Issue link: https://resources.system-analysis.cadence.com/i/1472078
Preparation of Geometry Models for Mesh Generation and CFD 12 www.cadence.com f Excess realism in the form of geometric details such as embossed or engraved text, fasteners, fillets, and chamfers. For many simulations, these details can be omitted without adversely affecting the suitability of the simulation results. However, in high-resolution simulations (such as adaptivity as described above) resolving these geometric details can become significant as the mesh is refined (see the comment above about geometry being a first-order effect). f Too idealistic in the sense that "non-manufacturable" geometry is created that includes sliver surfaces, cusps, knife-edges, and similar degeneracies. Figure 9: Illustrations of the challenges of using geometry models from MCAD systems from Gammon [33]. From L to R: complex topology, wetted surface extraction, excess realism, idealism. Beall et al. [40] address the issue of excessive geometry by classifying the entities as either intended features (e.g., fillets) or artifacts (e.g. slivers). The former are features that are directly created (by the designer) and can be suppressed relatively easily in the CAD software. The latter are features that are indirectly created (by the CAD software) and are unable to be suppressed (e.g., slivers). Excess and complex topology, as cited above, can be resolved by using higher-level topological abstractions (often called sheets or quilts) that account for the distinction between design features and artifacts of the modeling process or tool. Geometry model "repair" and "healing" are the umbrella terms used to address excessive detail. Repair functionality is available in many dedicated software tools (e.g., CADfix, TransMagic), interoperability libraries (e.g. Core Technologie, ACIS), and mesh generation and CFD software. Using Beall's terminology, one class of repair eliminates intended features such as fillets or holes using techniques based either on the geometry model's feature tree (if available) or on size. An overview of "defeaturing" techniques is provided by Wong [41]. [Importing and manipulating CAD geometry to make it more suitable for meshing is not always well understood. By editing and creating geometry and using solid model assembly, most models can be readily prepared for mesh generation. https://info. pointwise.com/webinar-mastering-geometry-cleanup] Gammon's "too idealistic" details (Beall's artifacts) are also addressed by model repair software and covers cases such as collapsing short edges and sliver faces, joining multiple curves and faces into larger entities, removing duplicates or isolated components, eliminating tangencies, and more. Some recent work exemplified by Owen [42] explores the use of machine learning to defeature a geometry model by accounting for the beneficial effects on the resulting mesh. [When working with realistic and complex geometry models, it is important to have various tools available to prepare those models for meshing. https://info.pointwise.com/webinar-tips-complex-geometries-large-grids] Rather than expending an effort to remove excessive details from a geometry model, a second option is to hide them using a shrink wrapping. As the name implies, a complex geometry model is fully encapsulated within a discrete representation that conforms to the original model to a prescribed degree while also covering holes, gaps, and other exterior imperfections. The resulting discrete geometry model is significantly smaller and less complex than the original. Insufficient Detail The problem of insufficient detail in a geometry model pertains mainly to geometry required by meshing that has nothing to do with the actual product definition as designed, such as outer boundaries (e.g., aircraft in a wind tunnel), farfield boundaries (e.g., aircraft in flight), closure of certain components (e.g., a plane at a jet engine nozzle's exit on which a boundary condition will be applied), prescribed meshing locations (e.g., a planar boundary that aligns with pressure taps or other measurement devices), and more.