Resolved Fluid Film

The Resolved Fluid Film model automatically uses the phase representation (fluid film or VOF) that is most suitable for the local flow conditions and mesh resolution.

The transition between the fluid film and VOF phase models is based on the volume fraction of liquid (the total of the fluid film phase and the VOF phase combined) at an interface and the transition volume fraction $α_{T R A N S}$ that is specified for the interface:

Fluid film transitions into the VOF phase when the total volume fraction in the neighbor cell is greater than the specified transition volume fraction $α_{T R A N S}$ .
The VOF phase transitions into fluid film when the total volume fraction is less than the specified transition volume fraction $α_{T R A N S}$ .

At every iteration, Simcenter STAR-CCM+ determines the most appropriate physics model by considering the volume fraction:

Figure 1. EQUATION_DISPLAY

α_{h} = \frac{V_{V O F} + V_{f ilm}}{V_{c ell}}

(2841)

where:

$V_{V O F}$ is the volume of the VOF liquid phase in the cell.
$V_{f ilm}$ is the volume of fluid film in the cell.
$V_{c ell}$ is the volume of the cell.

Values of $α_{T R A N S}$ from 0 through 1 give the expected mesh-dependent behavior. If $α_{h} > α_{T R A N S}$ , the fluid film is modeled as VOF; otherwise, it is modeled as fluid film. By default, $α_{T R A N S} = 0.8$ .

If $α_{T R A N S} \leq 0$ , the liquid is modeled as VOF only. If $α_{T R A N S} \geq 1$ , the fluid film model is used.

The transfer of mass between the fluid film and the VOF liquid phase is:

Figure 2. EQUATION_DISPLAY

\dot{m} = (α_{h} - α_{T R A N S}) \cdot \frac{\tilde{m}}{τ}

(2842)

where:

$\dot{m}$ is the mass transfer rate in $k g / s$ . This value is positive for fluid film $\to$ VOF mass transfer.
$\tilde{m}$ is the mass of liquid that is available for transfer.
$τ$ is the time scale. This value is of the order of the time-step size.

Film-VOF Velocity Equalization

When the Film-VOF Velocity Equalization option is activated in the Resolved Fluid Film model, a momentum source term is applied. This momentum term adjusts the velocity of the film in the shell region, and the velocity of the VOF mixture in the first volume cell layer next to the shell region (subsequently referred to as the VOF cell), by a factor proportional to the ratio of the film thickness and the height of the VOF cell. The effect of this velocity adjustment ensures that the velocity of the VOF cell is very similar to the film velocity at the transition volume fraction $α_{T R A N S}$ .