How to set BC at the interface
 

Hi Fluent users:‎ I have stratified air-water flow into a pipe , I consider water flow in the button of the pipe ‎and the water above it. I have a problem to represent the interface between the two phases ‎what type of BC should I used to set the interface . Anyone can guide me in this subject ‎please.‎ Thanks so much K Tamemy