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mike wilson April 8, 2008 10:29

Equivalent Sand Grain Roughness
 
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

BB April 8, 2008 14:16

Re: Equivalent Sand Grain Roughness
 
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?

mike April 8, 2008 16:06

Re: Equivalent Sand Grain Roughness
 
Im not to sure to be honest, I have an actual roughness height and need this converting to sand grain roughness, which is different.

Johnny April 8, 2008 22:59

Re: Equivalent Sand Grain Roughness
 
Try reading Boundary Layer Theory by Schlichting. It is described in detail there.

http://www.amazon.com/Boundary-Layer.../dp/3540662707

mike April 9, 2008 05:24

Re: Equivalent Sand Grain Roughness
 
Thanks for the reference but that book has been taken out of all the library's in Bristol, must be all us keen students!

zboud April 12, 2010 08:55

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

fedefrance July 31, 2014 08:08

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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