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Introductory GAMBIT Notes

GAMBIT v2.1 Jul 2003

Mesh Control through

Boundary Layers

and Face Vertex Types

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Face Vertex Type Basics

All vertices that are connected to a face are

assigned initial face vertex types based on

default angle criteria between the edges

connected to the vertex.

Combination of vertex types describes theface "shape".

Face vertex types are used automatically to determine

all quad face meshing schemes except the quad-pave scheme.

The tri meshing scheme also does not use face vertex types.

Changing vertex types can help you create a structured mesh orhelp facilitate generating a hex mesh.

For the Cooper to work, the side faces must be either mappable or

submapple and changing the vertex types may be required.

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Vertex Type Characteristics

End (E)

0 < Default Angle < 120

zero internal grid lines

Side (S)

120 < Default Angle < 216 one internal grid line

Corner (C)


two internal grid lines

Reverse (R)


three internal grid lines

E

E E

S SS

CC C

R R

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Modifying Face Vertex Types

Face Vertex Types can be changed fromdefault setting:

Automatically, by enforcing certain meshingschemes in face and volume meshing.

Can sometimes result in undesirable mesh.

Manually, by direct modification inthe Face Vertex Type form.

Select Face

symbols appear in graphics window

Select New Vertex Type

Select Vertices to be affected

Vertex Types can be applied to just Boundary Layers as option. A vertex can have multiple Types; one per each associated face.

For a given set of face vertex types, Gambit will choose which meshingscheme to use based on predefined "formulas".

SE

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Formula for Map Scheme

Map Scheme: 4*End + N*Side

Periodic Map Scheme: N*Side

Project intervals can be specified for

more mesh control.

E

E

E

ES+

E E

EE

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By manually changing vertex types

In Set Face Vertex Type form, change

vertices (default) to "Side" (example)

Open the Face Mesh form and pick the face

(GAMBIT should automatically select themap scheme)

By enforcing the Map scheme

In Face Mesh form, change the scheme

from default to "Map" and "Apply"

(GAMBIT will try to change the vertextypes so the scheme is honored)

How to Make a Face Mappable

E

SS

E

S

EE

S

Map: 4*End + 4*Side

E

EE

E

C

EE

CDefault

E

E

E

Sdefault Map: 4*End

E

E E

E

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Formula for Submap Scheme

Submap Scheme: 4*End + L*Side + M*(End + Corner) + N*(2*End +Reverse)

additional terms when interior loops exist

Periodic Submap Scheme: N*Side + M*(End + Corner) where M >2

additional terms when interior loops exist

E

EE

E

C

EE

CS

C

S

E

EE

E

E

C

C

C

CC

C E

E

E

C

E

E

CC

E

C

S S+ +

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How to Make a Face Submappable

By manually changing vertex types

Consider which vertex should be

changed to "Side"

In Set Face Vertex Type form,

change vertex (default type) to

"Side"

By enforcing the Submap scheme

In Face Mesh form, change the

scheme from default to "Submap"

and "Apply"

(GAMBIT will try to change thevertex types so the scheme is

honored)

User has less control - resulting

mesh may be undesirable

R

E

E

EE

E

E

E

default

Submap: 4*End + Side

+ (2*End + Reverse)

R

E

E

SE

E

E

E

R

E

E S

E

E

E

E

?

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Tri-Primitive Scheme

Tri-Primitive Scheme: 3*End + N*Side

To mesh a face with the tri-primitive scheme:

Manually, change one of the vertex types to "Side" in this example

The Tri Primitive scheme can not be enforced

E

S

E

E

E

E

E

E

E

E

E

S

default

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Meshing Faces with Quad and Tri Pave Schemes

Quad:Pave Scheme

All vertex types of the face are ignored

The sum of all elements on the edges of the face

must be even.

Mesh inner faces first if possible to prevent fromlocking into odd number intervals on boundaries.

No guarantee of a symmetric mesh on a symmetric

geometry

Tri:Pave Scheme

No even number restriction

Use boundary layers for better mesh near boundaries

Use sizing function andedge mesh grading for

controlling cell size distribution.

Edge mesh grading alone results in poor quality

mesh.

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Meshing Faces with Hybrid Quad/Tri Schemes

Quad/Tri: Tri-Map formula: 2*Triangle

The face vertex types need to be manually

changed to Triangle (T) and the Tri-Map

scheme must be selected.

Quad/Tri: Pave All vertex types are ignored except Trielement

(T) and Notrielement (N)

Trielement (T) will enforce a triangle

Notrielement (N) will avoid a triangle

Quad/Tri: Wedge Used for creating cylindrical/polar type meshes

The Vertex marked (T) is where rectangular

elements are collapsed into triangles

T T

E

N

T

S

T

E

E

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How to Make a Volume Mappable

Three options to map a volume:

Enforce the map

Manually change the vertex types on all faces so they are mappable

Enforce the map on the faces

E

E

E

EE

E

S

manually

change the

vertex types

S Mapdefault

E

EE

E

E

E

E

E

map the faces

enforce the map

Example:

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How to Make a Volume Submappable

Three options to submap a volume:

Enforce the submap

Manually change the vertex types on all faces so they are mappable

and/or submappable

Enforce the submap on the faces

EE

C

E

ES

S

EE

C

E

ES

E

Example: manually change the

vertex types

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How to Make a Volume Cooperable Three options to cooper a volume:

Manually change the vertex types on the side faces so they are mappable

and/or submappable

Pick the source faces

Enforce the map or submap on the side faces

E

E

S

S

S

S

E

E

E

E E

C

Example: manually change the

vertex types

3 Source Faces

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Boundary Layers Boundary layers are layers of elements growing out from a boundary

into the domain.

Produces high quality cells near boundary.

Allows resolution of flow field effects with fewer cells than would be

required without them.

In general, boundary layers are attached to: edges for 2D problems

faces for 3D problems

complicated 3D shapes may require boundary layer attachments to edges.

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Create Boundary Layers

Create Boundary Layer Form Show Option: toggles display of temp. boundary layer

Useful for complicated models

B.L. can be defined using Uniform or Aspect Ratio basedalgorithm

Definition Inputs (3 of 4 inputs required)

First row: height of first row of elements (a)

(or starting aspect ratio)

Growth factor: factor for geometric series (b/a)

Rows: total number of element rows

Depth: total height of boundary layer (D)

(or ending aspect ratio) Internal continuity and Wedge corner shape

Transition Pattern

Reduces number of elements in "flow" direction.

Not to be used with tets; watch for highly skewed cells.

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Attachment

General Attach to:

Edges for 2d problems

Faces for 3d problems

Arrows point to center of associated face or volume.

Boundary layers are initially displayed in orange to indicate that it is temporary. Temporary boundary layers update immediately with change in definition.

Boundary layer becomes permanent

(displayed as white) uponApply.

Edges

Boundary layer mesh in region nearvertices is defined by vertex type.

End: mesh overlaps

Side: angle bisected

Corner: angle divided into thirds

Reverse: angle divided into fourths.

E

E

ES

CE

E

E

R

E

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Boundary Layers and End Vertex Type

BL mesh butts up

against adjoining

edge

BL mesh uses

adjoining edge'smesh if pre-existing

If BL is attached to

adjoining edge on a

face, a block of

"overlap" elements is

created where thetwo BL meshes meet

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Wedge Corner Shape

At corner or reversal vertices, wedge corner shape option is

applicable.

ONWedge shape

OFF Block shape

ON OFF

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Attachment to Faces

A boundary layer attached to a face may "imprint" the adjoining faces.

The imprint is displayed graphically on adjoining faces.

Imprinting depends upon b.l. attachments of adjoining faces and state of

internal continuity.

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Attachment to Faces: Adjoining Faces do not

have boundary layer attachments Gambit checks angle bounded by attachment and adjoining faces

Angle controlled by

MESH.BLAYER.PROJECT_ADJACENT_ANGLE

Default is 135 degrees

If angle less than default angle: Imprints the adjoining faces

If angle greater than default angle: Does not imprint adjoining faces

Attachment face No imprinting

(140 > 135)

Default changed to

175 deg. produces

imprinting and a GAP

Default changed to 175 deg.

Vertex types modified (B.L.

only) to prevent GAP

140 deg

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Attachment to Faces: Adjoining Faces have

boundary layer attachments Imprinting and overlap region depends upon state

of internal continuity.

Internal Continuity allows boundary layers to be

formed with no crossover regions (vertices become

sides)

Internal Continuity "OFF" Internal Continuity "ON"

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