CFD Online Discussion Forums - VOF and the size of air region???

Hello everybody,

Actually, I am working with CFX to simulate the free-surface flow around a ship hull form, and I am using VOF method for doing this. When I have taken the region of air over the ship deck of nearly 0.5Ls (Ls is the ship length) with the top boundary condition of wall of free slip condition, the solution tends to be unstable and the produced size of wave is too large although I have taken 5Ls in front of ship and 3Ls behind the ship. The results became better (the produced waves is good and of reasonable height) when reducing the size of the air region to about 0.2Ls, but the solution is still divergent. Please, I want to know what are the main reasons for this improvement in the results with the reduction of air region size? (I think from the physical point of view the top boundary condition (wall) should be as far as possible from the free surface of water to make sure that there is no any affect from it on the results).

Many thanks for everyone in this forum