Rob
Ansys Employee

Have a look at the mrf examples for mixing tanks. You may, or may not, need a separate zone for the milk, but if you do have mutliple zones read up on share topology too.  Note, as milk and water mix it's a species problem, not multiphase. If you also have a head space (air) at the top then you have a multiphase model, with the liquid phase being a species mixture.