[snappyHexMesh] Face normals of non-planar faces

Teilhart · November 4, 2023, 06:41

How does snappyHexMesh (and I suppose OpenFOAM in general) define the face normal if a face fails to be planar? Is it kind of an (area-weighted) average of the triangle normals of some triangulation of the face? If so, which triangulation is used? I suspect that it might be independent of the chosen triangulation but I'm not quite sure.

cibanez · November 7, 2023, 03:49

Hi,

As you mention, in order to calculate the area and normal direction of a face, this one is divided into triangles by using its centroid.

The cross product of two vectors is perpendicular to those vectors and its magnitude is the area of a paralellogram with both vectors as sides. Half of this magnitude represents the area of a triangle. In OpenFOAM the sum of the cross product vectors on each subtriangle gives the normal direction to the entirety of the face. Half the magnitude of this last vector is the total area of the face. Also, the area weighted average of each triangle center gives the face center.

You can check the source code here:

https://develop.openfoam.com/Develop...shTools.C#L204

/Cristóbal

Teilhart · November 18, 2023, 07:31

Ok, the triangulation is based on the centriod. That clarifies ist. Thank you!

November 4, 2023, 06:41	Face normals of non-planar faces	#1
Teilhart New Member Join Date: Nov 2023 Posts: 4 Rep Power: 2	How does snappyHexMesh (and I suppose OpenFOAM in general) define the face normal if a face fails to be planar? Is it kind of an (area-weighted) average of the triangle normals of some triangulation of the face? If so, which triangulation is used? I suspect that it might be independent of the chosen triangulation but I'm not quite sure.

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November 7, 2023, 03:49		#2
cibanez New Member Cristóbal Join Date: Jan 2022 Location: Sweden Posts: 14 Rep Power: 4	Hi, As you mention, in order to calculate the area and normal direction of a face, this one is divided into triangles by using its centroid. The cross product of two vectors is perpendicular to those vectors and its magnitude is the area of a paralellogram with both vectors as sides. Half of this magnitude represents the area of a triangle. In OpenFOAM the sum of the cross product vectors on each subtriangle gives the normal direction to the entirety of the face. Half the magnitude of this last vector is the total area of the face. Also, the area weighted average of each triangle center gives the face center. You can check the source code here: https://develop.openfoam.com/Develop...shTools.C#L204 /Cristóbal

November 18, 2023, 07:31		#3
Teilhart New Member Join Date: Nov 2023 Posts: 4 Rep Power: 2	Ok, the triangulation is based on the centriod. That clarifies ist. Thank you!