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November 17, 2000, 13:31 
Solution method for transient problems

#1 
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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 underrelaxation for pressure correction max residual tolerance Thanks everybody, Roberto 

November 18, 2000, 04:02 
Re: Solution method for transient problems

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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 underrelaxation of the pressure is very important like any other relaxation. Any body correct me if I'm wrong.


November 18, 2000, 10:45 
Re: Solution method for transient problems

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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 100.0e6, you need 60 iterations, but if the time step is 10.0e6, 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 

November 19, 2000, 04:11 
Re: Solution method for transient problems

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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.


November 22, 2000, 10:13 
Re: Solution method for transient problems

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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.


November 27, 2000, 04:13 
Re: Solution method for transient problems

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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 

November 27, 2000, 06:23 
Re: Solution method for transient problems

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Hi, Roberto,
What you described is certianly Ok. You may need to use underrelaxation factor for PISO (this is used to underrelax the pressure correction) that would help in your problem. The underrelaxation 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. 

November 27, 2000, 10:10 
Re: Solution method for transient problems

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I've set the following values:
Max number of corrector stages = 1000 Reduction in residual for corrector stages = 0.1 Underrelax. 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 

November 27, 2000, 13:51 
Re: Solution method for transient problems

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(1). I found that the questions and answers are very interesting, that is, it is not going anywhere. (2). I think, the general rule is that, each CFD simulation run, including all of the parameters defined, is a "special case". (3). For this reason, it is important to provide as much information as possible in the question, if one is interested in getting the real help. (4). A commercial CFD code does not provide the solution, unless it also knows the user's problem ahead of the time and set the corresponding parameters accordingly. Unfortunately, such automatic codes are not available yet (may not be possible at all). (5). My suggestion to the one who asked the question is: (a). try to learn as much as possible the related methods used in the commercial codes, even if it is a black box and does not give adequate information. In this way, you will be able to ask the right question, (b). try to follow the sample tutorial cases closely first. These cases were worked out by the vendor's engineers, and supposed to have the repeatable solutions. (c). if your problem is in one of the sample tutorial cases, then the chances of getting your solution are rather high, because you don't have to change too much the control parameters. (d). when your problem is outside the sample tutorial cases, then you are on your own. It is your responsibility to put together all of the necessary steps and control parameters and find out whether this black box will generate a good solution for you or not. This is the common understanding of the commercial CFD codes. (6). AS matter of fact, these problems and issues must be considered long before one start running a code. Bye the way, running the code blindly will only make the life miserable. If I were you, I would first check into the listing of the sample tutorial cases, if the problem of interest is there, then go ahead and follow the sample. Otherwise, "No One In THE WHOLE WORLD KNOWS WHETHER YOUR PROBLEM HAS A SOLUTION OR NOT." A "general CFD code" does not mean that it will generate a solution for any CFD problem. It simply say that the "formulation" is general enough that, you can try it on other problems beside the sample tutorial cases. So, if you read this message thoroughly, and understand it, it will save you a lot of emotional problems.


November 27, 2000, 16:47 
Re: Solution method for transient problems

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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. 

November 28, 2000, 05:38 
Re: Solution method for transient problems

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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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