wall roughness height

[marco.mueller]

Hi,

can you clarify which roughness height is used in CFD. Is it the "absolute" value?

Is it the "epsilon" value that would be used in the Moody cart?

http://en.wikipedia.org/wiki/Moody_chart

Thanks

Marco

[OmkarJ]

Indeed,  it is average absolute roughness of the pipe and has a unit of length.

[marco.mueller]

any reference??? 😉

[OmkarJ · Accepted solution]

Sorry for late response, had been away on a vacation 🙂

Have a look at doco:

http://help.autodesk.com/view/SCDSE/2014/ENU/?url=/cloudhelp/2014/ENU/SimCFD/files/GUID-511AF0D1-6FB...

http://help.autodesk.com/view/SCDSE/2014/ENU/?url=/cloudhelp/2014/ENU/SimCFD/files/GUID-61C4EB55-362...

The second link states it is the height from wall with unit of lengthl. Hope this helps.

Omkar

[marco.mueller]

well thats all clear but I'm rather trying to correlate the "average rougness height" used in CFD to more common definitions like here:

http://en.wikipedia.org/wiki/Surface_roughness

or the roughness in the first link I posted.

Example: the customer has a pipe with Ra = 3.2. Would entering 0.0032 mm as wall roughness value be correct?

[OmkarJ · Accepted solution]

Well, here is the catch! Ra value just represents the texture abnormalities of the surface but the value "r" in the SimCFD documentation (in length unit) is the sandgrain roughness and both are not always comparable. Nikuradse has done extensive experiments to correlate the two:

Nikuradse, J. 1933 Laws of flow in rough pipes. VDI Forschungsheft 361. In translation, NACA TM 1292, 1950.

You can find additional information here, and more if you decide to do a bit of search:

http://ijmem.avestia.com/2012/008.html
http://rsta.royalsocietypublishing.org/content/365/1852/699.full
http://arrow.dit.ie/cgi/viewcontent.cgi?article=1030&context=engschmecart

I am not aware of any formula that will straightaway give the sandgrain roughness from Ra value and there exists a lot of literature around this. From one of the references above, the RzJIS value given in the Wiki link you posted comes closest in approximating the sandgrain roughness and Ra is the worst.

Will be interesting if Autodesk personnel have anymore to add on this...

[marco.mueller]

Thanks for your research, Omkar!

[eshtiwe266]

• Hello 
• I have a question, did you figure out how find roughness height from Ra. I have same problem I have Ra=0.55 but I don’t know the relationship or how to find roughness height for sand grain to put it in CFD fluent.
•  Thank you