# How to set boundary conditions where u multiplied by the boundary normal vector or u multiplied by the boundary tangent vector equals 0?

**URL:** <https://community.freefem.org/t/how-to-set-boundary-conditions-where-u-multiplied-by-the-boundary-normal-vector-or-u-multiplied-by-the-boundary-tangent-vector-equals-0/2698>\
**Category:** General Discussion\
**Created:** [September 11, 2023, 3:12am UTC](https://community.freefem.org/t/how-to-set-boundary-conditions-where-u-multiplied-by-the-boundary-normal-vector-or-u-multiplied-by-the-boundary-tangent-vector-equals-0/2698 "2023-09-11T03:12:21Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Kingexitroot](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/kingexitroot/32/1912_2.png) [@Kingexitroot](https://community.freefem.org/u/Kingexitroot)\
**Post date:** [September 11, 2023, 3:12am UTC](https://community.freefem.org/t/how-to-set-boundary-conditions-where-u-multiplied-by-the-boundary-normal-vector-or-u-multiplied-by-the-boundary-tangent-vector-equals-0/2698/1 "2023-09-11T03:12:21Z")

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Hello everyone. I have the following question: u is a 2D vector, and I need to impose the boundary conditions u_n=0 or u_t=0,  
where n and t are the boundary normal and tangential vectors, respectively. I couldn’t find the relevant syntax in the FreeFem documentation. How should I write this in FreeFem?