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Old   October 19, 2012, 09:35
Default Time step independence?
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David Gaden
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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.

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Old   October 20, 2012, 21:54
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Awais Ali
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Hi David,
are you comparing averaged pressure or just pressure at some time ?

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Old   October 22, 2012, 06:49
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Daniel P. Combest
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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 interFoam: timestep / mesh / PISO nCorrector dependency

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Last edited by chegdan; October 22, 2012 at 07:01. Reason: edit: bad grammar
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Old   November 5, 2012, 23:50
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David Gaden
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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!

Attached Images
File Type: jpg velocityDrivenPFull.jpg (10.4 KB, 11 views)
File Type: jpg velocityDrivenUInlet.jpg (64.2 KB, 12 views)
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