CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Computation can't achieve convergence

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   June 17, 2014, 05:01
Default Computation can't achieve convergence
  #1
New Member
 
Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
Rep Power: 3
dushechka is on a distinguished road
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.
Attached Images
File Type: jpg 1.jpg (97.8 KB, 12 views)
File Type: jpg 2.jpg (96.2 KB, 10 views)
File Type: jpg 3.jpg (95.9 KB, 9 views)
dushechka is offline   Reply With Quote

Old   June 17, 2014, 07:51
Default
  #2
Member
 
SanS
Join Date: Mar 2009
Posts: 42
Rep Power: 8
sans is on a distinguished road
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.
sans is offline   Reply With Quote

Old   June 17, 2014, 09:18
Default
  #3
New Member
 
Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
Rep Power: 3
dushechka is on a distinguished road
Quote:
Originally Posted by sans View Post
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
dushechka is offline   Reply With Quote

Old   June 17, 2014, 17:07
Default
  #4
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 500
Rep Power: 11
singer1812 is on a distinguished road
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?
singer1812 is offline   Reply With Quote

Old   June 17, 2014, 19:54
Default
  #5
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,960
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
This is a FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is offline   Reply With Quote

Old   June 18, 2014, 04:41
Default
  #6
New Member
 
Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
Rep Power: 3
dushechka is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post

Thanr you, ghorrocks. Exactly my situation. this link is very useful.
dushechka is offline   Reply With Quote

Old   June 18, 2014, 05:06
Default
  #7
New Member
 
Viktoriya
Join Date: Jun 2014
Location: Ukraine
Posts: 7
Rep Power: 3
dushechka is on a distinguished road
Quote:
Originally Posted by singer1812 View Post
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 View Post
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
dushechka is offline   Reply With Quote

Old   June 18, 2014, 06:35
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,960
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Convergence Centurion2011 FLUENT 24 May 9, 2015 08:02
Effect of convergence criteria for pressure loop on computation time mhdali101 Main CFD Forum 0 November 29, 2013 17:09
FSI/MFX: resutls after using fluid mass flux to achieve convergence gogohusky ANSYS 0 May 21, 2012 05:40
Force can not converge colopolo CFX 13 October 4, 2011 22:03
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17


All times are GMT -4. The time now is 06:54.