CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

strange solver behavior---Treatment of Rough Wall

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   December 11, 2007, 17:18
Default strange solver behavior---Treatment of Rough Wall
  #1
Yi
Guest
 
Posts: n/a
It is well established that if the roughness height is much higher than the first node off the wall, the solver will blow up or at least gives a warning message. However, the thickness of the first element in my mesh is 0.01m and I imposed a roughness height 0.14m, which is 14 times the size of the first element. The solver runs smoothly and the result looks reasonable. But I AM NOT HAPPY!!! BECAUSE IT IS NOT WHAT I EXPECT. Something must be wrong. Somebody help me, please! Why I still can get a solution by defining such a large roughness height??????
  Reply With Quote

Old   December 11, 2007, 17:21
Default Re: strange solver behavior---Treatment of Rough W
  #2
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

I see no reason why making the roughness height greater than the first node should make the solver fail. It means the results will be inaccurate as the mathematical model used to model the effects of roughness is not valid but the solver should still converge to a solution.

Regards, Glenn Horrocks
  Reply With Quote

Old   December 12, 2007, 18:00
Default Re: strange solver behavior---Treatment of Rough W
  #3
hannah
Guest
 
Posts: n/a
I always set my wall roughness height to 0.003ft, and first layer height of inflation to 0.001 inch, the solver never failed anyway. But How inaccuracy of the result would be? And is there any rule for this set up?

Thanks

  Reply With Quote

Old   December 13, 2007, 17:12
Default Re: strange solver behavior---Treatment of Rough W
  #4
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

How inaccurate? I have no idea. Do a benchmark study of flow in a pipe or flat plate with known roughness where you have good quality data to compare to (this should be easily available in the literature) and find out.

Glenn Horrocks
  Reply With Quote

Old   April 8, 2010, 16:15
Default
  #5
Member
 
prout
Join Date: May 2009
Posts: 32
Rep Power: 8
zboud is on a distinguished road
Does anyone know more about the behavior of CFX when the roughness is superior to the size of the first element, and when the SST model is used?
zboud is offline   Reply With Quote

Old   April 9, 2010, 13:24
Default
  #6
Senior Member
 
Join Date: Apr 2009
Posts: 517
Rep Power: 12
stumpy is on a distinguished road
The behaviour is as described above. Convergence is fine, but the results are not accurate because the model is not valid in this situation.
stumpy is offline   Reply With Quote

Old   April 14, 2010, 18:31
Default
  #7
Member
 
prout
Join Date: May 2009
Posts: 32
Rep Power: 8
zboud is on a distinguished road
I was looking for details, but I read the help files in between...

I do not understand why the model should not valid, given the rugosity treatment used by CFX. The rugosity effect is only a shift of the boundary layer velocity. Basically, the "under the roughness" nodes are extremely slowed down. And as the rugosity height used by CFX is the sand-grain equivalent rugosity, which is even higher, there's a good chance nodes will be under it. See the help which explains quite well...

Anyway, if someone has results showing the model is inaccurate, I'd be interested
zboud is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Thin Wall Heat Transfer BC for rhoSimpleFoam swahono OpenFOAM Running, Solving & CFD 12 October 4, 2013 11:49
Natural convection in a closed domain STILL NEEDING help! Yr0gErG FLUENT 3 June 12, 2013 02:12
Rough wall nellio75 CFX 4 November 19, 2009 06:23
Rough wall functions lourens OpenFOAM Pre-Processing 0 July 27, 2007 05:31
Wall BC's for a NS solver André Burdet Main CFD Forum 6 December 8, 2000 22:45


All times are GMT -4. The time now is 09:08.