How to change a patch name

Hey there,

It’s Robin from CFD Engine & after last week’s email I reckon we’re all set to deliver nicely-named .obj files to snappyHexMesh.

But what about when things go wrong? Someone’s made a typo in one of our patch names & we only spotted it after meshing 🤦‍♂️ It’s going to mess up our BCs, forces & post-pro, we can’t just leave it.

Do you know how to quickly change the name of a patch (or patches), on any sized mesh, even in parallel?

Yes? Awesome, see you next week 👋

No? Let’s go…

What’s in a name?

I wrote about the importance of naming things a while ago, it’s the foundation of automating your process. You can use part names to…

• control edge lengths, volume refinement & layering in snappyHexMesh;
• automatically assign boundary conditions;
• define your monitors (forces, mass-flows, moments etc);
• determine which bits are shown/hidden/exported in your post-pro;

You can build a really slick CFD process around a consistent naming convention, which makes it all the more annoying when a typo gets in the way – let’s fix it.

Easy Mode

If you’re working with a serial (non-decomposed) mesh, then just open /constant/polyMesh/boundary in your text editor, fix that typo & you’re done 👏

Hard Mode

But, if you’re dealing with a decomposed case spread across loads of processors (& you don’t fancy grappling with sed) then you might appreciate an alternative method.

Unfortunately, there’s no “rename my patch” functionality in OpenFOAM (AFAIK). Instead, we need to create a new patch, move the faces over from the misnamed patch & then delete it 😬

But don’t worry, createPatch has us covered…

Save the above text in /system/createPatchDict & run createPatch -overwrite in parallel (here’s a copy-&-paste-able version to save you some typing).

Take it for a quick test run on any old blockMesh – just edit your createPatchDict to change one (or more) of its boundary names & see what happens.

Note: You can check patch names any time using patchSummary -expand

Gotchas

A couple of things to remember…

Our new patch doesn’t inherit anything from the old one, so we need to specify its type (at a minimum) & whether it should be in any groups (optional).

You might also need to reset your boundary conditions after a name change. It depends how you manage your BCs, but it’s always worth checking.

Finally, if you want to leave out the -overwrite switch until you’re happy it’s all working nicely, then go for it.

There you have it

That’s how to rename a patch in OpenFOAM using createPatch.

Keep this for when your colleague next makes a typo, so that you can swoop in & save the day.

As an aside, this is exactly the kind of thing that gives people the hump with OpenFOAM. Should I be able to write an email about something as simple as changing a patch name? Probably not, but here we are.

Any other “simple” ops I should write about?

Or do you have a better way of renaming patches?

Drop me a note – I’m always keen to learn so that I can share it with everyone else 🙏

Until next week, stay safe,