Box in a box STL imports as only a single volume?

I’m trying to do a simple CHT test, so I made a simple box in a box. I tried both splitting and not splitting the STL, but either way I get just a single volume so I can’t assign separate fluid and solid regions.

How can I make separate volumes?


Hi @mmathiasmeier!

Can you share the project here? Makes it easier to make some suggestions on how you can proceed with your geometry.