[CIG-SHORT] A Question About Pylith

Fri Nov 18 13:20:07 PST 2016

I would also check your mesh quality.  That’s a huge number of iterations.

Cheers,
Charles

> On 19/11/2016, at 2:40 AM, Matthew Knepley <knepley at rice.edu> wrote:
> 
> On Fri, Nov 18, 2016 at 5:32 AM, zeynep yılmaz <ylmz.zeynep at gmail.com <mailto:ylmz.zeynep at gmail.com>> wrote:
> Thank you very much for taking time to respond my questions and for your detailed explanation. I am new at Pylith, your response helped me so much. 
> 
> I have implemented a model with the dimensions of 150 km for a 20x10 km fault plane as you have suggested. I also increased the resolution of the mesh. However, the linear solve converged after 1700 iterations. I would also like to ask you how I can choose the number of iterations after which Gram-Schmidt orthogonalization is restarted (ksp_gmres_restart).
> 
> --petsc.ksp_gmres_restart=500
> 
> You can also look at the solver slides
> 
>   https://wiki.geodynamics.org/software:pylith:cdm2016 <https://wiki.geodynamics.org/software:pylith:cdm2016>    at the bottom
> 
>  to try and improve that number I would hope you can keep it under 100.
> 
>   Thanks,
> 
>      Matt
>  
> Thanks a lot again.
> 
> Best wishes and kind regards,
> Zeynep
> 
> On Mon, Nov 14, 2016 at 11:27 PM, Charles Williams <willic3 at gmail.com <mailto:willic3 at gmail.com>> wrote:
> Dear Zeynep,
> 
> I am also sending my response to the cig-short mailing list.  For future requests, could you please make use of the mailing list?  This helps both by reducing the amount of e-mails we need to reply to, and it also helps everyone on the mailing list by answering questions that others might have.  You can join this mailing list at:  https://geodynamics.org/cig/about/mailing-lists/ <https://geodynamics.org/cig/about/mailing-lists/>.
> 
> In response to your question, there are several issues involved when comparing Okada’s analytical solution to finite element results:
> 
> 1.  The finite element solution and the analytical solution are modeling two different things, so the solutions will never agree completely.  The Okada solution represents displacements due to slip on rectangular patches with constant slip on each one, while the finite element solution represents slip at vertices that decreases either linearly or bilinearly (depending on element type) to zero at the surrounding vertices.  Obviously, the finite element solution is not capable of representing slip that varies over an infinitesimal distance (and this is actually not realistic in any case).
> 
> 2.  If you want to accurately represent an analytical solution, you must make sure that you have sufficient mesh resolution, particularly in regions where the solution is changing most rapidly.  Looking at the meshes you provided, this does not seem likely to be the case.  Your meshes are somewhat graded, but do not appear to have sufficient resolution in the vicinity of the fault.  Furthermore, in relation to point #1 above, if you want to accurately represent a constant-slip rectangular patch, you would need extremely high resolution around the fault edges.
> 
> 3.  Another issue when representing an analytical solution is insuring that the mesh boundaries are sufficiently far from the deformation source that they are not influencing the solution.  Again, this does not seem to be the case for your mesh.  As a general rule of thumb, the distance to the mesh boundaries should be at least a factor of 3 larger than the dimensions of the deformation source.
> 
> A final question is to determine why it is important to compare the results against the analytical solution.  If it is important to do this, then you probably want to evaluate both the analytical solution and numerical solution at an identical set of points and integrate the error over the volume.  Some of these questions were explored quite some time ago.  You can find some old benchmarks here:
> 
> https://geodynamics.org/cig/working-groups/short-term-crustal-dynamics/benchmarks/ <https://geodynamics.org/cig/working-groups/short-term-crustal-dynamics/benchmarks/>
> 
> Unfortunately, these benchmarks are extremely old and have not been updated for current versions of PyLith.
> 
> Good luck with your modeling, and please contact the cig-short mailing list if you need more help.
> 
> Cheers,
> Charles
> 
> 
> 
>> On 15/11/2016, at 1:55 AM, zeynep yılmaz <ylmz.zeynep at gmail.com <mailto:ylmz.zeynep at gmail.com>> wrote:
>> 
>> Dear Dr. Charles Williams,
>> 
>> I am working on calculation of static displacements for a reverse buried fault (with a dip of 70 degrees) by using Pylith Software. I would like to ask you how I can compare the accuracy of the FEM models (different size models attached below) with the Okada's analytical solution. Is there an easy way to compare the solutions by using Pylith? I have used Coulomb 3 software for Okada's solution, however it takes time to adapt. I have also downloaded Cigma package to calculate the erors. However I did not understand well if it is comparing the results with Okada's solution.
>> 
>> If you could have time to answer my question, I would be so glad. Thanks in advance.
>> 
>> Best Regards,
>> Zeynep Yilmaz
>> 
>> <model1_40x40x50.vtk><model2_80x80x100.vtk>
> 
> Charles Williams I Geodynamic Modeler
> GNS Science I Te Pῡ Ao
> 1 Fairway Drive, Avalon 5010, PO Box 30368, Lower Hutt 5040, New Zealand
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> 
> 
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Charles Williams I Geodynamic Modeler
GNS Science I Te Pῡ Ao
1 Fairway Drive, Avalon 5010, PO Box 30368, Lower Hutt 5040, New Zealand
Ph 0064-4-570-4566 I Mob 0064-22-350-7326 I Fax 0064-4-570-4600
http://www.gns.cri.nz/ <http://www.gns.cri.nz/> I Email: C.Williams at gns.cri.nz <mailto:your.email at gns.cri.nz>
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