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marupio October 19, 2012 09:35

Time step independence?
Basic stuff here.

I cannot get consistent results from pisoFoam. I have a simple cavity with a cross flow inlet / outlet. I vary the time step from 0.001 to 0.0000001 s and the resulting pressure fields vary by 20-60%.

Does anyone have any ideas?

So far I've observed:
  • Changing the cavity size has no effect (between 0.1m and 10m)
  • Changing the mesh density has no effect (10x10 vs 100x100)
  • Turbulence model has no effect (kEplison vs disabled)

Yeah, I'm an OpenFOAM developer... not a user.


owayz October 20, 2012 21:54

Hi David,
are you comparing averaged pressure or just pressure at some time ?


chegdan October 22, 2012 06:49


Are you sure that there is convergence between all of your fields at each timestep? If not, then this will have a big affect on your pressure field. Adding outer-correction will ensure convergence at each timestep. Have you looked at using pimpleFoam instead of pisoFoam? This is suggested on

marupio November 5, 2012 23:50

2 Attachment(s)

I am calculating an RMS error across the entire pressure field. Average pressure doesn't vary nearly as much.


Thanks for the suggestions, I will try implementing pimpleFoam into the solver.

I've attached some pictures of the pressure and flow field. I'm getting a pressure fluctuation at the inlet, and discontinuities at the corners. I'm sure this is a rookie problem, but it mystifies me. These images are for velocity driven flow. When I do pressure driven flow, though, it smooths out, but I still can't get timestep independence.

Thank you all for your suggestions. I will continue troubleshooting this between my full-time job and parenthood!


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