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Old   August 10, 2006, 05:43
Default Convergence on Local Time Scale Acceptable
  #1
Turbine
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Hi

I have a peculiar problem on hand. Whenever I run my model on Local Time Scale, it converges, buth when I switches to Physical Timescale, the run gives some mach number issues / floating point exceptions and will come out.

I was wondering if Convergence based on Local time scale is acceptable or not / If not what needs to be done.

Thanks

GT
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Old   August 10, 2006, 16:50
Default Re: Convergence on Local Time Scale Acceptable
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Glenn Horrocks
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Hi,

Convergence on local timescale factor is OK, I use it all the time. Depending on your simulation you may also need to converge on imbalances.

Glenn Horrocks
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Old   August 11, 2006, 07:55
Default Re: Convergence on Local Time Scale Acceptable
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Mike
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I disagree that convergence using a Local Timescale Factor is OK. It can give you non-physical converged results. You should use it to accelerate convergence, but always finish off with a constant timescale and run for at least one domain advection time. If the physical timescale is failing but local timescale is OK, then you probably just need to reduce your physical timescale. I guess if you have experience using a local timescale for a given type of simulation, and you know that finishing off with a physical timescale doesn't change the results, then you could just use the local timescale results; however I'd be very cautious about doing this. Hope this helps, Mike
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Old   August 11, 2006, 12:33
Default Re: Convergence on Local Time Scale Acceptable
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Robin
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Mike is absolutely right. Local Timescale Factor may be useful for getting through some startup transients, but should not be trusted for the final solution. If you job blows up when using a physical timescale (after trying to increase or decrease it), you probably have a problem with your physics or mesh. Local Timescale Factor will keep the solver stable and disguise these problems.

Regards, Robin
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Old   August 13, 2006, 17:51
Default Re: Convergence on Local Time Scale Acceptable
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Glenn Horrocks
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Hi,

If this is the case then it is an important point on using the local timescale option and really should be documented in the manual. I can't see any references in the manual about this issue.

Regards, Glenn Horrocks
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Old   August 14, 2006, 06:01
Default Re: Convergence on Local Time Scale Acceptable
  #6
Turbine
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Glenn, Mike, Robin

Thanks a lot for all your inputs. I believe now that first we can stabilise our runs with Local Time scale factor and when all the imbalances stabilise, we need to switch to Physical time scale .

Was wondering, if there is any particular use of Local Time Scale At all ?

Thanks

GT

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Old   August 14, 2006, 08:09
Default Re: Convergence on Local Time Scale Acceptable
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Robin
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Hi Glenn,

You're right, this probably could be explained better, although the documentation does warn that it is best used on meshes of uniform element size.

Regards, Robin
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Old   August 14, 2006, 08:13
Default Re: Convergence on Local Time Scale Acceptable
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Robin
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Early releases of the solver were not as stable and Local Timescale Factor could be useful to get through some of the initial startup transients. This is not generally the case anymore and I would personally say that there is no use for this. It can still help hold the solver together in some cases, but it is my experience that in the end, there was a problem with the mesh or physics that should have been addressed instead and the Local Timescale Factor only disguised the problem.

-Robin
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