[CIG-SHORT] Applying Gravitational Forces with Pylith

Wed Aug 1 20:59:50 PDT 2018

Thanks, Charles.

My error is due to the convergence of the nonlinear solver. I have made
some modifications but not sure if it is a good idea to decrease my
residual norms.

Best,
Niloufar

This is the setting I am using:

[pylithapp.timedependent]

formulation = pylith.problems.ImplicitLgDeform

[pylithapp.timedependent.formulation]

output = [domain,subdomain]

output.subdomain = pylith.meshio.OutputSolnSubset

solver = pylith.problems.SolverNonlinear

[pylithapp.timedependent.formulation.time_step]

total_time = 50000.0*year

dt = 2.0*year

[pylithapp.timedependent.interfaces]

fault = pylith.faults.FaultCohesiveDyn

[pylithapp.timedependent.interfaces.fault]

label = fault

edge = fault_edge

zero_tolerance = 1.0e-11

# Use the slip-weakening friction model.

friction.db_properties = spatialdata.spatialdb.UniformDB

friction.db_properties.label = Slip weakening

friction.db_properties.values =
[static-coefficient,dynamic-coefficient,slip-weakening-parameter,cohesion]

friction.db_properties.data = [0.6,0.4,0.2*m,0.0*Pa]

# ----------------------------------------------------------------------

# PETSc settings

# ----------------------------------------------------------------------

# NOTE: There are additional settings specific to fault friction.

# Set the solver options.

[pylithapp.petsc]

malloc_dump =

# Preconditioner settings.

pc_type = asm

sub_pc_factor_shift_type = nonzero

# Friction sensitivity solve used to compute the increment in slip

# associated with changes in the Lagrange multiplier imposed by the

# fault constitutive model.

friction_pc_type = asm

friction_sub_pc_factor_shift_type = nonzero

friction_ksp_max_it = 50

friction_ksp_gmres_restart = 30

# Uncomment to view details of friction sensitivity solve.

#friction_ksp_monitor = true

#friction_ksp_view = true

friction_ksp_converged_reason = true

# Reduce convergence tolerances.

ksp_rtol = 1.0e-16

ksp_atol = 1.0e-12

ksp_max_it = 4000

ksp_gmres_restart = 100

snes_rtol = 1.0e-9

snes_atol = 1.0e-8

snes_max_it = 4000

On Wed, Aug 1, 2018 at 2:50 PM, Charles Williams <willic3 at gmail.com> wrote:

> 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 ModelerGNS Science **I** Te Pῡ Ao*
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