand the

Sizing 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 meshing schemes 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 and

the 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

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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

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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 set

of 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 Cooper Scheme - permissible geometries

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

source faces

source faces

source faces

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Impermissible Cooper Geometries

Cooper Scheme - impermissible geometries

(a) Cannot construct logical cylinder, the side faces of which is mappable

(b) All source faces are meshed

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

(c)

(a)

(b)

A

B

C

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Cooper Application Cooper 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 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.

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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 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 willbe implemented in future.

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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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Sizing Functions Sizing 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.

Sizing Function accessed through Toolbar:

Without a Size Function With a Size Function

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Sizing Function Types

Sizing Function requires the specification of Type, Entities, and Parameters.

Sizing Function ‘Type’ controls method by which scope of sizing function is obeyed.

Fixed Scope is defined as a fixed region about

a source. Curvature

Scope is defined as a region near highly curved surfaces.

Proximity Scope is defined as a region within a

specified distance from objects.

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Sizing Function Definition Each Sizing Function Type requires the

specification of: Entities

Source entity defines shape and location of the ‘origin’ of affected region.

Attachment entities host the mesh that will be affected.

Parameters Defines growth rate of cells in affected region for

Fixed and Curvature Type. Defines number of cells in gaps for Proximity

Type. Defines extent of affected region in Attachment

entities. Defines maximum cell size allowed in affected

region.

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Fixed Sizing 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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Sizing Function - Attachments The attached entities host mesh to be affected.

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Fixed Sizing Function - Parameters

Start size Size adjacent to the source

Growth rate Ratio of two adjacent mesh-element

edge size Distance

Determine boundary of size function Size limit

Maximum allowable size for attachment entity

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Curvature Sizing Function Modifies size according to geometric

curvature Sources can only be face entities Parameters

Angle - Maximum allowable angle between outward pointing normals for any two adjacent mesh elements located immediately adjacent to the surface of a source

Others are as with Fixed

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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. Distance - maximum distance from the source at

which size function applies. Size limit

Limitations Becomes slow on large models Improper use may result in abrupt change in size Solutions

Use multiple size functions Specify large value for distance Increase resolution by changing the defaults for

background grids

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