Help in Open Channel Boundary

Tamime

Hi all: I tried to model two phase (oil-water) flow into a pipe using the open channel flow boundary which available in the VOF model . I consider the water flow in the button of the pipe and air above it. I consider water as the secondary phase and oil as primary. If the volume fraction of water at the inlet was given, hence how much the value of free surface level and button level should I used into this boundary at the inlet and outlet. And was there a need to patch the region of water to the free surface by using the open channel boundary in general?

Helps in this problem was highly appreciated Thanks K. Tamime