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-   -   Roughness in turbulent pipe flow (http://www.cfd-online.com/Forums/main/74882-roughness-turbulent-pipe-flow.html)

1000sh April 10, 2010 16:33

Roughness in turbulent pipe flow
 
Hello everyone,
I am trying to model and simulate a pipe with a turbulent flow and investigatng the effect of different levels of wall roughness to the skin friction coefficient, wall shear stress and drag.

I am using a k-epsilon model and different values for Ks for example 0.00167m, 0.00304m, 0.00501m.

When i analyze the results i obtain by using the XY plot, the plots for skin friction coefficient and wall shear stress are all the same for the 3 cases. can someone tell me what's wrong with my set up? My meshing is finer on the side of the wall.

I think the problem might be the meshing being too fine near the wall compared to my roughness.

I am using fluent and gambit.

Can someone tell me how to correctly mesh or simulate this flow please? i have been looking everywhere in forums and the manuals but dont understand what is wrong with my set up or how to change the meshing properly(if that is the problem).

thank you very much in advance for the help!

1000sh April 11, 2010 05:32

am i write in saying that i need a coarser mesh to model roughness in a turbulent flow?

and i can only test roughnesses of the height of half my first node right? i mean if i have the first node at a distance of 2cm from my wall (radius being 10cm), i can only test roughnesses to a height of 1cm right?

ztdep April 11, 2010 07:20

it will happen in pipe flow. the fow regime may be in the region that roughbess has no effects

1000sh April 11, 2010 09:00

thank you for your reply!

do you mean that in the laminar region, roughness has no effect?

when i mesh the geometry, do i have to make it coarse so that the value of y+ is high?


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