[kandelabr]

Hello.

I've been running an unsteady simulation of a pump with pimpleFoam. The results seem reasonable but from timestep to timestep minimum and maximum values 'jump' for more than 10%, however these 'jumps' seem to be getting smaller.

The question is, how long should I leave the case running? So far, 10 full revolutions have passed, that's 0.4 seconds of simulation time.

[sheaker]

Hello.

You could preview Your residuals in-time.
Take this file:

set logscale y
set title "Residuals"
set ylabel 'Residual'
set xlabel 'Iteration'
plot "< cat log | grep 'Solving for Ux' | cut -d' ' -f9 | tr -d ','" title 'Ux' with lines,\
     "< cat log | grep 'Solving for Uy' | cut -d' ' -f9 | tr -d ','" title 'Uy' with lines,\
     "< cat log | grep 'Solving for Uz' | cut -d' ' -f9 | tr -d ','" title 'Uz' with lines,\
     "< cat log | grep 'Solving for epsilon' | cut -d' ' -f9 | tr -d ','" title 'epsilon' with lines,\
     "< cat log | grep 'Solving for k' | cut -d' ' -f9 | tr -d ','" title 'k' with lines,\
     "< cat log | grep 'Solving for p' | cut -d' ' -f9 | tr -d ','" title 'p' with lines
pause 1
reread

Edit it for Your usage and put into Your case folder with any name, for example "Residuals".
Start simulation and write log file:

pimpleFoam | tee log

Type (in another terminal):

gnuplot Residuals -

Now You can check if Your case is still converging or not.

You should also set residuals in fvSolution file. Search for examples in /tutorials/


Have a nice day.
Sheaker

[kandelabr]

Thanks for your reply.

If I understand correctly, you're suggesting plotting the residuals to see the convergence more clearly?

How do I tell an unsteady case is converging?

[kandelabr]

I found out that this happens when I keep Courant Number too low (1.0 or even less). If I run the case with Co > 10, the oscillations are minimal.

Is there something to this?

[AndreasPe]

Hi!

Which OpenFOAM version are you using?

I assume you are doing a transient simulation with some kind of Rotor-Stator Interface? Depending on the version you are using, this is a GGI (foam-extend) or AMI (OpenFOAM). We are investigating on these unphysical oscillations, you are describing, too. Using foam-extend-3.1/3.2 these oscillations occur in our cases, too. As you mentioned, the oscillations decrease if you increase your timestep, we also made this observation.

Prof. Jasak has made some changes in the pimple-algorithm in foam-extend-4.0 which also seems to deal with this problem (have a look here: http://foam-u.fr/wp-content/uploads/...ance_Jasak.pdf ). However, this affected the convergence rate of the pressure equation in our cases. Till now, I did not exactly understand how the changes he made exactly eliminate the oscillations. It seems to have to do with the way of the flux interpolation interacting with moving meshes. Is there anyone who understood the changes made in the pimple-algorithm and the effect on those pressure oscillations? I just found this presentation of Professor Jasak here: http://openfoam-extend.sourceforge.n...s/courses.html