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!

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.

Enjoy,

Hrv

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!