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

what to monitor besides residuals?

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 4, 2009, 19:35
Default what to monitor besides residuals?
  #1
New Member
 
Franz Roman
Join Date: Jun 2009
Posts: 28
Rep Power: 17
franzdrs is on a distinguished road
Hi, I am running simulations of simple geometries with one velocity inlet and one pressure outlet and a porous zone. I make some variations to a base geometry. my goal is to find a geometry that gives the best (most uniform) velocity distribution at the outlet. For some geometries the residuals dont drop lower than say 3e-4 for epsilon (I am using k-epsilon realizable) and maybe not lower than 1e-5 for the other residuals. They go flat at this point. For other cases the residuals go flat at a lower value of the residuals. Well, the residuals look flat in the graph for hundreds or even thousands of iterations, but the values change a little bit as seen in the solver main window, so little that in the graph there is a horizontal line. Is that flat enough? or flat means absolutely no change in the values of the residuals?

I have read that it is good to monitor some value of interest to judge convergence. However, in my case I dont know which value this could be. I am not so much interested in any forces or pressures. I can only think of the mass flux, or the mass flow rate at the outlet. Would this be a good monitor for my case?

thanks in advance
Franz
franzdrs is offline   Reply With Quote

Old   August 5, 2009, 00:39
Default
  #2
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,297
Rep Power: 41
-mAx- will become famous soon enough
monitor the massflow rate, and iterate until you become also flat curve
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   August 5, 2009, 08:02
Default
  #3
New Member
 
Franz Roman
Join Date: Jun 2009
Posts: 28
Rep Power: 17
franzdrs is on a distinguished road
Hi mAx,
thanks for answering. A question though. Flat curves means that the residuals change so little around a value so that the curves "look" flat? or means that the residuals dont change one bit at all?
What about the massflow rate? does it have to be absolutely flat (no change in the values at all with ongoing iterations)? or is flat just how it looks in the graph, but with possible minor changes from one iteration to the next?
If the monitored quantities go flat, is it then ok if they do it at 1e-4 or 1e-5? or should they go flat at a lower level?
many thanks
Franz
franzdrs is offline   Reply With Quote

Old   August 5, 2009, 08:23
Default
  #4
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,297
Rep Power: 41
-mAx- will become famous soon enough
*flat residuals curves mean that residuals don t converge anymore
*massflow rate curves should be flat, but not absolutely. Value can smoothly change, but if you still get oscillations, then it is not ok.
It depends also on the precision you will reach.
*For me a flat monitoring with all residuals below 1e-3, it's ok. (in my case)
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   August 5, 2009, 08:26
Default
  #5
Member
 
Join Date: Mar 2009
Posts: 32
Rep Power: 17
udayrg is on a distinguished road
Hi
You can monitor mass flow rate at outlet, as well as total pr. at inlet as both the monitors show stabilization i.e. least variation and with your residuals fall in the plot(1e-3/4/5/6 your experience will tell you). You can conclude the run has formidably converged.
udayrg is offline   Reply With Quote

Old   March 21, 2013, 03:59
Thumbs up Monitor Plots
  #6
Member
 
Thiagu
Join Date: Oct 2012
Location: India
Posts: 60
Rep Power: 13
jthiakz is on a distinguished road
Monitor plots are the important are primary parameters, very closely related with your objective of the problem.(which one wants to verify using CFD tool) .

In simple single phase flow problems
1. Mass in & mass out
2. area ave pressure

# For Heat transfer problem
1. Min,Max ,Avg temperature
Example:
Air is heated by wall temp (imposed fixed wall temp BC), one can monitor (min/max/avg)temp of air at different location or in the region of interest, which will show solution fluctuation there. For steady state problem, it will be smooth. It is not necessary that it should be flat always, again it depends on physics at that location/local mesh refinement and calculation method (node avg/face avg)

To me , monitor plots = Experimental data (real time test rig readings from probes)
jthiakz is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[Tutorials] Tutorial of how to plot residuals ! wolle1982 OpenFOAM Community Contributions 171 February 20, 2024 02:55
Mesh size and solver residuals... Scott CFX 5 December 15, 2008 17:10
Convergence - scaled vs unscaled residuals HS FLUENT 1 November 7, 2005 05:45
residuals Karl CFX 2 June 24, 2003 23:13
Residuals? Tom Main CFD Forum 6 November 28, 2002 04:09


All times are GMT -4. The time now is 23:44.