Resolved Fluid Film Model Reference

Table 1. Resolved Fluid Film Model Reference
Theory	See Resolved Fluid Film.
Provided By	[phase interaction] > Models > Optional Models
Example Node Path	[phase interaction] > Models > Resolved Fluid Film
Requires	A VOF Multiphase simulation with the Fluid Film model activated. A Film-VOF Phase Interaction. In the Phase Interaction Model Selection dialog: Optional Models: Resolved Fluid Film
Properties	Mass Transfer Option, Film-VOF Velocity Equalization See Resolved Fluid Film Properties.
Activates	Interface Inputs	See Interface Settings.
	Child Nodes	Film-VOF Velocity Equalization, when Film-VOF Velocity Equalization is activated. See Film-VOF Velocity Equalization Properties.
	Field Functions	Film-VOF Mass Transfer Rate of [Film-VOF Phase Interaction] See Field Functions.

Resolved Fluid Film Properties

Mass Transfer Option

Determines how the mass transfer rate between fluid film and VOF is calculated.

Method	Corresponding Method Nodes
Source Based With this method the mass is transferred between fluid film and VOF via a source term, where a positive source term is applied to the receiving phase, and a negative source term for the sending phase. This source term is applied only to the first cell next to the wall boundary independent of the film thickness.	None.

Film-VOF Velocity Equalization

When activated, this option adjusts the velocity of the film and the velocity of the VOF mixture in the adjacent cell to ensure that the velocity of the VOF mixture is very similar to the film velocity at the transition volume fraction $α_{T R A N S}$ .

See Film-VOF Velocity Equalization.

Film-VOF Velocity Equalization Properties

This node is available when Film-VOF Velocity Equalization is activated.

Time Scale: The film velocity and the VOF mixture velocity are equalized in the specified time. For smaller time scales, higher momentum source terms are applied to both sides to equalize the velocities.

Interface Settings

Under the Phase Conditions > [phase interaction] > Physics Values node:

Resolved Fluid Film Volume Fraction

Specifies the value of $α_{T R A N S}$ for the interface. This value can be specified independently for each interface, and can be specified as a constant or as a field function. By default, $α_{T R A N S}$ has a constant value of 0.8.

For more information, see Specifying the Resolved Fluid Film Volume Fraction as a Field Function.

In some cases, using a particular value of $α_{T R A N S}$ for all boundaries does not produce the appropriate results. For example, if the mesh resolution is set for other purposes, such as to resolve the boundary layer. In these cases, you can ensure that certain boundaries are modeled as fluid film only, and other boundaries are modeled as VOF only, irrespective of the mesh that is used.

If you specify a value of $α_{T R A N S} = 0$ , the fluid phase is modeled with VOF only. If you specify a value of $α_{T R A N S} = 1$ , the fluid film model is used.

Specifying the Resolved Fluid Film Volume Fraction as a Field Function

When you specify $α_{T R A N S}$ as a field function, remember that the profile is evaluated on the interface. The Fluid Film phase exists on one side of the interface only, so a field function such as FluidFilmThickness is undefined on the VOF phase side of the interface.

To specify a field function profile that is defined on both sides of the interface, you can use one of the techniques described below:

Wrap the FluidFilmThickness field function into the alternateValue field function, for example:
alternateValue((${FluidFilmThickness} <0.0005) ? 2:0,0)

The interpolation across the interface returns the arithmetic mean.
Map the fluid film thickness to both the liquid side and the gas side of the interface at each iteration. As shown in the following example, the MappedFluidFilmThickness variable is available on both sides of the interface:
(${MappedFluidFilmThickness} <0.0005) ? 1:0

The solutions of these two examples are equivalent. However, using the mapper adds more computational cost.

Resolved Fluid Film Field Functions

The following field function is made available to the simulation when the Resolved Fluid Film model is selected in a Film-VOF Phase Interaction. To activate this field function, the Temporary Storage Retained option must be activated in the Segregated Flow solver.

Film-VOF Mass Transfer Rate of [Film-VOF Phase Interaction]: The rate of mass transfer is evaluated at the interface between the fluid film and the VOF domain. The mass flow rate is provided in kg/s, with the sign convention corresponding to the direction of mass transport across the interface. You can evaluate this field function on either the fluid film shell region or the surrounding fluid region. For an interface boundary of a particular region, a positive value indicates mass transfer across the boundary into the region.