Modeling Slug flow

Demetrios

I am trying to model slug flow and I have come up with two ideas on how to "set up" the model in Gambit but I have a few questions which I can't answer. If someone can help I would appreciate it very much.

1st Idea: Mesh only the fluid zone around the slug, leaving the slug "empty". In this case what are the boundary conditions that i should use for the interface (outer boundary of the slug) ? How am I going to make Fluent "understand" that the slug is filled with air and is not empty?

2nd Idea: Mesh the whole area of the tube and create another face inside the meshed tube wich will represent the slug. In this case what are the boundary conditions which I should use so that Fluent will understand that the face inside the tube is a slug filled with air?

Thanks in advance