[CIG-SHORT] [EXTERNAL] Applying Gravitational Forces with Pylith

[Niloufar Abolfathian]

Thanks, Brad for the comments. I will go through the 2D/gravitational
example and follow your instruction. I may bug you again with more detail
on the errors in case I could not fix them.

Best,
Niloufar

On Wed, Aug 1, 2018 at 8:43 AM, Brad Aagaard <baagaard at usgs.gov> wrote:

> On 07/31/2018 11:44 PM, Niloufar Abolfathian wrote:
>
>> Hello,
>>
>> I am intending to apply gravitational forces in my quasi-static model. My
>> model includes a fault and two materials, elastic and viscoelastic. In
>> order to get a steady model, I am running the model for more than 20,000
>> years.Going through the examples I am following what you are doing in the
>> example hex8/step17 and subduction/step8a-b-c. At the moment I am getting
>> my initial results with infinitesimal strain, gravitational forces, and no
>> initial stress.
>>
>
> If the geometry is such that there are shear stresses in the elastic
> material at the boundary between the elastic and viscoelastic layers, there
> will be shear stresses in the viscoelastic layer that cannot completely
> relax. This will result in a steady-state solution with nonzero velocities.
> If the viscous strains are not well resolved, (see #3), you may not be
> converging to the steady-state solution. See examples/2d/gravity for
> examples and the manual for the accompanying discussion.
>
> Since I talked with Brad at SSA, and he was warning about implying
>> gravitational forces, I like to know your suggestion regarding my model:
>>
>> 1. I am applying gravitational forces, from the manual you suggest
>> using finite strain rather than infinite strain. Is this always true? I am
>> having an error while running with lgImplicitDeform.
>>
>
> Without the entire error message, it is extremely difficult to help.
>
> 2. My understanding is that I have to run my model with lgImplicitDeform
>> and gravitational forces until I get steady state, and use the output
>> stress as input for the final round oflgImplicitDeform and gravitational
>> forces and initial stresses. What is your intuition on going through this
>> process?
>>
>
> Conceptually, what you describe is the reasonable approach. I am not sure
> what you are asking. Perhaps your question is answered in the manual in the
> section discussing examples/2d/gravity.
>
>
>> 3. What about adding few blocks around my model, with larger mesh size?
>> Can they reduce the probable effect (rotation) caused by the gravitational
>> forces?
>>
>
> By rotation, I assume you mean that there is a lateral variation in
> density that causes the model to want to rotate. If the lateral variations
> in density/geometry are near the boundaries, a larger model might help (so
> that the lateral variations do not occur near the boundary). A larger mesh
> size will introduce larger discretization errors and will likely make
> things worse. You should examine the viscous strains to analyze how they
> are decreasing and how well they are resolved.
>
> You may want to start with a model without any horizontal variations in
> density/geometry and iterate on your workflow until you get something that
> works. Then introduce the lateral variations in density/geometry one piece
> at a time so that you can identify at what stage problems arise.
>
> Regards,
> Brad
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