Choosing CAD for CFD

Do you have a lean CFD process yet? Are you cycling through build-measure-learn loops at a rate of knots. If so, you could consider CFD as your idea validation machine. Where any CFD project could be reduced to feeding the CFD machine (or CFD Engine ;-) with ideas until you find “the answer”. Throw your best ideas in one end & watch results / answers / inspiration come out of the other. But, before you can validate your latest idea, you have to get it out of your head into digital form. Enter CAD.

Before getting carried away I need to make an admission — I’m not a CAD guy (or CAD-ist or CAD jockey — whatever). I don’t do a lot of CAD. I don’t design for manufacture. I don’t do drawings. I don’t make assemblies. I’m not quite sure what PLM is, or do I mean PDM — who knows? CAD is a little like my holiday French — I can get by, but I’m not fluent.

So if you’re a CAD technician this might not be the post for you. But if CAD isn’t your first language & you’re thinking of getting a CAD package to help you translate ideas into food for your CFD process then this could be of interest.

The 3 big things I need from a CAD package to support my CFD habit are:

• Solid Import / Export Options;
• Quality Surfacing & Mesh Manipulation Tools;
• Accessible to a novice;

After much procrastination and much too much research, I chose Rhino as my CAD package. This is what Rhino has to offer with respect to my big 3…

1. Solid Import / Export Options

Other People’s CAD

I usually start projects with someone else’s CAD, so a variety of input formats is handy. Rock solid IGES & STEP importers are essential. This is stage one of a project, so if the import process is painful it can get things off on the wrong foot. The more import formats the better. Surely one of them will get the job done?

SnappyHexMesh-friendly OBJ Export

It helps shorten my workflow if the CAD output can go straight into snappyHexMesh. sHM doesn’t grok NURBS, so the export needs to be faceted. In my case, OBJ format. Rhino does a solid job of faceting almost any NURBS geometry. It also has plenty of options in the OBJ export panel which make it super simple to control mesh resolution & maintain your all important, surface naming convention. Not something that all CAD packages can manage.

2. Quality Surfacing & Mesh Tools

Dumb CAD Tools

By dumb I mean without history trees and parameterisation etc. Which is exactly what you get with models that arrive via IGES or STEP. I often need to get in there and perform some surgery. This could be identifying then fixing or replacing duff surfaces. Splitting, trimming or extending surfaces preferably to be joined together into solids. Rhino has intuitive tools for all of these plus strong boolean operations that work on surfaces as well as solids.

A range of mesh tools

Models don’t always arrive as surfaces. Sometimes it’s mesh, maybe from scanning or from a different modelling software. So a capable toolset for manipulating & fixing meshed geometry is essential. Whilst Rhino doesn’t have a huge mesh toolset, it does the most common operations well — filling holes, selecting geometry, separating parts, highlighting & closing free-edges — to name just a few.

Quality surfacing tools

Rhino is probably best known as a freeform surface modeller. As such, it’s got excellent tools for constructing flowing, smooth surfaces with all the usual continuity options you might expect. Great for constructing external aero (or hydro) shapes. It’s an added bonus that these tools also work well with the dumb models I mentioned earlier.

3. Accessible

Intuitive

I’m probably not using it right, but I can at least get things done in Rhino. I don’t use CAD every day, so I need software that is relatively intuitive & that doesn’t enforce an unfamiliar workflow that I’m likely to forget. If I ever get the time then I’ll probably take some training and try to level up — if I ever get the time.

Runs on a Mac

This is pretty specific to me & knocks a lot of CAD packages out of the running. But as my workaday machine is a Mac (more on that another day perhaps?) a Mac version is essential. Rhino for Mac? Check.

Lightweight

The aforementioned Mac is a laptop so it’s handy that Rhino works well on a decent spec laptop. I don’t have any local CFD crunch power, so it would be at odds to need a workstation just for occasional CAD use.

What if I were choosing again, right now?

Sometimes the best tool is just the one you’re most comfortable with. But things change fast in the software world, even in the CAD world. So if I were looking for CAD for CFD right now I’d definitely give the following options a second look.

Onshape

If you haven’t heard of (or tried) Onshape then do it now. Billed as “The Future of CAD” it’s cloud CAD in your browser. Whatever the objection that just popped into your head — relax, they’ve addressed it already. Seriously impressive. But it’s got other tricks too. Including native editing of several file formats, powerful collaboration tools, direct modelling, mobile apps & reasonable monthly pricing. I’m definitely watching how this one develops. From a cloudy point of view, Onshape + Simscale = very interesting.

Autodesk Fusion360

Fusion360 is Autodesk’s full-featured, multi-platform, pseudo-cloud (you download software & install something locally) CAD package. It has great licensing - free for startups & educational use - monthly/yearly subscription for the rest of us. All the features you’d expect plus some really nifty looking mesh tools. Not to mention that it’s from the stable of awesomeness that is Autodesk (check out A360 & Flow Design if you haven’t seen them yet). With more simulation and mesh handling options coming in the near future this is one to keep an eye on.

Caeses

You might not of heard of Caeses (yet) but it’s the only truly CFD-focussed CAD tool that I’ve ever come across, which automatically makes it interesting. It has a clear focus on generating parameterised, simulation-ready geometry for optimisation. It exports to OpenFOAM. It has built in analysis/optimisation tools, Windows and Linux versions and it’s pricing is pretty reasonable. You can even have a sneak peek at the capability via their demonstration web-apps - very impressive.

No affiliations

This isn’t meant to be an advert for any particular CAD package. I have no affiliation with any vendor. The suitability of a CAD package will depend heavily on what you simulate & how your design process works. Rhino is a great fit for me. But check out all 4 if you’re looking for CAD for CFD — I’m sure at least one will fit your workflow.