Issue link: https://resources.system-analysis.cadence.com/i/1472078
Preparation of Geometry Models for Mesh Generation and CFD 10 www.cadence.com Interoperability by CAD Embedding Interoperability issues can be avoided if the simulation software is embedded in the CAD software. No files have to be exchanged; no direct interface must be implemented. The geometry model is directly accessed in its native format by the simulation software. Most MCAD software includes embedded CFD applications. Examples include Autodesk CFD, SolidWorks Flow Simulation, and Creo Simulation Live. For an introduction to CAD-embedded CFD, see the article by Resolved Analytics [32]. An alternative to CAD-embedded CFD is meshing and/or CFD software with a suitable level of geometry modeling capability. Intersections, Trimming, and Tolerances Consider the primary source of geometry models for industrial CFD: analytic B-Rep models created in MCAD software. As described above, a patchwork of spline surfaces is used to define the geometry of an object. Trimming process removes subsets of the geometric surfaces that are not part of the topological model (e.g., the portion of a wing that penetrates the fuselage). B-Rep topology is used to unite the trimmed surfaces into a unified whole (a solid model) by describing their relative adjacencies. The main point to be made here is one of awareness. The geometry model created in the MCAD software and delivered for meshing and CFD is considerably more complex than what appears on the screen (Figure 6). Figure 6: A B-Rep NURBS geometry model is considerably more complex than one sees on the computer screen. Image from Urick & Marussig [38]. Perhaps more revelatory is that the intersection of surfaces in a model comprised of analytically defined splines is approx- imate, inexact, and based on a tolerance. This is not a mathematical necessity but one of practicality. Consider that the analytically derived intersection of two bicubic B-Spline surfaces results in an intersection curve of polynomial degree 324. It is impractical for MCAD software to compute, store, and edit curves of this complexity for memory usage, speed, and flexi- bility issues. As the polynomial degree increases, the density of the array of control points in parametric space (Figure 1) increases accordingly. Therefore, the intersection is computed approximately using a process involves point sampling on each surface to within a tolerance and then fitting the resulting collection of points into a curve. An intersection curve that does not precisely conform to either of its parent surfaces is a byproduct of this computation. The intersection tolerance may be thought of as the radius of a "tube of uncertainty" within which the curve is considered to be precise. [The majority of engineers find analysis-suitable model preparation from CAD data to be a tedious and time-consuming task that consumes up to 73 percent of their time (by one study), for reasons not often understood. A natural question arises as to why such a barrier exists between the CAD geometry and analysis model when the CAD system accurately represents the intended design. https://blog.pointwise.com/2017/11/29/why-cad-surface-geometry-is-inexact/]