In this final installment in our series of posts about the paper Preparation of Geometry Models for Mesh Generation and CFD, we take a peek at what the paper has to say about geometry model suitability. Or more accurately, the lack of suitability which leads to the CFD Vision 2030 Study’s assessment that most CFD processes are “onerous.”
It’s worth noting that the practitioner’s options for geometry modeling are often limited. Their organization may have standardized on a particular MCAD platform. They may receive all models from outside sources. Or their tools, especially their meshing software, may be limited in the types of geometry models it can accept.
All of the geometry modeling techniques described [in the paper] can produce geometry models that are perfectly suitable for mesh generation and CFD simulation. In practice, however, their use often presents challenges for the downstream user. Rather than cast this discussion in terms of “quality” – or its counterpart, sloppiness – it is presented in terms of suitability. The same geometry model may be suitable for one type of meshing such as CFD and unsuitable for another. Suitability has to be assessed within the greater context of one’s unique simulation requirements and unique simulation toolchain.
A geometry model’s suitability begins and ends with interoperability: how does the model get from the CAD software to the meshing software. There are two major facets to interoperability: representation and translation. The former pertains to how the sending and receiving systems represent the geometry in their own data structures. Lest one think that a circle is a circle, there are distinct differences between an analytic circle, a NURBS circle, and a faceted circle. Beyond geometry, the manner in which topology is represented is even more critical. And for boundary representations, it comes down to trimming and intersection curves.
Therefore, the intersection is computed approximately using a process that involves point sampling on each surface to within a tolerance and then fitting the resulting collection of points into a curve. A byproduct of this computation is an intersection curve that does not precisely conform to either of its parent surfaces. 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 concept of the “tube of uncertainty” is explained in another blog post, “Why CAD Surface Geometry is Inexact.” The paper concludes with descriptions of excessive and insufficient detail, repair and healing, defeaturing and refeaturing.
Download the 20-page paper, Preparation of Geometry Models for Mesh Generation and CFD today. The paper and its 43 references should give you a solid understanding of what’s waiting for you the next time someone sends you a geometry model for a CFD simulation.