VOF Model for slug flow in ahorizontal pipe.
 

I am trying to simulate a VOF model for the two phase flow of air and water in a horizontal pipe.

the volume fraction=0.5 for air
Since both the fluids are flowing and there's no region strictly defined by air or water so I did not patch any region.
I am not getting the required interface
Do I need to patch specific regions for air and water before the inlet?