Hi Nima,

Answer to your question depends the certainity with which we know the value of flow parameters on these boundaries.

Example: (1) if you know the velocity at the inlet, then use the 'velocity-inlet' and 'outflow' condition; (2) If you know the pressure values at both the places, use 'pressure-inlet' and 'pressure-outlet' boundary conditions.

Feel free to ask if still in doubt.

Regards,

Sagar

Thank you Sagar,

As I said, the known values are: discharge (that can be converted to the mass flow rate), and the water surface level at outlet.

I have experience in modelling the two-phase flow (water+air) through an emergency gate in a dam. I use the same BCs (Mass flow inlet (or velocity) for the inlet and zero pressure outlet for the outlet). Also, I guess in your case, since it is a river, you need to specify the top boundary with pressure outlet. Cheers