[CIG-SHORT] Dynamic faulting under gravity

[Birendra jha]

Hi all

I removed gravity (and initial stress), improved the aspect ratio of my grid (condition number is much better now), still the nonlinear solver is not converging (linear solver converges in <50 iterations). SNES Function Norm decreases for first 2 iterations then increases (from 2.8E-02 to 3.08E-02 in 4 iterations).

I have the attached the 3 files necessary to run the problem.

Problem description: a slanted fault in a box domain. Dynamic fault. Static friction model with 0 cohesion.
Boundary condition: roller boundary on -x, +y, -y. Dirichlet displacement on +x of -1 cm to apply compression on the fault.

Please help!

Thanks and regards
Birendra

--- On Thu, 11/22/12, Brad Aagaard <baagaard at usgs.gov> wrote:

> From: Brad Aagaard <baagaard at usgs.gov>
> Subject: Re: Dynamic faulting under gravity
> To: "Birendra jha" <bjha7333 at yahoo.com>
> Cc: cig-short at geodynamics.org
> Date: Thursday, November 22, 2012, 3:18 AM
> Birendra,
> 
> You have very flat hex cells in your mesh. This results in a
> very poorly 
> conditioned system. I think this prevents getting a decent
> guess for the 
> slip, so the nonlinear solve fails to converge.
> 
> It looks like you are trying to resolve very small scale
> geometric 
> features with a coarse hexahedral mesh. I think you will
> need a mesh 
> with much closer to cube-like cells to get the nonlinear
> solve to converge.
> 
> Regards,
> Brad
> 
> 
> On 11/20/2012 11:32 AM, Birendra jha wrote:
> > Hi,
> >
> > With that, the linear solver converges but nonlinear
> solver didn't converge in 100 iterations.
> > To improve the gravity modeling, I also modified the
> boundary conditions such that now I have zero normal
> displacement on +x,-x,+y,-y,-z faces except the fault nodes
> on +y,-y,-z faces. With this setup, in absence of fault
> model in pylith, the case runs fine with almost 0
> displacement everywhere except on the fault nodes which are
> free. This is as expected. But with fault friction, it does
> not converge. The unconverged solution also shows large
> displacements on the fault nodes, I am just not sure if the
> numbers are accurate.
> >
> > The modified files are attached.
> >
> > Thanks
> > Birendra
> >
> > --- On Tue, 11/20/12, Brad Aagaard <baagaard at usgs.gov>
> wrote:
> >
> >> From: Brad Aagaard <baagaard at usgs.gov>
> >> Subject: Re: Dynamic faulting under gravity
> >> To: "Birendra jha" <bjha7333 at yahoo.com>
> >> Cc: cig-short at geodynamics.org
> >> Date: Tuesday, November 20, 2012, 10:28 PM
> >> Birendra,
> >>
> >> The linear solve did not converge. If the linear
> solves
> >> within the
> >> nonlinear solve doesn't converge, the nonlinear
> solve won't
> >> converge.
> >> You are using ASM for the preconditioner. Try using
> the
> >> field split
> >> preconditioner with ML on the elastic part and the
> custom
> >> fault
> >> preconditioner. See examples/3d/tet4/step04 for
> PETSc
> >> settings.
> >>
> >> Brad
> >>
> >> On 11/19/2012 09:36 PM, Birendra jha wrote:
> >>> Dear all
> >>>
> >>> I have a relatively simple case of a slanted
> fault
> >> plane (along y axis) inside a layered box. I am
> using
> >> FaultCohesiveDyn with static friction model (please
> see the
> >> attached file). +x, -x boundaries are fixed in x,y
> >> directions and bottom is fixed in z direction. y
> faces are
> >> free.
> >>> I have gravity on and I use
> initial_stress.spatialdb
> >> with a density of 2400 kg/m3.
> >>>
> >>> The nonlinear solver is not converging. Can you
> please
> >> help? What am I missing? There are four thin layers
> in the
> >> grid sandwiched between thick layers above and
> below. Is
> >> that the problem?
> >>>
> >>> I am using the Pylith 1.8.0 windows binary.
> >>>
> >>> Thanks and regards
> >>> Birendra
> >>>
> >>
> >>
> >
> 
>
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