Is it possible to set an outlet boundary condition that only one phase can pass through during a VOF simulation? For example, gas can pass through but water can not. Many thanks for the help!

You can easily do it through udf. You can forcefully impose the volume fraction of your desired fluid (which you would like to pass through outlet boundary) is 1. but forceful imposition may not have valid physics.

You can execute that piece of udf every iteration.

-Mubasshir

Many thanks Mubasshir

Could you give me some details? Which Macro should I use?

Thanks a lot!

-Zhi