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 Roberto Ciardulli November 17, 2000 13:31

Solution method for transient problems

Can anybody tell me how the values of the following parameters affect the solution?

maximum number of corrector stages

reduction in residuals for corrector stages

under-relaxation for pressure correction

max residual tolerance

Thanks everybody,

Roberto

 A.Hassaneen November 18, 2000 04:02

Re: Solution method for transient problems

Roberto, The values of the corrector stages and the residuals works like a criteria for convergence of the inner iteration for each time step. i.e. the solution terminate the inner iteration when any of these two parameter fulfilled. The under-relaxation of the pressure is very important like any other relaxation. Any body correct me if I'm wrong.

 X.Ye November 18, 2000 10:45

Re: Solution method for transient problems

Hi,

Because of Calculation high transient flow with Star CD, I just want to ask: If you calculate it with a smaller time step, do you need only a smaller number of iteration for each time? For example, If the time step is 100.0e-6, you need 60 iterations, but if the time step is 10.0e-6, do you need only 6 iteration?

I ask this question, because I want to compare the calculation efficiency with the explicit method. StarCD has the implicit method.

X. Ye

 A.Hassaneen November 19, 2000 04:11

Re: Solution method for transient problems

I don't have definite answer for your question, but you may try it and look at the file (case.info) to see how the solution is going.

 J. Y. Luo November 22, 2000 10:13

Re: Solution method for transient problems

The maximum no. of correctors is very similar to the maximum no. of iterations for steady flow calculations - if the equations converges earilier, this no. is then unimportant: the most important criterion is how much the residuals of all equations are reduced, normally PISO will converge within 2 to 4 correctors, so the default 20 correctors are quite irrelevant.

 Roberto Ciardulli November 27, 2000 04:13

Re: Solution method for transient problems

So is it ok if I use a very high value for MAX NUMBER of CORRECTOR STAGES (ex. 100) and a very low value for REDUCTION in RESIDUAL for CORRECTOR STAGES (ex. 0.1)

I still get the neg. density error during transient state runs and I would like to know if these parameters affect the solution's stability.

Thanks everybody,

Roberto

 J. Y. Luo November 27, 2000 06:23

Re: Solution method for transient problems

Hi, Roberto,

What you described is certianly Ok. You may need to use under-relaxation factor for PISO (this is used to under-relax the pressure correction) that would help in your problem. The under-relaxation factor would not affect the final results at each time step, but may need a few more correctors for PISO to converge. Typically a value of 0.5 is common.

 Roberto Ciardulli November 27, 2000 10:10

Re: Solution method for transient problems

I've set the following values:

Max number of corrector stages = 1000

Reduction in residual for corrector stages = 0.1

Under-relax. for press. correction = 0.1

but I got the NEGATIVE DENSITY error even before the first time step!!!!!

What can I do??? I really need to solve this problem!!

Thanks,

Roberto

 John C. Chien November 27, 2000 13:51

Re: Solution method for transient problems

 Sreenadh Jonnavithula November 27, 2000 16:47

Re: Solution method for transient problems

Hi Roberto,

John's right - you need to tell us more about the actual problem to allow someone to figure out whats going on. My guess is that the problem has nothing to do with corrector stages or residual tolerance, but something either in the setup or the mesh. For example, if you have a large outflow velocity imposed, you might cause the cells next to it to "empty out" over your time step, with the code reporting it as negative density. What exactly are you trying to simulate? I would also encourage you to contact user support directly.

 Ahmed Hassaneen November 28, 2000 05:38

Re: Solution method for transient problems

Roberto, As I said before, your problem is related to either one of these four reasons:

1. the length of the time step (make it smaller) 2.an event of cell deletion or activation which is not properly set up. 3.your mesh size 4.initial or boundary values.

Could you give us an idea about your boundries and initial values??

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