The VOF formulation relies on the fact that two or more fluids (or phases) are not interpenetrating . For each additional phase that you add to your model, a variable is introduced: the volume fraction of the phase in the computational cell. In each control volume, the volume fractions of all phases sum to unity. The fields for all variables and properties are shared by the phases and represent volume-averaged values, as long as the volume fraction of each of the phases is known at each location. Thus the variables and properties in any given cell are either purely representative of one of the phases, or representative of a mixture of the phases, depending upon the volume fraction values. In other words, if the th fluid's volume fraction in the cell is denoted as , then the following three conditions are possible:
Based on the local value of , the appropriate properties and variables will be assigned to each control volume within the domain.