[CIG-SHORT] Sign of shear stress changes from one fault node to another

[Brad Aagaard]

Romain,

The first thing to check is the behavior of the linear and nonlinear 
solves. Are they converging at every time step? Are the absolute and 
relative tolerances set to appropriately relative to the friction zero 
sliding tolerance?

The relative sizes of the absolute KSP and SNES tolerances are very 
important. This is what works in most of our examples. They may need to 
be tuned slightly for your particularly problem, such as greater 
relative separation (here there is one order of magnitude difference).

[pylithapp.problem.interfaces.fault]
zero_tolerance = 1.0e-11
[pylithapp.petsc]
# Linear solver tolerances
ksp_rtol = 1.0e-20
ksp_atol = 1.0e-12
# Nonlinear solver tolerances
snes_rtol = 1.0e-20
snes_atol = 1.0e-10

Regards,
Brad


On 11/05/2015 05:20 AM, Romain Jolivet wrote:
> Hello,
>
> I am running a 2d, fully elastic experiment in which a fault
> (pylith.faults.FaultCohesiveDyn) with Rate and State friction
> (pylith.friction.RateStateAgeing) cuts through the entire rectangular domain.
> Gravity is on.
> Left boundary of the model has a pylith.bc.DirichletBC dirichlet BC set so that
> it moves horizontally, with the entire domain in compression.
> Surface is stress free while bottom boundary is set to compensate gravity with a
> pylith.bc.Neumann BC. One point at the surface is fixed so the problem can be
> solved.
> Initial pylith.faults.TractPerturbation on the fault is set so that shear and
> normal stresses are 0.
> The fault is entirely rate-strengthening a>>b and I just wait until the fault
> slips at steady state (simulation runs 10000 years with 10 year time steps).
>
> The final distribution of stresses along the fault is very awkward (see
> attached) as, for some reason, Shear Stress is negative for certain nodes, while
> it should be positive everywhere.
> I looked at the orientation of the normalDir, but it is constant along the
> fault, so it does not seem related.
> Any thoughts?
>
> Cheers,
> R
>
> PyLith version: git repository downloaded on sept 30th 2015
> PETSc version: git repository downloaded on sept 30th 2015
>
>
> —————————————————————————————————————
> —————————————————————————————————————
> Romain Jolivet
> Postdoctoral Fellow
>
> University of Cambridge
> Department of Earth Sciences
> Bullard Labs
> Madingley Rise
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> Cambridge CB3 0EZ
> United Kingdom
>
> email: rpj29 at cam.ac.uk <mailto:rpj29 at cam.ac.uk>
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