Fuel tank simulation

Hi,

I have a project about fuel tank simulation. The thing that I try is similar to the sloshing tank example. However, since the fuel tanks are closed [ :) ] I need to limit the deforming mesh. Has anyone got any idea how to simulate a fuel tank?

So far I have put another rectangle on the existing rectangle and defined air for the properties. But program did not allow me to use the boundary between two rectangles. Thus, I could not define any thing for that boundary and the simulation crashed.

I need to put an upper boundary for the system to stop floating :)