Now that I have your attention through deliberate (and all too common) misuse of the phrase “for all intents and purposes,” let’s talk about computer aided design and geometry models. 

What’s in a name?

We all know that CAD is the acronym for “computer-aided design” which is defined as the use of computer systems for the creation and modification of a design. Well-known CAD software includes SolidWorks, CATIA, NX, and Creo.

But how many times do we hear people say or say ourselves things like

• I’m working with the CAD.
• Where did you get the CAD model?
• There’s a problem with the CAD.

In cases like those above, what we really mean when we say CAD is “geometry model.”

As the folks here at Pointwise will tell you, I repeatedly insist on the rigorous adherence to a standard vocabulary in our communications about our software and mesh generation in general.

Loose and inaccurate terminology threatens our ability to communicate efficiently. It also threatens to lighten my wallet because my friend Nigel Taylor wants to collect $10 every time I slip up and loosely use the term “CAD.” And as Sophocles reminded us, the beginning of wisdom is the definition of terms.

Types of CAD and Suitability

In addition to CAD, I’m certain we’ve all heard a couple of variations on that theme, namely MCAD (Mechanical CAD) and ECAD (Electronics CAD). MCAD is a subclass of CAD used for the design of mechanical objects.

During his presentation at last week’s Pointwise User Group Meeting 2018, Bob Haimes introduced a new way to think of MCAD that resonated with me because it’s an important distinction for us CFD people.

• mCAD  is “manufacturing CAD” because its primary purpose is to design an object so it can be manufactured.
• aCAD is “analysis CAD” because its primary purpose is to create a geometry model that can be used to simulate the design’s performance with CFD, FEA or more.

Not only do these terms dovetail with previous thoughts on the topic (MCAD vs. preCAD in Reference 1) but they inject the concept of suitability for a particular purpose.

Just like our meshes have to be generated for a particular purpose (i.e. a particular flow solver, for a particular type of simulation, to obtain a particular type of result, to a certain level of accuracy), geometry models must be created for a particular purpose: manufacturing or simulation or something else.

We all know what unsuitability for CFD looks like in a geometry model: missing surfaces, unnecessary detail, lack of watertightness.

Bottom view of the DrivAer geometry model.

A rose by any other name…

Even assuming we all agree on the definitions of CAD, mCAD, and aCAD, the practical situation hasn’t changed. We CFD folks are often given geometry models that require a bit of preprocessing before they’re suitable for simulation.

But at least if we use terms consistently, our collaborative dialog on how to address these issues will be made just a tad easier.

And regarding whether the rose would smell as sweet, next on my terminology hit list are the unsweet adjectives “sloppy” and “dirty” as applied to geometry models.

What next?

If you’d like to read other thoughts on geometry models for simulation, here are some relevant posts here on Another Fine Mesh.

• Why CAD Surface Geometry is Inexact
• Survey Results – Mesh Generation and CAD Interoperability

Do not hesitate to leave a comment if you want to share your own insights on this topic. And feel free to subscribe to Another Fine Mesh by email using the “Sign me up!” button or by clicking on “RSS – Posts” link.

References

1. Geometry, Mesh Generation, and the CFD 2030 Vision, by John R. Chawner, John F. Dannenhoffer III, and Nigel J. Taylor, AIAA paper no. 2016-3485.
2. Industrial Perspective on Geometry-Handling for Aerodynamics, by Nigel Taylor, AIAA paper no. 2015-3408.
3. Geometry Modelling: Underlying Concepts and Requirements for Computational Simulation, by Nigel J. Taylor and Robert Haimes, AIAA paper no. 2018-3402.