Difficult BCs about Freesurface Simulation by VOF
There is an actual difficulty on the boundary treatment when I try to simulate a practical air-water freesurface problem using VOF method. Would you please give me some suggestions?
The problem is: A partially filled water delivery tunnel, with the water level and velocity at the inlet given, the outlet water level is disturbing by the effect of tailrace water. To simulate the wave propagation in the tunnel, the VOF module is used. Because we know the water level history,the outlet boundary condition is set as pressure-outlet and the pressure distributions and water fractions (which are spatial and temporal dependent) are imposed by UDFs. But the VOF fractions can't be used by the solver in the simulation because the flow is not backflow. Only Backflow VOF Fraction can be imposed at the pressure-outlet by Fluent, but giving the VOF fraction at pressure-outlet of forward flow is also a posed problem.
The question is: How can I impose the VOF fraction at the outflow pressure boundary?
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