Uses of virtual topology include:
Before performing analysis of a CAD model, you may want to group faces/edges together to form virtual cells. In such cases, virtual topology can aid you in reducing the number of elements in the model, simplifying small features out of the model, and simplifying load abstraction.
You can split a face to create two virtual faces, or split an edge to create two virtual edges. For example, in the case of a rectangular face in which a single edge on one side of the face corresponds to two edges on the opposite side of the face, you can split the single edge so that node alignment across the face can have similar spacing.
When needed, you can create virtual hard vertices to facilitate split face operations.
Virtual topology can be helpful for handling fillets for MultiZone and sweep meshing. See Using Virtual Topology to Handle Fillets in MultiZone Problems.
A CAD Model has two parts:
Topology: The connectivity of a CAD model, meaning: vertices are connected to edges, which are connected to faces, which are connected to volumes. Each one of these entities is referred to as a cell.
Geometry: The geometry of the CAD model is the underlying mathematical definition of the aforementioned cells.
A virtual cell in the Mechanical application or the Meshing application modifies the topology of only the local copy in the Mechanical application or Meshing application. Your original CAD model remains unchanged. New faceted geometry is also created with virtual topology. However, the mesher may project the nodes back to the original geometry where applicable.
You can use Virtual Topology to simplify the geometry to aid in meshing. There are several ways to simplify the topology using either automatic, manual, or a combination of automatic and manual approaches. The best approach to use is generally based on the meshing objectives (number of elements desired) and the cleanliness of the CAD model.
Virtual cells are often created to:
Reduce the element count (increase the mesh size).
Fix the topology of bodies that are not sweepable, to have mappable faces so that the bodies are now sweepable.
Fix meshing problems.
To reduce the element count, it is often a good idea to first use Automatic or Repair operations to reduce the number of faces/edges, and then use Manual virtual topology operations to ensure important topology is respected and/or features that could create mesh quality problems are removed.
To fix the topology of bodies make them sweepable. Automatic and/or Repair operations might be helpful, but it is often very dependent on the geometry. Manual virtual topology operations give you more control and are often the better approach.
Meshing problems sometimes occur because of the topology. When a meshing failure occurs it generally points to the offending topology. Using Manual virtual topologies is a good way to fix such problems.
For more information about the Automatic and Manual approaches to creating virtual cells, see:
Note:
There are geometric limitations to creating virtual cells, including those related to cells that would have too much curvature, or other limitations in trying to represent a group of faces by a single face.
The tesselation of models from CATIA4 may not be appropriate for virtual topology, which could prevent the creation of virtual cells for these models.