How to avoid un-oriented triangles?

[xusho]

When creating triangles manually by Mesh->Create Triangles tool, surface mesh may have un-oriented triangles and Dual Domain analyses may fail.

Why some triangles are not oriented?

For Dual Domain meshes, nodes of each triangle are stored in counter clockwise direction as image (a) below, and triangle normal vectors point outwards (right hand rule).

Suppose we have a hole on a triangular mesh, as image (b) below, and we want to fill the hole by creating triangles manually.

As in image (c), an existing triangle A has nodes with labels 1, 2, 3.

Now if we create triangle B next to triangle A, users may click 3 nodes either in clockwise order (4,1,2) or counter clockwise order (4,2,1). Internally, nodes for triangle B will be stored as (4,2,1), and it is oriented automatically based on triangle A. In this case, no un-oriented triangles will be in the mesh.

However, if we create triangle C first, it does not share edges with any existing triangles, so it is impossible to orient triangle C automatically. In this case, the mesh may have un-oriented triangles, depending on how we click nodes to form triangle C.

How to avoid un-oriented triangles?

1. If possible, create new triangles next to existing triangles and grow layer by layer.
If you need to fill a hole, start to create triangles at the boundary of the hole. Avoid creating triangles that do not share edges with existing triangles.

2. If you have to create triangles without sharing edges with others, click Orientation after all triangles are created.

3. For Dual Domain meshes, click nodes in counter clockwise order when using Create Triangles tool.

4. For midplane meshes, before creating new triangles, find an existing triangle as reference, click Utilities->Query, its node labels will be displayed, and you can figure out whether they are in clockwise order or counter clockwise order. Click nodes in the same order to form new triangles.

5. Click Mesh->Orientation at end of manual editing.

6. If you import triangular meshes from 3rd party, click Orientation before analyses.