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 |
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?
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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.
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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 |
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!
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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 |
here is an article with conversion factor and also it has good references
http://ijmem.avestia.com/2012/PDF/008.pdf |
i have used 30 micron sand grain size for rotor 67 so how much it equivalent to in microns rms.
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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.
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