The multi-fluid VOF model provides a framework to couple the VOF and Eulerian multiphase models. This allows the use of discretization schemes and options suited to both sharp and dispersed interface regimes while overcoming some limitations of the VOF model that arise due to the shared velocity and temperature formulation.
Modeling a Sharp Interface Regime
For cases operating in a sharp interface regime, the Multi-Fluid VOF model provides interface sharpening schemes such as Geo-Reconstruct, CICSAM, and Compressive along with the symmetric and anisotropic drag laws.
The symmetric drag law, which is used as the default in the sharp interface regime, is isotropic and tends to approach the behavior of the VOF model for high drag coefficients. The anisotropic drag law helps to overcome a limitation of the symmetric drag law by allowing higher drag in the normal direction to the interface and lower drag in the tangential direction. This facilities continuity of normal velocity across the interface while allowing different tangential velocities at the interface.
The anisotropy is characterized by the anisotropy ratio, defined as:
Conceptually, this ratio can be very high, however higher values sometimes lead to numerical instability. Therefore, it is recommended that you use anisotropy ratios up to approximately 1000.
Two types of drag formulations exist within the anisotropic drag law: one that is based on the symmetric drag law and the other is based on different viscosity options.
Formulation 1
This is based on the symmetric drag law, where the effective
drag coefficient in the principal direction 
is described as follows:
 | (14–503) |
where 
is the friction
factor vector in the principal direction. 
is the isotropic drag coefficient
obtained from the symmetric drag law.
Formulation 2
The effective drag coefficient in the principal direction 
is described as follows:
 | (14–504) |
where 
is the volume fraction for phase 
and 
is the volume fraction for phase 
.
The viscous drag component in the principal direction 
is
 | (14–505) |
where the viscosity options can be any one of the following:
= 
= 
= 
= 
= 
= 
and 
is the length scale.
Note: Both the symmetric and anisotropic drag laws used in the sharp interface regime are numerical drags. Therefore, droplet diameter in the symmetric law and length scale in the anisotropic law can be considered as numerical means to provide the drag.
Modeling a Sharp/Dispersed Interface Regime
For cases operating in the sharp/dispersed interface regime, the Multi-Fluid VOF model provides interface capturing schemes such as Compressive and Modified HRIC along with all options for all applicable drag laws.
To learn how to use the multi-fluid VOF model and the two drag formulations, refer to Including the Multi-Fluid VOF Model in the User's Guide.