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

1. CAD/CAE Data Exchangeand Geometry Cleanup(Virtual Geometry)

2. 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.

3. 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

4. CAD Data Exchange - Standard Options • Standard Translation Options • Translation uses an intermediate, neutral or standard, file format. • Applicable for all CAD/CAE systemsthat 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

5. 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

6. 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

7. 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.

8. Virtual Geometry • Three kinds of geometry in GAMBIT: • Real • 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.

9. 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.

10. 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.

11. 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.

12. 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:

13. + + + + between these faces collapse this face 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

14. 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

15. + + + + + 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

16. + + 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 withother 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

17. Edge Connect (Virtual) • Edge Connect • 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 aninterpolated ‘average’ of the picked edges. • Use Preserve first edge shape to force result toassume shape of first edge in pick list. • Preserve first vertex location is available for vertex connects.

18. Invoking too early may result in very small edges unconnected real edges/faces connected virtual edges/faces Edge Splits 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: Option On Original Option Off

19. tolerance Merge Connect Unite Comparison of Face Unite, Merge, and Connect • Unite • 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

20. Importing IGES Files • File  Import  IGES • Summary • 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

21. 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

22. Merge example: 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