PISO - new questions

Benedikt Flurl

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!