Steady state VOF problem
 

Hi,
I am solving a 3d almost cubic tank with a water inlet on one of the sides, a water outlet at the bottom. The head space of the tank is filled with air. The are a few baffle-like obstacles in the tank that I would like to look at the turbulent flow around them. I only need the steady state solution, and since the geometry is big and have many meshes because of odd scaling of the obstacles compared to the tank dimensions, I am assuming that unsteady state would take ages.

So I chose st.st, implicit VOF all default settings. The first issue I have is the boundary conditions. I will only have my primary phase(water) in the inlet and outlet and most of the tank, and air only in the head space. Should I choose mixture in the boundary conditions? Apparently I am not able to choose different phases for different boundaries I have created. e.g. inlet, outlet, head space interior etc.

Could someone please shed a light?