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.
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.
If you’d like to read other thoughts on geometry models for simulation, here are some relevant posts here on Another Fine Mesh.
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.
- 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.
- Industrial Perspective on Geometry-Handling for Aerodynamics, by Nigel Taylor, AIAA paper no. 2015-3408.
- Geometry Modelling: Underlying Concepts and Requirements for Computational Simulation, by Nigel J. Taylor and Robert Haimes, AIAA paper no. 2018-3402.
You have picked a hard one! In my young punk days, I used to hate when people used words like “CAD” or “design” or “engineering” in ways that didn’t fit my preconceived definition. Today, l’m tickled when someone explains to me that CAD is computer aided drafting 🙂
Lately, its been a problem so I fully support you giving proper descriptive names to things. I’ve had to switch to MCAD and ECAD a lot more with those silos starting to dissolve and the two groups needing to talk to each other. Using my old standard of “CAD” just confuses people.
Now maybe I could go with mCAD and aCAD too but it seems kind of demeaning for the manufacturing and analysis folks. Those are important people! Maybe you could embrace the future with Unicode or (gasp) emoticons.
The ones I want to be demeaning toward are those people making faceted, holey, mis-stitched surface abominations. Please come up with a name for them! :-p
Mark, I don’t speak emoji and I’m too old to learn. When I heard Bob Haimes use the terms mCAD and aCAD I saw them more as designations on how to focus our thinking about geometry models and the topic of suitability for a particular purpose. A couple years ago when I first presented the idea of preCAD, someone in the audience pointed out that adding yet another term might not be the best solution. After all, the demarcation between the two is blurry.
Of course, the really interesting bit buried in mCAD versus aCAD is the technical reason why geometry models built for those disparate purposes differ.
If we ignore incompetence, geometry models are the way they are for known reasons that we either fix or work around.
P.S. Anyone know why we refer to people who run the CAD software as “CAD operators?” I keep getting a mental image of an old-style telephone switchboard with ladies handling dozens of wires.
John, I’m not sure that this was intended because of the nature of the post stating how CAD is used incorrectly, but the phrase is “for all intents and purposes” and not “for all intensive purposes.” https://www.dictionary.com/e/for-all-intents-and-purposes-for-all-intensive-purposes/
Joe, did you read the first paragraph? SMH. You’re gonna owe me a beer for that one.
For some grammatically correct fun reading over the Thanksgiving holiday, I offer you some more mondegreens and eggcorns: https://grammarist.com/mondegreens/
Thanks. But you still owe me a drink.
I’m looking forward to it. 🙂
Ha ha. I was THIS close to correcting you on the title. Good article.
Thanks for reading. Sorry about the clickbait.