[alimea]

Dear foamers,

I used the rheological data presented in the paper entitled (attached):

http://webx.ubi.pt/~pjpo/coreia2004b.pdf

for numerical simulation of forced-convection heat transfer of PTT viscoelastic fluid. I want to calculate the material functions (Trouton ration, N1, N2 vs shear rate). In addition I want to Show the prediction of PTT model for stress ration (S = ratio of first normal stress to the shear stress), and the effect of epsilon (first parameter of PTT model) on the Trouton ratio.

My questions:

1- I used rheoTestFoam to obtain the requested material functions. I ran the problem for the FENE-CR model using rheoTestFoam in transient form and plotted the extensional viscosity against time (t/lambda). Also, I ran it for the exponential PTT (the 5-modes fluid introduced in the paper of Alves et .al) without any problem.
However, when I wanted to run the problem in ramp mode, the problem diverges. I have changed many of the parameters of the numerical solution, but I could not solve the problem.

2- I could not run the problem for a simple shear flow to obtain the shear viscosity, N1, and N2 vs shear rate. When I run it, gives me zero viscosity! It seems that the solver cannot realize the PTT model, because I ran the Herschel Bulkley without any problem.

3- A question about the material functions:
How can I calculate the stress ratio (tau_xx/tau_xy)? In simple shear flow, tau_xx is zero and in pure extensional flow, tau_xy is zero!

Thanks.