gambit 3 edgeface

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Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002

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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 you Apply, 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 are picked 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 local Apply button for that option will be 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 enabling Apply 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 mesh

hard 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 or after 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 1best worst

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Examining the Mesh

Examine 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.

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.

Tri’s Skewness should not exceed 0.85.

Tet’s 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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Linear/Quadratic Elements (FIDAP/POLYFLOW USERS ONLY)

General tools Higher-order elements

For FEM codes (FIDAP and POLYFLOW), the element order can be changed at all three meshing levels

Only linear and quadratic elements are directly available A 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

gambit 3 edgeface

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