How to implement non-Newtonian fluid with pimpleFoam
 

Hello everyone, I hope you're all well. I'm new to OpenFOAM so I hope to be relatively clear.

I'm working on implementing a polymer in OpenFOAM and I'd like to do it using pimpleFOAM (the case with simpleFOAM works fine).

My case is simple: I'm looking at the flow of a polymer in a cylindrical pipe (to simulate a capillary viscometer). The mesh is good.
For this, I'm using the BirdCarreau model in transportModel with a very large zero shear viscosity. I apply an initial velocity to the fluid.

The problem lies in the convergence of the velocity. In my log.pimpleFOAM, very quickly the noIt. reaches 1000. By decreasing Ufinal's relaxationFactors to 0.9 instead of 1, I achieve convergence. Nevertheless, it seems to me that this is not very physical (you can change the relaxationFactors of U but not of UFinal).
By leaving this factor at 1 and increasing the nOuterCorrectors to 2000, I manage to converge, but it takes a long time and my results don't agree with those obtained using simpleFOAM.

Do you have any idea of the problem?

Thanks in advance and have a nice day.