Rob
Ansys Employee

Periodic has some limiations with multiphase, and will keep the same direction vector to the flow (ie +x) rather than -x and +x depending on which side of the domain.  Not sure you can have the same flow in reality as the phases may interact differently, but will leave that for you to consider. UDFs may work, but could also make the model unstable as you may excite any oscillatons in the flowfield.