Setting Up Free Surface Mesh Refinement

Free Surface Mesh Refinement is an Adaptive Mesh Refinement (AMR) criteria designed for VOF multiphase simulations. Adaptive Mesh Refinement refines or coarsens cells in the volume mesh based on specified criteria, which means that you use a fine mesh only in the areas where the simulation requires it. Solution quantities are automatically interpolated to the adapted mesh.

In a VOF multiphase simulation, you can use AMR to improve the resolution of the interface between phases. The Free Surface Mesh Refinement criterion identifies the cells near the interface and refines the mesh accordingly. As the interface develops and moves during the simulation, AMR continually regenerates the mesh to ensure that the interface is resolved and tracked efficiently.

The HRIC scheme that is used in VOF simulations requires that the CFL number be kept below 0.5 in order to preserve a sharp interface between the fluid phases. For more information, see High-Resolution Interface Capturing (HRIC).

To keep the CFL number below 0.5 you must specify a sufficiently small time-step. Alternatively, you can use the Adaptive Time Step Model.

The Free Surface Mesh Refinement criterion is not suitable for steady VOF cases and so can only be selected for unsteady cases.

To set up Free Surface Mesh Refinement:

  1. In the [physics continuum], select the Adaptive Mesh model from the Optional Models group box.
  2. Right-click the Adaptive Mesh > Adaptive Mesh Criteria node and select New > Free Surface Mesh Refinement.

    This criterion is intended for unsteady simulations that model the interface between VOF phases. The User Defined Mesh Adaption criterion is also available for VOF simulations. You can use either or both of these criteria in a VOF multiphase simulation.

    For more information on the available adaptive mesh criteria, see Adaptive Mesh Refinement.

  3. Select the Adaptive Mesh node and set the appropriate properties.

    The important properties are:

    • Transition Width

      Specifies the width of any transitional zones between different refinement levels. This value is the number of cells of refinement level N that separate cells of level N+1 from cells of level N-1.

      The AMR algorithm attempts to avoid large volume changes between cells, so the refinement levels of two adjacent cells cannot differ by more than one.

    • Limit Cell Size

      When activated, you can specify a minimum cell size (Min Adaption Cell Size).

      During the mesh refinement process, the predicted size of each child cell is compared with this value. A cell is not refined if the child cell sizes would be below the specified limit.

    For more information on these, and the other global Adaptive Mesh model properties, see Adaptive Mesh Properties.

  4. Select the Adaptive Mesh > Adaptive Mesh Criteria > Free Surface Mesh Refinement node and set the appropriate properties.

    An important property is Max Refinement Level. This value specifies the maximum number of times that a cell can be refined by the Free Surface Mesh Refinement criterion.

    The refinement level should be chosen in conjunction with the initial mesh. Choose the lowest value that is sufficient to resolve the features of interest. If you have a relatively fine initial mesh, do not specify a high value for this property as the cell count can grow rapidly to an impractical number. You are advised to start with the default value of 2 and, if necessary, increase the level incrementally until you obtain reasonable results.

    For more information on this property, and the other properties, see Free Surface Mesh Refinement Properties.

To ensure that a sharp interface between phases is maintained, the Free Surface Mesh Refinement criterion avoids refining cells that already contain the interface. However, when other model-driven or user-defined criteria insist on refinement, these refinement requests have highest priority. Also, when the interface starts showing small scale features that need to be resolved, a refinement of interface cells might be required. By default, Simcenter STAR-CCM+ maps the volume fraction field from coarse to fine cells using a simple injection scheme (zero order interpolation). However, when interface cells are refined, this interpolation strategy can smear the interface over several cells. For applications that show a significant amount of smearing due to the default injection scheme, you can apply an interpolation strategy that is based on interface reconstruction. This strategy maintains a sharp interface on the refined mesh but is computationally more expensive than the default strategy. Additionally, the sharp interpolation strategy requires that the interface is well-resolved on the coarse mesh.

To apply the sharp interpolation strategy:

  1. Select the Models > Volume Of Fluid (VOF) > Adaptive Mesh Interpolation node and set Option to Sharp Reconstruction.
    For more information on this property, see Free Surface Mesh Refinement.

By default, mesh refinement is triggered every five time-steps for unsteady flow. You can modify this frequency to suit the requirements of your simulation. More frequent mesh refinement means that fewer cells are refined at each AMR event, but the AMR computation overhead can be more significant. The optimal AMR period depends on the application. The default setting of 5 time-steps should be suitable for most cases.

If your simulation uses adaptive time-stepping, the time-step is not known in advance, so it can be difficult to predict the most suitable update frequency. To be conservative, you should update the mesh every one or two time-steps.

  1. Select the Solvers > Adaptive Mesh > Trigger node and set Trigger Mode to one of the following:
    • Time Step

      Triggers mesh refinement every n time-steps.

    • Delta Time

      Triggers mesh refinement when the simulation time advances by a specified amount.

    The Iteration and Update Event triggers are not supported for Free Surface Mesh Refinement. The Iteration criterion applies to steady simulations only. The Update Event criterion cannot be used, as mesh refinement is based on a prediction of the free surface location at the time that AMR is triggered. For an Update Event trigger, this point in time is unknown so such a prediction is not possible.
  2. Select the corresponding child node and set the trigger properties:
    • Time-Step Frequency

      Set the Frequency, the number of time-steps between mesh refinements. If you set this value to 0, mesh refinement is not triggered automatically. You can manually perform an AMR pass using the Refine/Coarsen Mesh Once context menu option on the Adaptive Mesh solver.

      To ignore the first few time-steps, set the appropriate Start value. To end mesh refinement after a particular number of time-steps, activate Enable Stop and set the appropriate Stop value.

    • Delta Time Frequency

      Mesh refinement is triggered when the simulation time advances by the amount that you specify in the Delta Time property.

      Set the appropriate Start Time and Delta Time. To end mesh refinement at a particular time, activate Enable Stop and set the appropriate Stop Time.

    For more information on the solver properties and context menu options, see Adaptive Mesh Solver Reference.

By default, AMR is enabled on all regions. If necessary, you can disable mesh refinement on specific regions.

  1. To deactivate AMR on a region, select the Regions > [region] > Physics Conditions > Adaption Option node and deactivate Enable Adaption.