Modeling Volume of Fluid Flow

The Volume of Fluid (VOF) Multiphase model is used in cases when each phase constitutes a large structure, with a relatively small total contact area between phases. This model is used to solve problems involving immiscible fluid mixtures, free surfaces, and phase contact time.

In such cases, there is no need for extra modeling of inter-phase interaction, and the model assumption that all phases share velocity, pressure, and temperature fields becomes a discretization error. A good example of this type of flow is sloshing flow in a water tank, where the free surface always remains smooth.

To set up a VOF multiphase simulation:

  1. Set up the Volume of Fluid (VOF) model in the physics continuum.
    See Setting Up the Volume of Fluid (VOF) Model.
  2. Define the composition of the multiphase mixture.
    The Eulerian Multiphase model controls the definition of Eulerian phases within the physics continuum. You specify the bulk properties and phase composition of the fluid mixture.

    See Defining the Eulerian Phases.

  3. Specify the material components for each phase, and set the material properties.
    You set the material properties for each individual phase and set the material properties for the mixture. For a multi-component phase, you also set the material properties for each individual component of the phase.

    See Specifying the Material Components.

  4. Define the phase interactions.

    See Controlling Phase Interactions in VOF Simulations.

  5. Set the initial volume fraction of each phase.
    See Setting Initial Conditions.
  6. Set the Phase Source Option for each fluid region.
    See Setting Region Conditions.
  7. (Optional) Set up any porous regions.

    You specify the porosity of the region, the porous inertial resistance and the porous viscous resistance for each phase, and any volume fraction sources that are required.

    See Porous Regions Workflow.

If you want to simulate effects such as diffusion due to capillary effects or osmotic pressure, specify the appropriate momentum sources for each phase.
  1. Select the Regions > [physics continuum] > Phase Conditions > [phase] > Physics Conditions > Momentum Source Option node and set Momentum Source Option to Specified.

    See Region Settings.

  2. Set the boundary conditions for the flow.
    See Setting Boundary Conditions.

If you want to reduce the run-time of your simulation without affecting quality, you can use Multi-Stepping and/or Adaptive Time-Stepping. By sub-stepping the volume fraction transport equation with a reduced time-step, Multi-Stepping allows you to increase the global time-step to reduce computational costs. Adaptive Time-Stepping allows you to control the time-step based on physics or numerical conditions.

  1. If you want to use multi-stepping to improve the interface resolution without increasing the global time-step:
    1. Select the Solvers > Segregated VOF node and set Solution Strategy to either Implicit Multi-Step or Explicit Multi-Step (Deprecated).

      When this option is selected, the Segregated VOF solver performs multiple steps per time-step. See Multi-Stepping Guidelines.

    2. Select the Implicit Multi-Step or Explicit Multi-Step (Deprecated) node and specify the Explicit Multi-Step (Deprecated) Solver Properties or Implicit Multi-Step Solver Properties .
  2. If you want to apply automated time-step control, select the Adaptive Time-Step model and set the Adaptive Time-Step solver properties.

    See Setting Up Adaptive Time-Stepping.

  3. If you want to use automated mesh refinement to improve the interface resolution or reduce the computation time, select the Adaptive Mesh model.

    Activate the Free Surface Mesh Refinement criteria and specify the appropriate properties.

    See Setting Up Free Surface Mesh Refinement.

  4. Set the appropriate stopping criteria.

    You can optimize the computational time of your VOF simulation by automatically adjusting the number of inner iterations per time-step.

    See Setting Up Automatic Inner Iterations.