# Non-penetration condition at solid wall

**URL:** <https://community.freefem.org/t/non-penetration-condition-at-solid-wall/369>\
**Category:** General Discussion\
**Created:** [April 24, 2020, 1:24am UTC](https://community.freefem.org/t/non-penetration-condition-at-solid-wall/369 "2020-04-24T01:24:21Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Hong](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hong](https://community.freefem.org/u/Hong)\
**Post date:** [April 24, 2020, 1:24am UTC](https://community.freefem.org/t/non-penetration-condition-at-solid-wall/369/1 "2020-04-24T01:24:21Z")

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I impose a Navier slip boundary condition at an inclined wall to solve the liquid slip flow over hydrophobic surface. But the slip velocity penetrates into solid wall because u_N.x + v_N.y = 0 is not satisfied. How to implement the non-penetration condition (u_N.x + v_N.y = 0) at solid boundary. I have tried many methods, but they does not work.  
Many thanks.