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Valued Contributor
marco.mueller
Posts: 77
Registered: ‎05-16-2012
Accepted Solution

wall roughness height

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10-21-2013 02:28 AM

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

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OmkarJ
Posts: 426
Registered: ‎10-02-2012

Re: wall roughness height

10-22-2013 07:32 AM in reply to: marco.mueller

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

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Valued Contributor
marco.mueller
Posts: 77
Registered: ‎05-16-2012

Re: wall roughness height

10-30-2013 08:45 AM in reply to: OmkarJ
any reference??? :smileywink:
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OmkarJ
Posts: 426
Registered: ‎10-02-2012

Re: wall roughness height

11-07-2013 01:34 AM in reply to: marco.mueller

Sorry for late response, had been away on a vacation :smileyhappy:

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

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Valued Contributor
marco.mueller
Posts: 77
Registered: ‎05-16-2012

Re: wall roughness height

11-13-2013 05:13 AM in reply to: OmkarJ

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?

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OmkarJ
Posts: 426
Registered: ‎10-02-2012

Re: wall roughness height

11-13-2013 07:10 AM in reply to: marco.mueller

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...

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Valued Contributor
marco.mueller
Posts: 77
Registered: ‎05-16-2012

Re: wall roughness height

11-13-2013 07:11 AM in reply to: OmkarJ
Thanks for your research, Omkar!
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