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Choosing convergence criteria on monitor point

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Old   June 3, 2012, 00:00
Default Choosing convergence criteria on monitor point
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I am monitoring torque in a rotating component. The torque value I am expecting is between 1000-2000 N*m, depending on rotational velocity. I would need to find the value within 25 N*m. What should I use to determine when this happens, as it seems there will always be a slight change if i let it go on forever. Would something like a change of less than .01% over 20 iterations likely be sufficient for what I'm looking for?
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Old   June 3, 2012, 06:59
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Glenn Horrocks
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You are looking for quiet accurate results (2.5 on torque) so you are going to have to be very careful with accuracy to achieve this. I think you are going to have to be a bit more clever than that to achieve your target accuracy reliably.

I would have a look at the key reference text on CFD accuracy "Computational Fluid Dynamics" by Roache. Things like the link on this FAQ are a good summary

A thought on your specific case of judging convegernce: If you let your case converge for a while you may find that the torque returned is following a relationship like error=1/x^n or error=e^-nx or something. If you can pick what path it is taking for convergence you can get a far more accurate torque number than you simple tolerance approach. Note that this is a similar approach as Richardson extrapolation is to mesh refinement studies.
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