Issue link: https://resources.system-analysis.cadence.com/i/1472078
Preparation of Geometry Models for Mesh Generation and CFD 11 www.cadence.com This process of going from NURBS to intersections to trimmed surfaces to a B-Rep model is perfectly reasonable as long as the downstream user of the model recognizes a) the model is only accurate within a tolerance and b) the tolerance must be accurately conveyed and interpreted from sending to receiving application. Figure 7: The intersection of the red and blue surfaces comprises of a fit of sampled points in each surface's parametric space. Image from Urick & Marussig [38]. Problems may still arise even when the role of tolerances is understood and accurately conveyed. Nowhere is this more apparent than in mesh adaptation. The mesh cell sizes may be refined below the tolerance of surface-surface intersections (i.e., smaller than the radius of the "tube of uncertainty"). At these levels of mesh refinement, these artifacts of the geometry modeling become features of the geometry model as illustrated in Figure 8. This figure also illustrates an important point: that the gaps between surfaces in a geometry model aren't necessarily between coplanar surfaces. Figure 8: When a mesh is refined through adaptation or other methods, the cell sizes can become small enough that they begin to resolve tolerance issues on the geometry model. Image from Park et al. [39]. Image used with the permission of the author. Details – Too Many, Too Few Excessive Detail The geometry model's primary use in manufacturing is often a complete product definition. The adjective "complete" that can pose a challenge for CFD simulation. Gammon [19] identifies ten fundamental differences between CAD and CAE (or CFD) geometry models, several of which fall into excess detail category. f Excess (and complex) topology results from combining the underlying mathematics and usage practices. Because surface meshing often keys off the geometry model's surface topology, complications in the topology become complications in the mesh. f Geometry in excess of the wetted surfaces or OML (i.e., the boundaries of a CFD domain). In a realistically complex product definition model, extracting the wetted surfaces can be challenging.