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continuity residual in porous media

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Old   June 8, 2013, 12:33
Default continuity residual in porous media
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Hi everyone,
for two month I occupy with cfd (fluent). So I have less experience in using fluent. I have to model flow through porous media. I struggle with bad convergence behavior of continuity. While the other residuals converge, continuity stays constant at round about 0.01. I read a lot about the continuity convergence behavior and similar problems, but I donít found any helpful advice. The net difference of the mass flow rate in inlet and outlet is much less than 0.5%, but I do not know whether the solution is correct. Here some facts about my problem.
- Tetra mesh
- Acceptable mesh metrics
- Cells: 7 million
- Pressure based solver, steady
- Standard k-epsilon
- Porous media model
- One mass flow inlet, one pressure outlet
- Coupled scheme + pseudo transient option
I tried varying solution controls for pressure and momentum, but it does not help. Also using the simplec scheme does not help. Does somebody have an advice? Thank you very much.
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Old   June 14, 2013, 10:51
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What are you analysing? How are you defining you porous media? Are you monitoring any other variables apart from the default?

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Luke
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Old   June 16, 2013, 12:08
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Hi Luke,

I try to model a desufurization process, that works with a granulat. I use the porous media model to calculate the pressure drop of the system. In addition to velocities and chemistry the pressure drop is the most important property for me.
I have a look at different surface monitors. The surface integral of pressure difference between inlet and outlet seems to be stationary after round about 400 iterations. Also seems the mass imbalance to be stationary. The net imbalance is of 10^-6. This is 0.001% of the smallest mass flux of the system. I think it is a good value. But I am not sure about it.

Thank you for your reply.
Regards
student

Last edited by student2008; June 17, 2013 at 04:52.
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Old   June 17, 2013, 16:00
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How did you calculate/derive your resistance coefficients for you porous medium?
Monitor the pressure just before and after your porous medium and compare the drop to what you expect it should be.
Have you tried reducing your relaxation parameters?

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Luke
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Old   June 17, 2013, 22:33
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How are your residuals normalized/scaled? I recommend local scaling for most cases.
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Old   June 18, 2013, 03:49
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Hi Luke,

I derived the coefficients by the the equations recommended for packed bed by the ansys users guide:

D=1/((D_p≤/150)*(e≥/(1-e)≤))

C=3.5/D_p*((1-e)/e≥)

Where D is the mean particle diameter and e the packed bed porosity.
I have a look at the pressure before and after the porous media and the pressure drop seems to be realistic.

I reduced the under relaxation for momentum and pressure, but it did not help.
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student
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Old   June 18, 2013, 03:52
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Hi Chris,

I use the default settings for the residuals. I will try the lokal scaling.
Thank you.

Regards
student
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