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Equivalent Sand Grain Roughness

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Old   April 8, 2008, 10:29
Default Equivalent Sand Grain Roughness
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mike wilson
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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
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Old   April 8, 2008, 14:16
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BB
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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?
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Old   April 8, 2008, 16:06
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mike
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Im not to sure to be honest, I have an actual roughness height and need this converting to sand grain roughness, which is different.
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Old   April 8, 2008, 22:59
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Johnny
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Try reading Boundary Layer Theory by Schlichting. It is described in detail there.

http://www.amazon.com/Boundary-Layer.../dp/3540662707
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Old   April 9, 2008, 05:24
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mike
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Thanks for the reference but that book has been taken out of all the library's in Bristol, must be all us keen students!
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Old   April 12, 2010, 08:55
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prout
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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...k_{s}=3k_{RMS} 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
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Old   July 31, 2014, 08:08
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federico
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here is an article with conversion factor and also it has good references

http://ijmem.avestia.com/2012/PDF/008.pdf
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