# How i set Neumann B.C in every cell of exterior surface?

**URL:** <https://fenicsproject.discourse.group/t/how-i-set-neumann-b-c-in-every-cell-of-exterior-surface/2253>\
**Category:** Uncategorized\
**Created:** [January 20, 2020, 7:55am UTC](https://fenicsproject.discourse.group/t/how-i-set-neumann-b-c-in-every-cell-of-exterior-surface/2253 "2020-01-20T07:55:05Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![odysgros](https://avatars.discourse-cdn.com/v4/letter/o/ba9def/32.png) [@odysgros](https://fenicsproject.discourse.group/u/odysgros)\
**Post date:** [January 20, 2020, 7:55am UTC](https://fenicsproject.discourse.group/t/how-i-set-neumann-b-c-in-every-cell-of-exterior-surface/2253/1 "2020-01-20T07:55:05Z")

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Hi everyone. I solving Poisson equation in 3d box and i would like to set Neumann boundary condition in every cell of exterior surface. I thought to use Raviart Thomas element but i dont know how to set these boundaries conditions. Any help would be much appreciated. Thank you !!!

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**Author:** ![volkerk](https://avatars.discourse-cdn.com/v4/letter/v/9d8465/32.png) [@volkerk](https://fenicsproject.discourse.group/u/volkerk)\
**Post date:** [January 20, 2020, 9:55am UTC](https://fenicsproject.discourse.group/t/how-i-set-neumann-b-c-in-every-cell-of-exterior-surface/2253/2 "2020-01-20T09:55:10Z")

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You can consider the method used in this demo:  
[https://fenicsproject.org/docs/dolfin/latest/python/demos/neumann-poisson/demo\_neumann-poisson.py.html](https://fenicsproject.org/docs/dolfin/latest/python/demos/neumann-poisson/demo_neumann-poisson.py.html)