GAmbit- mesh control

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Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002

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Mesh Control throughBoundary Layers

and Face Vertex Types

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Face Vertex Type Basics

u All vertices that are connected to a face areassigned initial face vertex types based ondefault angle criteria between the edgesconnected to the vertex.

u Combination of vertex types describes theface ‘shape.’

u Face vertex types are used automatically to determineall quad face meshing schemes except the quad-pave scheme.l The tri meshing scheme also does not use face vertex types.

u Changing vertex types can help you create a structured mesh,control the mesh or help facilitate generating a hex mesh.l For the majority of models, vertex types don’t need to be changed.

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Vertex Type Characteristics

u End (E)l 0 < Default Angle < 120l zero internal grid lines

u Side (S)l 120 < Default Angle < 216l one internal grid line

u Corner (C)l 216 < Default Angle < 309l two internal grid lines

u Reverse (R)l 309 < Default Angle < 360l three internal grid lines

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E E

S S S

CC C

R R

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Modifying Face Vertex Typesu Face Vertex Types can be changed from

default setting:l Automatically, by enforcing certain meshing

schemes in face and volume meshing.n Can sometimes result in undesirable mesh.

l Manually, by direct modification inthe Face Vertex Type form.n Select Face

s symbols appear in graphics window

n Select New Vertex Typen Select Vertices to be affectedn Vertex Types can be applied to just Boundary Layers as option.

u A vertex can have multiple Types; one per each associated face.u For a given set of face vertex types, Gambit will choose which meshing

scheme to use based on predefined ‘formulas.’

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Formula for Map Schemeu Map Scheme: 4*End + N*Side

u Periodic Map Scheme: N*Sidel Project intervals can be specified for

more mesh control.

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u By manually changing vertex typesl In Set Face Vertex Type form, change

vertices (default) to “Side” (example)l Open the Face Mesh form and pick the face

n (GAMBIT should automatically select themap scheme)

u By enforcing the Map schemel In Face Mesh form, change the scheme

from default to “Map” and “Apply”n (GAMBIT will try to change the vertex

types so the scheme is honored)

How to Make a Face Mappable

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Map: 4*End + 4*Side

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CDefault

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Sdefault Map: 4*End

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Formula for Submap Schemeu Submap Scheme: 4*End + L*Side + M*(End + Corner) + N*(2*End +

Reverse)l additional terms when interior loops exist

u Periodic Submap Scheme: N*Side + M*(End + Corner) where M >2l additional terms when interior loops exist

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S S+ +

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How to Make a Face Submappableu By manually changing vertex types

l Consider which vertex should bechanged to “Side”

l In Set Face Vertex Type form,change vertex (default type) to“Side”

u By enforcing the Submap schemel In Face Mesh form, change the

scheme from default to “Submap”and “Apply”n (GAMBIT will try to change the

vertex types so the scheme ishonored)

n User has less control - resultingmesh may be undesirable

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default

Submap: 4*End + Side+ (2*End + Reverse)

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?

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Tri-Primitive Scheme

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

u To mesh a face with the tri-primitive scheme:l Manually, change one of the vertex types to “Side” in this examplel The Tri Primitive scheme can not be enforced

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default

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Meshing Faces with Quad and Tri Pave Schemesu Quad:Pave Scheme

l All vertex types of the face are ignoredl The sum of all elements on the edges of the face

must be even.n Mesh inner faces first if possible to prevent from

‘locking’ into odd number intervals on boundaries.l No guarantee of a symmetric mesh on a symmetric

geometry

u Tri:Pave Schemel No even number restrictionl Use boundary layers for better mesh near boundaries

u Use sizing function and edge mesh grading forcontrolling cell size distribution.l Edge mesh grading alone results in poor quality

mesh.

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Meshing Faces with Hybrid Quad/Tri Schemes

u Quad/Tri: Tri-Map formula: 2*Trianglel The face vertex types need to be manually

changed to Triangle (T) and the “Tri-Map”scheme must be selected.

u Quad/Tri: Pavel All vertex types are ignored except Trielement

(T) and Notrielement (N)l Trielement (T) will enforce a trianglel Notrielement (N) will avoid a triangle

u Quad/Tri: Wedgel Used for creating cylindrical/polar type meshesl The Vertex marked (T) is where rectangular

elements are collapsed into triangles

T T

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N

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How to Make a Volume Mappableu Three options to map a volume:

l Enforce the mapl Manually change the vertex types on all faces so they are mappablel Enforce the map on the faces

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manuallychange thevertex types

S Mapdefault

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map the faces

enforce the map

Example:

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How to Make a Volume Submappableu Three options to submap a volume:

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

and/or submappablel Enforce the submap on the faces

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CE

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Example: manually change thevertex types

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

l Manually change the vertex types on the side faces so they are mappableand/or submappable

l Pick the source facesl Enforce the map or submap on the side faces

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Example: manually change thevertex types

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

into the domain.l Produces high quality cells near boundary.l Allows resolution of flow field effects with fewer cells than would be

required without them.

u In general, boundary layers are attached to:l edges for 2D problemsl faces for 3D problems

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

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Create Boundary Layersu Create Boundary Layer Form

l Show Option: toggles display of temp. boundary layern Useful for complicated models

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

l Definition Inputs (3 of 4 inputs required)n First row: height of first row of elements (a)

s (or starting aspect ratio)

n Growth factor: factor for geometric series (b/a)n Rows: total number of element rowsn Depth: total height of boundary layer (D)

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

l Transition Patternn Reduces number of elements in ‘flow’ direction.n Not to be used with tets; watch for highly skewed cells.

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Attachmentu General

l Attach to:n Edges for 2d problemsn Faces for 3d problems

l Arrows point to center of associated face or volume.l Boundary layers are initially displayed in orange to indicate that it is temporary.l Temporary boundary layers update immediately with change in definition.l Boundary layer becomes permanent

(displayed as white) upon Apply.

u Edgesl Boundary layer mesh in region near

vertices is defined by vertex type.n End: mesh overlapsn Side: angle bisectedn Corner: angle divided into thirdsn Reverse: angle divided into fourths.

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Boundary Layers and End Vertex Type

u BL mesh butts upagainst adjoiningedge

u BL mesh usesadjoining edge’smesh if pre-existing

u If BL is attached toadjoining edge ona face, a block of“overlap” elementsis created wherethe two BL meshesmeet

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Wedge Corner Shapeu At corner or reversal vertices, wedge corner shape option is

applicable.l ON ® Wedge shapel OFF ® Block shape

ON OFF

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Attachment to Facesu A boundary layer attached to a face may ‘imprint’ the adjoining faces.

l The imprint is displayed graphically on adjoining faces.l Imprinting will depend upon b.l. attachments of adjoining faces and state of

internal continuity.

u If adjoining faces do not have b.l. attachments-l Attaching boundary layer to face:

n Gambit checks angle bounded by attachment and adjoining faces.n Will imprint (become visible) the adjoining faces if angle is less than default

angle (135o).n Will not imprint adjoining faces if angle is larger than default.

s b.l. is still created but may not impact mesh.

u If adjoining faces have b.l. attachments-l Imprinting and overlap region depends upon state of internal continuity.

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Internal Continuityu Internal Continuity

l Allows boundary layers to be formed withno crossover regions (vertices becomesides)

l Must be ON for tet/hybrid meshing

Internal Continuity “ON”

Internal Continuity “OFF”

PrismGrowth inBoundaryLayer

GAmbit- mesh control

Documents

affectedn vertex types

vertex types dont

given set of face vertex

vertex typesl

face mesh form

facen gambit

applyn gambit

set face vertex type