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fferroni January 11, 2012 04:22

Non-convergence for smaller mesh spacings

I'm running some simulations of MHD duct flow using mhdFoam, and I ran some a case with a square duct cross section mesh of 20x20 and 40x40 elements and they both converged. I tried running a 100x100 case and now it doesn't converge! How is this possible? I swear I haven't changed anything else. I ran checkMesh and everything was good, and it looks alright on Paraview too... :confused:

Anyone wish to point out possible reasons?



akidess January 13, 2012 03:07

You have to keep an eye on the Courant number. If you refine your mesh without reducing the time step, you are increasing the Courant number. If it gets too large, the solution algorithm will become unstable.

fferroni January 13, 2012 06:42

Ah ok. So for an evenly spaced mesh, the time-step needs to be reduced proportionally to the decrease in mesh spacing?
Is this the condition you are referring to?

Thank you. I will see if it works!



akidess January 13, 2012 08:18

Yes, except that in this case the limitation is not due to an explicit time integration scheme, but to maintain pressure-velocity coupling with the PISO-algorithm.

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