Gambit Tips and Tricks

Gambit 2.2 Tips & Tricks Thomas Scheidegger  June 8, 2004

Outline

• Data exchang ange wi with other CAD sy system tems • Cons onstruc ruction ion and clean ean-up tools ols • Meshing tools • Miscellaneous ti tips an and tr tricks

Data exchange with other CAD Systems • CAD data import • Checking connectivity • Chec Checki king ng real real geom geomet etry ry/t /top opol olog ogy y • Tolerant modeling • Heal Healin ing g and and smoo smooth thin ing g rea reall geo geome metr try y • Data ata ex exchang ange vi via me mesh im import

CAD Data Import New InterOP R12 translators for IGES, STEP, Parasolid, and Catia 4 (add-on) in Gambit 2.2  – Warning is issued if the model size exceeds 10000 units

Geometry

Mesh

Faceted Geometry

•ACIS (R12)

•I-DEAS UNV

•I-DEAS FTL

•Parasolid (v15.0)

•FLUENT/TGRID

•STL

•Catia V4

•CGNS (v2.0)

•VRML

•STEP (AP203/214)

•PLOT3D (formatted)

•Optegra visualizer 

•IGES

•ANSYS

•Direct Pro/E Interface

•PATRAN •NASTRAN

Geometry (Real/Virtual) Geometry (Faceted)

Geometry (Faceted)

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

Color by entity

Color by connectivity

Checking Real Geometry/Topology •

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 reconstructing entities

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  – all entities need to be disconnected before making tolerant

Tolerant Modeling •

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 < 0.01

Maximum tolerance 0.0001

< 0.1

0.001

< 1.0

0.01

< 10.0

0.1

> 10.0

1.0

Tolerant IGES import

Healing Real Geometry •

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

•

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 Effective tool to connect (stitch) faces into a volume Face healing can result in multiple volumes Stitch faces

Stitch faces, Repair geometry

Healing Real Geometry •

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 •

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

•

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  • 64-bit utility available for large mesh files

Data Exchange via Mesh Import

•

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

Data Exchange via Mesh Import •

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

 Automatic angle: 15 deg

 Automatic angle: 25 deg  Additional edges added manually

Construction and Clean-up Tools

• Real (ACIS) geometry tools • Real Boolean operations • Imprinting • Virtual geometry tools • Clean-up tools • Decomposing virtual geometry

Real Face From Wireframe

•

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 •

Create real face from wireframe in Gambit 2.2

Real face creation from non-coplanar edges

Tolerant real face creation from noncoplanar edges Tolerance value reported in transcript window

Real Volume Stitch

•

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 Three possible volumes

•

1

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

2

3

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

 – Clean geometry through mesh export/import

Imprinting in Boolean Operations Unite cylinder with left cube

Global imprinting

GEOMETRY.VOLUME.BOOLEAN_METHOD = 0

Local imprinting

GEOMETRY.VOLUME.BOOLEAN_METHOD = 1

• Default Default algori algorithm, thm, more more stable stable • Should Should not require require to discon disconnect nect geomet geometry ry before operation • Requires subsequent bi-directional bi-directional split

Virtual Geometry •

‘Overlay’ te technology  – Tool of choice choice to clean-up and simplify imported geometry  – Needed when tolerant modeling modeling and healing/smooth healing/smoothing ing are not sufficient  – Modifies size size and shape shape of boundary surfaces while while being constrained by underlying ‘host’ geometry definition • Repres Represent ents s same same geo geomet metry ry but but diff differ erent ent topolo topology gy • Can be der derive ived d from from real, real, virtua virtual, l, or facete faceted d geome geometry try

 – Limited use for Boolean operations operations  – Semi-automatic Semi-automatic virtual clean-up tool available

Three real faces merged into one virtual face

Virtual Geometry Operations •

Merg Merge: e: Repl Replac aces es two two conn connec ected ted enti entiti ties es (edg (edge/ e/fa face ce/v /vol olum ume) e) with a single virtual entity  – Only volumes volumes can be merged in real

•

Spli Split: t: Par Partit titio ions ns an an indi indivi vidu dual al enti entity ty (ed (edge ge/fa /face ce/v /vol olum ume) e) int into o two two or more connected virtual entities

•

Conn Connec ect: t: Comb Combin ines es two two indi indivi vidu dual al unco unconn nnec ecte ted d enti entitie ties s (vertex/edge/face) into a single virtual entity  – Connect entities entities by connecting connecting lower order topology  – Virtual connect connect can bridge bridge relatively large gaps in in model, larger  larger  than ACIS tolerance of 1.0e-6

•

Coll Collap apse se:: Spl Splits its a fac face e and and merg merges es the the res resul ultin ting g wit with h two two neighboring faces

•

Simp Simpli lify fy Fac Faces: es: Rem Remov oves es dang dangli lin ng ed edges ges

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

 – Select Clean Up Domain

Semi-automatic Clean-up Tool •

Clean Up Duplicate Edges in Gambit 2.2  – Includes edges which are coincident in part (T-junction connect)

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  problematic import

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

required to connect the faces defining the sliver volume

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 GEOMETRY.VERTEX.CONNECT_REMOVE_SHORT_EDGE = 1

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

Virtual Geometry Operations

•

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

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

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

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

Decomposing Virtual Geometry •

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 • Unselect ‘Lower topology’ toggle to retain visibility of lower topology

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 Without Size Function

Controlled growth rate with Size Function

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 Non-component source

Component sources

Fixed Size Function •

 Attachment face

Requires Attachment specification  – Attachment is the mesh to be affected • Fixed Size Function can be attached to edges, faces, and volumes  Attachment volume • 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

•

No Size Function

Parameter specification  – Angle • Maximum allowable angle between any two adjacent face normals on source • 5-30 degrees is reasonable value

With Curvature Size Function

 – Growth rate  – Size limit 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

Gap defined by one source face

 – Cells/gap • Minimum number of mesh cells in gap

 – Growth rate  – Size limit

Gap defined by two source faces

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  – Only growth rate and size limit are required

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

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

Ideal background grid

Underresolved BGRID

Resolved BGRID

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 SF start size = 0.01

Preview start size 0.02222

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 • Cannot directly control first cell height

2D Boundary Layers •

Boundary layer shape near vertices depends on vertex type  – End (E) type

E

BL mesh butts up against adjoining edge

E

Overlapped mesh where two BL meet E

 – Side (S) type

E

R 

C

E

 Angle at vertex is bisected Continuous BL ‘Internal continuity’ enforced by manually changing

E S

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 Wedge corner shape off

Wedge corner shape on

E

3D Boundary Layers

•

Internal continuity  – Allows boundary layers to be formed without crossover region

•

Imprinting on adjacent face

Internal continuity ‘on’

 – 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 Gambit 2.1 Internal continuity ‘off’

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  – Modify existing BL instead

• Dangling 3D BL are not supported

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 Can grow many layers with good quality

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

Boundary Layer Defaults

•

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 = 0.0 o) 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 MESH.BLAYER.QUICK_N_DIRTY 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  – Can be misleading when attaching BLs

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 PERIODIC 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  – Default periodic type is ‘translational’ in Fluent 6

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

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 = 1 PROJECT_TO_SURFACE = 0

•

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 •

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  – Fix: Split nose cone and create separate face for first element

Meshing Tips •

Example 2: Self-intersecting volume  – Tet/TGrid initialization fails due to self-intersecting volume Difficult to diagnose for complex geometries  – Fix: Split volume

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 Gives access to more memory and allows larger models

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

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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

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‘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 conditions if  FILE_IO.FLUENT5.EXPORT_SURFACE_MESH_ONLY = 1

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

GEOMETRY.EDGE.MAX_NUM_FACETS = 500 GEOMETRY.TOLERANCE.EDGE_FACET = 0.000001

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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

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] 0 – color by ID

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1 – color by type

Change the default black background color   – GRAPHICS.GENERAL.WINDOWS_BACKGROUND_COLOR

Picking Entities •

Picking by dragging a box over entities Picking ‘partially included’ entities when dragging box diagonally down

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Using mouse and cursor:

Shift +

Picking ‘completely included’ entities when dragging box diagonally up

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 – Left mouse: initial pick, additional picks  – Middle mouse: modify pick  – Right mouse: ‘Apply’ or next list box

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Using picking filter   – Picking based on real/virtual meshed/unmeshed entities

Journal Files

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Extracting lost journal file from existing database file gambit –res(torejournal) journalfile –id database

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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 = 0 to use old Boolean/imprinting method  – Set GEOMETRY.GENERAL.REAL_LABEL_CHANGE_MINIMIZATION = 0 when running Gambit 2.0 journal files  – Set GEOMETRY.FACE.VIRTUAL_FACE_SPLIT_METHOD = 0 to get old behavior 

Miscellaneous Tips and Tricks

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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

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Most Gambit documentation is in the Modeling Guide!  – Use Quick Help to directly access the complete documentation