Defining the Phase Interactions

You require phase interactions to describe how the phases – water and air – mutually affect each other. The shape and position of the interface between the water film and air is influenced significantly by surface tension effects.

For this hybrid VOF-Fluid Film simulation, the water film is defined both as a Eulerian (VOF) phase and as a Fluid Film phase with the same material properties. Therefore, you require two separate phase interactions both with surface tension activated to describe the interdependency between the water film and air:

  • Film-VOF phase interaction between the air VOF phase and the water fluid film phase

  • VOF-VOF phase interaction between the air VOF phase and the water VOF phase

To model the transition of the water fluid film phase to the water VOF phase depending on mesh resolution, a third phase interaction is needed:

  • Film-VOF phase interaction between the water fluid film phase and the water VOF phase. This phase interaction invokes the Resolved Fluid Film model.

    The Resolved Fluid Film model automatically uses the model (fluid film or VOF) that is the most appropriate. The fluid film model is used in areas where a liquid film is present and where the numerical mesh is not sufficiently fine to resolve the flow and the distribution of phases with the VOF model. As the fluid film gets thicker, or as the film enters a region with a finer mesh, the flow of liquid film is modeled using the VOF model. Similarly, if a VOF liquid phase enters a region with a coarse mesh, or if the film thickness gets smaller, the flow of liquid is modeled using the fluid film model.

To define the phase interactions:

Create the Film-VOF phase interaction between the Air and the WaterFilm phases.
  1. In the Physics 1 continuum, right-click the Models > Multiphase Interaction > Phase Interactions node and select New > Air > WaterFilm.
  2. Rename the Phase Interaction 1 node to Fluid Film Surface Tension.
  3. For the Fluid Film Surface Tension phase interaction, open the Fluid Film Surface Tension Model Selection dialog and, in the Optional Models group box, select Surface Tension.

    The Multiphase Material model is selected automatically.

  4. Click Close.
A minimum film thickness of 1.0E-5 m and the default surface tension value for the air-water interface, 0.072 N/m, are appropriate for this simulation.
  1. Edit the Fluid Film Surface Tension > Models node and set the following properties:
    Node Property Setting
    Multiphase Material > Material Properties > Surface Tension > Constant Value 0.072 N/m
    Surface Tension Minimum Film Thickness 1.0E-5 m

Create the VOF-VOF phase interaction between the Water and the Air phases. When you create this type of phase interaction, you select the primary phase first, and then the secondary phase.

  1. Right-click the Models > Multiphase Interaction > Phase Interactions node and select New > Water > Air.
  2. Rename the Phase Interaction 1 node to VOF Water Surface Tension.
  3. For the VOF Water Surface Tension phase interaction, open the VOF Water Surface Tension Model Selection dialog and, in the Optional Models group box, select Surface Tension Force.

    The Multiphase Material model is selected automatically.

  4. Click Close.
The surface tension for the air-water interface must be set to the same value for both phase interactions.
  1. Select the VOF Water Surface Tension > Models > Multiphase Material > Material Properties > Surface Tension > Constant node and set Value to 0.072 N/m.
Create the Film-VOF Phase Interaction between the Water and the WaterFilm phases.
  1. Right-click the Models > Multiphase Interaction > Phase Interactions node and select New > Water > WaterFilm.
  2. Rename the Phase Interaction 1 node to Resolved Fluid Film.
  3. Reopen the Resolved Fluid Film Model Selection dialog and, in the Optional Models group box, select Resolved Fluid Film.
  4. Click Close.

    The Phase Interactions node appears as shown below:



  5. Save the simulation.