Inlet and Outlet at the same time - what BCs?
 

I have a box with a mass flow inlet and outlet next to each other on the same side. Around this box is a huge enclosure (cuboid) and I want to simulate the overall flow pattern inside the enclosure. I don't know where the air for the box inlet gets sucked in from nor where the air from the box outlet leaves the enclosure.

So I think I need to set the BCs of the enclosure walls as inlet & outlet at the same time because both are possible. I am new to CFD in general, so I am a little bit lost.

Right now I radomly put pressure inlets & outlets on the enclosure walls. I get "Reversed flow"-errors during the simulation, I guess that makes sense due to the situation I described above. But I am not sure if it's right...

Reversed flow is not an error but simply states that the pressure at that wall would be lower then your specified pressure in the outlet - thus mass will flow from the outlet into your compute domain. If you want to avoid that you can check "prevent reverse flow" - then the sover will relace it with a wall in case mass flow from the outlet to your compute domain will occure.

For your generall setup it would be nice if you can maybe send a picture of your setup / explain with it what you want to get from the simulation because just based on "general flow field" im a not certain what you are looking for.

@alphaNOVA
Thank you for the response- got to know it is not an error and okay. I don't want to avoid the reversed flow since I think it is like that in reality.

I attached a 2D picture of the model. Actually, it's 3D, but I think the 2D picture explains enough.

So there's a (black) box and there is one inlet and one outlet on the side of the box, both (very low) mass flows. Both are right next to each other. As I said, I am not interested in modeling what's happening inside the box, but in what's happening around. So I want to know, how the air around the box gets sucked into the box. There is no other force around for the air to move except the force due to the inlet and outlet. It's like the box is just surrounded by normal atmospheric air, gauge pressure = 0.

For that, I put an enclosure around the box. But I do not know where the air enters or leaves the enclosure.

I hope this makes it understandable? If not, please ask.

Then both pressure outlet and pressure inlet should work imo. Just make sure you dont prevent backflow and that the pressure and thermal match your case.