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August 18, 2011, 16:18 |
Very small residual
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#1 |
Member
Join Date: Aug 2011
Posts: 30
Rep Power: 15 |
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. |
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August 18, 2011, 17:07 |
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#2 |
New Member
Ertan Karaismail
Join Date: Apr 2009
Posts: 17
Rep Power: 17 |
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) |
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August 18, 2011, 17:37 |
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#3 |
Member
Join Date: Aug 2011
Posts: 30
Rep Power: 15 |
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? |
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August 18, 2011, 18:03 |
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#4 |
Senior Member
Vieri Abolaffio
Join Date: Jul 2010
Location: Always on the move.
Posts: 308
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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...
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August 19, 2011, 12:40 |
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#5 |
New Member
Ertan Karaismail
Join Date: Apr 2009
Posts: 17
Rep Power: 17 |
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 |
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Tags |
residual, small |
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