Theory
Before meshing, surfaceFeatureExtract scans the combined surface (every patch together, so the seams between patches count) and writes the feature edges to an .eMesh file. Its setting includedAngle is this value: an edge is marked when the included angle between its two faces is smaller than it, equation (1). OpenFOAM's annotated dictionary gives the end points: 0° selects no edges, 180° selects every edge.
The feature edges are used in two places:
- Refinement. Cells that the edges cross are refined one level beyond the surfaces during castellation (level in uniform sizing, the finest wall level in curvature sizing).
- Snapping. With explicit edge snapping (see Edge snapping) the mesh points are also pulled onto these edges. With implicit snapping (the default) the mesher finds the edges from the surface itself, and the file is used for the refinement only.
This setting is neither the mesher's resolveFeatureAngle (the curvature threshold, set from Cells across a pipe diameter or 30°) nor the layer stage's featureAngle (fixed at 60°, beyond which layers stop at a convex edge). The three angles have different meanings and are set separately.
Equations
| where | meaning | unit |
|---|---|---|
| Included angle between the two faces at the edge (180° = flat) | ° | |
Feature angle (includedAngle) | ° | |
| Unit normals of the two faces | - |
Default and rule
- Default
- 60°Measured
- Rule
60°: only edges sharper than 120° of fold are features.
- Source
Validation on case V2 (a two-inlet single-chamber muffler with internal inlet and outlet tubes), all other settings equal:
- 60°: maximum skewness 4.69, layer coverage 99.2 %, 210k cells;
- 150°: maximum skewness 9.83, layer coverage 97.7 %, 255k cells (+22 %).
150° extracts more edges, and hence more cells, but on a real multi-body model each extra edge is another snapping constraint, and the mesh quality degrades. An earlier, separate comparison showed the same trend. The included-angle reading of
includedAngle, equation (1), is the program's interpretation; it is consistent with the end points 0° and 180° given in OpenFOAM's annotatedsurfaceFeatureExtractDict. The surface feature tools are part of the mesher's workflow (OpenFOAM user guide, 2026).
Valid range 0° (no edges) to 180° (every edge)
Values
0° to 180°. The feature edges are extracted again with this value every time a mesh is built.
High values
Above 90° every right-angled port rim, pipe end and baffle joint becomes a feature: more cells and, on V2, a worse mesh.
One model
The comparison of 60° and 150° was made on one muffler.