How to know the number of cells at the interface in a 2 phase flow using VOF?
I am trying to model the Laser melting of a solid block using solidification/melting and VOF. The 2 phases are the inert gas and the solid metal block. I am using a UDF to apply the laser heat source and want to apply this heat source at the top surface of the metal block which will be the interface of the gas and the metal block. I have patched the volume fraction of metal block to be 1 for the given block dimension.
Now, as I understand FLUENT does not create a sharp interface between 2 phases and the volume fraction change takes place over a number of cells. So, I cannot exactly use 0.5 as volume fraction to apply the source terms. I am taking a range of volume fraction for the applying the source terms(0.05<C_VOF<1). Next I understand that I should also use the cell centre z coordinates to identify these cells which have a volume fraction in that range? So, how to know the number of cells at the interface?