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© 2006 Fluent Inc.

Introductory GAMBIT TrainingGAMBIT 2.3 June 2006

Volume Meshing

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ApproachA high-quality hex mesh is generally preferred over a tet mesh.

Reduced discretization error and false numerical diffusion for a given mesh size.Significantly lower cell count

Example:Compare the cell count for a 10×10×10 cube using hex and tet with a cell size of 1.

Hex mesh generates 1,000 cells.Tet mesh generates 7,726 cells!

For a hex mesh, geometries typically need to be decomposed into simpler ones so that one of the hex meshing schemes can be used.In some cases, the geometry can be very complex.

Hex meshing can be expensive or impractical.In these cases, a tet or hybrid mesh is preferred in order to reduce meshing effort.

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Volume MeshingUpon 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 combinationsHex

Map, Submap, Tet Primitive, Cooper, StairstepHex/Wedge

CooperTet/Hybrid

TGrid, HexCore

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Volume Meshes - Hex ExamplesHex – Map

Hex -- Submap

Hex – Tet Primitive

Hex – Cooper

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

Tet/Hybrid: TGrid

Tet/Hybrid: HexCore

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Hex Meshing – MapA mappable volume:

Is a logical cubeHas all faces either mappable or submappableHas topologically matching mesh on all faces.

submap face

Mesh

Mesh

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Hex Meshing – SubmapA submappable volume:

Has all faces either mappable or submappable.Has topologically matching opposite faces.

Mesh

Mesh

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Hex Meshing – Tet Primitive Tet-Primitive scheme

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

How the tet primitive scheme worksConnect center points on edges, faces and the volumeMesh the four subvolumes using the map scheme.

Mesh

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Hex Meshing – CooperThe Cooper scheme 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 need not be premeshed.At least one source face must not be meshed and must span the entire cross section.

Faces that intersect the source faces are referred to as side faces.Side faces must be either mappable or submappable

Cooperdirection

Source Faces Side Faces (two hidden)

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Cooper Examples

Volume Containing

Multiple Holes

source faces

source faces

source faces

source faces

Multiple Source Faces and Multiple Interior

LoopsSource Faces Not

Parallel

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Cooper Tool MethodologyWhen 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 verify that GAMBIT has chosen the correct faces.If necessary, change the source faces selection.

GAMBIT may not be able to resolve the 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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Troubleshooting the Cooper ToolA

B

C

Problem:Source faces A, B, and C are premeshed. The Cooper tool fails. Why? How can this volume be meshed?

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Troubleshooting the Cooper ToolA

B

C

Solution:The mesh on source faces A and B cannot be projected onto face C (the source faces are overconstrained. Delete the mesh on face C in order to generate the volume mesh.

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Troubleshooting the Cooper Tool

A

B

C

Problem:A brick is split as shown. The Cooper tool fails. Why? What can be done to generate a volume mesh?

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Solution:Cooper tool fails because no logical axis exists. If faces A and B are source faces, then face C must be either mappable or submapple. Face C contains a void and can only be paved. Split the volume with a face as shown. Use Face A1 as one source face for volume 1 and use face C2 as one source face for Volume 2.

A1

Volume 1

Volume 2

C1

A

B

C

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A

B

Interior loops

Problem:The Cooper tool fails because the interior loops on source faces A and B either overlap or are close.

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A

B

A1 A2

Interior loops

Solution:Split source face A as shown. Neither face A1 nor A2 contain closed interior loops.

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How to Make a Volume CooperableThree options to use the Cooper Tool:

Manually change vertex types on the side faces to make them mappable or submappable.Manually select the source faces. GAMBIT will attempt to make side faces mappable or submappable.Enforce the map or submap scheme on the side faces.

Example: manually change the vertex types

3 Source FacesS

E

S

E

C C

E

E

E

E

E

E

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Tet/Hybrid MeshingTetrahedral/Hybrid Mesh Scheme - TGrid

Most volumes can be meshed without decomposition, regardless of complexity.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 transition layer of pyramids.

Tet: TGrid

3 Hex/WedgeCooper

21 Hex Cooper

Pyramidlayer

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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 appropriately sized triangular mesh.

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

Low-quality pyramid

Prism layer

small angle

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HexCore MeshingCombines Tet/Hybrid mesh with Cartesian mesh in the core.Fewer cells with full automation and geometric flexibility.Important HexCore defaults:

Hexcore_Offset_LayersThe number of offset layers (cell layers between wall and hexahedral core); default value is 3.Hexcore_Quad_Surface_SplitControls quad/tri splitting and eliminates pyramid cells when turned on; see AppendixHexcore_MethodControls the method used to create HexCore –Standard or TGrid HexCore.

TGrid HexCore requires specification of buffer layers.

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Assigning Boundary and Continuum TypesBoundary 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

Continuum Type formContinuum types are defined in a similar way as boundary types.Multiple fluid/solid zones can be defined.Unspecified continuum zones are always assigned the fluid type.

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Example: Flow over a Heated Obstacle

BoundaryName = inlet

Type = VELOCITY_INLET

ContinuumName = obstacle

Type = SOLID

BoundaryName = outlet

Type = PRESSURE_OUTLET

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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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Meshed Size Function from Boundary Layer Cap

Meshed Size Function starting from boundary layer cap improves size transition between the boundary layer and volume mesh.

Useful for external aerodynamics applications.Specify the Growth Rate and Max. Size for the mesh growing from the last prism layer into the volume.Example: 3D wing profile with 12 boundary layers; the meshed size function is used for smooth transition to the tetvolume mesh.

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Hex-Core Meshing – Surface Split Options

Geometry: CylinderEdit Default: Mesh.Cartesian.Hexcore_Quad_Surface_Split

1 (default)Split boundary quad into 2 triangleshanging edges created (NOT allowed in FIDAP)Smooth boundary hexes with larger hexcore

0Boundary quads are NOT splitPyramid (transition) elements createdBoundary hexes not smoothed

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FIDAP 8 Example: Flow over a Heated Obstacle

Continuum: Name = step

Type = SOLID

Boundary: Name = outlet

Type = PLOT

Boundary: Name = outlet

Type = PLOT

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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 FIDAP

Edge 3-node 3-node

Face 8-node quad 9-node quad

Volume 21-node brick 27-node brick

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