Volume Meshing

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Introductory GAMBIT TrainingGAMBIT 2.2 February 2005

Volume Meshing

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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 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 combination

Available element/scheme type combinationsHex

MapSubmapTet-PrimitiveCooperStairstep

Hex/WedgeCooper

Tet/HybridTgridHex-Core

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Volume Meshes - Hex ExamplesHex: Map

Hex: Submap

Hex: Tet-Primitive

Hex: Cooper

Hex: Stairstep

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Hex/Wedge and Tet/Hybrid ExamplesHex/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 - SubmapVolumes 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) volumeVolumes directly meshable using Tet-Primitive scheme

How the Tet Primitive Scheme worksConnect center points on edges, faces and the volumeMap the four sub-volumes

Tet-Primitive scheme

Tet Primitive

Mesh

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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 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. If GAMBIT chooses the sources faces

Check the source face list and visually check for an intelligent selectionIf necessary, change the source faces selected by GAMBIT.

If GAMBIT fails to pick a set of source faces Manually select the source facesIf necessary, manually change the vertex types (discussed in lecture 3) 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 on opposing source Faces A and BThe Cooper tool fails if the interior loops (when projected onto a single face) intersect or are "close".

Work around: Split Face A. Neither of the faces A1 and A2 have interior loops.

A

B

Interior loops

A1 A2

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Problem: No logical cylinder exists: If Faces A and B are source faces, then Face C must be either mappable or submapple. 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.

AB

C

A1

C2

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How to Make a Volume CooperableThree options to cooper a volume:

Manually change the vertex types on the side faces so they are mappableand/or submappablePick the source facesEnforce the map or submap on the side faces

EE

S

SS

S

EE

E

E E

C

Example: manually change the vertex types

3 Source Faces

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Tetrahedral/Hybrid MeshingTetrahedral/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).Use 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: TroubleshootingQuality 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 sizedtriangular 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 MeshingTetrahedral/Hybrid Mesh Scheme –Hex - Core

Combines Tet/Hybrid mesh with core Cartesian meshFewer cells with full automation and geometric flexibilityNon Conformal Meshes Created with:

Size FunctionsHexcore_Quad_Surface_Split Default (split quads into tri elements)

The number of offset layers (cell layers between wall and hexahedral core) is controlled by the GAMBIT default Hexcore_Offset_Layers.

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Hex – Core Meshing : Surface Split

1 (default)Split boundary quad into 2

triangleshanging edges created

(NOT allowed in FIDAP)Smooth boundary hexes

with larger hexcore0

Boundary quads are NOT split

Pyramid (transition) elements created

Boundary hexes not smoothed

Geometry: CylinderEdit Default: Mesh.Cartesian.Hexcore_Quad_Surface_Split = 1 (default) or 0

Hex Core Tets Pyramids

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Assigning Boundary and Continuum Types The Boundary Type Form

Enter entities to be grouped into single zone in entity list box.

First choose entity type as face or edge.Select boundary type for zone (entity group).

Available types depend on SolverName zone if desired.Apply defines zone and boundary type.

Can also modify and delete zone/boundary.By default,

External faces/edges are wallsInternal faces/edges are interior

The Continuum Type FormSimilar operation.All continuum zones are by default, fluid.

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FLUENT 5/6 Example: Flow over a Heated Obstacle

Boundary: Name = inlet

Type = VELOCITY_INLET

Boundary: Name = outlet

Type = PRESSURE_OUTLET

Continuum: Name = step

Type = SOLID

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FIDAP 8: Example: Flow over a Heated Obstacle

Boundary: Name = inlet

Type = PLOT

Boundary: Name = outlet

Type = PLOT

Continuum: Name = step

Type = SOLID

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Defaults: Example: Flow over a Heated Obstacle

By default, the 4 remaining external faces have the Name and Type:

Boundary: Name = wall

Type = WALL

By default, the one remaining volume has the Name and Type

Continuum: Name = fluid

Type = FLUID

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Linear/Quadratic Elements (FIDAP/POLYFLOW USERS ONLY)

General toolsHigher-order elements

For FEM codes (FIDAP and POLYFLOW), the element order can be changed at all three meshing levelsOnly linear and quadratic elements are directly availableA change to quadratic element type at one level will automatically change the element type in other levels The following table presents the most commonly used and recommended quadratic element types for FEM - solvers

POLYFLOW FIDAPedge 3-node 3-nodeface 8-node quad 9-node quadvolume 21-node brick 27-node brick

Documents

Volume Meshing