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Surface roughness and correction of y+

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Old   September 14, 2015, 06:08
Question Surface roughness and correction of y+
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I like to solve a surface roughness problem in a tube. I'm using the k-omega sst model, y+=1 and the ks+ have some different values (between 2 and 8). So that I'm not in the visous sublayer all the time.
Now I found a correction for the mesh in case of surface roughness in the fluent documentation ( Law-of-the-Wall Modified for Roughness):

(y+) = (y+) + (ks+)/2

Question: Should i correct my mesh for ever different (ks+) or could i stay on y+ = 1?
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Old   September 15, 2015, 21:59
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The treatment is general and you do not really need to change your mesh. The y+ is being corrected in order to account for the influence of the wall roughness. But the virtual shift is more applicable to situations where the roughness height is thicker than the viscous sublayer.

Notice though that the corrected y+ is y+ corrected = y+ + Ks/2. It is a bit counter-intuitive why there is a + if the effective size of the channel has been reduced. The reason there is a positive sign (+) is because the roughness induced blockage. Note that the u+ curve shifts downward (towards higher y+) if the wall is rough instead of smooth.
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correction, ks+, surface roughness

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