Hi

I'm trying to model the open channel water flow with STAR using VOF. Water flows at 0.4m/s.

What could be the best outlet boundary condition for water and air if I already set air and water inlet BC using velocity.

If anyone knows other necessary parameters dealing with VOF model,that could be very helpful as well.

Anyone has an idea?

Use a pressure boundary with prescribed pressure varying with water depth. Also, coarsen the mesh towards the exit. You would not want to have wave reflections from the boundary.

If you are interested in the development of turbulent flow, use cyclic boundary conditions with prescribed mass flow.

I've tried the built-in prescribed pressure boundary with static-mean on and static-environment on option for the water pressure boundary. But both didn't work.

I've generated a subroutine for water pressure outlet by using rho x g x h (hydrostatic pr). In this case what's necessary treatment for dynamic prssure due by velocity(0.4m/s) ?

You need the subroutine to make it work. But you do not need to specify the dynamic pressure (why would you?). The thing is that you want to keep the see-level at a constant height through the outlet boundary, so it is sufficient to impose the condition on static pressure.