I am trying to set up a flow simulation of a submerged heat exchanger. Within the internal flow of the heat exchanger piping, I have R22 (copper piping) and Ambient Air (aluminum). I've created two fluid subdomains for these, and set up the inlet and outlet boundary conditions.
However, I want to see what happens if I have this piping configuration submerged within a volume of water. I've attempted to model this by creating a box with the desired volume, and adding water as a fluid subdomain within the box. When I try to run the simulation, I keep getting the error "Faces of this feature belong to different regions". I assume this is saying that the piping configurations are not allowed to be contained within two regions of fluid (ie. air and water).
How could I go about resolving this issue? I have attached images of my configuration, both with and without the box showing. Basically, I want to run my heat exchanger simulation as if it was submerged in a volume of water.