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

If my solution is converged ?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 8, 2009, 04:35
Wink If my solution is converged ?
  #1
New Member
 
Join Date: Jun 2009
Posts: 21
Rep Power: 8
jing113cn is on a distinguished road
I want to use the time-dependant solver to get the steady state. And I also monitor some important parameters, such as temperature, vof...
After some iterations, I found these parameters almost do not change, but the residuals remains at a high value(10-2) and goes flat. I wonder if I my problem is converged ?
The residula and monitor look like this >
vof.JPGresidual.JPG
jing113cn is offline   Reply With Quote

Old   September 9, 2009, 08:28
Default
  #2
Member
 
JP
Join Date: Mar 2009
Posts: 57
Rep Power: 9
srjp is on a distinguished road
Residuals becoming flat may indicate:
1. Your solution has converged, or
2. This is the minimum residual possible with your current grid.

For further reduction in residuals, you may have to refine or adapt the grid.
srjp is offline   Reply With Quote

Old   September 9, 2009, 14:08
Default
  #3
New Member
 
Sean Delfel
Join Date: Aug 2009
Posts: 27
Rep Power: 7
delfel is on a distinguished road
Yeah, I agree with srjp. You're going to need to refine your grid. I would recommend plotting contours of mass imbalance (since your continuity residuals didn't converge) and refining where they are high.

-sean
delfel is offline   Reply With Quote

Old   September 10, 2009, 01:39
Default
  #4
New Member
 
Join Date: Mar 2009
Posts: 25
Rep Power: 8
denizen is on a distinguished road
you both wrong.
this is transient solution and if one decrease the time step the residuals will decrease too.
you execute 1 ineration on 1 time step?
denizen is offline   Reply With Quote

Old   September 10, 2009, 02:03
Default
  #5
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 2,964
Rep Power: 30
-mAx- will become famous soon enough
the continuity residuals are too high.
reduce your time step
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   September 10, 2009, 09:16
Default
  #6
Member
 
JP
Join Date: Mar 2009
Posts: 57
Rep Power: 9
srjp is on a distinguished road
Sorry, I thought it was steady state simulation.
May be after frustration of seeing so many flat residuals !!!
Agree with mAx on the timestep reduction.
srjp is offline   Reply With Quote

Old   September 10, 2009, 11:00
Default
  #7
New Member
 
Sean Delfel
Join Date: Aug 2009
Posts: 27
Rep Power: 7
delfel is on a distinguished road
ditto.

-sean
delfel is offline   Reply With Quote

Old   September 11, 2009, 02:51
Default
  #8
New Member
 
Join Date: Jun 2009
Posts: 21
Rep Power: 8
jing113cn is on a distinguished road
Yes, 1 interation on 1 time step at beginning, because I want save computation time. And then I increase the interations each time step, and the residuals also remain flat.

Quote:
Originally Posted by denizen View Post
you both wrong.
this is transient solution and if one decrease the time step the residuals will decrease too.
you execute 1 ineration on 1 time step?
jing113cn is offline   Reply With Quote

Old   September 11, 2009, 03:39
Default
  #9
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 2,964
Rep Power: 30
-mAx- will become famous soon enough
forget 1 iteration per time step
try the default settings, eg: 20 iterations per time step
I would also recommend you to start your unsteady computation from a well converged steady solution
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   September 11, 2009, 17:46
Default
  #10
Member
 
Ivan
Join Date: May 2009
Posts: 85
Rep Power: 8
ivanbuz is on a distinguished road
Most likely you are using a too big time-step. The time in your pictures is 2.4e4. Imagine if time step=1e-6, how many steps it will be....
ivanbuz is offline   Reply With Quote

Old   September 16, 2009, 09:21
Question Character of success iteration???
  #11
Senior Member
 
teguhtf's Avatar
 
teguh hady
Join Date: Aug 2009
Location: Surabaya, Indonesia
Posts: 153
Rep Power: 7
teguhtf is on a distinguished road
Hello!!!
I'm a new user in FLUENT. I've read your Post. I wanna know what is the indication that we have success iteration??
I've thought that FLAT graph on the Residual graph is the indication!!!
Am I wrong???
teguhtf is offline   Reply With Quote

Old   September 17, 2009, 04:07
Default
  #12
New Member
 
Join Date: Jun 2009
Posts: 21
Rep Power: 8
jing113cn is on a distinguished road
I think when the we get converged solution, the residual will go flat. But at the same time, the residual should fulfill the criterion, such as 10e-4,10e-5...
jing113cn is offline   Reply With Quote

Old   September 17, 2009, 06:59
Default
  #13
New Member
 
Join Date: Mar 2009
Posts: 25
Rep Power: 8
denizen is on a distinguished road
Flat or not it is no matter. If iteration converged then it is. I think so.

Regards.
denizen is offline   Reply With Quote

Old   September 28, 2009, 03:29
Default
  #14
Senior Member
 
teguhtf's Avatar
 
teguh hady
Join Date: Aug 2009
Location: Surabaya, Indonesia
Posts: 153
Rep Power: 7
teguhtf is on a distinguished road
Is there an equation to prove it?? just to know how much convergence value!!!
teguhtf is offline   Reply With Quote

Old   September 29, 2009, 01:33
Default
  #15
New Member
 
Join Date: Mar 2009
Posts: 25
Rep Power: 8
denizen is on a distinguished road
there is navier-stokes equation.
and http://en.wikipedia.org/wiki/Residua...rical_analysis)

regards.
denizen 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
Solution converged, and then diverge again. jrg FLUENT 2 April 1, 2013 23:10
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58
Calculate a converged flow solution kumar FLUENT 2 May 24, 2007 01:33
Could anybody help me see this error and give help liugx212 OpenFOAM Running, Solving & CFD 3 January 4, 2006 19:07
converged solution or correct solution SAM FLUENT 1 October 12, 2004 15:38


All times are GMT -4. The time now is 16:22.