Issue With Specifying Boundary Conditions at Interfaces
 

I am working on a simple heat transfer model. Everything is being modeled as a solid. However, I seem to only be able to properly apply boundary conditions to boundaries that are not interfaces. When I apply boundary conditions to a boundary that is also part of an interface, the boundary condition doesn't actually have any effect on my model's solution. ie: I set it to have a certain heat flux or static temperature, but it simply doesn't have that effect (or any effect for that matter) once I run the simulation.

Am I missing something fundamental here?

I can also apply interface conditions to interfaces successfully but the only options there are Heat Flux and Heat Source, whereas I would like to apply a constant temperature to a shared boundary.

Thanks,
Jonathan

edit: I changed the physics region for the water to be liquid instead of solid. Now the boundary conditions between the solid/liquid works whereas before when it was a solid/solid boundary it did not.
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Update: Thanks for you reply LuckyTran, that explanation helped quite a bit.

Yes. Boundary conditions can only be applied to boundaries. Interfaces are interfaces and not boundaries. Boundaries exist at ends of domains. Interfaces exist between domains.

The original surfaces are kept when converting boundary surfaces into interfaces. The original boundary is disjoint from the domain and replaced with the interface. You are allowed some interfacial settings, such as heat generation rate, but it should not be confused with a boundary condition.

Even if the interface is physical, if you want to impose a boundary condition then you must treat the physical interface as a modelled boundary. Example: Any inlet or outlet is a modelled boundary condition but physically it is an interface between your test section and the rest of the universe.