I'm studying the flow in the upper plenum of a pressurized water reactor. I need to specify inlet boundary conditions at windows that are located on each face of square tubes. I first specified a Total Pressure inlet boundary condition. But during the calculation, a message in the solver says that a wall has been placed at these inlets in order to prevent fluid from flowing out of the domain.

That's why I now specified an Opening pressure and direction boundary condition. I want to know if that condition is realistic for my situation and what happens if the fluid flows out of that inlet (is that flow lost like with an outlet condition or does it still flow in the domain).

Thank you

I realised my question is maybe not comprehensible.

So I need to specify an inlet condition at each window of a square tube. There are two windows on each face of the tube (8 in total). So I want to know if I specified an opening pressure with directions of the flow out of the tube, what happens to the flow that enter the boundary on the opposite direction (the flow that enters in the tube) : does it that flow, after entering the tube, leaves it through another window or does CFX consider it as an outlet?

Have you read the documentation on openings?