Modeling a Fluid Film with Different Characteristic Length Scales

You can use the Resolved Fluid Film model to simulate free-surface flows in cases where the fluid film model is not adequate in all places or over the entire simulation time, and where using the VOF model requires fine meshes and unacceptably large computing times.

The two modeling approaches are combined so that the fluid film approach is used in regions where a thin liquid film is present and where the numerical mesh is not sufficient to resolve the flow and distribution of phases with the VOF model. As the fluid film gets thicker, or as the film enters regions with finer meshes, the flow is modeled using the VOF model. Similarly, if a VOF liquid phase enters a region with a coarse mesh, or if the liquid thickness gets smaller, the flow of liquid is modeled using the fluid film model.

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 the interface and the transition volume fraction ${\alpha }_{TRANS}$ that is specified for the interface:

• Fluid film transitions into the VOF phase when the total volume fraction in the neighbor volume cell is greater than the specified transition volume fraction ${\alpha }_{TRANS}$ .
• The VOF phase transitions into fluid film when the total volume fraction is less than the specified transition volume fraction ${\alpha }_{TRANS}$ .

To ensure stability, mass is transferred at a finite rate: the transition of mass from one phase to the other takes place over a few iterations rather than in a single time-step. For more information, see Resolved Fluid Film.

Due to the no-slip condition for the film at the wall boundary, the film velocity can be different to (usually less than) the velocity of the VOF mixture in the first cell next to the boundary. In this scenario, when mass is transferred from the film to the VOF mixture, it decelerates the bulk mixture flow. To reduce this unphysical effect, you can equalize the velocity of the film phase and the VOF phase before the transition.

The Film-VOF Velocity Equalization 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 ${\alpha }_{TRANS}$ . For more information, see Film-VOF Velocity Equalization.

The Resolved Fluid Film model can be used with both single-component and multi-component fluids.

However, the following conditions apply:

• A complete mixture mapping is required. Every component in the fluid film must be mapped to the corresponding component in the liquid VOF phase.
• Both the fluid film and VOF phases must consist of the same material components. However, the mass fractions of the components can be different in the initial material composition of the two phases.
• A fluid film or VOF phase can use only a single instance of the Resolved Fluid Film model.

  A particular phase cannot be included in two different Film-VOF phase interactions. For example, you cannot model two fluid film phases that are flowing into a single VOF phase.