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

titio July 6, 2009 13:20

Problem using PISO
 
Hi 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

alberto July 7, 2009 02:26

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.

titio July 7, 2009 05:48

Quote:

Originally Posted by alberto (Post 221708)
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.

Thanks Alberto,

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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