Defining the Phase Interactions and Flow Regime Transitions

Phase interactions describe the mutual influence of the two materials (phases) air and water on each other. In a hybrid multiphase setup, you also use phase interactions to control the transition of water into different flow regimes.

Overall, you require five phase interactions:
Phase Interaction Phases Flow Regime Transition Mechanism
VOF-VOF Water and air -
VOF-Lagrangian Water and water droplets Primary break-up: large droplets (VOF blobs) and ligaments are resolved by the VOF model. Ligaments subsequently break up into droplets. Only spherically shaped VOF blobs with a minimum blob sphericity of 0.7 and which are smaller than the maximum blob diameter of 5 cm are converted to Lagrangian droplets. The blob sphericity is defined as the ratio of surface area of a perfect sphere to actual surface area of the blob with the same blob volume. The Resolved Eulerian-Lagrangian Transition model handles this transition. When Lagrangian droplets hit the water free surface, these droplets are converted to VOF water by the Impingement model.
Film-Lagrangian Water droplets and water film The Lagrangian water droplets impinging on a water film surface are converted to thin unresolved fluid film. The fluid film absorbs the mass and the energy of the Lagrangian droplets. For this tutorial case, transition in one direction only is considered, that is, transition from droplet to film. Transition from fluid film to droplet by stripping is not considered.
Film-VOF Water film and water Depending on the volume fraction of water at the phase interface between air and water, VOF water transitions to fluid film water and the other way around. This transition is also influenced by the mesh resolution at the interface.
Film-VOF Water film and air -

The shape and position of the interface between water and air is influenced by surface tension effects. Therefore, you activate the surface tension force for all phase interactions that describe the relation between water and air.

To define the phase interactions:

  1. Create a phase interaction between the free surface water and air phases:
    1. Right-click the Physics 1 > Models > Multiphase Interaction > Phase Interaction node and select New > Air > Water.
      This action creates a VOF-VOF Phase Interaction between Air and Water..
    2. Rename the Phase Interaction 1 node as Air-Water.
    3. Right-click the Air-Water > Models node and select the following models:
      Group Box Model
      Optional Models Multiphase Material
      Surface Tension Force
    4. Click Close.
  2. Create a phase interaction between the VOF free surface water phase and the Lagrangian water droplets:
    1. Right-click the Multiphase Interactions > Phase Interactions node and select New > Water > Water Droplets.
    2. Rename the phase interaction as Water <-> Water Droplets.
    3. Select the Water <-> Water Droplets node and select the following models:
      Group Box Model
      Optional Models Resolved Transition
      Impingement
    4. Click Close.
    5. Edit the Resolved Transition > Eulerian-Lagrangian Transition Criteria node and set the following properties:
      Node Property Setting
      Blob Diameter Criterion Minimum Blob Diameter 0.0 m
      Maximum Blob Diameter 0.05 m
      Blob Shape Criterion Minimum Blob Sphericity 0.7
      Minimum Blob Inertia Tensor Eigenvalue Ratio 0.0
  3. Create a phase interaction between the Lagrangian water droplets and the water film:
    1. Right-click the Phase Interactions node and select New > Water Droplets > Water Film.
    2. Rename the phase interaction as Water Droplets -> Film.
    3. Right-click the Water Droplets -> Water Film > Models node and select the following properties:
      Group Box Model
      Optional Models Multiphase Material
      Impingement
    4. Click Close.
  4. Create a phase interaction between the VOF free surface water phase and the water film:
    1. Right-click Phase Interactions and select New > Water > Water Film.
    2. Rename the phase interaction as Water <-> Water Film.
    3. Right-click the Water <-> Water Film node and select the following models:
      Group Box Model
      Optional Models Multiphase Material
      Resolved Fluid Film
    4. Click Close.
  5. Create a phase interaction between the VOF air phase and the water film:
    1. Right-click the Phase Interaction node and select New > Air > Water Film.
    2. Rename the phase interaction as Air - Water Film.
    3. Right-click the Air - Water Film node and select the following models:
      Group Box Model
      Optional Models Multiphase Material
      Surface Tension
    4. Click Close.
  6. Save the simulation.