Tim Weber

I see, with coincide, I meant that the end of both surfaces is on the same plane the means no material/volume of one part is inside the other, there faces are just touching. But I understand your explanation.

Just to follow up on the multibody solution. When I combine all the parts into one body and then proceed to the fluent setup, it creates interior boundary conditions of the connection/touching of two bodies. If those bodies got assigned two different materials, I get a warning that same bodies have different materials.

But if I make them walls, I have to decide on a material of that wall, and since there are two different materials touching, there is no unique solution. Or does it just not matter what kind of material that is chosen at that wall?