Discussion Groups

## Simulation CFD

Valued Contributor
Posts: 61
Registered: ‎05-16-2012

# wall roughness height

201 Views, 6 Replies
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

Valued Mentor
Posts: 340
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.

Valued Contributor
Posts: 61
Registered: ‎05-16-2012

# Re: wall roughness height

10-30-2013 08:45 AM in reply to: OmkarJ
any reference???
Valued Mentor
Posts: 340
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

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

Valued Contributor
Posts: 61
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?

Valued Mentor
Posts: 340
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:

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...