[CIG-MC] CitcomCU Rheology Question

[Eh Tan]

Ethan Fahy wrote:
> Hello Eh,
>
> Thanks for the email link, it was very helpful.  I do have a couple
> followup questions though.  I have been trying to implement the
> Rheol=2 viscosity law for a typical mid ocean ridge system (from
> 0-410km).  I have used reasonable non-dimensional numbers for all
> related parameters which I'll list below.  I am limiting the variation
> in viscosity using the VMIN and VMAX parameters, and have found that
> variations greater than roughly 2 orders of magnitude start producing
> velocities that are very unrealistic, sometimes up to a hundred times
> greater than the driving plate velocity.  
> -Ethan Fahy
>   

Hi Ethan,
(cc to cig-mc)


The symptom you described is typically due to premature stop of the
iterative solver. This might be caused by either:

1) tole_compressibility is not small enough. For high resolution model,
you will need a smaller tole_compressibility to get correct result.

2) piterations is not large enough. For a model with large viscosity
variation, the Uzawa solver will need more iterations to converge.

3) accuracy is set too small. For a model with sharp viscosity
variation, the stiffness matrix has a large condition number. The solver
cannot solve the problem to satisfy the accuracy so it keeps iterating.
Eventually, it stops at the wrong result.

You will need to change these parameters to ensure convergence. If you
still have trouble in getting convergence, please send your input file
and *.log0 to me. I will try to reproduce your result and come up some
suggestion.

Cheers,

-- 
Eh Tan
Staff Scientist
Computational Infrastructure for Geodynamics
California Institute of Technology, 158-79
Pasadena, CA 91125
(626) 395-1693
http://www.geodynamics.org



> On Wed, May 27, 2009 at 6:06 PM, Eh Tan <tan2 at geodynamics.org> wrote:
>   
>> Hi Ethan,
>>
>> You might want take a look of this message:
>> http://geodynamics.org/pipermail/cig-mc/2009-March/000125.html
>>
>> Although the message is about CitcomS, it should apply equally well to
>> CitcomCU.
>>
>> Cheers,
>> Eh
>>
>> Ethan Fahy wrote:
>>     
>>> Hello,
>>>
>>> In the CitcomCU input file, how are viscE (activation energy) and
>>> viscZ (activation volume) non-dimensionalized?  Is viscE equal to
>>> E/(R*T), where E is activation energy [J/mol], R is the gas constant
>>> in [J/K/mol], and T is a reference temperature in [K]?  If so, is the
>>> reference temperature given by "ReferenceT" from the input file?  The
>>> plume-input file from the CIG website has  default viscE value of
>>> 6.907753.  This would seem to correlate to  very low values for
>>> activation energy compared to standard values from the literature I've
>>> seen.  Any advice would be most appreciated.  Thanks in advance.
>>>
>>> -Ethan Fahy
>>> _______________________________________________
>>> CIG-MC mailing list
>>> CIG-MC at geodynamics.org
>>> http://geodynamics.org/cgi-bin/mailman/listinfo/cig-mc
>>>
>>>       
>>