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physical velocity formulation and porous media

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Old   June 9, 2003, 11:29
Default physical velocity formulation and porous media
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Hello Gurus -

I have a 3D sim with a porous bundle zone. The flow regime is laminar at low Reynolds number. I am using the new physical velocity formulation for the porous media in FLUENT6.1 and have had successful (and rapid) convergence under steady state conditions. However, I am unable to get convergence for a time dependent simulation with fixed timestepping. I'm using the segregated solver with PRESTO! pressure interpolation, PISO P-V coupling, 2nd order momentum discretization and 1st order implicit time discretization. All solver parameters (underrelaxation factors and the like) are defaults. I begin from the steady state solution described above and have taken the timestep down to as low as 0.025 seconds - after one or two successful timesteps the residuals blow up. The size of the timestep doesn't seem to be the issue (although I welcome any "enlightenment" from you). Does anyone else have experience with a similar simulation setup or have some suggestions on what might be the problem?

Best regards - Ken
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Old   June 10, 2003, 05:42
Default Re: physical velocity formulation and porous media
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May I knwo what system do you simulate?? e.g. liquid-gas, gas-gas or.....

do you grid size is fine enough??

I have tyr porous media with gas-gas, it seem no divergence problem to me...

but did you check the concentration of you system?? do its tarry with any experimental or theory result??
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Old   June 10, 2003, 10:14
Default Re: physical velocity formulation and porous media
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Ivan -

It's all liquid - the porous media is in place for pressure drop and fluid flow effects. I am not modeling a mixture or multiphase problem at this time.

As for the grid - I achieve rapid convergence with the default solver settings for the steady-state solution so it appears to be fine by that functional metric. I believe the cell skewness and volume limits are ok - its a tet mesh so it ain't perfect, but it does appear adequate.

Comparison with experimental data for the steady state systems has been good with past simulations.

I am basically looking for the time-dependent simulation to converge to the steady state solution or thereabouts after a fair number of timesteps. All models used are the same but the time-dependent simulation diverges even with a very short timestep.

I welcome further comments and thanks for the questions and insight.

Regards - KLG
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