Flat epsilon residuals

pipolaki · April 22, 2015, 07:44

Hello,

I currently have a problem with my DPM simulation on ANSYS Fluent V15. Epsilon residuals are stuck around 1.0 during the computation (Image here :
https://drive.google.com/folder/d/0ByiqwBpvr2T0VXR2VkJhYVl3elE/edit ).
As soon as I uncheck the 2-way turbulence coupling option, the epsilon residual is decreasing.
Is there any explanation about this ?
Is it normal to have convergence issues concerning epsilon ?
According you, can I trust the results of my simulation with such high epsilon residuals ?

Thank you for your help.

pipolaki · June 1, 2015, 08:19

I'm still wondering why the epsilon residual remains flat when 2-way turbulence coupling is active... Any ideas ?

arctan · February 10, 2017, 02:47

I've got the same problem, epsilon residual flat around 1.0. Have you worked it out?

pipolaki · February 10, 2017, 09:57

I completely forgot this topic : thank you for your question.

In order to avoid the epsilon residual problem, Ansys support suggested me to use a first order discretization scheme for turbulence and it worked : no more issues on epsilon residuals.

Moreover, I found that in my study the option "two-way turbulence coupling" was improving my results when I compared them with our experimental datas.

Please, let me know if this solution is helping you or not.

Regards.

Quote:

Originally Posted by arctan

I've got the same problem, epsilon residual flat around 1.0. Have you worked it out?

arctan · February 12, 2017, 22:18

Hi Pipolaki,
Thanks for your reply!
I tried to switch to first order discretization scheme for turbulence, unfortunately, it wouldn't work for me. Then I tried a smaller under-relaxation factor for turbulent epsilon and the epsilon residual did decrease by an order of magnitude, but it remained there. As I unchecked the two-way turbulence coupling, the epsilon was decreasing.
Is it normal to have convergence issues concerning epsilon, with two-way turbulence coupling option ?
Can I trust the results of my simulation with such high epsilon residuals ?
Regards,

pipolaki · February 17, 2017, 03:21

Sorry to hear that the first order scheme didn't work for you. I have no other suggestion.

I also noticed that if the two-way turbulence coupling was unchecked, the epsilon residual decreased and this behavior is still mysterious to me.

My personal opinion (which is not very valuable !) is that if the residual is high, it's not good for your results. If I were you, I would perform two computations :
- one with the 2-way turbulence coupling checked and its high epsilon residual ;
- another with the 2-way turbulence coupling unchecked and all the good residuals.
Then I would compare the results, especially those which concerns the turbulence properties. Maybe it could help you to choose between the two solutions.

Good luck.

April 22, 2015, 07:44	Flat epsilon residuals	#1
pipolaki New Member Join Date: Jul 2012 Location: France Posts: 26 Rep Power: 13	Hello, I currently have a problem with my DPM simulation on ANSYS Fluent V15. Epsilon residuals are stuck around 1.0 during the computation (Image here : https://drive.google.com/folder/d/0ByiqwBpvr2T0VXR2VkJhYVl3elE/edit ). As soon as I uncheck the 2-way turbulence coupling option, the epsilon residual is decreasing. Is there any explanation about this ? Is it normal to have convergence issues concerning epsilon ? According you, can I trust the results of my simulation with such high epsilon residuals ? Thank you for your help. Last edited by pipolaki; April 27, 2015 at 06:54.

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June 1, 2015, 08:19		#2
pipolaki New Member Join Date: Jul 2012 Location: France Posts: 26 Rep Power: 13	I'm still wondering why the epsilon residual remains flat when 2-way turbulence coupling is active... Any ideas ?

February 10, 2017, 02:47		#3
arctan New Member Outside US and Canada Join Date: Apr 2015 Location: Beijing Posts: 9 Rep Power: 11	I've got the same problem, epsilon residual flat around 1.0. Have you worked it out?

February 12, 2017, 22:18		#5
arctan New Member Outside US and Canada Join Date: Apr 2015 Location: Beijing Posts: 9 Rep Power: 11	Hi Pipolaki, Thanks for your reply! I tried to switch to first order discretization scheme for turbulence, unfortunately, it wouldn't work for me. Then I tried a smaller under-relaxation factor for turbulent epsilon and the epsilon residual did decrease by an order of magnitude, but it remained there. As I unchecked the two-way turbulence coupling, the epsilon was decreasing. Is it normal to have convergence issues concerning epsilon, with two-way turbulence coupling option ? Can I trust the results of my simulation with such high epsilon residuals ? Regards,

February 17, 2017, 03:21		#6
pipolaki New Member Join Date: Jul 2012 Location: France Posts: 26 Rep Power: 13	Sorry to hear that the first order scheme didn't work for you. I have no other suggestion. I also noticed that if the two-way turbulence coupling was unchecked, the epsilon residual decreased and this behavior is still mysterious to me. My personal opinion (which is not very valuable !) is that if the residual is high, it's not good for your results. If I were you, I would perform two computations : - one with the 2-way turbulence coupling checked and its high epsilon residual ; - another with the 2-way turbulence coupling unchecked and all the good residuals. Then I would compare the results, especially those which concerns the turbulence properties. Maybe it could help you to choose between the two solutions. Good luck.