Phase interaction
 

What if we do not turn on the phase interaction for multi-phase model like VOF. You can consider there are two phases in my system, air and water. What physics do we miss if we turn off phase interaction?

What phase interaction are you specifically talking about here? In VOF there is only one phase interaction which is inherently modelled, that is surface tension. But this is nothing you can switch on or off, its always on.

Ok on second thought, you can actually turn off surface tension modelling. But in my mind it's not usefull since surface curvature and hence bubble shape, deformation etc. is what you want to study.

Thank you for your reply. I understand that we will miss surface tension if we do not consider surface tension. Is there any effect on momentum transfer between phases?

If you neglect surface tension, drag will likely be unphyiscal hence momentum is calculated incorrectly.