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

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?

Im not to sure to be honest, I have an actual roughness height and need this converting to sand grain roughness, which is different.

Try reading Boundary Layer Theory by Schlichting. It is described in detail there.

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

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]

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

[fedefrance]

here is an article with conversion factor and also it has good references

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

[prashant9397]

i have used 30 micron sand grain size for rotor 67 so how much it equivalent to in microns rms.

[ghorrocks]

Have you read the description of wall roughness definition in the CFX documentation? This is discussed there. See Solver Modelling Guide, section 4.2.4.

[evcelica]

Originally Posted by prashant9397

i have used 30 micron sand grain size for rotor 67 so how much it equivalent to in microns rms.

Look at Fedefrance's post right above yours, that reference is about as good as it gets. Keep in mind there is no exact "equivalent", just an approximate relationship.