# Assigning surface roughness to a fairing

**URL:** <https://www.simscale.com/forum/t/assigning-surface-roughness-to-a-fairing/87769>\
**Category:** Fluid Flow / CFD\
**Created:** [February 21, 2020, 5:38am UTC](https://www.simscale.com/forum/t/assigning-surface-roughness-to-a-fairing/87769 "2020-02-21T05:38:34Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![OTsyganov](https://www.simscale.com/forum/letter_avatar/otsyganov/32/5_5575768a8748004e209b776fc1b2916d.png) [@OTsyganov](https://www.simscale.com/forum/u/OTsyganov)\
**Post date:** [February 21, 2020, 5:38am UTC](https://www.simscale.com/forum/t/assigning-surface-roughness-to-a-fairing/87769/1 "2020-02-21T05:38:34Z")

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Let’s say I want something to be pretty rough, about 1mm of variance.  
As I understand, I need to create a custom boundary condition, no-slip, Turb. kinetic energy wall function with surface roughness, 0.001m scale. What does turbulence wall setting of ‘wall function’ vs ‘full resolution’ means and what should I use? Any other settings I should touch? (What roughness constant is, for instance?)

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**Author:** ![Ricardopg](https://www.simscale.com/forum/user_avatar/www.simscale.com/ricardopg/32/26909_2.png) [@Ricardopg](https://www.simscale.com/forum/u/Ricardopg)\
**Post date:** [February 21, 2020, 1:52pm UTC](https://www.simscale.com/forum/t/assigning-surface-roughness-to-a-fairing/87769/2 "2020-02-21T13:52:33Z")

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Hey!

I recall reading about the implications of surface roughness in the modeling somewhere but can’t quite remember where it was 😐. Make sure to check out this [video](https://www.youtube.com/watch?v=vYbRUmVTmGM) about the topic.

Wall function versus full resolution is related to [yplus](https://www.simscale.com/forum/t/what-is-y-yplus/82394). If you’re interested in yplus, also make sure to check this [thread](https://www.simscale.com/forum/t/wall-distance-experience-do-these-calculations-look-right-y-yplus/86796).

/Ric