smoothMesh

OpenFOAM mesh smoothing tool to improve mesh quality. Moves internal mesh points by using the Centroidal smoothing algorithm (a version of the Laplacian smoothing algorithm, which uses surrounding cell centers instead of the neighbour point locations to calculate the new point position). Optional heuristic quality constraint options exist to constrain the smoothing, to avoid self-intersections. No changes to mesh topology are made.

smoothMesh demo video illustrates mesh smoothing on an example mesh, cross-section from which is shown below. Without quality constraints, centroidal smoothing would move the point highlighted in blue to the location highlighted with green, which is outside of the domain, and therefore smoothing tends to create self-intersecting mesh in the concave part of the geometry in this case. Self-intersections like this can be avoided by using additional quality constraints, which restrict the movement of vertices.

Current features and restrictions

• Works on 3D polyhedron meshes
• Requires a consistent (not self-intersecting or tangled) initial mesh with "good enough" quality
• Smoothes internal mesh points only (boundary points are frozen)
• Developed on OpenFOAM.com v2312

Warning: This tool is still under active development!

Compilation instructions

. /usr/lib/openfoam/openfoam2312/etc/bashrc
cd smoothMesh/src
wclean; wmake

Command line options

Basic options

• -centroidalIters specifies the number of smoothing iterations (default 20).

• -maxStepLength is the maximum length (in metres) for moving a point in one iteration (default 0.01). Adjust this value for your case. Smoothing process seems to be stable when this value is smaller than about one tenth of minimum cell side length.

• -minEdgeLength defines edge length below which edge points are fully frozen at their current location, but only if edge length would decrease during smoothing (default 0.05). Adjust this value for your case.

• -totalMinFreeze option causes mesh points on all edges shorter than -minEdgeLength to freeze (default false). This option is useful to keep boundary layers in the mesh unmodified, and smooth the large cells only, if the boundary layer related options below are not used.

Quality constraint options

The following options are related to additional heuristic quality control constraints for smoothing. The constraints work by disallowing movement of point (freezing of points) if the movement would cause quality of the mesh would suffer too much. Without constraints, centroidal smoothing may squish cells and create self-intersecting cells e.g. near concave geometry features, depending on the mesh details. Have a look at the algorithm description document for details.

Note: The old -qualityControl option has been superceded by the options below.

• -faceAngleConstraint boolean option enables an additional quality control which restricts decrease of smallest and largest face-face angle (default is true). When this option is enabled, the -minAngle option defines the minimum angle (in degrees, default value 35), and the -maxAngle option specifies the maximum angle (in degrees, default value 170).

• Note: -minAngle value causes point freezing only if the angle is below this value and if the angle would decrease in smoothing. Points are allowed to move if the angle value increases with smoothing, regardless of this value. The same applies for the -maxAngle option: Freezing takes place only if angle is above the specified value and if the angle would increase in smoothing.

Boundary layer related options

The options below are related to handling of prismatic cells near mesh boundaries, to either preserve or improve the orthogonality and the thickness of boundary layer cells in the mesh. If the mesh contains prismatic boundary layers, the unconstrained centroidal smoothing will tend to bloat the boundary layer cells into normal size. That can be avoided using the options below. These options affect only the prismatic cell edges near the mesh boundaries.

Warning: This is an experimental feature (WIP)!

• -boundaryMaxBlendingFraction is the maximum fraction (0 <= value <= 1) by which boundary layer edge length and edge direction are blended with the centroidal smoothing locations. Zero value disables the effect of all other boundary related variables below (default 0).

• -boundaryEdgeLength specifies the target thickness for the first boundary layer cells (default: 0.05).

• -boundaryExpansionRatio specifies the thickness ratio by which boundary layer cell thickness is assumed to increase (default: 1.3).

• -boundaryMinLayers is an integer value specifying the number of boundary layers which experience a full force of boundary blending specified with the -boundaryMaxBlendingFraction option (default: 1).

• -boundaryMaxLayers specifies the number of boundary cell layers beyond which boundary blending options above ceases to affect smoothing, and only centroidal smoothing is applied (default: 4).

Description of the algorithm

Please view the algorithm description document.

Usage examples

Adjust at least the -maxStepLength and -minEdgeLength options according to your mesh cell size.

• Parallel run example: mpirun -np 3 smoothMesh -centroidalIters 20 -maxStepLength 0.01 -minEdgeLength 0.05 -parallel
• Serial run example: smoothMesh -centroidalIters 20 -maxStepLength 0.01 -minEdgeLength 0.05

Test case

The folder testcase contains an artificial test case which contains skewed and non-orthogonal cells, as well as variance in geometric shell shapes and topology. This is meant to be a challenging (but not impossible) task for centroidal smoothing.

Getting help and feedback

Please use Github issues section for asking help. If you like this tool, please star this repository in Github!