gambit-mesh cleanup & dataexchange_7

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Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002

Fluent User Services Center

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CAD/CAE Data Exchangeand Geometry Cleanup

(Virtual Geometry)

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Introductionu Several translation methods available to enable data exchange with

CAD/CAE systems.l Appropriate approach depends upon source.

u Translation can:l return incomplete, corrupt, or disconnected geometryl return geometry details unnecessary for CFD analysis

u Geometry cleanup refers to processes required to prepare geometry formeshing.l Fix incomplete or corrupt geometry and connect disconnected geometryl Remove unnecessary detailsl Decompose geometry into meshable sections

u Gambit’s Virtual Geometry operations can help with the cleanupprocess.

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CAD Data Exchange - Direct Optionsu Direct Translation Options

l ACIS-based CAD programs:n e.g., AutoCad, Cadkey, TurboCadn can export ACIS files (.sat or .sab)

which can be imported into Gambit.l Parasolids-based CAD programs:

n e.g., Unigraphics, SolidWorks, PATRAN, ANSYSn can export Parasolid files (.x_t and .xmt_txt) which

can be imported into Gambit.l CAD programs using proprietary geometry kernel

n e.g., I-DEAS, Catia, Pro-Engineer, CADDSn Direct (single-stage) translators purchase from third

party vendors e.g., Catia/ACIS translators see spatial.com website for more information

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CAD Data Exchange - Standard Optionsu Standard Translation Options

l Translation uses an intermediate, neutralor standard, file format.

l Applicable for all CAD/CAE systemsthat can output:n STEP files

s Pro/E supports STEP export at no additional cost.s Other systems support STEP as add-on.

n IGES filess Common format supported by most systems.

l STEP (Standard for Exchange of Product model data)n International standard defining format for geometry

and model information.n Gambit supports AP203 and AP214n Preferred over IGES import

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CAD Data Exchange - Standard Options (2)u Standard Translation Options (continued)

l IGES (Initial Graphics Exchange Specification)n Topology/connectivity information is lost when CAD programs export IGES

surface data only.s e.g., faces associated with volume, etc.s implies that volumes must be recreated from imported faces (tedious)

n Some CAD packages export IGES-solids as well as IGES-surfaces.s I-DEAS and CADDSs Topology/connectivity information maintained.

n Gambit provides two options for IGES imports Spatial

– Imported geometry comes in as all real, supports solidss Native (Fluent)

– Original IGES translator, does not support solids– Trimmed surfaces come in as virtual geometry

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Import Mesh and Import CAD

u Import Mesh and some Import CAD options result in faceted geometry.l Least preferred approach

u Import/CAD Pro/E (Direct)l Gambit directly accesses

Pro/E’s geometry enginen Eliminates geometry

translation lossesn User works in

Gambit environmentl Need special Gambit and

valid Pro/E licensel Solid models alone are supported

n assemblies are not

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u Geometry imported from other CAD systems can lack the required accuracyand precision to render valid or connected ACIS geometry.l This results from numerical limitations in original CAD system or neutral file

formats, or differences in tolerances between CAD systems and ACIS.

u Use visual inspection and check command to verify integrity ofgeometry/topology.l Fix geometry problems by healing if real geometry.

n Healing can be invoked at time of import.l Fix topology problems by deleting and reconstructing entities with virtual

geometry.

u Connect disconnected geometry using healer or virtual geometry operations.l Healing corrects tolerance problems in the model and attempts to connect

coincident edges and form volume from connected faces.n If unsuccessful, resort to virtual geometry operations.

Check and Heal Real Geometry

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Virtual Geometry

u Three kinds of geometry in GAMBIT:l Real

n Defined by the ACIS library of geometry creation/modification routines.n Geometry defined by mathematical formulae.

l Virtualn A Fluent Inc. library of routines providing additional functionality by

redefining topology.n Derive their geometrical descriptions by references to one or more real

entities (called the Hosts).

l Faceted geometryn Treated like virtual geometry.n Derived from importing a mesh or faceted geometry into GAMBIT, split

mesh operations, or stairstep meshing scheme.

Two objects that share the same underlying geometry but different topologies.

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Virtual Geometry: Uses

u Virtual geometry and the operations that create them are used tosimplify, clean, and connect existing geometry.l Simplify/Clean:

n remove details from the model unnecessary for CFD analysis.n merge faces/edges to increase mesh quality.n decompose geometry into smaller, meshable components.

l Connect:n Connect geometry that becomes disconnected during import process.

u Virtual geometry provides additional flexibility in operations thataffect geometry and mesh.l Merges edges to enable non-coplanar face to be created.l Modify the mesh by repositioning nodes on virtual face.

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Creating Virtual Geometry

u In general, virtual geometry is created as a result of a virtual geometryoperation on a real entity.l Can also be created from a ‘native’ IGES import operation.

u Virtual geometry operations:l are accessed:

n by selecting virtual option on a real geometry panel andn through dedicated virtual operation panels.

l employ any combination of real, virtual, and/or faceted entities.l result in the creation or modification of virtual (typical) and real entities.l Some real geometry operations will not work with virtual geometry.

n e.g., boolean operations and some split operations will not work withvirtual geometry

n Take care when planning to use virtual geometry operations.

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Characteristics

u Virtual entities:l entities are colored differently from real entities.l naming convention: v_vertex, v_edge, v_face, v_volume.

u When performing a virtual geometry operation:l Directly connected lower and upper geometry will become virtuall Underlying real geometry (host) will become invisible and inaccessible (or

put in the “background”)

u Deleting virtual geometry:l Will not delete host geometry.l Typically, lower order entities (virtual) remain undeleted.

u Meshing and Boundary Assignments:l Meshing and boundary assignment operations are unaffected by virtual

geometry.

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Virtual Geometry Operations-1

u Merge - replaces two connected entities with a single virtual entity

u Split - partitions an individual entity into two separate, connected virtualentities (recall: a real face can only be split with another real face)

u Connect - combines two individual, unconnected entities such that the lowergeometry is shared at common interfaces (unrestricted by ACIS tolerances)

++

++

Example:

Example:

Example:

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u Create - creates independent virtual entitiesl Use host entities for shape definition

u Collapse - splits a face and merges the resulting pieces with two ormore neighboring faces


+ + + +

collapsethis face

betweenthese faces

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u Convert - converts non-real entities to reall Applicable to vertices, edges, faces, and volumes.l Edges are sampled and real spline (NURBS) curve generated.

n sampling controlled by geometry.edge.VIRTUAL_NUM_SAMPLING_POINTS

l Face conversions require that a map mesh first be generated on face (no Side verticesallowed).

l Volume conversions require that all lower topologies can be convertedl Topology and any existing mesh are preserved.

u Face Simplifyl Removes dangling edges

and hard points from a face.l Result is virtual face


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Edge/Face Merge

u Virtual Edge/Face Merge optionsl Virtual (Forced)

n Create one single edge/face from all edges/faces

++

max. edge =

min. angle = 135

+ +

++

+

face merge

edge merge

l Virtual (Tolerance)n Merge all entities shorter than Max. Edge/Face Lengthn Merge all entities of higher entity angle than Min. Anglen No input will merge all vertices connected to two edges only

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Face and Volume Splits (Virtual and Faceted)u Virtual Face and Volume Splits

l Face Splitsn Using two vertices

n Using an edgen Note: Faceted faces can be split with

other faceted facesl Volume split (with face)

n All edges of the face have to beconnected to the volume

++

one volume two virtual volumes

connected face

virtual volumesplit

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Edge Connect (Virtual)u Edge Connect

l Also available in Vertex and Facel Virtual (Forced)

n Pick two or more edges you want to connectl Virtual (Tolerance)

n Every picked edge within the tolerancewill be connected

n 10 % of shortest edge is recommended (default)n The shortest edge is shown by clicking the

“Highlight shortest edge” buttonn The shape of the connected edge is an

interpolated ‘average’ of the picked edges.s Use Preserve first edge shape to force result to

assume shape of first edge in pick list.– Preserve first vertex location is available

for vertex connects.

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T-Junctions Optionu T-Junctions - splits edges by vertices that exist within a specified tolerance of

the edges and then connects the split entities.

u Use Preserve split-edge shape option to get following result:

connected virtual edges/facesunconnected real edges/faces

Edge Splits

Invoking too early may resultin very small edges

Original Option Off Option On

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Comparison of Face Unite, Merge, and Connect

u Unitel Operates on real geometryl Faces must have equal

tangents at edgel No unite for edges

u Mergesl Operates on real/non-real

geometry ® virtuall Faces must share edge but they

need not be tangent

u Connectl Operates on real/non-real

geometry ® real or virtuall Replaces selected entities with

single entity

Merge

Unite

Connect

tolerance

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Importing IGES Filesu File ® Import ® IGES

l Summaryn Review important information in the form

before importing the file.n Validity of information varies.

l Optionsn Native or Spatial Translatorn Ability to scale the IGES file at import (Scale

model between the dimensions of 1e-6 and1e+4, preferably around 1)

n Remove stand alone entitiesl Virtual Cleanup

n Enables automated cleanup sequence using:s connect tolerances edge merge tolerances angle merge tolerance

– geometry.edge.VIRTUAL_MERGE_MIN_ANGLE

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Virtual Geometry Cleanup Strategy-1u 1. Delete all unnecessary geometryu 2. Check validity of imported geometryu 3. Correct invalid geometry (Heal and/or reconstruction)u 4. Check connectivity by color coding

l Helps distinguish between connected and unconnected entities.n White - Stand-alone entitiesn Orange - Unconnected faces (Edge connected to one Face)n Dark Blue - Connected faces (Edge connected to two Faces)n Light Blue - Multiple connections (internal Face)

u 5. Connect Geometry (can be automated using Virtual Cleanup option)l a. Merge edges based on length and angle tolerances to eliminate short edges.l b. Real/Virtual connect of vertices, edges, and faces, in steps, based on increasing

connect tolerancel c. Connect with T-Junction Option.l d. Use forced connect operation for entities out of tolerance

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Virtual Geometry Cleanup Strategy-2

u 6. Create additional geometry, if necessary, and form volume.l Some of this may need to be done before resorting to virtual geometry

commands so that real boolean operations are available.l Bridge real and existing virtual geometry together using virtual

geometry.l In 3D, use face stitch command to create virtual volumes.

u 7. Simplify facesl Merge small edges and faces with neighbors to eliminatel Remove sharp angles for better meshing.

u 8. Decompose volume, if necessary.u 9. Mesh

Merge example:

gambit-mesh cleanup & dataexchange_7

Documents

n gambit

information fluent

fluent user services

corrupt geometry

faceted geometry

geometry formeshing

step standard

geometry details unnecessary