[W.N.]

The control gate of the flow through the open channel. I know the discharge quantity which will go through the gate, but I don't know how to define it.
I tried to define the gate as inlet B.C., but the result as shown in the picture. The flow go on two directions, but I want it go from left to right. esam.JPG

[karachun]

You can reduce the domain and model only section between the pressure outlet to the gate, now your gate becomes inlet.
Or may cut thin brick inside the gate. Now you have two surfaces that represent the gate cross section. You can set the upstream surface as mass outlet and downstream as mass inlet with same mass flow. But this is rather cheating than simulation.

[ghorrocks]

More fundamentally, if you know the flow through the gate it does not make sense to have this in the middle of your simulation. The flow upstream and downstream are decoupled (as the coupling is the known flow between the two), so you should consider doing a simulation the upstream bit with the gate as an outlet boundary; and a separate simulation of the downstream bit with the gate as an inlet boundary.