In PHOENICS code, the density field can be calculated in SEM or in HOL methods as follows:

RHO = RHOG * (1-VFOL) + RHOL * VFOL (it's logical)

G: for Gaz

L: for Liquid

VFOL: Volume Fraction of Liquid

but

why ? the viscosity field is calculated as follows:

ENUL = ENULG * (1-VFOL) + ENULL * VFOL

Look to the one-fluid formulation. To be prefectly correct the Indicator function (VFOL) is suppose to be a heavyside (step) function . so RHO is NOT a mixture but is EITHER a gas or liquid. this is also true for viscosity. Now we introduce our VOF, replace our step function with a volume intergral and people attempt to place some physics where we have averaged them all out.

so... ----------------------------------------------------- Q.. why ? the viscosity field is calculated as follows:

A.. That is just what the final form of the equations

came to when we put all of our assumptions.. -----------------------------------------------------