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September 29, 2011, 10:22 
How to judge convergence?

#1 
New Member
Akash
Join Date: Sep 2011
Posts: 12
Rep Power: 6 
Dear All,
How are you all doing! I believe doing awesome! I have a problem in understanding the convergence of my problems. I solve the model without cavitation and then with caviatation. I set my RMS residuals to 1e06. I check for 1) Mass and Momentum to be below 1e06 2) Turbulence to be below at least 1e4 3) Volume fractions to be below 1e4 4) Imbalance of all and most important is mass imbalance and Pvolum imbalance almost near to zero 5) Flow  In  Mass boundary flow on in(viscor) and PMass boundary flow on in to that of Out  Mass boundary flow on Out (viscor) and PMass boundary flow on Out to be equal over the number of iterations. Kindly let me know whether I am right or please correct me if I am wrong. My Imbalance especially mass imbalance and Pvolum imbalance oscillates at zero between 10 to 22 and I stop it at zero, when it has run long Please some on solve me or suggest me the better way. Best Regards Akash A V 

September 29, 2011, 14:15 

#3  
Super Moderator

Quote:
Last edited by Far; September 29, 2011 at 14:17. Reason: wrong command in latex 

September 29, 2011, 16:11 

#4 
New Member
Akash
Join Date: Sep 2011
Posts: 12
Rep Power: 6 
Dear Sir,
Thanks for your kind response. I usually see to that all the residuals are below the set value(1e06), but some times I even compromise at 1e04. Regarding the mass imbalance and Pvolum imbalance oscillates (+/) at zero between 10 to 22% and then flantens up to zero. I wanted to ask that, is it right thing to stop the solution when the residuals have come down the set value and before it converges fully. Or if not should we allow it run still it come to halt. Are there some kind of divergence and bisection graphs where upon we can make out the solution is converging or not. please let me know on this. As this is making me confused with when to stop the solution, although the residuals have come below the set values(1e06) Once again thank you! Best Regards Akash A V 

September 29, 2011, 16:17 

#5 
Super Moderator

better to give some time, to be on the safe side, to see that solution does not change for next 2050 iterations, If you have difficulty in getting the residuals below 1e04, try to reduce the time step


September 29, 2011, 17:50 

#6 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,559
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My preferred approach is to do a sensitivity analysis. Compare the results of a parameter you care about (eg drag, maximum pressure, whatever is important in your analysis) with a range of convergence tolerances. When the parameter asymptotes out to within a tolerance you are happy to accept then that defines your convergence critereon.


September 30, 2011, 13:51 

#7 
New Member
Akash
Join Date: Sep 2011
Posts: 12
Rep Power: 6 
Dear All,
Thanks a lot. I am doing out sensitive test, with the mesh as well as the parameters. I see with change in the mesh size and the values things change horribly and a time I felt, what ever I did was all crap. But soon realized that nothing much changes with these changes, only certain output changes but need to again get hold upon it based uopon sensitive tests. I will stick on to 1E06 and is the best approach. Reduce all residuals below 1e06 and see to the imbalane graph and check all variable are at atleast 0.1% +/ between zero. Check other parameters like mass flow in and going out (must be all most the same) and the mass viscor, turbulence Rms all below 1e04. Send me your email ID I will send the convergence graphs Please correct me if I am wrong Best Regards Akash A v 

October 3, 2011, 17:47 

#10  
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,559
Rep Power: 97 
Quote:


October 4, 2011, 00:04 

#11  
Super Moderator

Quote:
PS. In external aerodynamics CFD I put less stringent convergence criteria than for the internal flows, specially for turbomachinery 

October 4, 2011, 00:09 

#12  
Super Moderator

Quote:
1. Saves alot time in future 2. Confidence level in results is higher, and every body shall trust your CFD, even yourself!!!!! 

October 4, 2011, 01:12 

#13 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,559
Rep Power: 97 
Your 0.01% was actually the mass residual? From your initial comment I suspected you were talking about a mass imbalance. 1e4 is a loose residual (but often OK for design purposes) but 1e4 as a mass imbalance is very tight.
But again, the best approach is to do a sensitivity study and find out how tight your exact case requires. 

October 4, 2011, 02:07 

#14  
Super Moderator

Quote:
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