# Normal boundary condition

**URL:** <https://community.freefem.org/t/normal-boundary-condition/341>\
**Category:** General Discussion\
**Created:** [April 10, 2020, 1:55pm UTC](https://community.freefem.org/t/normal-boundary-condition/341 "2020-04-10T13:55:58Z")\
**Posts on this page:** 1\
**Showing post:** 6

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**Author:** ![felric](https://avatars.discourse-cdn.com/v4/letter/f/5f8ce5/32.png) [@felric](https://community.freefem.org/u/felric)\
**Post date:** [May 6, 2022, 10:18pm UTC](https://community.freefem.org/t/normal-boundary-condition/341/6 "2022-05-06T22:18:37Z")

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Dear all,

I have the same problem as @michaellevitin : I have a 3D elasticity problem with symmetry conditions on a equilateral triangle. Displacement vector is [ux,uy,uz]. On the base “1” of the triangle (y=0, x=[-a/2 a/2]) I can simply add …+on(1, uy=0). However on the other two sides of the triangle, being not aligned to x or y axis, I tried …+on(2, ux_N.x+uy_N.y) but of course is not working. In fact I should put into relations ux with uy and therefore it is not possible to simply putting the diagonal elements to a tgv value.  
Is there a way to impose symmetry conditions (e.g., Dirichlet boundary condition for the normal component of a vector field) on boundaries that are NOT aligned to x, y and z axis?  
Thanks very much in advance

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