In Both Creo Simulation live Plus and Creo Flow analysis, we can't easily create the fluid domain if one of the openings is surrounded by multiple surfaces. It forces the users to create the fluid volume manually with surfaces copies, merges, ... which is long and might be unstable in case of modifications.
It can happen quite often as Creo divides all cylindrical surfaces in 2 separates surfaces.
In the attached screenshot, I've recreated a simplified representation of the limitation. A simple fluid vein represented by the radial hole made in the tube cannot be created because the hole crosses the 2 half cylinders.