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November 26, 2009, 13:44 
first order convergedsecond order diverged

#1 
New Member
Shengyi Wang
Join Date: Mar 2009
Posts: 22
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Dear all,
In my case, I use first order upwind scheme and get a very well converged solution; however, when I switch to second order upwind scheme, totally diverge!!! Any one has some suggestion to resolve this problem? Thx in advance. Sy 

November 27, 2009, 01:35 

#2 
Senior Member

Would you please tell me the implementation method of the second order upwind scheme? and have you try to change the relaxation parameter


November 27, 2009, 10:48 

#3  
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Shengyi Wang
Join Date: Mar 2009
Posts: 22
Rep Power: 9 
Quote:
I am sorry I do not quite understand what you mean by the implementation. Could you please give a more detailed explanation? Underralaxation factor has been tuned down, but it doesn't work. I am using sst kw with transition model, y+ is less than 1. The blade is actually a NACA0012 blade with an angle of attack of 8 degree. Thank you for reply! R, Shengyi 

December 23, 2009, 06:57 
RE

#4 
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Ioan
Join Date: Dec 2009
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Dear gmwsy,
To get a stable secondorder simulation you should do the following trick: start the simulation for the first let say 100 timesteps with a firstorder scheme and after switch to the secondorder one. The problem behind is that for highorder schemes is very hard to start from "bad" initial solution (the initial solution is illconditioned)... 

December 23, 2009, 13:45 

#5  
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Shengyi Wang
Join Date: Mar 2009
Posts: 22
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Quote:
Thank you for your reply. But what i am doing is actually a steady case...so there does not exist time steps or initial solution. 

December 25, 2009, 02:34 

#6 
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Zhao
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Location: China
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December 25, 2009, 09:39 

#7 
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Shengyi Wang
Join Date: Mar 2009
Posts: 22
Rep Power: 9 

December 28, 2009, 04:50 

#8 
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Join Date: Apr 2009
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Dear gmwsy
I would suggest to carefully check your mesh. Second order usually relies on an enlarged stencil, and will be much more sensitive to the mesh quality. Try to find where the convergence problem comes from, and readjust your mesh in the area. You may also fix it using some classical mesh quality criteria (determinant, skewness, etc.). BR koo 

December 30, 2009, 05:37 

#9  
New Member
Shengyi Wang
Join Date: Mar 2009
Posts: 22
Rep Power: 9 
Quote:
Thank you for your reply. I have checked the mesh and obviously the problem occurs at the boundary layer. Because I am not using the wall function and thus y+ is around 1, so the aspect ratio of the cells is very big. But this is usual in my area. I do not know how to change the mesh. Best wishes to the new year, Shengyi 

January 1, 2010, 18:50 

#10  
Senior Member
Join Date: Jul 2009
Posts: 245
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Quote:
What meshing program are you using? What solver? NB FLUENT has a very useful mesh adaption criterion specially for boundary layers_: Boundary layer adaption 

January 1, 2010, 19:08 

#11  
New Member
Shengyi Wang
Join Date: Mar 2009
Posts: 22
Rep Power: 9 
Quote:
Thanks for reply. I know about the adaption function in Fluent, but it might not help a lot. I recently found a potential solution for this problem: using a ke model to get a solution and then switch to kw. 

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