I have a situation where we have a simplified version of a burner in a chamber. The burner shape could be thought of as say a rectangular donut floating in space in the chamber. The purpose of this simulation is to find the level of mixing of hot and cold airstreams. So basically, the cold air travels around as well as through the center empty region of burner, as the burner creates hot air in basically a square-ish donut shape. We want to start the simulation at the exit of the burner, as opposed to upstream and flowing over the burner. So I have three distinct regions with boundary conditions on my inlet face.
For a simplified version, think of a 1/2" pipe within a 2" pipe, all within a 4" pipe. I want to be able to control the center region and the outer region together with the same flow, but that middle region inside the 2" pipe but outside the 1/2" pipe needs to be independant of the others.
The problem I am having is that when I void fill my burner, that center region that is NOT burner is always filled as well. And when I go to apply boundary conditions, that center region will need entirely different conditions than the burner. I cannot seem to find a way to select and outer and inner boundary for a surface, but JUST an outer boundary.
How about adding more volumes in CAD? You could even create the air there (or cap the model off, leaving it hollow, CFD will automatically do the rest), which saves worrying about void filling.
Could you share a screenshot to help me have a clearer picture of what you are trying to achieve?
Bear in mind also, that you cannot have internal surface boundary conditions, so any internal inlets where you have temp/flow need to eminate from a suppressed volume.