Computation can't achieve convergence

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June 17, 2014, 05:01
Computation can't achieve convergence
#1
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Viktoriya
Join Date: Jun 2014
Location: Ukraine
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Hi everyone!
First of all, sorry for my language mistakes. English isn't my native language.

I simulate turbulent flow in centrifugal pump channel using CFX and standard k-e model. Meshing is done using icem cfd.

the graph of RMS start to rise after 500 iterations. Is it OK? I think that I have problems with mesh..but how can I check it?

And I don't like the precession. I've tried to use different value of physical timescale, but it didn't help.

Hope you will help me. If you need some information for this I will answer the questions about my simulation.
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 June 17, 2014, 07:51 #2 Member   SanS Join Date: Mar 2009 Posts: 41 Rep Power: 10 Have you checked your plots? may be you have a separation in your domain that's causing this. Plot some variables on interest and look at their variation vs. iterations.

June 17, 2014, 09:18
#3
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Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
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Quote:
 Originally Posted by sans Have you checked your plots? may be you have a separation in your domain that's causing this. Plot some variables on interest and look at their variation vs. iterations.
Thank you for the answer

I have no separation in domain

 June 17, 2014, 17:07 #4 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 14 Have your solution export the residuals. Plot the residuals in Post. Using Isovolume you can plot those that are above your criteria (whatever you want it to be). But, this could be a transient issue. Are you sure you arent getting separation or vortex in the domain? Residuals are looking like they have a cycle to them. Might try running this as a transient case for a bit to see if it steadies out. Are you comparing this to experiment or analytical solution? Hows the Head value you are listing compare?

 June 17, 2014, 19:54 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,982 Rep Power: 107 This is a FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

June 18, 2014, 04:41
#6
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Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
Rep Power: 5
Quote:
 Originally Posted by ghorrocks This is a FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

Thanr you, ghorrocks. Exactly my situation. this link is very useful.

June 18, 2014, 05:06
#7
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Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
Rep Power: 5
Quote:
 Originally Posted by singer1812 Are you sure you arent getting separation or vortex in the domain? Residuals are looking like they have a cycle to them.
Now I'm not so sure.It's really look like I get vortex or smth. Should I check it in Post. right?

Quote:
 Originally Posted by singer1812 Are you comparing this to experiment or analytical solution? Hows the Head value you are listing compare?
I don't have experiment data. But I have solution after coarse mesh and Head=100,68 m. And Head=100,231 m after this simulations

 June 18, 2014, 06:35 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,982 Rep Power: 107 The FAQ explains how to handle this. Try a larger time step. That may get the transient behaviour under control. If not then you may be forced to consider a transient simulation - but there are lots of other things to consider..... read the FAQ There is also an FAQ on accuracy - have a read of that while you are at it.

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