Wall roughness effect on laminar flow

hannesjk · November 21, 2016, 09:01

Hello!

I have a flow with very low Reynold number (below 1) in a axisymmetric pipe simulation and I have no slip condition on the pipe wall. When I change the wall roughness height of the pipe wall I get different pressure losses (lower pressure loss with higher wall roughness height) even though i should not? The pipe diameter is 0.3 mm and I vary the wall roughness height from 5 micrometers and up. Do you know why this is happening?

Thanks in advance

sbaffini · November 21, 2016, 11:01

In order to modify the roughness parameters for a wall you necessarily need to have a turbulence model on, which seems wrong at your low Re number.

Moreover, roughness effects are models as much as turbulence models are. That is, they model something below the scale you can actually reach with your grid. So, in practice, they are only introduced trough wall functions and not for fully resolved walls (or, nonetheless, the model becomes quite questionable for fully resolved walls).

So, in practice, you are actually seeing the effect of the roughness trough a wall function and a turbulence model, both of which seem pretty wrong to me for your specific case.

November 21, 2016, 09:01	Wall roughness effect on laminar flow	#1
hannesjk New Member Hannes Join Date: Oct 2016 Posts: 6 Rep Power: 9	Hello! I have a flow with very low Reynold number (below 1) in a axisymmetric pipe simulation and I have no slip condition on the pipe wall. When I change the wall roughness height of the pipe wall I get different pressure losses (lower pressure loss with higher wall roughness height) even though i should not? The pipe diameter is 0.3 mm and I vary the wall roughness height from 5 micrometers and up. Do you know why this is happening? Thanks in advance

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November 21, 2016, 11:01		#2
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,151 Blog Entries: 29 Rep Power: 39	In order to modify the roughness parameters for a wall you necessarily need to have a turbulence model on, which seems wrong at your low Re number. Moreover, roughness effects are models as much as turbulence models are. That is, they model something below the scale you can actually reach with your grid. So, in practice, they are only introduced trough wall functions and not for fully resolved walls (or, nonetheless, the model becomes quite questionable for fully resolved walls). So, in practice, you are actually seeing the effect of the roughness trough a wall function and a turbulence model, both of which seem pretty wrong to me for your specific case.