[CIG-SHORT] pseudo plane-strain in Pylith

[Brad Aagaard]

Lucas,

Solving plane strain problems with meshes one cell think causes 
logistical problems like you are running into. My suggestion would be to 
keep the front and back planes with Dirichlet BC but also apply a 
Neumann BC to the points intersecting the fault to create a 
pseduo-Dirichlet BC. As long as the out of plan deformation is caused by 
a constant traction, this should work reasonably well. A better solution 
would be to use PyLith in 2-D and your other code in 3-D and map the 
solution back/forth.

Brad

On 4/5/13 1:07 PM, Lucas Abraham Willemsen wrote:
> Hello,
>
> Currently I am trying to investigate the geomechanical response of a
> reservoir with a single fault when CO2 is injected. The fluid-flow part
> of the problem is performed by a code which is coupled to Pylith. The
> results from Pylith are fed into the flow code, which marches time
> forward and the  new fluid pressures are then fed back into Pylith, etc.
>
> The CO2 sequestration geometry I am considering is plane-strain, but the
> complication is that the flow-code (currently experimental) only works
> with hexahedrals. So I turned the 2D problem into a 3D-problem that is
> 1-element thick in the out-of-plane direction (y-direction). To enforce
> plane strain conditions I put zero displacement in the y-direction on
> the front and back plane. But the problem is that the fault intersects
> these front and back planes. This is a natural consequence of turning
> the 2D geometry with fault into a 1-element thick 3D geometry with fault.
>
> All of the input parameters are independent of 'y' since I am trying to
> model a plane strain problem. So there should be no reason for slip in
> the y-direction on any of the fault nodes (intuitively speaking). But
> Pylith does not allow me to explicitly constrain the y-displacements of
> the fault where they intersect the front and back plane. Therefore, the
> only way to run the model is by excluding the nodes on the fault from
> the front- and back- plane nodesets. But this way the boundary condition
> on these fault nodes is effectively zero normal stress, and elastic
> extrusion of material takes places. (Applying Neumann B.C. with normal
> stress to these fault nodes on the front and back plane to somehow
> reduce the extrusion does not seem to affect the fault nodes either. It
> would not be a great solution anyway).
>
> Is there a way to deal with faults in this 'pseudo plane-strain'
> geometry I described?
>
> sincerely,
> Lucas
>
>
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