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benedikt flurl May 13, 2005 06:07

Please Answer (PISO)
This is already the third time that I am asking about PISO, but strange things are happening and perhaps anybody can help me: I implemented the PISO and it is running but it doesn't hold what for example Issa is promising: -In steady state cases I can realy calculate very fast, sometimes four times as fast as SIMPLE (then also iteratively in PISO), but in unsteady cases (for example Karman Vortex Street) I cannot see advantages. Some Information about my Program: -I used for the second Pressure Correction Equation the same coefficients as in the first one. They contain the Neumann-Boundary-Conditions for all Boundaries. Have I to change them in any way? -In the unsteady cases Issa shows, it is possible to run PISO without Iteration for small time steps. What is SMALL? How can I see, that the approximated solution is good, what can I use as Residuum (the normally used residuum is remaining high...) -I do not use underrelaxation for Momentum Equations and no underrelaxation Pressure Correction.

Thank you all for your help, I hope anybody understands my problems, it's difficult to formulate them, and I am a CFD beginner. Greatings, Benni!

Hrvoje Jasak May 13, 2005 06:56

Re: Please Answer (PISO)
Keep the Courant number at around 0.5 and limit the number of PISO correctors to about 2. You should see a drop in the initial residual for the pressure equation between the first and second PISO corrector of about a factor of 10 and everything will be fast and fine.



Benedikt Flurl May 14, 2005 03:12

Re: Please Answer (PISO)
Thank you, But tell also shortly, if my assumed boundary conditions are correct (see first mail). (I already studied your Thesis about PISO and your Homepage,-very nice) Benni!

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