Gambit Tips and Tricks

Gambit 2.2 Tips & Tricks

Gambit 2.2 Tips & Tricks

Thomas Scheidegger 

Thomas Scheidegger 

June 8, 2004

Outline

Outline

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Data exchange with other CAD

Data exchange with other CAD

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Geometry (Real/Virtual)

CAD Data Import

Geometry

Mesh

Faceted Geometry

•ACIS (R12)

•Parasolid (v15.0) •Catia V4

•STEP (AP203/214) •IGES

•Direct Pro/E Interface

•I-DEAS FTL •STL •VRML •Optegra visualizer  •I-DEAS UNV •FLUENT/TGRID •CGNS (v2.0) •PLOT3D (formatted) •ANSYS •PATRAN •NASTRAN Geometry (Faceted) Geometry (Faceted)

New InterOP R12 translators for IGES, STEP, Parasolid,

and Catia 4 (add-on) in Gambit 2.2

Checking Connectivity

•

Check import of volumes/faces using entity based coloring

 – Vertices (white), edges (yellow), faces (light blue), volumes (green), groups (dark green)

 – Virtual/faceted geometry identified by label (v_*/f_*), picking filter, and color coding

• Vertices (darkkhaki), edges (peru), faces (dodgerblue), volumes (springgreen)

•

Check connectivity using connectivity based coloring

 – White: Stand-alone vertex or edge

 – Orange: Vertex connected to only one edge, or edge connected to only one face (i.e. unconnected face!)

 – Blue: Vertex connected to exactly two edges, or edge connected to exactly two faces (i.e. connected face)

 – Magenta: Vertex connected to three or more edges, or edge connected to three or more faces (e.g. internal face)

Checking Connectivity

Checking Real Geometry/Topology

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Checks for geometrical and topological inconsistencies

 – Topology problems (very rare) must be fixed before meshing  – Detailed reporting through change in defaults

• GEOMETRY.GENERAL.REAL_GEOMETRY_CHECK_SUMMARY = 0

 – Geometry not C1 or G1

• Indicates lack of parametric/geometric continuity • Real operations (booleans, splits, sweeps) may fail • Can use virtual operations as fallback

• Geometry errors do generally not prevent meshing. If the model can be shaded, then it can likely be meshed

•

How to fix problems

 – Fix real geometry problems by healing the unconnected entities, smoothing (new in Gambit 2.2), or by regenerating entities

 – Fix topology problems by deleting and reconstructing entities  – Fix virtual/faceted geometry problems by deleting (hiding) and

Tolerant Modeling

•

Tolerant modeling in Gambit

 – Real connectivity using an automatically evaluated local tolerance value (instead of 1.0e-6 real ACIS tolerance) assigned to vertices and edges

 – Allows use of less precise CAD data and still ensures proper  topology

 – Most construction tools (Booleans and splits) are tolerant  – Requires less virtual clean-up

 – Tolerant modeling will attempt to stitch volume  – Make geometry tolerant

• globally during geometry import

 – all entities can be connected

 – Automatic sliver/short edge removal during tolerant import, controlled through defaults

GEOMETRY.TOLERANCE.HEAL_REMOVE_SLIVER_FACE = -1 GEOMETRY.TOLERANCE.HEAL_REMOVE_SHORT_EDGE = 0.001

Print out of entity labels being eliminated

• controlled inside heal face/volume forms

•

Assigns local tolerances instead of geometry modification

•

Tolerances based on face size or edge length (body box size).

Stitching stops at maximum tolerance

Body box size Maximum tolerance < 0.01 0.0001 < 0.1 0.001 < 1.0 0.01 < 10.0 0.1 > 10.0 1.0

Tolerant Modeling

Healing Real Geometry

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ACIS is a high precision modeler and needs all entities to satisfy

stringent data integrity check

•

Geometry imported from other CAD systems often lacks the

required accuracy

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Healing is an operation designed to detect and correct accuracy

problems in ACIS models

•

Optional healing processes

 – Geometry simplification

• Spline to analytic geometry conversion  – Stitch faces

 – Repair geometry

• Heals inaccuracies in model

•

Healing always includes tolerant modeling

•

Auto or manual tolerances

Healing Real Geometry

Stitch faces Stitch faces, Repair geometry

Effective tool to connect (stitch) faces into a volume

Face healing can result in multiple volumes

Healing Real Geometry

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Heal model

 – globally during geometry import • all entities can be connected • no control over heal options

 – controlled inside heal face/volume forms

• all entities need to be disconnected before healing • manual control over heal options and tolerances

• often easier to export ACIS model and heal globally during re-import

•

Healing repairs geometry near boundaries. Healing does not fix

geometry defects distant from edges

•

Healing can further corrupt poor geometry. Carefully inspect and

check geometry after healing!

Smoothing Real Geometry

•

Geometry (edge, face) smoothing in Gambit 2.2

 – Smoothes discontinuous edges

 – Reduces geometry discontinuity defects for B-spline surfaces  – Can aid in subsequent Boolean operations

 – Two smoothing options • Remove discontinuities

 – Remove G1 discontinuity (default) • Reduce complexity

 – Simplified NURB representation

Face smoothing

Data Exchange via Mesh Import

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Import of surface or volume mesh files results in faceted

geometry, similar to virtual geometry

 – Use virtual geometry operations to work with entities  – Convert virtual geometry to real if needed

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Effective method to ‘clean’ corrupt meshable geometry

 – Mesh the defect geometry in Gambit with sufficiently fine spacing  – Assign boundary zones to faces as necessary, export mesh

 – Import mesh back into Gambit and work with faceted geometry

•

Boundary types are preserved during import of mesh files into

Gambit

 – Except periodic boundary types, periodic_shadow will have type ‘wall’  – Import of mesh with hanging nodes (e.g. adapted mesh, hex core

mesh) is not supported

 – PLOT 3D files with Iblanking or of binary type need to be converted to Fluent/Fidap mesh file using utility fe2ram 

•

Specify a Feature Angle to automatically build

geometry edges based on face normal

changes, i.e. the feature angle

 – Default feature angle is 135o

 – Conversion based on boundary information only for feature angle 0o

•

Enable the angle based edge splitting option

 – MESH.MODIFY.EDGE_SPLIT = 1

 – MESH.MODIFY.EDGE_SPLIT_ANGLE = 135

•

Complete mesh is imported

 – Importing volume mesh through ‘Surface’ import generates geometry from all interior face zones  – Delete cell zones in Tgrid first if only faces need

to be imported from volume mesh

•

Adjust imported topology using the mesh modify

forms (face/volume)

 – Allows to split faces without loss of connected mesh

• Modify meshed volume replaces old volume with new one • Modify meshed face adds modified face to original face

 – Unclick the ‘Keep original edge’ toggle to remove existing edges

 – Use Automatic Angle to add geometry edges

• Similar to feature engle during mesh import

 – Fine tune geometry edge definition by manually adding or  deleting individual mesh edges

• Spurs removed in single pick

Data Exchange via Mesh Import

 Automatic angle: 15 deg  Automatic angle: 25 deg

Construction and Clean-up Tools

• Real (ACIS) geometry tools

• Real Boolean operations

• Imprinting

• Virtual geometry tools

• Clean-up tools

Real Face From Wireframe

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Create face from wireframe

 – Coplanar edges

• Any number of coplanar edges within ACIS tolerance can be stitched into a real face

 – Non-coplanar edges

• Any 3 or 4 sided non-planar loop can be stitched into real face

• Loops with 5 or more non-planar edges can be stitched into real face if 

 – Edge loop is convex

 – Edges don’t turn ‘excessively’

 – New planar tolerant real face creation in Gambit 2.2 for non-coplanar  edge loops

• Tolerant option ignored if ACIS is able to create non-tolerant face • Maximum tolerance reported in transcript window

• Edges should be close to coplanar and connected • Tolerance should be smaller than intended mesh size

 – Turn off final projection of mesh from facets to geometry if tolerance is larger  than mesh size (MESH.FACE.PROJECT_TO_SURFACE = 0)

Real Face From Wireframe

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Create real face from wireframe in Gambit 2.2

Real face creation from non-coplanar edges

Tolerant real face creation from non-coplanar edges Tolerance value

reported in transcript window

Real Volume Stitch

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Create volume by stitching faces in Gambit 2.2

 – Real tolerant face stitch with tolerance option • Automatic or manual tolerance control

• Tolerance should be smaller than intended mesh size  – Both real and virtual creation in single step

 – May produce unwanted short edges • Healing can be tried in place of stitching

• GEOMETRY.VOLUME.SPLIT_AT_CVXTY_PTS_IN_FACE_STITCH = 0

 – Option to stitch single or multiple volumes

• Automatic addition of missing (connected) faces for single volume • Extra faces automatically discarded for multiple volumes

 – Single volume stitch can handle voids and dangling faces in Gambit 2.2

Real Volume Stitch

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Single volume

 – Searches automatically for missing (connected) faces  – Builds smallest possible volume from selected faces

•

Multiple volumes

 – Builds volumes for each watertight set of faces found  – Discards extra faces not belonging to a closed set

Real Boolean Operations

•

Why Boolean operations can fail

 – Poor geometric quality, complex topology

 – NURBS surfaces do not represent cylinders and spheres exactly. Boolean between ‘coincident’ analytic and NURBS surfaces are unpredictable

 – Difficulties in finding imprints of connected entities • Two imprinting algorithms available

GEOMETRY.VOLUME.BOOLEAN_METHOD = 0, 1

•

What to do if split operation of entity A with B fails

 – Copy/Heal option

• Copy B (and heal if needed), delete the original B

• Perform the split operation, reconnect before meshing  – Intersection/Subtraction option

• Intersect A with B while retaining A

• Subtract the result (intersection) from A while retaining the tool • Connect before meshing

Imprinting in Boolean Operations

Imprinting in Boolean Operations

GEOMETRY.VOLUME.BOOLEAN_METHOD = 1 GEOMETRY.VOLUME.BOOLEAN_METHOD = 1

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before operation

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GEOMETRY.VOLUME.BOOLEAN_METHOD = 0 GEOMETRY.VOLUME.BOOLEAN_METHOD = 0

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Virtual Geometry

Virtual Geometry

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 – Semi-automaSemi-automatic virtual tic virtual clean-up tool clean-up tool availableavailable

Three real faces merged Three real faces merged into one virtual face

Virtual Geometry Operations

Virtual Geometry Operations

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(vertex/edge/face) into a single virtual entity

(vertex/edge/face) into a single virtual entity

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than ACIS tolerance of 1.0e-6

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Semi-automatic Clean-up Tool

•

Quickly identify, zoom-in, highlight areas that cause connectivity

and mesh quality problems

 – Graphics color coding set to connectivity  – Graphics window pivot set to mouse

 – Suggests appropriate tools (standard virtual tools) to fix problems  – Easy switch between clan-up tools and regular geometry tools

•

Nine available clean-up tools

 – Clean Up Short Edges  – Clean Up Holes

• Option for real face creation in Gambit 2.2 • Easy access to additional face creation tools

 – Clean Up Cracks

 – Clean Up Sharp Angles  – Clean Up Large Angles  – Clean Up Small Faces  – Clean Up Hard Edges  – Clean Up Fillets

 – Clean Up Duplicate Geometry in Gambit 2.2

• Removal of duplicate volumes, faces, edges, and vertices

Semi-automatic Clean-up Tool

•

Clean Up Duplicate Edges in Gambit 2.2

Semi-automatic Clean-up Tool

•

Clean Up Duplicate Faces in Gambit 2.2

 – Connects or deletes faces  – Two search options

• Topology based

 – All lower topology has to be identical

• Centroid based

 – Less accurate, but helpful in detecting duplicate faces with different lower topology

Semi-automatic Clean-up Tool

•

Clean Up Duplicate Volumes in Gambit 2.2

 – Connects or deletes duplicate volumes • Duplicate entities due to modeling errors or 

Semi-automatic Clean-up Tool

•

Clean Up Short Edges in Gambit 2.2

 – Edge merge and face merge in addition to vertex connect

• Automatic selection of edges and faces in merge list, controlled by defaults

TOOLS.CLEANUP.FACE_MERGE_MIN_ANGLE = 135 TOOLS.CLEANUP.EDGE_MERGE_MIN_ANGLE = 135

• Angles sharper than the defaults are cleaned up using vertex connect

Manual Virtual Clean-up Operations

•

Eliminate short edges

 – Edge merge with adjacent edges

• Virtual (Tolerance) option allows to automatically merge short edges below a specified edge length

• Virtual (Tolerance) option with no value specified for  ‘Max. Edge Length’ and ‘Min. Angle’ will merge all

possible edges and automatically eliminate all vertices connected to exactly two edges

 – Connect vertices (collapsing short edge between)

• GEOMETRY.VERTEX.CONNECT_REMOVE_SHORT_EDGE

must be set to 1 in the defaults to connect the vertices defining the shortest edge

 – Identify short edges using the ‘Highlight shortest edge’ button in the edge and face connect forms

Manual Virtual Clean-up Operations

•

Eliminate sliver faces, small faces, sharp face angles

 – Merge face with neighbor face

• Virtual (Tolerance) option allows to automatically merge faces which form angles larger than ‘Min. Angle’

• Avoid merging faces which form sharp angles (≤ 90°). Sharp angle merge can cause meshing problems

 – Set GEOMETRY.FACE.SHARP_ANGLE_MERGE in defaults to 1 in order  to merge faces which form sharp angles

• Use ‘merge edges’ option in face merge GUI to automatically remove short edges

 – Connect edges (collapsing sliver face between)

• GEOMETRY.EDGE.CONNECT_REMOVE_DEGENERATE_FACE = 1

required to connect the edges defining the sliver face

•

Eliminate sliver volumes

 – Connect faces (collapsing sliver volumes between)

• GEOMETRY.FACE.CONNECT_REMOVE_DEGENERATE_VOLUME = 1

Example: Eliminate Short Edge

•

Eliminate short edge by

 – Splitting off small face

 – Merging small face with adjacent face  – Merging lower topology (merging edges)

 – Alternative method: Connect vertices using

Face connect with T-junction splits

•

New functionality in Gambit 2.2, to ensure connectivity between

non-aligned faces

 – Utilizes projections, splits, and connects

 – For real and virtual geometry, resulting geometry is always virtual  – Easy connecting of poorly matching faces

•

Tolerance used for virtual connect should be smaller than mesh

size

 – If tolerance is larger than intended mesh size, or if mesh size is smaller than merged/collapsed faces, meshing may fail

 – Mesh within the tolerance region gets pushed back to edge

Virtual Geometry Operations

For faces that overlap by a large amount, split each face with the intruding edge and form new face in center 

For faces separated by a large gap, create new face to fill the gap

Connect Preserving Mesh

•

Preserve higher topology mesh during edge and face connect

 – Mesh on unconnected entities must be topologically equivalent, but can have different grading

 – Might require mesh smoothing after connect operation

•

Allows easier meshing of geometry of repeated patterns

 – Copy geometry using ‘mesh unlinked’ option before connecting

‘Preserve first edge shape’ applies to mesh as well

•

Face splits by

 – Face (Virtual)

• Set GEOMETRY.FACE.VIRTUAL_FACE_FACE_SPLIT to 1 in defaults • Bi-directional option

• Try exporting surface meshes and splitting imported faceted faces if virtual face split fails

 – Edges (Virtual)

• Endpoints of splitting edge need to be connected to edges of face • Tolerance for edge proximity to face

 – Vertices (Virtual)

• Tolerance option for vertex proximity to face • Can produce dangling edges

• ‘Shaped edge’ option

 – Locations (Virtual)

• Locations snap to edges near boundaries

• Manually drag and position temporary ‘Locations’ • ‘Shaped edge’ option

Decomposing Virtual Geometry

•

Volume splits by

 – Faces (Virtual)

• All split tool faces must be connected together  • Must share boundary edges with volume boundary

• If volume split fails (rarely), make volume invisible (keep lower topology visible) and stitch together desired volumes

 – Locations (Virtual)

• Easy tool to split virtual volumes

• Manually drag and position temporary ‘Locations’ • Splitting volumes with voids not supported

 Adjusting Virtual Geometry

•

Adjust virtual geometry using the ‘slide virtual vertex’ tool

Vertices can be adjusted after meshing (might require subsequent smoothing of  mesh)

Tips for Virtual Geometry

•

Partially deleting virtual geometry is tricky and not

recommended!

 – Host geometry can become partially visible and make stitching of  new geometry tricky

 – Hide virtual geometry instead using the ‘Specify Display Attributes’ form

Meshing Tools

• Size Functions

• Boundary Layers

• Miscellaneous meshing tools

• Meshing tips

Size Functions

•

Control mesh distribution on faces and in volumes

 – Eliminate the need to premesh edges manually

 – Automatically capture important geometry and flow features  – Ensure high-quality meshes

•

Primarily designed for unstructured meshing

•

Speed and robustness improvements in Gambit 2.2

Size Function Types

•

Fixed

 – Controls mesh element size in a region as a function of distance from a given location

•

Curvature

 – Controls mesh element size as a function of geometric curvature of  a face or edge and growth of mesh away from source

 – Good for highly curved surfaces or edges

•

Proximity

 – Controls the number of of mesh elements in a gap (edge gaps and face gaps) and growth of mesh away from gap

•

Meshed

 – Uses existing mesh as initial size on source entity  – New Size Function type in Gambit 2.2

Fixed Size Function

•

Requires Source specification

 – Source is origin of Size Function, center of region • Vertices, edges, faces, and volume can be sources for 

Fixed Size Function

• Source entity type determines shape of size function

 – Spherical shape around vertex

 – Cylindrical shape around straight edge

• Sources can be topologically part of the attachment (component source) or independent of attachment (non-component source)

• Sources can be internal or external

Component sources

Fixed Size Function

 Attachment volume  Attachment face

•

Requires Attachment specification

 – Attachment is the mesh to be affected

• Fixed Size Function can be attached to edges, faces, and volumes

• Attachment is ideally the whole model to ensure high quality mesh

•

Parameter specification

 – Start size

• Mesh element size at source  – Growth rate

• Geometric stretching of mesh away from source • Cannot be less than 1.0

 – Size limit

• Maximum allowable mesh size for attachment • No further growth if size limit is reached

Curvature Size Function

•

Source specification

 – Curvature of source entities (edges, faces) used to determine start size

•

Parameter specification

 – Angle

• Maximum allowable angle between any two adjacent face normals on source

• 5-30 degrees is reasonable value  – Growth rate

 – Size limit

No Size Function

With Curvature Size Function

Edges as source entities new in Gambit 2.2

Proximity Size Function

•

Source specification

 – Source entities which contain gap

• Gap is volumetric region between specified faces, or  area between opposing edges of specified face

•

Parameter specification

 – Cells/gap

• Minimum number of mesh cells in gap  – Growth rate

 – Size limit

Gap defined by one source face

Meshed Size Function in Gambit 2.2

•

Ensures that mesh grows controlled away from a

premeshed source entities (edges or faces)

 – Very useful after importing mesh

 – Local start size taken from premeshed source entities

•

Parameter specification

Background Grid

•

Use of the background grid default parameters is key

to obtaining the desired meshes

 – TOOLS.SFUNCTION.BGRID_MAX_TREE_DEPTH controls the maximum refinement of the background grid

• Increase the default value (16 in Gambit 2.2) until no cells hit the tree depth as reported in the transcript window • A value of –1 puts no limits on the background tree depth,

but makes Size Functions slow for larger models  – TOOLS.SFUNCTION.NONLINEAR_ERR_PERCENT

controls the allowable deviation of the local mesh from the prescribed mesh size

• Default is 25%, can vary between 3 and 25%

• Number of cells above the prescribed tolerance are reported in the transcript window

 – TOOLS.SFUNCTION.REPORT_BGRID_INFO = 1 turns reporting in the transcript window on (on by default)

Background Grid

•

Use the Size Function reporting in the Transcript window

 – If the mesh is noticeably changing as BGRID_MAX_TREE_DEPTH is increased, it is a sign that too many background grid cells are hitting the limit

Ideal background grid Background grid level reached maximum value specified Size Function not sufficiently resolved

Working with Size Functions

•

Use multiple Size Functions as needed to resolve geometry and

flow features

 – Use Size Functions for surface meshing and combine with Boundary Layers

•

Fixed Size Function is most robust

 – Can substitute for Curvature and Proximity Size Function

•

Size Functions on virtual geometry can yield unexpected results

 – Host of virtual face is used as source. Causes difficulties when host face extends beyond virtual face

• Use vertices or edges as source entities instead of virtual faces to obtain reliable results

• Restitch source face from edges to have different host geometry (making a copy of the source face is not sufficient)

Real source face.1, attachment face.5

Virtual source v_face.9 and attachment v_face.10 have same host

Working with Size Function

•

Size function preview uses simple background grid

 – Increase TOOLS.SFUNCTION.VISUAL_GRID_MAX_LINE_NUM (default value = 25) if the size limits in the size function preview do not

match the specified limits

Boundary Layers

•

Produce high quality cells near boundary

 – Attachment to edges for 2D BL  – Attachment to faces for 3D BL

•

Uniform or aspect ratio based BL

 – Uniform

• First row height as parameter 

• Difficulties on highly curved geometry

 – Aspect ratio based

• Aspect ratio of first cell is parameter  • Does well in concave corners

E E E S C E E E R  E

2D Boundary Layers

•

Boundary layer shape near vertices depends on vertex type

 – End (E) type

BL mesh butts up against adjoining edge Overlapped mesh where two BL meet

 – Side (S) type

 Angle at vertex is bisected Continuous BL

‘Internal continuity’ enforced by manually changing vertex type to S

 – ‘Wedge corner shape’ option for corner (C) and reversal (R) vertices Wedge corner shape applies to 2D BL only

Can be combined with Cooper tool for quasi-3D BL

3D Boundary Layers

•

Internal continuity

 – Allows boundary layers to be formed without crossover region

•

Imprinting on adjacent face

 – Default MESH.BLAYER.PROJECT_ADJACENT_ANGLE removed in Gambit 2.2

 – Single default MESH.VERTEX.MAX_END_ANGLE (=120o by default) for consistent imprinting of 2D and 3D BL

Internal continuity ‘on’

Internal continuity ‘off’ Gambit 2.1

3D Boundary Layers

• Different 3D BL settings allowed on

adjacent surfaces in Gambit 2.2

• Imprinting of 3D BL fails if adjacent

volume has already BL attached

 –

Boundary Layer Defaults

•

Angle smoothing for 3D BL in Gambit 2.2

 – MESH.BLAYER.ANGLE_SMOOTH_FACTOR (0.0….1.0) controls amount of smoothing for both 2D and 3D BL

Gambit 2.1

 ANGLE_SMOOTH_FACTOR only for 2D BL Can only grow few layers

Gambit 2.2

Consistent for 2D BL and 3D BL

Boundary Layer Defaults

•

Height transitioning for 3D BL

 – MESH.BLAYER.HEIGHT_TRANSIT_RATIO controls how fast the BL height changes when sides of BL attach to premeshed edges

> 1 = number of cells over which height changes from premeshed edge to specified height

< 1 = fraction of edge length

Gambit 2.1 Only for 2D BL

Gambit 2.2

HEIGHT_TRANSIT_RATIO consistent for  2D and 3D BL

•

Use normal and offset smoothing when BL generates

skewed elements in concave corners

 – Normal Smooting

• Direction of advancing normals is smoothed to avoid collapsing of BL NORMAL_SMOOTH_ITER (default = 10) NORMAL_SMOOTH_ANGLE (default = 10o) NORMAL_SMOOTH_CONVERGED (default = 0.0o) NORMAL_SMOOTH_LOCALLY (default = 1)  – Offset Smoothing

• Eliminates dips in the advancing layers

OFFSET_SMOOTH_ITER (default = 0)

OFFSET_SMOOTH_CONVERGED (default = 0.01)

Boundary Layer Tips

•

Use

to control the

graphics display during BL creation and attachment

= 0 Project BL using MESH.BLAYER.USE_FACET_EVALS and take

attached Size Functions into account

1 Do not project BL, but use Size Functions (default) 2 Do not project, disregard Size Function (fastest)

•

Deactivate the display of the temporary BL as it is being

defined

•

Arrow of temporary BL points toward the label (ID) of the

face/volume into which the BL grows

Meshing of Periodic BC

•

Hard-link face meshes before meshing

 – Use periodic option to ensures periodicity of mesh (ensures periodicity even when one of the edges is split)  – Reverse orientation active as default

 – Can hard-link multiple pairs of periodic faces in same volume

•

Assign

boundary type

 – Multiple hard-linked face pairs can be picked as one periodic zone

•

Export mesh

•

Fluent 6 correctly recognizes node pairing of 

periodic boundary conditions

Misc. Map Meshing Tools

•

Auto-match distribution of premeshed edges on opposite edges

 – MESH.MAP.MATCH_PREMESHED_EDGE_GRADING = 1  – Auto-match doesn’t propagate

to edges of adjacent faces when map-meshing volumes

•

Sweep and revolve of faces generates 3D meshes from 2D meshes

 – Use ‘With mesh’ option in sweep and revolve GUI  – Uses grading and spacing of premeshed

sweep path

 – Same option when sweeping or revolving edges into faces

•

Poor mesh by mapper on highly curved surfaces

 – Projection to the surface distorts the mesh

 – Smooth the face mesh using ‘Winslow’ scheme (only available for quad elements)

Meshing Tips

•

Surface meshing done on faceted face representation by default

 – MESH.FACE.EXACT_MESH_EVALS = 0

•

Final exact projection from facets to surface by default

 – MESH.FACE.PROJECT_TO_SURFACE = 1

 – Turn final projection off and leave mesh on facets if projection generates skewed or degenerated mesh on bad/corrupt surfaces

PROJECT_TO_SURFACE = 0 PROJECT_TO_SURFACE = 1

Meshing Tips

•

If Tet/TGrid fails to initialize the volume mesh

 – Inspect quality of surface mesh

 – Turn on verbosity in defaults MESH.TETMESH.VERBOSITY = 1 • Reports number of uninitialized nodes

• Likely fixable if only few nodes fail to initialize

 – Export surface mesh and try meshing volume in TGrid • TGrid allows to visualize uninitialized nodes

•

Example 1: Nose cone of missile

 – Initialization fails due to collapsed surface elements

Meshing Tips

•

Example 2: Self-intersecting volume

 – Tet/TGrid initialization fails due to self-intersecting volume Difficult to diagnose for complex geometries

Meshing Tips

•

Default limit for tri elements on single face is 100’000

 – Increase limit in defaults MESH.TRIMESH.MAX_FACES or use a value of –1 for no restriction

•

Mesh size limits

 – Memory estimate for single volume with 1 million cells Hex mesh: 190MB

Tet mesh: 340MB (about 6 million tet cells max. on 2GB machine)  – Access additional swap space on UNIX/Linux platforms (requires

3GB of swap space on disk)

setenv FLUENT_MMAP_PATH “/tmp/filename” setenv FLUENT_USE_MMAP 1

Meshing Tips

•

Mesh delete

 – Delete volume or face mesh using ‘Remove unused lower mesh’ removes edge mesh, but does not reset assigned edge grading and spacing

 – Delete edge mesh without using ‘Reset to default values’ deletes the edge mesh, but retain the assigned grading and spacing

• ‘Reset to default values’ restores the default spacing specified in

MESH.INTERVAL.COUNT MESH.INTERVAL.SIZE

Mesh Export

•

Native Fluent 6 export

 – Exports mixed (volume + surface) mesh

 – Exports surface mesh without specifying the boundary conditions • Unspecified boundaries will be ‘wall’

 – ‘Export 2d Mesh’ option

• Ensures proper 2D mesh format for Fluent 6

• Default FILE_IO.MESH.EXPORT_2D_TOLERANCE = 0.0001 is used to

determine whether mesh is in xy-plane

•

‘Utility’ export option available through defaults

 – FILE_IO.FLUENT5.EXPORT_USING_UTILITY = 1  – For rare cases where direct Fluent 6 exports fails  – Exports only volume mesh

 – Exports surface mesh only for surfaces with specified boundary

Miscellaneous Tips and Tricks

• Geometry display and faceting

• Boundary zone coloring

• Picking entities

• Journal files

Geometry Display and Faceting

•

Geometry display and meshing use faceted representation for 

increased speed

 – Use

GEOMETRY.EDGE.MAX_NUM_FACETS (default = 100) GEOMETRY.TOLERANCE.EDGE_FACET (default = 0.01) before the geometry creation to control the edge facets

 – For existing geometry, change the defaults, save and restore database

•

If view manipulation is too slow

 – Turn visibility of silhouettes off using the display attribute form

 – Turn the silhouette generation for new faces automatically off using

GRAPHICS.GENERAL.ADD_SILHOUETTES_AT_FACE_CONSTRUCTION = 0 GEOMETRY.EDGE.MAX_NUM_FACETS = 500

Color by Zone Type/ID

•

Option to color zones by zone type or by zone ID

 – Similar to Fluent and TGrid

 – GRAPHICS.GENERAL.ZONECOLOR_TYPE = [0, 1]

•

Change the default black background color 

 – GRAPHICS.GENERAL.WINDOWS_BACKGROUND_COLOR 0 – color by ID 1 – color by type

•

Picking by dragging a box over entities

•

Using mouse and cursor:

_…

 – Right mouse: ‘Apply’ or next list box

•

Using picking filter 

 – Picking based on real/virtual meshed/unmeshed entities

Picking Entities

Picking ‘completely included’ entities when dragging box diagonally up Picking ‘partially included’

entities when dragging box diagonally down

Journal Files

•

Extracting lost journal file from existing database file

gambit –res(torejournal) journalfile –id database

•

Tips to minimize impact when running old journal files

 – Use default GUI.GENERAL.JOURNAL_ENTITY = 1 or 2 to generate ID independent labels, if possible

0 – journal original labels (default) 1 – journal lastid, i.e. creation order  2 – journal entity location

 – Set GEOMETRY.VOLUME.BOOLEAN_METHOD = 0to use old Boolean/imprinting method

 – Set GEOMETRY.GENERAL.REAL_LABEL_CHANGE_MINIMIZATION = 0when running Gambit 2.0 journal files

 – Set GEOMETRY.FACE.VIRTUAL_FACE_SPLIT_METHOD = 0 to get old behavior 

Miscellaneous Tips and Tricks

•

Use Edit Defaults to customize default settings

 – For example, set GLOBAL.GENERAL.SOLVER = 64 for Fluent 6 use  – Save defaults in ~/Gambit.ini file

 – More information on the defaults is in the Gambit Defaults Guide at www.fluentusers.com

•

Most Gambit documentation is in the Modeling Guide!