Setting Up Adaptive Mesh Refinement
In Simcenter STAR-CCM+, the Adaptive Mesh model provides the general framework for automatic mesh adaption. For VOF multiphase simulations an additional option is provided that supplies the Adaptive Mesh model with adaption criteria based on the position of the free surface.
Here, you activate adaptive mesh refinement using the model-based adaption criterion for free surface flows. This adaption 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 a sharp interface is resolved and tracked efficiently. AMR not only refines cells but also coarsens those that were previously refined when the interface moves to a different location.
To set up adaptive mesh refinement with the free surface criterion:
- Reopen the Physics 1 Model Selection dialog and select the Adaptive Mesh model from the Optional Models group box.
-
Right-click the node and select .
The corresponding node is added under the Adaptive Mesh Criteria node.
Set the global Adaptive Mesh model properties. The important properties are Transition Width and Limit Cell Size.
The 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. In this simulation, you want to resolve the flow field near the interface, as well as the interface itself, so you specify a transitional zone thickness of five cells. This setting also ensures that the mesh does not change suddenly from a coarse mesh to a fine mesh, which can cause numerical stability issues.
When Limit Cell Size is activated, the Min Adaption Cell Size specifies the minimum size for a refined cell. A cell is not refined if the child cell sizes would be below the specified limit. The default value of 1.0E-6 m is acceptable in this simulation.
The Prism Cell Refinement default setting is appropriate for this simulation. The initial mesh does not contain any prism cells.
-
Select the node and set the following properties:
Property Setting Transition Width 5
Limit Cell Size Activated
Min Adaption Cell Size 1.0E-6 m
Set the Free Surface Mesh Refinement properties. The most important property is Max Refinement Level, which specifies the maximum number of times that any cell can be refined. The refinement level is chosen in conjunction with the initial mesh—choose the lowest value that is sufficient to resolve the feature of interest. If the initial mesh is relatively fine, a low value is appropriate. The default value of 2 is suitable for this simulation.
-
Select the node and set the following properties:
Property Setting Max Refinement Level 2
Enabled Activated.
The default value for Swept Distance Estimation Factor is acceptable because AMR is triggered at every time-step, so the interface is unlikely to run beyond the refined mesh zone.
By default, adaptive mesh refinement is triggered every five time steps for unsteady flow. In simulations that use adaptive time-stepping, the time-step is not known in advance, so you cannot predict the most suitable refinement frequency. In this simulation, take a conservative approach and refine the mesh at every time-step.
- Select the node and set Frequency to 1.
You can perform adaptive mesh refinement on specific regions. By default, adaptive mesh refinement is enabled on all regions.
- Select the node and check that Enable Adaption is activated.
- Save the simulation.