CAD/CAE Data Exchange and Geometry Cleanup (Virtual Geometry)

Introduction Several translation methods available to enable data exchange with CAD/CAE systems. Appropriate approach depends upon source. Translation can: return incomplete, corrupt, or disconnected geometry return geometry details unnecessary for CFD analysis Geometry cleanup refers to processes required to prepare geometry for meshing. Fix incomplete or corrupt geometry and connect disconnected geometry Remove unnecessary details Decompose geometry into meshable sections Gambit’s Virtual Geometry operations can help with the cleanup process.

CAD Data Exchange - Direct Options Direct Translation Options ACIS-based CAD programs: e.g., AutoCad, Cadkey, TurboCad can export ACIS files (.sat or .sab) which can be imported into Gambit. Parasolids-based CAD programs: e.g., Unigraphics, SolidWorks, PATRAN, ANSYS can export Parasolid files (.x_t and .xmt_txt) which can be imported into Gambit. CAD programs using proprietary geometry kernel e.g., I-DEAS, Catia, Pro-Engineer, CADDS Direct (single-stage) translators purchase from third party vendor e.g., Catia/ACIS translator see spatial.com website for more information Please mention that GAMBIT can read any older versions of the .sat file format. We also don’t have very much experience using the STEP translators, and we can’t import STEP directly into GAMBIT currently. What experience we do have is not all positive (we have a guy with some files from UG via STEP and Steptools STEP to ACIS translator - he brought them to training and there are problems with some of the files.)

CAD Data Exchange - Standard Options Standard Translation Options Translation uses an intermediate, neutral or standard, file format. Applicable for all CAD/CAE systems that can output: STEP files Pro/E supports STEP export at no additional cost. Other systems support STEP as add-on. IGES files Common format supported by most systems. STEP (Standard for Exchange of Product model data) International standard defining format for geometry and model information. Gambit supports AP203 and AP214 Preferred over IGES import Please mention that GAMBIT can read any older versions of the .sat file format. We also don’t have very much experience using the STEP translators, and we can’t import STEP directly into GAMBIT currently. What experience we do have is not all positive (we have a guy with some files from UG via STEP and Steptools STEP to ACIS translator - he brought them to training and there are problems with some of the files.)

CAD Data Exchange - Standard Options (2) Standard Translation Options (continued) IGES (Initial Graphics Exchange Specification) Topology/connectivity information is lost when CAD programs export IGES surface data only. e.g., faces associated with volume, etc. implies that volumes must be recreated from imported faces (tedious) Some CAD packages export IGES-solids as well as IGES-surfaces. I-DEAS and CADDS Topology/connectivity information maintained. Gambit provides two options for IGES import Spatial Imported geometry comes in as all real, supports solids Native (Fluent) Original IGES translator, does not support solids Trimmed surfaces come in as virtual geometry

Import Mesh and Import CAD Import Mesh and some Import CAD options result in faceted geometry. Least preferred approach Import/CAD Pro/E (Direct) Gambit directly accesses Pro/E’s geometry engine Eliminates geometry translation losses User works in Gambit environment Need special Gambit and valid Pro/E license Solid models alone are supported assemblies are not Currently the boundary conditions are lost when importing mesh files. This is scheduled to be changed for the 1.2 release.

Check and Heal Real Geometry Geometry imported from other CAD systems can lack the required accuracy and precision to render valid or connected ACIS geometry. This results from numerical limitations in original CAD system or neutral file formats, or differences in tolerances between CAD systems and ACIS. Use visual inspection and check command to verify integrity of geometry/topology. Fix geometry problems by healing if real geometry. Healing can be invoked at time of import. Fix topology problems by deleting and reconstructing entities with virtual geometry. Connect disconnected geometry using healer or virtual geometry operations. Healing corrects tolerance problems in the model and attempts to connect coincident edges and form volume from connected faces. If unsuccessful, resort to virtual geometry operations. Currently the boundary conditions are lost when importing mesh files. This is scheduled to be changed for the 1.2 release.

Virtual Geometry Three kinds of geometry in GAMBIT: Real Virtual Defined by the ACIS library of geometry creation/modification routines. Geometry defined by mathematical formulae. Virtual A Fluent Inc. library of routines providing additional functionality by redefining topology. Derive their geometrical descriptions by references to one or more real entities (called the Hosts). Faceted geometry Treated like virtual geometry. 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.

Virtual Geometry: Uses Virtual geometry and the operations that create them are used to simplify, clean, and connect existing geometry. Simplify/Clean: remove details from the model unnecessary for CFD analysis. merge faces/edges to increase mesh quality. decompose geometry into smaller, meshable components. Connect: Connect geometry that becomes disconnected during import process. Virtual geometry provides additional flexibility in operations that affect geometry and mesh. Merges edges to enable non-coplanar face to be created. Modify the mesh by repositioning nodes on virtual face.

Creating Virtual Geometry In general, virtual geometry is created as a result of a virtual geometry operation on a real entity. Can also be created from a ‘native’ IGES import operation. Virtual geometry operations: are accessed: by selecting virtual option on a real geometry panel and through dedicated virtual operation panels. employ any combination of real, virtual, and/or faceted entities. result in the creation or modification of virtual (typical) and real entities. Some real geometry operations will not work with virtual geometry. e.g., boolean operations and some split operations will not work with virtual geometry Take care when planning to use virtual geometry operations.

Characteristics Virtual entities: entities are colored differently from real entities. naming convention: v_vertex, v_edge, v_face, v_volume. When performing a virtual geometry operation: Directly connected lower and upper geometry will become virtual Underlying real geometry (host) will become invisible and inaccessible (or put in the “background”) Deleting virtual geometry: Will not delete host geometry. Typically, lower order entities (virtual) remain undeleted. Meshing and Boundary Assignments: Meshing and boundary assignment operations are unaffected by virtual geometry.

Virtual Geometry Operations-1 Merge - replaces two connected entities with a single virtual entity Split - partitions an individual entity into two separate, connected virtual entities (recall: a real face can only be split with another real face) Connect - combines two individual, unconnected entities such that the lower geometry is shared at common interfaces (unrestricted by ACIS tolerances) Example: + + Example: + + Example:

Virtual Geometry Operations-2 Create - creates independent virtual entities Use host entities for shape definition Collapse - splits a face and merges the resulting pieces with two or more neighboring faces + collapse this face between these faces

Virtual Geometry Operations-3 Convert - converts non-real entities to real Applicable to vertices, edges, faces, and volumes. Edges are sampled and real spline (NURBS) curve generated. sampling controlled by geometry.edge.VIRTUAL_NUM_SAMPLING_POINTS Face conversions require that a map mesh first be generated on face (no Side vertices allowed). Volume conversions require that all lower topologies can be converted Topology and any existing mesh are preserved. Face Simplify Removes dangling edges and hard points from a face. Result is virtual face

Edge/Face Merge Virtual Edge/Face Merge options Virtual (Forced) Create one single edge/face from all edges/faces face merge edge merge + + Virtual (Tolerance) Merge all entities shorter than Max. Edge/Face Length Merge all entities of higher entity angle than Min. Angle No input will merge all vertices connected to two edges only + + max. edge = min. angle = 135 + +

Face and Volume Splits (Virtual and Faceted) Virtual Face and Volume Splits Face Splits Using two vertices Using an edge Note: Faceted faces can be split with other faceted faces Volume split (with face) All edges of the face have to be connected to the volume + connected face virtual volume split one volume two virtual volumes

Edge Connect (Virtual) Also available in Vertex and Face Virtual (Forced) Pick two or more edges you want to connect Virtual (Tolerance) Every picked edge within the tolerance will be connected 10 % of shortest edge is recommended (default) The shortest edge is shown by clicking the “Highlight shortest edge” button The shape of the connected edge is an interpolated ‘average’ of the picked edges. 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.

Invoking too early may result in very small edges T-Junctions Option T-Junctions - splits edges by vertices that exist within a specified tolerance of the edges and then connects the split entities. Use Preserve split-edge shape option to get following result: unconnected real edges/faces Edge Splits Invoking too early may result in very small edges connected virtual edges/faces Original Option Off Option On

Comparison of Face Unite, Merge, and Connect Operates on real geometry Faces must have equal tangents at edge No unite for edges Merges Operates on real/non-real geometry  virtual Faces must share edge but they need not be tangent Connect Operates on real/non-real geometry  real or virtual Replaces selected entities with single entity Merge Connect tolerance

Importing IGES Files File  Import  IGES Summary Options Review important information in the form before importing the file. Validity of information varies. Options Native or Spatial Translator Ability to scale the IGES file at import (Scale model between the dimensions of 1e-6 and 1e+4, preferably around 1) Remove stand alone entities Virtual Cleanup Enables automated cleanup sequence using: connect tolerance edge merge tolerance angle merge tolerance geometry.edge.VIRTUAL_MERGE_MIN_ANGLE

Virtual Geometry Cleanup Strategy-1 1. Delete all unnecessary geometry 2. Check validity of imported geometry 3. Correct invalid geometry (Heal and/or reconstruction) 4. Check connectivity by color coding Helps distinguish between connected and unconnected entities. White - Stand-alone entities Orange - Unconnected faces (Edge connected to one Face) Dark Blue - Connected faces (Edge connected to two Faces) Light Blue - Multiple connections (internal Face) 5. Connect Geometry (can be automated using Virtual Cleanup option) a. Merge edges based on length and angle tolerances to eliminate short edges. b. Real/Virtual connect of vertices, edges, and faces, in steps, based on increasing connect tolerance c. Connect with T-Junction Option. d. Use forced connect operation for entities out of tolerance

Virtual Geometry Cleanup Strategy-2 6. Create additional geometry, if necessary, and form volume. Some of this may need to be done before resorting to virtual geometry commands so that real boolean operations are available. Bridge real and existing virtual geometry together using virtual geometry. In 3D, use face stitch command to create virtual volumes. 7. Simplify faces Merge small edges and faces with neighbors to eliminate Remove sharp angles for better meshing. 8. Decompose volume, if necessary. 9. Mesh Merge example: