Free Surface Mesh Refinement Reference

The Free Surface Mesh Refinement criterion is used to control Adaptive Mesh Refinement with respect to the interface between phases in a VOF multiphase simulation. To improve the resolution of the interface between the phases, this criterion lets AMR identify the cells near the interface and refine 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 Free Surface Mesh Refinement criterion does not refine the mesh away from the interface. If you want to resolve flow features away from the interface, you can add a User-Defined Mesh Adaption criterion, or specify a larger transition width. For more information on these settings, see Adaptive Mesh Model Reference.

Table 1. Free Surface Mesh Refinement Reference
Theory	See Free Surface Mesh Refinement.
Provided By	Right-click Continua > Physics 1 > Models > Adaptive Mesh > Adaptive Mesh Criteria and select New > Free Surface Mesh Refinement
Example Node Path	Continua > Physics 1 > Models > Adaptive Mesh > Adaptive Mesh Criteria > Free Surface Mesh Refinement
Requires	Time: Implicit Unsteady Material: Multiphase. Multiphase Model: Volume of Fluid (VOF).
Properties	Enabled, Max Refinement Level, Resolution Criterion for Interface Detection, Swept Distance Estimation Factor [Upstream, Downstream], Limit Cell Size. See Free Surface Mesh Refinement Properties.
Activates	Field Functions	VOF Transported Interface Marker See Field Functions.

Free Surface Mesh Refinement Properties

Max Refinement Level

Each time a cell is refined, child cells are generated with half of the original base size. The refinement level can increase (refine) or decrease (coarsen) by only one level at each AMR event.

This value specifies the maximum number of times that a cell can be refined by the Free Surface Mesh Refinement criterion. This criterion does not request refinement for any cells for which the specified limit is reached. However, the cells can be refined by another active criterion at the next AMR event, if the other criterion has a higher limit specified.

Enabled

Activates the Free Surface Mesh Refinement criterion in the simulation.

Resolution Criterion for Interface Detection

A sensitivity parameter that is used to identify the cells that contain the interface. An interface that is resolved by this number of cells or less can be detected and used to trigger mesh refinement. However, an interface that is smeared over more cells than this number is no longer considered to be an interface, so does not trigger mesh refinement at the next AMR event.

This value is $r_{i}$ in Eqn. (2600). Larger values generally give more tolerant behavior, allowing smeared interfaces to be detected.

Swept Distance Estimation Factor [Upstream, Downstream]

These values are scaling factors that multiply the estimated distance that is swept by the interface in the upstream and downstream flow directions between two successive AMR events. Downstream refers to the cells that are in front of the interface, as seen relative to the flow field; Upstream refers to the cells that are behind the interface. For example, the default value of 1.5 for the downstream direction means that the interface marker is transported for 1.5 times as long as the estimated time until the next AMR event.

These values are $C^{U p}$ and $C^{D o w n}$ in Eqn. (2602). If the interface tends to run out of the refined mesh zone, you should increase these values.

Limit Cell Size

When enabled, at each AMR event, the size of each cell that is flagged for refinement is compared with the value that is specified in the Min Adaption Cell Size node. The Free Surface Mesh Refinement criterion does not request refinement for any cells that would become smaller than the specified limit after refinement. However, the cells can be refined by another active criterion at the next AMR event, if the other criterion has a lower minimum size limit specified.

The default minimum cell size is 0.0 m. The minimum cell size profile can be specified as a constant or a field function.

This setting is similar to the Min Adaption Cell Size that is set in the Adaptive Mesh model. However, this property can be specified more generally, such as by a field function that can vary spatially.

Field Functions

The following field function is available when the Free Surface Mesh Refinement criteria is activated:

VOF Transported Interface Marker: The transported marker $I$ that is determined as $I = \max (I^{D o w n}, I^{U p})$ . For more information on this value, see Free Surface Mesh Refinement.