# Equivalent Sand Grain Roughness

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 April 8, 2008, 10:29 Equivalent Sand Grain Roughness #1 mike wilson Guest   Posts: n/a Hi, I am trying to figure out how to calcualte the required sand grain roughness to use on a surface. I have a real roughness height of 0.0005m and would like to know how to enter this into CFX. I found the equations K+=yR(Rho/Mu)u* ...etc... but dont really see how they help me! I have calculated values for y+, ut, u+, u*, tauW, y* but dont know where my roughness height of 0.0005 comes into this? Cna anyone shed some light or talk me through it, Cheers

 April 8, 2008, 14:16 Re: Equivalent Sand Grain Roughness #2 BB Guest   Posts: n/a I had a case that needs the roughness too. After spending a little time, I just assumed the roughness published in most books for material is the equivalent sand grain roughness required in CFX. Is this right?

 April 8, 2008, 16:06 Re: Equivalent Sand Grain Roughness #3 mike Guest   Posts: n/a Im not to sure to be honest, I have an actual roughness height and need this converting to sand grain roughness, which is different.

 April 8, 2008, 22:59 Re: Equivalent Sand Grain Roughness #4 Johnny Guest   Posts: n/a Try reading Boundary Layer Theory by Schlichting. It is described in detail there. http://www.amazon.com/Boundary-Layer.../dp/3540662707

 April 9, 2008, 05:24 Re: Equivalent Sand Grain Roughness #5 mike Guest   Posts: n/a Thanks for the reference but that book has been taken out of all the library's in Bristol, must be all us keen students!

April 12, 2010, 08:55
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Quote:
 Originally Posted by BB ;87359 I had a case that needs the roughness too. After spending a little time, I just assumed the roughness published in most books for material is the equivalent sand grain roughness required in CFX. Is this right?
No, it's not! The sand grain roughness is higher. But I don't know if there is any obvious relation between the actual roughness and the sand grain equivalent... seems OK in certain cases, though.

See "Extensions of the Spalart–Allmaras turbulence model to account for wall roughness"B. Aupoix and P. R. Spalart, doi:10.1016/S0142-727X(03)00043-2

And "Turbulent flow in smooth and rough pipes", J.J Allen, M.A Shockling, G.J Kunkel and A.J Smits, doi: 10.1098/rsta.2006.1939

 July 31, 2014, 08:08 #7 New Member   federico Join Date: Jan 2013 Posts: 13 Rep Power: 6 here is an article with conversion factor and also it has good references http://ijmem.avestia.com/2012/PDF/008.pdf Freeman, JLMulder, sagarmore and 1 others like this.

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