# Very small residual

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

 August 18, 2011, 15:18 Very small residual #1 New Member   Orxan Shibliyev Join Date: Aug 2011 Posts: 18 Rep Power: 6 Hi everyone! For common residual definition which is, r = b - A*y I get a very small value even at the beginning (about e-5). So I'm not sure how I can set a criterion of convergence. Waiting your precious answers.

 August 18, 2011, 16:07 #2 New Member   Ertan Karaismail Join Date: Apr 2009 Posts: 17 Rep Power: 8 One option could be using normalized value for setting convergence criterion; Rn_i=R_i/R_1 < tolerance where, Rn_i= normalized residual from the ith iteration R_i = residual from the ith iteration R_1= residual from the 1st iteration (generally max residual)

 August 18, 2011, 16:37 #3 New Member   Orxan Shibliyev Join Date: Aug 2011 Posts: 18 Rep Power: 6 Mr. Ertan Thank you. But in this case what should be the proper tolerance? How can I be sure that, chosen tolerance is enough for accurate results?

August 18, 2011, 17:03
#4
Senior Member

Vieri Abolaffio
Join Date: Jul 2010
Location: Always on the move.
Posts: 308
Rep Power: 8
Quote:
 Originally Posted by orxan.shibli Mr. Ertan Thank you. But in this case what should be the proper tolerance? How can I be sure that, chosen tolerance is enough for accurate results?
you would need to check at the history of some physical quantities of the flow plotted vs the number of iteration. for example lift, drag, heat transfer, etc...

 August 19, 2011, 11:40 #5 New Member   Ertan Karaismail Join Date: Apr 2009 Posts: 17 Rep Power: 8 That depends on how much accuracy you need. For unsteady flows, it should be safe to can pick a tolerance (convergence criterion) of 0.001 or less for the normalized residuals. For steady flows, I would suggest setting the convergence criterion to a very low value, such as 10^-10. This way you could minimize the iteration error in your calculations. (If you plot residuals, you should be able to see them level off (flatten) after a while). HTH

 Tags residual, small

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Unseen OpenFOAM Running, Solving & CFD 7 April 16, 2014 03:38 xiuying OpenFOAM Running, Solving & CFD 8 August 27, 2013 15:33 jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 05:24 Lord Kelvin OpenFOAM 6 April 12, 2011 11:24 carsten OpenFOAM Bugs 11 September 12, 2008 11:16

All times are GMT -4. The time now is 06:35.