Open channel flow with submerged outlet
 

Hi everybody.
I'm trying to simulate an open channel condition (VOF, 2 phase, air and water, ecc.).
The first part of the channel works in free flow condition. The second part is a closed submerged circular conduit.

There's no way to make it converge. I've tried almost every boundary conditions, without success.
I've activated the "open channel" option in the model, and set a mass flow inlet as a inlet boundary condition, with "open channel" flag.
As outlet condition I've used a pressure outlet, setting the expected pressure at the end of the conduit.

In the open channel part, the sky is set as pressure outlet, with pressure=0 (atmosphere).

So the air mass flow inlet should exit through the sky, while the water should go by the conduit.

As say, no way to make it converge.

Any suggestion?