Surface roughness and correction of y+

Stansen · September 14, 2015, 07:08

Hello,

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 (6.3.14.2.9. 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?

LuckyTran · September 15, 2015, 22:59

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.

September 14, 2015, 07:08	Surface roughness and correction of y+	#1
Stansen New Member Join Date: Sep 2015 Posts: 4 Rep Power: 11	Hello, 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 (6.3.14.2.9. 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?

September 15, 2015, 22:59		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	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. devesh.baghel and randolph like this.