[CIG-SHORT] on the description of power-law-coefficient

[Charles Williams]

Dear Ikuo Cho,

If I'm understanding your question correctly, you will not be able to do what you are planning with the current released version of PyLith (1.4.2).  In that version, it is not possible to spatially vary the power-law parameters.  The reason is what you have already noticed -- the power-law-coefficient units are dependent on the power-law-exponent.  This is a problem in most power-law implementations, and this has been fixed (but not extensively tested) in the version of the code we're working on now.  We have changed the specification so that power-law materials now require 3 parameters -- a reference stress, a reference strain rate, and the power-law exponent.  This makes the units constant, and you can therefore have spatial variations in these parameters.

Right now, you have two options to solve your problem.  You can either define a separate material for each set of parameters using the current version of the code, or you can check out the version from the subversion repository.  The documentation in the repository has been updated to reflect the changes in the power-law implementation, but we have not yet put in documentation for the example problem.  I think it will be at least a month or two before our next release, so those are your only options for now.

I hope this has answered your question.  Good luck with your modeling, and please e-mail back with any questions you might have.

Cheers,
Charles

On 25/02/2010, at 10:44 PM, Ikuo Cho wrote:

> Dear Pylith Developers:
> 
> I have a question on a description of the parameter
> power-law-coefficient in a spatial database: Does the dimension corresponding to the power-law-coefficient have a
> meaning? I'm asking this because I want to solve a problem of spatially variable power-law exponents.
> More concretely, is the following description correct or not?
> 
> 0. 0. 0. 2500. 3000. 5500 1.e-37 3.0
> 0. 0. 20. 2500. 3000. 5500 1.e-37 2.0
> 
> with 
> 
> value-names = density vs vp power-law-coefficient power-law-exponent 
> value-units= kg/m**3 m/s m/s Pa**(-3.0)/s None 
> 
> when the flow law is given as
> 
> (strain rate) = At (stress)**3.0, where At=1.e-37 [Pa**(-3.0)/s], at
> (x,y,z)=(0,0,0)
> 
> and 
> 
> (strain rate) = At (stress)**2.0, where At=1.e-37 [Pa**(-2.0)/s], at
> (x,y,z)=(0,0,20).
> 
> 
> Ikuo Cho
> ----------------------------------------------------------
> Geological Survey of Japan,
> National Institute of Advanced Industrial Science and Technology
> Tsukuba Central 7, Tsukuba 305-8567 Japan
> Tel +81-29-861-3891, Fax +81-29-861-3682
> ----------------------------------------------------------
> 
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Charles A. Williams
Scientist
GNS Science
1 Fairway Drive, Avalon
PO Box 30368
Lower Hutt  5040
New Zealand
ph (office): 0064-4570-4566
fax (office): 0064-4570-4600
C.Williams at gns.cri.nz
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