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 April 11, 2002, 09:07 Piso for steady state #1 Andrea Guest   Posts: n/a Hi. Since my calculations (a simple adiabatic steady state air flow) have great problems of convergence with SIMPLE, I'm going to try with PISO algorithm. Anybody can tell me something about the use of it in steady state? One problem I have, for example, is that when I try to set the values in the window of solution method ,the "Maximum Number of corrector Stages" is fixed to a value very near to zero; is it normal? Why? Thanks for your answers.Andrea.

 April 12, 2002, 03:58 Re: Piso for steady state #2 john YL Guest   Posts: n/a If it's a simple air flow simulation, STAR should not encounter convergence problem. Maybe you can provide a bit more detail? As for the PISO scheme, the default max. no. of corrector stages is 20.

 April 12, 2002, 06:36 Re: Piso for steady state #3 Andrea Guest   Posts: n/a I'm studying a flow of air in the pipe that leads to a close coupled converter of a car; the geometry of this pipe is a little bit complex(it has an elbow, the section varies, and it hasn't symmetry planes),but not so much.I 've used PROAM to mesh the CAD, so there are some trimmed cell. I' ve used the porous model for the converter, and inlet-pressure as boundaries; I've already tried to lower underrelaxation factors, but the residuals remain on values of 10e-1,10e-2.I don't really know what else to do, and I don' think the mesh is too coarse. If you have some ideas.... Thank you a lot. Andrea PS: Other informations on PISO for steady state? Thank you again.

 April 15, 2002, 21:57 Re: Piso for steady state #4 raymond Guest   Posts: n/a It is a little bug for the wrong number of correcter stage of PISO. It only happened in the windows version. YOu can define it typing. Furhter, for the unconvergence of your problem. It might be the mesh qulity is too poor. You can get convergence with further low under-relaxtion factor, especially for the pressure in SIMPLE. For the PISO, it might take longer time to get convergence than SIMPLE. You must define carefully the location of reference pressure in PISO when you have the pressure boundary involved. Otherwise, it might divergence.

 April 16, 2002, 03:03 Re: Piso for steady state #5 bosko Guest   Posts: n/a Reference pressure isn't of any real consequence for incompressible calculations, PISO or SIMPLE. Setting a 'wrong' value of pressure certainly shouldn't cause the solution to diverge. If you have a pressure boundary in the solution then the pressure field is tied down to that and the reference pressure only influences post processing. I don't think PISO is ever a good alternative to simple for steady state calculations. It is usually slower to converge and takes much longer per timestep. I think you'd be much better concentrating on finding out WHY the original solution didn't work and sticking with SIMPLE. B.

 April 16, 2002, 07:20 Re: Piso for steady state #6 marco Guest   Posts: n/a Hallo Andrea! Having a poor convercency, I wouldn't use piso. May be you can check following points: 1. Do you have recirculation at the pressure boundary? If so, you should extend your calculation domain. 2. What's the magnitude of the ALPHA and BETA values of the porous media? The difference between the biggest and smallest value shouldn't be more then 3 orders of magnitude. 3. Try reducing the value of the maximum residuum for the pressure equation.

 April 16, 2002, 14:53 Re: Piso for steady state #7 Andrea Guest   Posts: n/a Thank you for your help; I just can't understand what do you mean with "reducing the value of the maximum residuum for the pressure equation".About the other suggestions,I've reduced today the maximum values of alfa and beta, but nothing has changed with residuals.What is strange in my calculation is that, with high values for TE and ED underrelaxation factors(0.6),TE and ED residuals fall down, but there is no way to make go down u,v,w,p res.,both with small and high underrelaxation factors. If you've got other ideas.. Thank you a lot again.Andrea.

 April 16, 2002, 15:03 Re: Piso for steady state #8 Andrea Guest   Posts: n/a Thank you a lot for you answer.I've tried with PISO but residuals are always the same;I've passed back to SIMPLE modifying the values of underrelaxation factors hoping that this experimental method (monitoring step by step their influence on calculation and restarting it with new values when it doesn't go well)will take me to a convergence.Thanks again.Andrea.

 April 18, 2002, 04:36 Re: Piso for steady state #10 Andrea Guest   Posts: n/a Hallo Lars! If you don't mind, I would like to make you a pair of questions, since you seem to know a lot of things more than I know:1)how is it possible that the flow I study remains transient forever, if the boundaries are costant in time?: there will be a time in wich the values in each cell become costant after the transient, or I'm wrong?2)You talk about error estimation:I've already tried to use the tool to do this in the StarGuide but I couldn't:can you teach me how to do? Thank you for your previous help;sorry for my repeared questions, but I've got to take my degree in a month and I have still a lot of problems. Thank you again,Andrea.

 April 19, 2002, 07:33 Re: Piso for steady state #12 marco Guest   Posts: n/a Hallo Andrea, my suggestion was to reduce the residual tolerance, mainly for the pressure equation. You can do this with the comand resi 0.05 0.05 0.05 0.01 (defaults: 0.1 0.1 0.1 0.05) or in the panel: Analysis Controls->Sol. Controls -> Eq. Behavior -> Primary Variables. May be you need to encrease the number of (pressure) sweeps, but usually not. This could help to reach a lower value of the final resuduals also in the case your're computing a kind of average solution of a flow which in reality is transient (s. Lars's message). The real improvement in the solution is not big. If you've the kind of problem cited by Lars, you should check the convergence monitoring the macroscopic values you're interested in (for example mass flows or averadge pressure at given sections). It's up to you to judge if the neglected transient effects are relevant for your problem. Marco

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