[CIG-SHORT] Maxwell bulk rheology behavior in v1.3.1 versus v1.4.2

[Brad Aagaard]

Ikuo-

We found a couple issues related to the test problem you supplied.

(1) The time step in your simulation using v1.3.1 is too large so the 
simulation isn't accurately modeling the viscoelastic deformation.

PyLith v1.3.1 did not have a check to make sure the time step is 
sufficiently small relative to the shortest Maxwell time. This was added 
in v1.4, which is why if you try to run v1.4.2 with a time step of 
1.0*year you will get an error message, but not in v1.3.1.

Because the Maxwell time in your problem is so short, the stresses relax 
very quickly. In using a time step of 1.0*year with v1.3.1 the stresses 
are relaxing artificially slowly because the time step is so large.

(2) The elastic solution in v1.3.1 for the Maxwell bulk rheology is 
incorrect.

It appears that v1.3.1 is using the viscoelastic elastic constants 
instead of the elastic constants. The elastic solution at the first time 
step (-dt to 0) should match the solution for a purely elastic material. 
In v1.4.2 this is true, but in v1.3.1 it is not. This is subtle in most 
problems with large Maxwell times, but your test problem has such a 
short Maxwell time that this bug shows up. I don't know the precise 
location in the code where this bug occurs in v1.3.1, but v1.4.2 looks 
correct.

Regards,
Brad


Ikuo Cho wrote:
> Dear PyLith developers
> 
> I calculated linear viscoelastic responses of a medium, under the
> setting MaxwellIsotropic3D, while giving three-dimensional
> inhomogeneities to its viscosities using the SimpleDB spatial database.
> *info.vtk indicates that medium properties have been read correctly, but
> the way stresses attenuate apparently doesn't reflect the given
> inhomogeneities and absolute values of the viscosity coefficients. An
> earlier version (PyLith 1.3.1) produced intuitively expected results,
> but PyLith 1.4.2 gave counter intuitive outputs, no matter which one I
> used--binary for XP, binary for Linux or the source code to be compiled
> by the user (see attached figures). 
> 
> You can validate what I am saying using the  setting files attached to
> this email: Here is what I did.
> 1) Place four files (dislocation_3d_mat1.4.2.cfg,
> dislocation_3d_mat1.4.2.cfg, 3d_mat1.3.1.spatialdb and
> 3d_mat1.4.2.spatialdb) in the exercise directory
> pylith-1.4.2/examples/3d/hex8.
> 
> 2) Execute PyLith1.4.2 by typing:
> cd pylith-1.4.2/examples/3d/hex8
> pylith dislocation_3d_mat1.4.2.cfg
> 
> 3) Execute PyLith1.3.1 by typing:
> mv pylithapp.cfg pylithapp.cfg.org
> pylith dislocation_3d_mat1.3.1.cfg
> 
> The following is the only difference in "spatialdata" that describes material properties.
> <   value-units =  kg/m^3  m/s  m/s Pa^s // units
> ---
>>   value-units =  kg/m**3  m/s  m/s Pa*s // units
> 
> I have no idea what the problem is. Did I miss any important changes in
> the way input should be given? 
> I'd be happy to receive any advise.
> 
> ----------------------------------------------------------
> Ikuo Cho  ( ikuo-chou at aist.go.jp )
> 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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