# [snappyHexMesh] symmetryPlane in sHM

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 March 30, 2017, 07:43 symmetryPlane in sHM #1 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,708 Blog Entries: 6 Rep Power: 51 Hi everybody, I have a question to you. Maybe one got to the same problem. I am meshing a complex geometry with sHM but I have problems with the symmetry planes. The faces are not 100% aligned to the patch normal and therefore I get e.g. displacements in y direction based on the surface normal direction. Question 1 Is there a way to flatten the symmetry patches in order that all points are aligned within one plane (like flattenMesh for 2D). Note 1 symmetry vs. symmetryPlane I read that people just change it without thinking about the differences. The difference should be:symmetryPlane has to be really flat and one single patch (no multi patch assembly) symmetry can be used for non-100% aligned flat patches and multi patch assemblies I got this from https://openfoam.org/release/2-3-0/numerics/ However, using the symmetry BC in my simulation lead to the problem that I get motion in the direction I want to constrain (like y-direction). This is based on the fact that I do not get a 100% aligned flat patch after snapping with snappyHexMesh. Question 2 Does anybody know how to get a flat patch using sHM Maybe there is a tool out which can be used to flatten a patch (like flattenMesh for 3D) I was expecting that after defining the patchType in the snappyHexMeshDict to be symmetry or symmetryPlane, the meshing application takes care of adjusting the points. assign the symmetryPlane patch type. I get the problem, that the application tells me the following: Code: ```Symmetry plane 'sym1' is not planar. At local face at (0.00702909 0.00026875 0.0282749) the normal (1 3.22739e-15 1.60222e-15) differs from the average normal (0.428571 5.18688e-16 -6.34413e-17) by 0.326531 Either split the patch into planar parts or use the symmetry patch type``` However, the normal vector at point (0.00702909 0.00026875 0.0282749) is fine. It should direct to the x direction which it probably do but I have no idea why it is complaining about that point because it is not on the sym1 patch. __________________ Keep foaming, Tobias Holzmann

 April 25, 2017, 06:58 #2 Senior Member     Kal-El Join Date: Apr 2017 Location: Finland Posts: 150 Rep Power: 9 You can use the outer limits of your blockMesh to create very flat surfaces.

 April 25, 2017, 07:59 #3 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,708 Blog Entries: 6 Rep Power: 51 Hello Kai, this will not help, or if you think, please offer an example. very flat surfaces are not flat at all. Using snappy will produce non-flat surfaces at the patches during the snap process. The solution is to move the patch points to a plane by yourself. The program which can handle that could be found in my repositories. __________________ Keep foaming, Tobias Holzmann

 April 25, 2017, 08:46 #4 Senior Member     Kal-El Join Date: Apr 2017 Location: Finland Posts: 150 Rep Power: 9 I ment something like this https://sites.google.com/site/snappy.../cylinder-case (inlet-boundaries, with "small blockMesh")

 April 25, 2017, 08:59 #5 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,708 Blog Entries: 6 Rep Power: 51 Did you ever checked if the patches (inlet/outlet) could be set for symmetryPlane? In addition my geometry is a chainsaw motor block. So no simple geometry. However, thanks for your reply. __________________ Keep foaming, Tobias Holzmann

 April 25, 2017, 09:11 #6 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,708 Blog Entries: 6 Rep Power: 51 Okay I checked your geometry ... inlet and outlet can be specified as symmetryPlane However, I tried a lot of things (even similar to yours) without success for the complex geometry __________________ Keep foaming, Tobias Holzmann

 April 25, 2017, 09:19 #7 Senior Member     Kal-El Join Date: Apr 2017 Location: Finland Posts: 150 Rep Power: 9 That is not my geometry. Just something I found.