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Introductory GAMBIT Notes

GAMBIT v2.1 Jul 2003

Fluent User Services Center

www.fluentusers.com

Edge and Face Meshing

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Meshing - General

To reduce overall mesh size, confine small cells to areas where

they are needed (e.g., where high gradients are expected).

Controlling cell size distribution

Edges, Faces and Volumes can be directly meshed A uniform mesh is generated unless pre-meshing or sizing functions

are used.

Pre-meshing

Edge meshes can be graded (varying interval size on edge)

Graded edge mesh can be used to control distribution of cell size in

face mesh.

Controlling distribution of cell size in face mesh also controls

distribution of cell size in volume mesh.

Sizing Functions

New feature to control cell size in faces and volumes directly.

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Edge Meshing

Edge mesh distribution is controlled through

the spacing and grading parameters.

Using the Edge meshing form

Picking

Temporary graphics

Links, Directions

Grading/Spacing

Special characteristics

Apply and Defaults

Invert and Reverse

Options

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Picking Edges for Meshing

Picking

Temporarily meshed edges

When you pick an edge, the edge is

temporarily meshed using white nodes

Displayed edge mesh is based on current grading and spacing parameters If you modify the scheme or spacing, the temporary mesh will be

immediately updated

When youApply, the mesh nodes will turn blue

Sense

Sense is used to show direction of grading Every picked edge will show its sense direction using an arrow

The sense can be reversed by a shift-middle click on the last edge picked

(this is in addition to the "next" functionality) or by clicking on the

Reverse button

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Soft Links

Picking and soft links

Pick with links

By enabling this option, Soft-linked edges can be selected in a single pick

Linked edges share the same information and can be picked in a single

pick

Modifying soft links

You can anytime:

Form links

Break links

Maintain links

By default, GAMBIT will form links between unmeshed edges that arepicked together

By default, GAMBIT will maintain links between meshed edges that are

picked together

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Grading Controls mesh density distribution along an edge.

Grading can produce single-sided or double-sided

mesh

Doubled-sided mesh can be symmetric or asymmetric.

Symmetric schemes produce symmetric mesh about

edge center.

Asymmetric schemes can produce asymmetric mesh

about edge center.

Single-sided grading:

Uses a multiplicative constant, R, to describe the ratio of

the length of two adjacent mesh elements, i.e., R = l(i+1) / li

R can be specified explicitly (Successive Ratio) or

determined indirectly

Gambit also uses edge length and spacing information to

determine R.

Single-sided grading

Symmetric grading

Asymmetric grading

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Double Sided Grading

Symmetric grading schemes implicitly

generate double sided grading that is

symmetric.

Asymmetric schemes are accessible when

Double-Sided Option is used with: Successive Ratio, First Length, Last Length,

First-Last Ratio, and Last-First Ratio

The mesh is symmetric if R1 and R2 are

equal.

The mesh is asymmetric if R1 and R2 are not

equal.

Edge center is determined automatically.

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Spacing

Spacing

In all meshing forms, the following spacing functions can be specified:

Interval count - recommended for edge meshing only

A value of 5 creates 5 intervals on the edge (6 nodes, including ends)

% of edge length - recommended for edge meshing only An edge length of 10 and a value of 20 creates 5 intervals on the edge

Interval size - the default setting

Identifies the interval size relative to overall dimensions of geometry

Identifies "average" interval size if used with grading

An edge-length of 10 and a value of 2 creates 5 intervals on the edge

Average size of elements/grid is 2

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First Edge Settings

Use first edge settings enabled

First edge selected in pick list updates form

Useful to copy settings from one meshed

edge to other edges.

Use first edge settings disabled Any time you pick two or more meshed edges

where there is a difference in:

the Type

the Spacing

the localApply button for that option willbe turned off

This allows you to maintain pre-existing grading and/or spacing settings for

each edge.

Enforce a change in grading and/or spacing by enablingApply button.

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Mesh Options

Apply without meshing

This option is useful in cases where you

want to impose a scheme without fixing

the number of intervals

The higher level meshing scheme will decide

(and match) the intervals Example

Specify fixed interval and no grading

Specify double sided grading and

Apply without Meshing on bottom edges

Face meshing will automatically match mesh

Remove Old Mesh Deletes old mesh

Ignore Sizing Function

Sizing function has precedence on meshing

unless this option is enabled.

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Face Meshing Face Meshing form

Upon picking a face

GAMBIT automatically chooses Quad elements

GAMBIT chooses the Type based on the

Solver/face vertex types

Available element/scheme type combinations Quad

Map

Submap

Tri-Primitive

Pave

Tri

Pave

Gambit also has quad-to-tri conversion utility.

Quad/Tri

Map

Pave

Wedge

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Face Meshing - Quad Examples

Quad: Map

Quad: Submap

Quad: Tri-Primitive

Quad: Pave

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Face Meshing - Quad/Tri and Tri Examples

Quad/Tri: Map

Quad/Tri: Pave

Quad/Tri: Wedge

Tri: Pave

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Deleting Old Mesh

Existing mesh must be removed before remeshing.

Mesh can be deleted using

delete mesh form.

Lower topology mesh can also be deleted (default)

Existing mesh can also be removed in all Create

mesh forms without the need for Delete mesh

Remove mesh

Leaves all lower topology mesh Remove mesh + lower mesh

Removes all lower mesh that is not shared

with another entity

Undo after meshing operation also works!

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Hard Linking

Mesh Links (Hard Links) Mesh linked entities have identical mesh

created for periodic boundary conditions

Applicable to Edge, Face, and Volume entities

Best to use soft links for edge meshing

To link volume meshes, all faces must be meshhard linked first.

Setting up Hard Links for Faces

Select faces and reference vertices

Edge sense will appear

Reverse orientation on by default for sense Periodic option can be used for split edges

Mesh one face before orafter hard link is defined

mesh on second face generated automatically

Multiple pairs of hard links can be created.

+ + + +

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Assessing Mesh Quality

Default measure of quality is based on EquiAngle Skew.

Definition of EquiAngle Skew:

where:

max = largest angle in face or cell

min = smallest angle in face or cell

e

= angle for equiangular face or cell

e.g., 60 for triangle, 90 for square

Range of skewness:

e

mine

e

emax ,180

max

min

max

0 1

best worst

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Examining the MeshExamine Mesh Form Display Type

Plane/Sphere

View mesh elements that fall in plane or sphere.

Range

View mesh elements within quality range. Histogram shows quality distribution.

Show worst elementautomatically zooms into

worst element

Select 2D/3D and Element Type

Select Quality Type

Display Mode

Change cell display attributes.

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Striving for Quality

A poor quality grid will cause inaccurate solutions and/or slow convergence.

Minimize EquiAngle Skew:

Hex and Quad Cells

Skewness should not exceed 0.85.

Tris Skewness should not exceed 0.85.

Tets

Skewness should not exceed 0.9

Minimize local variations in cell size:

e.g., adjacent cells should not have size ratio greater than 20%.

If Examine Mesh shows such violations:

Delete mesh

Perform necessary decomposition and/or pre-mesh edges and faces.

Remesh

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Mesh Smoothing

Gambit generally incorporates default smoothing scheme after initial

mesh on face or in volume is created.

Default scheme can be changed in Edit - Defaults

Different scheme can be selected and

applied after meshing. Face mesh smoothing

Length-weighted Laplacian

Centroid Area

Winslow

Volume mesh smoothing Length-weighted Laplacian

Equipotential

Boundary mesh by default is fixed.

Option allows frees boundary mesh.

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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 Solver

Name zone if desired.

Apply defines zone and boundary type.

Can also modify and delete zone/boundary.

By default, External faces/edges are walls

Internal faces/edges are interior

The Continuum Type Form

Similar 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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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 remainingvolume has the Name and Type

Continuum: Name = fluid

Type = FLUID

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