Interior vs. Coupled Wall between Different Solids
 

Hello all,

My question is about the quite popular warning you get when you have an interior defined between two different solid materials. It goes like "Warning: zone of type interior found between different solids".

FLUENT gets really unhappy when you define solid boundaries as interior. It seems like the error disappears when you define them as coupled walls, hence the command slit-interior-between-different-solids.

So, is there any problem (numerically) with having an edge set as interior? Or is this warning just the result of some legacy coding in Fluent? What condition gets imposed when you set a boundary between solids as internal vs. when you set it as a coupled wall?

I compared two heat transfer cases, one with internal and one with coupled wall bcs, and saw that except for the nodes laying on the edge itself there was no major temperature difference.

I'm planning on setting up a simpler test case using two 2D blocks and comparing results of both setups.

Any ideas, tips or advice? It would be really nice if I someone could point me in the right direction for some theoretical explanation about this.

I would also be really unhappy if there is an interior zone between two different materials and beg you to use a coupled wall like you should.

The solution far from these zones where the interior/coupled-wall are will of course be mostly unaffected.

Interior types don't have boundary or interface conditions, they simply pass information along. Values at interior faces follow the interpolation and reconstruction rules.

Coupled walls are similar in that they mostly pass information along but they have interface conditions (they match fluxes).

So it is a pretty big deal that you are using the wrong type unless this interface is in another galaxy, a region you don't care about at all.

Thanks a lot for you explanation. I was a bit confused since Fluent does not give the same error when both solids have the same k. Now it is clear to me that you need to match fluxes since you have different thermal conductivity values.