free boundary
 

Hi:

I want to ask a question about free boundary condition. The question is in a face there may be inflow and outflow. For example, the opening of fire compartment. In the upper part, the air could flow out while in the lower paer the air enters the room. How can we define the opening? Who can tell me whether the pressure inlet can be used as free boundary? If it can be used, How can use?

Thanks