jump of residual in transient sim

Flashidashi · June 26, 2015, 06:42

Hey guys,

i noticed a quite interesting residual-phenomenon in my results. In this case i simulated the flow around a vertical cylinder (transient).

What i mean is the jump in picture (b) after 500 iterations.

Anyone knows, which effect(s) is/are responsible for it?

Maybe Ghorrocks?

Thanks in Advance Flashi

ghorrocks · June 26, 2015, 07:02

If this is a transient simulation it is probably doing an additional coeff loop per time step to achieve the convergence tolerance.

By the way: reporting convergence tolerances like this is a bit pointless for transient simulations. You need to establish a suitable convergence tolerance and just do as many coeff loops as is required to make it. This means that your residuals should be flat from beginning to end as it just achieves the tolerance. It will take more coeff loops in the tricky to converge bits and less in the easy bits, but the residual it achieves in the end is roughly the same from beginning to end of the simulation.

Flashidashi · June 26, 2015, 08:28

For these simulations i used 2 to 6 inner coef. loops and a residual of 1*10^-(4) for the global mesh (with Courant_max = 1) and initial files from steady state sims. So i got these residuals for the finer mesh-regions. After a certain number of iterations, only 2-3 coef. loops were neccessary. So i think, i did what you say in your post.

Greets Flashi

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June 26, 2015, 07:02		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If this is a transient simulation it is probably doing an additional coeff loop per time step to achieve the convergence tolerance. By the way: reporting convergence tolerances like this is a bit pointless for transient simulations. You need to establish a suitable convergence tolerance and just do as many coeff loops as is required to make it. This means that your residuals should be flat from beginning to end as it just achieves the tolerance. It will take more coeff loops in the tricky to converge bits and less in the easy bits, but the residual it achieves in the end is roughly the same from beginning to end of the simulation.

June 26, 2015, 08:28		#3
Flashidashi New Member Andreas Join Date: Dec 2014 Posts: 16 Rep Power: 11	For these simulations i used 2 to 6 inner coef. loops and a residual of 1*10^-(4) for the global mesh (with Courant_max = 1) and initial files from steady state sims. So i got these residuals for the finer mesh-regions. After a certain number of iterations, only 2-3 coef. loops were neccessary. So i think, i did what you say in your post. Greets Flashi