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Old   June 13, 2006, 09:26
Default VOF Speed!!!!!!!!
  #1
soy
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Hi, i am using the VOF (Geo-R, unsteady formulation) with NITA solver and Variable Time Step. The geometry contains arround 1 milion cell.

I need more than 15 hours to simulate just 1 physical second!!!

Can anybody help me to reduce the CPU time?

Thanks in advance
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Old   June 14, 2006, 01:37
Default Re: VOF Speed!!!!!!!!
  #2
Gernot
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Hard to say without any more information. What about mesh quality, and computerpower (RAM)?
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Old   June 14, 2006, 02:37
Default Re: VOF Speed!!!!!!!!
  #3
soy
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Hi, thanks for reply

mesh quality: tétra with max skew 0.85 and max aspect ratio 5.

I runs calculation on server IBM HPC. 15h for 1 sec is from calculation on 8 proc.

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Old   June 14, 2006, 02:48
Default Re: VOF Speed!!!!!!!!
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Michael Bo Hansen
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Convert your mesh to polyhedral (beta feature at the moment)
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Old   June 14, 2006, 03:07
Default Re: VOF Speed!!!!!!!!
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soy
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is it possible with fluent6.2.16 (and tgrid 4.0.6)?
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Old   June 14, 2006, 03:14
Default Re: VOF Speed!!!!!!!!
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Michael Bo Hansen
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No. You have to download the beta version 6.3.17, but be carefull.... By the way. The conversion from tet to polyhedral in FLuent is memory demanding limiting the use to apprx 350000 tet cells. Hopefully They will change that when the new official version is released.

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Old   June 14, 2006, 03:27
Default Re: VOF Speed!!!!!!!!
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soy
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thanks a lot for help, i will try it

if someone know an other way to decrease the CPU time, i am still interested.

thanks.
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Old   June 17, 2006, 11:12
Default Re: VOF Speed!!!!!!!!
  #8
Andrew Wick
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Well you could...

-Try increasing the under-relaxation factors -Try increasing the NITA correction factors -Try increasing the NITA residual tolerance -Try changing the scheme from geo-reconstruct to Euler-Explicit with the Modified HRIC discretization scheme

Unfortunately, changing any of the first three could and probably will affect your solution stability and/or final results. I would compare any changes you make to the original to make sure you are properly converging. Also, the euler-explicit scheme is much faster than geo-reconstruct, but the interface is not as sharp so you'll want to compare the changes to this as well. Overall the EE scheme isn't too bad though.

Also, since you're using NITA, try the Fractional Step Method. It's a little bit quicker (depending on how much under-relaxation is needed for stability)

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