residuals of bin fraction remains constant

melj · March 10, 2020, 11:26

Hello

I am modeling a multiphase bubble column using Euler-Euler; RSM for turbulence; and PBM. The residuals of bin-fraction appear to be constant after around 30 iterations. Why does this happen? Does this mean that I could run the simulations with lesser no. of iterations per time step?

I am using 350 iterations/time step currently.

thank you

vinerm · March 11, 2020, 03:17

Are the residuals low enough? If yes, then it is good.

The idea behind number of iterations per time-step and the size of time-step is to ensure such a value of time-step so that no more than 30 iterations are required for convergence in each time-step. So, 350 iterations is really too high a value. In case the simulation takes more than 30 iterations to converge, reduce the time-step. So, in your case, it is good that it is converging within 30 iterations.

melj · March 12, 2020, 15:39

I wanted to get the bin fraction residuals below 10^-3. But, it does not go below that and so it does not converge. The value of residual reduces to a constant. I feel that its pointless to let it run for 350 iterations and so rather run it for 50 iterations for every time step.

Or would you suggest to work with the URF?

vinerm · March 13, 2020, 04:27

Computers are dumb, so, they can only consider it 1e-3 if it really is 1e-3. But we can observe it better. So, if it is close to 1e-3, such as 2e-3 or even 3e-3, it is alright. However, if it is at least an order larger, then you can play with URFs as well as gradient limiters. Residuals gets stalled with standard limiters. You can try using cell-to-cell limiters and see if that improves the residuals.

melj · March 13, 2020, 17:06

thank you vinerm.

cultivate · June 17, 2022, 11:47

hi,I meet the same question:the residuals of bin fraction is high and it cannot go down with iterations.why?
thanks in advance

March 10, 2020, 11:26	residuals of bin fraction remains constant	#1
melj Member mln Join Date: Dec 2019 Posts: 39 Rep Power: 6	Hello I am modeling a multiphase bubble column using Euler-Euler; RSM for turbulence; and PBM. The residuals of bin-fraction appear to be constant after around 30 iterations. Why does this happen? Does this mean that I could run the simulations with lesser no. of iterations per time step? I am using 350 iterations/time step currently. thank you

March 11, 2020, 03:17	Residual and Convergence	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	Are the residuals low enough? If yes, then it is good. The idea behind number of iterations per time-step and the size of time-step is to ensure such a value of time-step so that no more than 30 iterations are required for convergence in each time-step. So, 350 iterations is really too high a value. In case the simulation takes more than 30 iterations to converge, reduce the time-step. So, in your case, it is good that it is converging within 30 iterations. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

March 13, 2020, 04:27	Residuals	#4
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	Computers are dumb, so, they can only consider it 1e-3 if it really is 1e-3. But we can observe it better. So, if it is close to 1e-3, such as 2e-3 or even 3e-3, it is alright. However, if it is at least an order larger, then you can play with URFs as well as gradient limiters. Residuals gets stalled with standard limiters. You can try using cell-to-cell limiters and see if that improves the residuals. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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March 12, 2020, 15:39		#3
melj Member mln Join Date: Dec 2019 Posts: 39 Rep Power: 6	I wanted to get the bin fraction residuals below 10^-3. But, it does not go below that and so it does not converge. The value of residual reduces to a constant. I feel that its pointless to let it run for 350 iterations and so rather run it for 50 iterations for every time step. Or would you suggest to work with the URF?

March 13, 2020, 17:06		#5
melj Member mln Join Date: Dec 2019 Posts: 39 Rep Power: 6	thank you vinerm.

June 17, 2022, 11:47		#6
cultivate New Member wu pei peng Join Date: Jul 2021 Posts: 4 Rep Power: 4	hi,I meet the same question:the residuals of bin fraction is high and it cannot go down with iterations.why? thanks in advance