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- - **Problem using PISO**
(*https://www.cfd-online.com/Forums/openfoam-solving/66111-problem-using-piso.html*)

Problem using PISOHi Foamers,
I am having some problems using PISO to simulate the flow of complex fluids, in particular polymers, where some extra stresses have to be considered. I can already simulate the flow in a simple system, infinite parallel, but when I compare the analytical solution with the predictions of OpenFoam, I manage to obtain the correct velocity profile but the predicted pressure gradient is smaller than expected. On possible problem is the analytical solution is wrong, but I doube check it and it is correct. I am thinking on the the possibility of doing more PISO steps, to improve the calculation of the pressure field, or changing the PISO algorithm implemented in icoFoam, that I am using. Has anybody any ideas of what is going on? Any help will be much appreciated. Regards, Titio |

Hi Antonio,
first, an elementary question: is the pressure gradient wrong or is the value of pressure simply different? I ask because if you use an incompressible solver in OF, the actual value of the pressure you find depends on the reference value. However in such a case the gradient should be OK. About your specific case, without knowing more on the model you use to describe the non-Newtonian stress tensor, and how you implemented it, it is impossible to reply. Best, A. |

Quote:
The pressure value is simply different, by at must 10%, I tried to change the mesh and the problem remains. I am testing the possibility of iterating several times in a PISO step, this time including the momentum predictor. Do you have more ideas? Regards, Titio |

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