What is an "Opening" boundary condition. When I use "Inlet" and "Outlet" I get message to use "Opening" which will allow my flow to go "IN" and "OUT" at the same time.

Any idea will be good. Thank you, REALLY, thanks

Generally speaking, "inlet" allows fluid flow into domain, and "outlet" outflow. If the B.C is set to "opening", the flow may leave the domain and then come back again, i.e. backflow. It is allowed if the B.C have been set to "opening" other than "outlet".

Inlets and Outlets are exclusive; they only allow flow in or out and the solver puts up walls if the flow tries to go in the wrong direction. This is done primarily to help convergence, but can also be used to simulate the action of a valve. The only exception to this is a velocity specified inlet, whereby the velocity direction is respected regardless of whether flow goes out of the domain.

An opening, on the other hand allows flow to move in or out of the domain. This is similar to a pressure boundary in many other codes (though most other codes do not have exclusive inlet/outlet boundary conditions as CFX does). If the flow direction is not known, then an opening may be appropriate. For details on the options I recommend reviewing the CFX documentation.

-CycLone