Open atmosphere B.C. in Fluent
 

Hi everybody,
I'm trying to simulate a 3D model of oil drop rising in water (Using VOF multiphase scheme).
My geometry is a simple tank with upper surface open to atmosphere. What is the best boundary condition for the top surface.
I have tried Pressure outlet and I get backflow in some of the faces, is it normal, can it ruin the results?
Thanks...

If you are not studying the effect on surface, outflow or pressure outlet will work. Otherwise, extend the domain to include some air column and give pressure outlet at the exit.