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Old   September 29, 2006, 10:08
Default convergence RMS
  #1
anna
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Hello,

I would like to ask you if you can advise me something. My RMS is set to 1.0E-4. The solver is running without problem and finishing successfully the 100 iterations defined at the beginning. But the RMS of 1E-4 is not reached. There is a little pick at the very beginning by 1.E-2, goes down to 1E-3 and keeps almost constant 1e-3 till the end of the iterations. Do you have any ideas?
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Old   September 29, 2006, 10:35
Default Re: convergence RMS
  #2
Joe
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Read "Judging convergence" in the manual.
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Old   October 2, 2006, 03:02
Default Re: convergence RMS
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TobiasZ
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Hi Anna, Reduce your physical time step by for example 5 and try again and see if the RMS gets lower. Depending on your simulation model the total number of required iterations might need more iterations. If for example the fluid flow is very slow iterations might need many iterations for convergence to obtain the required convergence criterion. Another reason for the high RMS could also be the quality and size of your mesh. Regards. Tobias
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Old   October 3, 2006, 03:40
Default Re: convergence RMS
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TB
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What turbulence model are you using? This could also be due to the inherent unsteady phenomenon in your flow. Run the transient simulation and check it out.
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Old   October 3, 2006, 03:46
Default Re: convergence RMS
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anna
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Hi, I am using SST model.
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Old   October 3, 2006, 04:26
Default Re: convergence RMS
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TB
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Have you got the same problem using standard ke model?
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Old   October 3, 2006, 04:35
Default Re: convergence RMS
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anna
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Yes, just at the beginning there is a small jump in the residuals plot and even after 500 iterations the residuals stay almost stable. But lateron by comparing the results in the output file, Global imbalance, MAX, RMS residuals everything seem to be withing normal limits. My mesh is not perfect, because I made it with ANSYS and I imported it in CFX and because of the boundary layer region I made the volumes next to it with hex mesh. So, to be more clear I have a multi-layers coil placed in a tube with running water. Next to the coil surface the Yplus for example is about 7 and next to the tube surface the Yplus is approx. 18 but eventhough I think the SSt model is the most appropriate one.

What is your opinion? Thanks a lot. Anna
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Old   October 3, 2006, 04:50
Default Re: convergence RMS
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TB
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SST model is very sensitive to the mesh quality, esp if the flow inside the domain is complex. I suggest you to include the following things in your checklist:

1.mesh (I'll try hexa if tet doesn't work properly) 2.unsteady flow effect (I found that SST model sometimes captures the unsteady phenomenon that ke model fails to show in the solution, this will cause convergence problem)

You need to solve the convergence problem before you can say anything about the accuracy.
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