strange solver behavior---Treatment of Rough Wall

December 11, 2007, 16:18

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??????

December 11, 2007, 16:21

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

December 12, 2007, 17:00

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

December 13, 2007, 16:12

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

zboud · April 8, 2010, 16:15

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?

stumpy · April 9, 2010, 13:24

The behaviour is as described above. Convergence is fine, but the results are not accurate because the model is not valid in this situation.

zboud · April 14, 2010, 18:31

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

December 11, 2007, 16:18	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??????

December 11, 2007, 16:21	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

December 12, 2007, 17:00	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

December 13, 2007, 16:12	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

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Natural convection in a closed domain STILL NEEDING help!	Yr0gErG	FLUENT	4	December 2, 2019 00:04
Thin Wall Heat Transfer BC for rhoSimpleFoam	swahono	OpenFOAM Running, Solving & CFD	12	October 4, 2013 11:49
Rough wall	nellio75	CFX	4	November 19, 2009 05:23
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 21:45

April 8, 2010, 16:15		#5
zboud Member prout Join Date: May 2009 Posts: 32 Rep Power: 16	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?

April 9, 2010, 13:24		#6
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	The behaviour is as described above. Convergence is fine, but the results are not accurate because the model is not valid in this situation.

April 14, 2010, 18:31		#7
zboud Member prout Join Date: May 2009 Posts: 32 Rep Power: 16	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