Phase Field Model
The Phase Field Model node adds the equations described in The Equations for the Phase Field Method. The node defines the associated phase field parameters including surface tension and interface thickness.
Phase Field Parameters
Define the following phase field parameters. Enter a value or expression for the:
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Surface tension coefficient σ (SI unit: N/m).
Parameter controlling interface thickness epf (SI unit: m). The default expression is pf.ep_default, a value proportional to the maximum element size with a proportionality factor dependent on the ratio of the maximum and minimum element sizes in the domain.
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Mobility tuning parameter χ (SI unit: m·s/kg). This parameter determines the time scale of the Cahn–Hilliard diffusion and it thereby also governs the diffusion-related time scale for the interface. Select User defined (default) or Calculate from velocity. For User defined enter a value or expression for the mobility tuning parameter χ. The default is 1 m·s/kg, which is a good starting point for most models.
For Calculate from velocity enter a value for the expected velocity at the interface U (SI unit: m/s). χ is then computed as:
where hmax is the largest mesh cell size.
Keep the χ parameter value large enough to maintain a constant interface thickness but still low enough to not damp the convective motion. A too high mobility can also lead to excessive diffusion of droplets.
External Free Energy
Add a source of external free energy to the phase field equations. This modifies the last term on the right-hand side of the equation:
The external free energy f (SI unit: J/m3) is a user-defined free energy. In most cases, the external free energy can be set to zero. Manually differentiate the expression for the external free energy with respect to ϕ and then enter it into the ϕ-derivative of external free energy field ∂f/∂ϕ.
Convection
Enter values or expressions for the components (u, v, and w in 3D, for example) of the Velocity field u (SI unit: m/s). The applied velocity field transports the phase field variables through convection.
Additional Sources of Free Energy