[CIG-SHORT] Inconsistency Between Linear and Powerlaw Viscoelastic Gravitational Forces

[Charles Williams]

There are definitely significant differences between how the two rheologies are done.  You would need to look in the manual to see how to correlate the two.  The power-law rheology is controlled by reference stress and reference strain rate values, rather than a viscosity coefficient.  You can use the utility code powerlaw_gendb.py to convert the A-value for power-law to these values, but you will still need to watch out for factors of 2 and sqrt(3.0).

Charles

p.s.  i'm at a workshop right now and don't have time to put together an example, but I may try to do that after the workshop.


On 13/10/2010, at 2:18 PM, Brad Aagaard wrote:

> Jeff-
> 
> Can you explain more specifically what differences you see in the 
> stresses? Please specify if this occurs for the elastic solution (first 
> time step) or at latter time steps. Is the displacement field the same 
> but only the stress fields are different? Are the strains different or 
> just the stresses?
> 
> When I use the pylith trunk, I get identical deformation (displacements, 
> strains, and stresses) for the elastic solution, but then the 
> deformation (all fields) seem to differ between the two problems.
> 
> Another issue to keep in mind is that the physical properties as well as 
> the initial state variables must be equivalent for the deformation to be 
> the same.
> 
> Charles- Is there anything tricky about differences in the state 
> variables between the MaxwellIsotropic3D model and the PowerLaw3D model 
> that could be responsible for this issue.
> 
> Brad
> 
> 
> On 10/12/2010 01:58 PM, Jeffrey Thompson wrote:
>> Dear CIG Developers,
>> 
>> I've been running into difficulties with the magnitude of gravitational
>> stresses using a PowerLaw3D rheology.  As a consistency check, I ran two
>> models with the same material parameters, using a MaxwellIsotropic3D
>> rheology first and a PowerLaw3D rheology (with n=1) second.  I obtained
>> wildly different values of stress within the two model runs, with the
>> magnitudes of stresses varying by about 10 orders of magnitude.  I've
>> attached a tarball with my model parameters for both cases, and was
>> wondering if you could take a look at this problem.
>> 
>> I'm currently running PyLith 1.5.1 using the tarballed version available off
>> of the CIG website (i.e. not an svn version).
>> 
>> Thanks,
>> Jeffrey Thompson
>> Seismolab
>> California Institute of Technology
>> 
>> 
>> 
>> 
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Charles A. Williams
Scientist
GNS Science
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PO Box 30368
Lower Hutt  5040
New Zealand
ph (office): 0064-4570-4566
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C.Williams at gns.cri.nz
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