Local Meshing

Local meshing speeds up the re-execution of mesh generation after changes are made to the geometry or mesh settings (and you already have an initial completed mesh). With this feature, Simcenter STAR-CCM+ remeshes surfaces or volume within a defined location without regenerating a complete mesh.

Depending on your meshing strategy, you can generate the first mesh without any local refinements, and then take advantage of local meshing to focus only on specific areas as you test different refinements. Similarly, if you expect to modify geometry at a particular location, you can surround the location with a volume that permits Simcenter STAR-CCM+ to remesh the surfaces or volume within it.

With local meshing, you set limits on where you want remeshing to occur by defining local extents. There are two type of extents that you can define:

Surface Extent — accepts a set of part surfaces that define the remeshing zone. When used with volume remeshing, the mesher extracts volume extents based on the selected surfaces.
Volume Extent — accepts any part within the simulation that defines a volume. When used with volume remeshing, the mesher extracts volume extents based on the selected surfaces. For surface meshing, within the volume are candidates for remeshing.

You can additionally extend the influence of the extents by setting inflation properties on each extent.

Local surface meshing: Local surface meshing re-runs the surface remesher only on surfaces specified by surface extents or that lie within the volume defined by volume extents.

Local volume meshing

Local volume meshing re-runs the volume mesher only on volumes specified by local extents without regenerating a complete mesh. In cases where there are design changes, you can use local volume meshing for the meshing process. For example, if you wanted to change the design of a wing mirror and rear wing of a car, you can specify a volume around each part and remesh only that part without re-meshing the entire vehicle.

Note	Local volume meshing is only available for the trimmed cell mesher and prism layer mesher. Local remeshing only operates in serial mode, hence you are advised to run the initial global mesh generation using parallel, and then switch to serial for the remeshing.

To use local meshing:

Create an automated mesh operation, select the required meshers, and generate a global mesh using parallel mode. To set this mode, select the automated mesh operation and set Mesher Execution Mode to Parallel.
Select the automated mesh operation, set Mesher Execution Mode to Serial and activate the Perform Local Meshing option.
The Local Extents node is added to the automated mesh operation.
Create extents that define the portions of mesh for which remeshing is active:
- For surface extents:
  1. Right-click the Local Extents node and select New > Surface Extent.
    A Surface Extent node is added to the Local Extents node.
  2. Select the Surface Extent node and set Surfaces to include all part surfaces that you want to remesh. You can select multiple part surfaces from different bodies.
- For volume extents:
  1. Create a shape part that encloses the part surfaces that you want to remesh. You can create multiple shape parts to cover different areas on the geometry.
    In general, set up volume extents so that boundaries pass through well-resolved surface and volume cells. When using volume extents in part-replacement scenarios, ensure that the extents cover both the old (removed) parts and the new (added) parts.
  2. Right-click the Local Extents node and select New > Volume Extent.
    A Volume Extent node is added to the Local Extents node.
  3. Select the Volume Extent node and set Parts to the shape part you created earlier. You can select multiple shape parts.
Use the inflation properties to extend the scope of the extent into surrounding surfaces. This extension can help to increase the quality of the remeshed surface, as any parts of surfaces that fall within the extended zone are included in the remeshing.

Typically, you should aim to have 5 layers of triangles between the surface parts that are remeshed and the surfaces that remain unchanged. You can control this transition either through inflation distance, in which case you have to roughly convert the existing mesh size to an absolute distance, or with inflation layers, for which you select the number of layers to grow.

When regenerating volume mesh, the local volume mesher extracts cells within volume extents. Two layers of cells are added to the extracted set by default. In some cases, extra cells or cell layers are added automatically if the local volume mesher identifies problems. Hence, in some cases, cells outside the volume extent can change without anything being added due to the Inflation Distance or Inflation Layers properties.

To set the extent parameters, see Local Meshing Extents Properties.
Update the geometry or mesh settings. For example, you might create a custom mesh control on surfaces within an extent.
Re-execute the mesh pipeline.

If you experience difficulty in generating a local mesh, try the following steps:

Avoid passing extent boundaries through negative volume cells. Simcenter STAR-CCM+ immediately grows extents to avoid negative volume cells.
Avoid passing extents through locations where automatic surface repair made changes.

Local Meshing Extents Properties

Inflation Distance — specifies a distance beyond the extent. Any surfaces falling within this distance are included in remeshing, regardless of whether they are connected to the original surface parts covered by the extent.
Inflation Layers — specifies a number of adjacent bands of triangles by which the extent can grow. A negative value grows the stitching inside the extent and a positive value grows the stitching outside the extent. The default value is -1. For this option, Simcenter STAR-CCM+ only enlarges the extent along connected surfaces. If there are two disconnected parts close to each other, the inflation layer does not grow from one part to the other.
Best practices when using Inflation Layers:
- Choose inflation layers such that it stays within the inflation distance. For example, if there is space for only 4 layers inside the pseudo-volume extent generated based on the inflation distance, then specifying inflation layers to 5 or above has no additional effect compared with 4 layers.
- Choose to grow in or out depending on where the geometry is cleaner. If there are multiple surfaces and small features close to the outside of the surface extent then choose a positive value.
- Do not use Inflation Layers to 0 or 1 if the surface extent touches the other parts at a sharp angle. With these settings the mesher can try to stitch edges on the sharp perimeter and have trouble finding the correct orientation if triangle qualities are poor.
Note When you grow stitching outside the geometry, the mesher can inadvertently modify faces which are outside the defined extent.