Problem with convergence monitors

Ryan_Yang · March 21, 2016, 08:49

Hi, everyone,

I am trying to judge the convergence of one simulation with one facet average value on the monitoring surface. I activated the the monitor in the 'convergence manager' and defined the stop criterion as '0.001'. The default residuals monitor were deactivated.

It worked fine when I did the steady case and the calculation stopped exactly when the residual of the monitoring value is lower than 0.001. But when I switched the case into transient, the stop criterion failed to work and the calculation went on working until the max iterations per time step was reached

.

Does anyone have same experience and know how to fix the problem in transient calculation?

thanks!

Ryan

jpina · March 21, 2016, 09:29

As far as I know, in a transient simulation, this criteria is used not for finishing the simulation but for stopping the iterations of the calculated time step.

Ryan_Yang · March 21, 2016, 09:36

Quote:

Originally Posted by jpina

As far as I know, in a transient simulation, this criteria is used not for finishing the simulation but for stopping the iterations of the calculated time step.

Hi, Jpina,

thanks for reply!

To stop the iterations of calculating time step is what I want to do. But the calculation won't stop even if the stop criterion are reached.

jpina · March 21, 2016, 09:50

Maybe not all the stop criterions are satisfied: Courant number for explicit, truncation errors, other residuals...?

Ryan_Yang · March 21, 2016, 10:05

Hi, Jordi,
Monitors on the other residuals were turned off. Are Courant number and truncation error checked for convergence in FLUENT?

jpina · March 21, 2016, 10:09

Only if you use variable time step!

Ryan_Yang · March 21, 2016, 10:17

The time step is fixed...So I guess the surface-monitored value should be the only criterion that checked for convergence.

Ryan_Yang · March 21, 2016, 10:51

It seems to me that in transient calculation, FLUENT calculates the residual of user-defined monitoring value at end of each TIME STEP, which based on the value of current iteration and last iteration at pre-time step. Once the criterion is reached, the whole calculation will be stopped.

what is the point of doing so? why not applying the same convergence monitoring method of default criteria (continuity, x,y,z-velocity,.....), which calculates the residual at each iteration and jump to next time step when convergence is reached.

I am now in big confusion

.

March 21, 2016, 08:49	Problem with convergence monitors	#1
Ryan_Yang New Member ryan Join Date: Jun 2011 Location: Jülich,Germany Posts: 19 Rep Power: 14	Hi, everyone, I am trying to judge the convergence of one simulation with one facet average value on the monitoring surface. I activated the the monitor in the 'convergence manager' and defined the stop criterion as '0.001'. The default residuals monitor were deactivated. It worked fine when I did the steady case and the calculation stopped exactly when the residual of the monitoring value is lower than 0.001. But when I switched the case into transient, the stop criterion failed to work and the calculation went on working until the max iterations per time step was reached. Does anyone have same experience and know how to fix the problem in transient calculation? thanks! Ryan

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March 21, 2016, 09:29		#2
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	As far as I know, in a transient simulation, this criteria is used not for finishing the simulation but for stopping the iterations of the calculated time step.

March 21, 2016, 09:50		#4
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	Maybe not all the stop criterions are satisfied: Courant number for explicit, truncation errors, other residuals...?

March 21, 2016, 10:05		#5
Ryan_Yang New Member ryan Join Date: Jun 2011 Location: Jülich,Germany Posts: 19 Rep Power: 14	Hi, Jordi, Monitors on the other residuals were turned off. Are Courant number and truncation error checked for convergence in FLUENT?

March 21, 2016, 10:09		#6
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	Only if you use variable time step!

March 21, 2016, 10:17		#7
Ryan_Yang New Member ryan Join Date: Jun 2011 Location: Jülich,Germany Posts: 19 Rep Power: 14	The time step is fixed...So I guess the surface-monitored value should be the only criterion that checked for convergence.

March 21, 2016, 10:51		#8
Ryan_Yang New Member ryan Join Date: Jun 2011 Location: Jülich,Germany Posts: 19 Rep Power: 14	It seems to me that in transient calculation, FLUENT calculates the residual of user-defined monitoring value at end of each TIME STEP, which based on the value of current iteration and last iteration at pre-time step. Once the criterion is reached, the whole calculation will be stopped. what is the point of doing so? why not applying the same convergence monitoring method of default criteria (continuity, x,y,z-velocity,.....), which calculates the residual at each iteration and jump to next time step when convergence is reached. I am now in big confusion .