[Raijin Thunderkeg]

Hi, All.

I'm modelling the distribution of dye in the water of a water-gas system. The volume fraction distribution of this 2-phase system is as shown below. I insert
a water-specific additional variable and a transport equation is set for it and a kinematic diffusivity has been set. After 20 s, the distribution of dye is shown
as below. It can be seen from the contour that the dye spreads into the gas phase. This is repugnant to common sense because this additional variable can
exist only in water phase in physics.

My question is : What is the reason for this ? Interphase transfer ? How to solve it?