[CIG-SHORT] Applying Gravitational Forces with Pylith

[Charles Williams]

Dear Niloufar,

The reason you would want to use finite strain for gravitational problems is because with an infinitesimal strain formulation, the stresses at each time step do not update to conform to the deformed shape.  This means that when body forces (e.g., due to gravity) are computed, they are computed for the original shape of the body, and therefore do not take into account any vertical deformation that may have occurred.  The end result is that you would generally end up overpredicting the amount of vertical deformation, since the net result of gravity in a viscoelastic problem is to damp the vertical displacements.  If the vertical displacement field is not important to you, this may not be an issue.

If you need the vertical displacements, you should probably use finite strain.  What error are you getting when you use ImplicitLgDeform?

As far as reaching steady state, that may not be possible, but you may be able to achieve quasi-equilibrium.  For example, if you have horizontal gradients in density, the model would attempt to deform until everything is ‘flat’.  I think you will need to do some experimentation with this.

I think you would definitely want to extend the outer boundaries of your mesh, compared to the example problems.  The example problems were meant to demonstrate the concept, but not to provide an entirely realistic model.  This is generally true of any model, and not just models with body forces.

Cheers,
Charles


> On 1/08/2018, at 6:44 PM, Niloufar Abolfathian <niloufar.abolfathian at gmail.com> 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.
> 
> 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.
> 
> 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 of lgImplicitDeform and gravitational forces and initial stresses. What is your intuition on going through this process?
> 
> 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?
> 
> Thanks,
> Niloufar
> 
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Charles Williams I Geodynamic Modeler
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