fluent5
TRANSCRIPT
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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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
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 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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
Volume Meshes - Hex Examples Hex: Map
Hex: Submap
Hex: Tet-Primitive
Hex: Cooper
Hex: Stairstep
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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
Hex/Wedge and Tet/Hybrid Examples Hex/Wedge: Cooper
Tet/Hybrid: Tgrid
Tet/Hybrid: Hex-Core
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Introductory GAMBIT Notes
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Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
Getting the Cooper Tool to Work (2)
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
Getting the Cooper Tool to Work (3)
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
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 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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Fluent User Services Center
www.fluentusers.com
Hex - Core: With Size Function
Hanging nodes created
(NOT allowed in FIDAP)
Geometry: Elliptical Cylinder
Edit Default: Hexcore_Quad_Surface_Split = 0
Hex Core Tets Pyramids
Boundary cells split
at transition only
(NOT allowed in FIDAP)
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Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Fluent User Services Center
www.fluentusers.com
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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Fluent User Services Center
www.fluentusers.com
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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GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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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Introductory GAMBIT Notes
GAMBIT v2.1 Jul 2003
Fluent User Services Center
www.fluentusers.com
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