Understanding Surface Mesh Diagnostics for Multi-Part Cases
Whether the surface belongs to single or multiple parts affects the diagnosis of non-manifold edges, proximity and self-intersection.
For example, if one of these errors is on an interface between parts, diagnosing a single part does not report the error. However, the diagnostics tool calculates free edges on a per-part basis, which is independent of the number of parts that are selected for diagnostics. See Changing the Parts in the Active Model.
The main consideration in multiple parts cases is whether an interface exists between two neighboring parts. This section aims to provide an overview of what to expect when diagnosing multiple part cases, and how to recognize and fix surface errors. Some surface errors, such as free edges, do not affect the surface remesher. However, having any form of non-closed or non-manifold surface causes the volume meshers to produce an error message and does not generate a mesh.
This section uses a simple example, two cube surfaces, to highlight the different scenarios possible when dealing with multiple parts or parts. The diagram below illustrates the terminology:
The examples below focuses on whether each cube is closed (that is, has a surface at the interface plane) and whether an interface or contact exists between the two cubes. Additionally, this section covers an example where two parts or parts share edges, due to merged vertices.
It is important to note, the surface diagnostics tool identifies errors regarding the active model and not on a per-part basis, except for free edges as noted earlier. Therefore, select all required parts in the Surface Repair Options dialog before staring surface repair, otherwise the diagnostics tool does not identify certain errors.
Two Connected Boundaries with No Interface
This situation is the simplest, where the two cubes are closed and no interface exists between the boundaries at the interface plane.
The surface diagnostics do not report any free edges or non-manifold edges in this instance. However, the diagnostics tool identifies interface boundaries as being close and piercing one another. Attempting to volume mesh in this situation results in an error message reporting self-intersecting surfaces (providing you are not using per-part meshing).
To fix these surface errors, create an interface between the two boundaries at the interface plane. This fix allows the volume mesher to proceed and creates a conformal mesh between the two parts, if supported by the meshing model. See Creating a Boundary Interface.
Two Connected Boundaries with an Interface
This situation is the same as the previous situation, but with an in-place interface created between the boundaries at the interface plane.
Surface diagnostics report no issues in this instance, and the meshers are able to create a volume mesh. This situation is the ideal situation for using the Simcenter STAR-CCM+ meshing pipeline.
One Surface with No Interface
This section considers two situations. The first situation uses cubes without shared edges, that is, two disconnected cubes. The second situation uses cubes with shared edges, that is, two connected cubes.
In the first situation, the diagnostics tool marks the edges on the part containing the open surface as free edges. Attempting to volume mesh in this situation results in an error message reporting one part as open. To fix this error, close the open part by filling the hole, using the surface repair tools. Then create an interface between the two boundaries, which provides the ideal situation that is described above. The meshers are then able to create a volume mesh.
In the second situation, the diagnostics tool marks the edges on one part as free edges, since the surface only belongs to one part. Attempting to volume mesh in this situation produces the same error as above, reporting one part as open. To fix this error, close the open part by filling the hole, using the surface repair tools. See Repairing Surfaces.
However, this fix appears to swap one problem, free edges, for another, non-manifold edges. Therefore, create an interface between the two boundaries, which resolves the non-manifold edges, and allow volume meshing.
No Interface Boundaries, with No Interface/ Between Parts
In this situation, both cubes do not have an interface, and the surface diagnostics marks the open ends of both parts as free edges.
To fix this error, close each part separately by starting surface repair with only one part that is selected as the active model. Then create an interface between the two parts, which allows the volume mesher to run.