[alexey7783]

Hi forum members

Although I already did some simulations with OpenFOAM I would call me a real newbie, especially as my programming skills are still under development. So I hope that someone of you can help me:

for my PhD thesis (which is experimental) my professor wants me to implement some numerical simulations. It comprises the outflow of a non-newtonian fluid from a cylindro-conical vessel. I did some first experiments with newtonian fluids (water and a water/glycerol mixture at 22°C) and simulated them with OpenFOAM. The results are really really promising, as for example the outflow times are nearly the same (only 2-4% error) or the massflux at the orifice (1-3% error). You see, I was really happy at that point

But now the point came, where I had to do the "real" work, which means try all of that with a non-newtonian fluid. I chose a very simple one, namely the glycerol/water mixture mentioned above to which I added some carboxymethylcellulose (CMC). I recorded the flow curves with our rheometer in the range from shear stresses starting from approx. 0 to 500 1/s, because these are the shear rates I extracted from the newtonian simulations. Than I chose the Cross-Power-Law because the curve fits showed excellent agreement and (which is the more important reason) is already implemented in the interFoam solver (as the fluid is flowing out, air is coming in from above, that's why I need a twophase solver).

When I simulate the outflow with the parameters I computed for the Cross-Power-Law the fluid is flowing far to quick. For example, in reality it takes the different mixtures between 37-56 seconds, in the corresponding simulations between 15-20 seconds. That is really a huge deviation. And of course, the mass fluxes at the orifice are therefore also too high. Do you think this is the fault of the solver? I tried to do the following first: I changed the Cross-Power-Law-Coefficients in that way, that newtonian flow with the viscosity of the pure glycerol/water mixture was present (m=0, n and nuInf can be whatever) and Lo and behold! the same results as in the pure newtonian case. This means for me, that the solver is working right, but I had an idea:

I than tried to analyse the structure of the solver with my basic c++ knowledge and I understand it the following way: the solver is calculating the viscosity value of each cell by using the velocities that are currently present. Than he stores it as viscosity value of this cell and than he basically solves the NSE acting like the viscosity of this cell is constant (newtonian behaviour so to say) in this time step (the one calculated at the beginning). But isn't it more correct to calculate the viscosity iteratively, because when the velocity changes, the calculated nu should also?
I say so, because this for me could explain the faster outflow.

My question to you with more experience is: do you think it is worth spending so much time (for me it will take a lot of time, due to my lack of programming knowledge till now) on implementing the calcNu() in the iteration, or are the wrong results due to something else, for example the wrong shear rate range in which I determined my fit parameters for the Cross-Power-Law model?

I would really appreciate your help and I am looking forward to your answers

My best regards

Alex