[peob]

I am encountering a peculiar problem in a number of my simulations with pimpleFoam and rhoPimpleFoam. When I run my cases at reasonably large Courant numbers (maxCo from 50 to 10000, say) they are nicely stable, but I don't have the time-accuracy I would like. So I lower the maxCo down to something on the order of 1 or smaller (e.g. 0.3) to improve my time-accuracy, however, my solution then goes unstable. This is the exact opposite behavior from what I would expect: stable at small maxCo and unstable at large maxCo.

Attached is an example pimpleFoam case that demonstrates the behavior. Unfortunately I cannot include the grid, because it exceeds the file size limitations. However, if you happen to have cfMesh on your system, all the files are set up for you to simply run "cartesianMesh" and you should then be able to generate the grid. NOTE... you will also have to scale the grid from millimeters to meters before running the case. Simply execute the command: transformPoints -scale '(0.001 0.001 0.001)'. Boundary conditions and all of the other settings are already there; no need to change them.

If you set the maxCo to a large value (say 100) then the solution converges just fine. However, if you set the maxCo to a small value (try 1.0, or 0.5 or smaller) the solution goes unstable. It may take a few hundred iterations to see the instability grow, but as the solution continues to run, the instability worsens. I have defined a couple of animation slices (y and z-direction) where you can plot the progression of the instability in the U-field.

I'm running OpenFOAM version 3.0.1 on Ubuntu 14.04 LTS. In the attached case, I used cfMesh to generate the cartesian mesh. CheckMesh indicates that I have a good quality mesh.

Has anyone else encountered this problem? I welcome any thoughts or suggestions.

Thanks.
Phil