Free surface approach
We wanted to simulate Rayleigh-Marangoni instabilities with Fluent.
This instabily can be observed in a frying pan, while slightly heating
oil. in 3D hexagonal structures are observed, and the height of oil (interface
between air and water to bottom of the pan) varies with the oil velocity.
A first approach to this was trying to make a 2D simulation using two fluids. Trying to work out the different methods of using two liquids, it showed out none of the multiphase solvers could support heat transfers. These should be implemented in the VOF (Volume Of Fluid) model, in version 6 of Fluent (out in 2000).
A possible solution to that problem was to create a user defined function (UDF), creating a scalar energy, tranfering it, and having the density vary with the energy defined. Unfortunatly this is not possible, since the density cannot be defined by an UDF.
This made impossible the simulation of Rayleigh-Marangoni instabilities.