[aspect-devel] problem with ASPECT - the iterative Stokes solver did not converge

[Rene Gassmoeller]

Hi Petar,
from a first glance the parameters you used for your model look good,
but there is one thing you should test. In your parameter file you did
not set the Adiabatic surface temperature, because it is not needed for
the simple material model, and it defaults to 0 (@Wolfgang: We should
maybe change this to a reasonable value like 1600? Many of the more
complex plugins rely on a good adiabatic profile). The Steinberger
material model, and also your harmonic perturbation initial condition
rely on the initially computed adiabatic profile to compute their
properties, therefore this might mess up you calculation. I would guess
this is already the problem.

However there are a few other things that you might try to find the
reason for the convergence problems if the above did not help. First you
could test the convergence in 2D, in your model setup that is as simple
as setting Dimension = 2. And you could try a higher resolution in 2D.
Both options should show significantly better convergence, so you have
at least a starting point.

Then you could test a temperature dependency in the simple material
model (e.g. set Thermal viscosity exponent to something like 10). If
this turns out to be a problem then your resolution might be too low to
resolve the viscosity changes you are prescribing.

If it turns out to be a resolution issue, try setting the global
refinement in the 3D model to 5 and the adaptive refinement to 0. Aspect
will still coarsen and adaptively refine again later on, just the
initial convergence should be much better.

You mentioned that you tested a constant viscosity profile with the
Steinberger material model, but in the parameter file it looks like you
only modified the radial dependence? Did you also set the temperature
dependence to 0? You could do that by setting 'Lateral viscosity file
name' to our test file in
data/material-model/steinberger/test-steinberger-compressible/test-viscosity-prefactor.txt.
With a constant radial and lateral viscosity the convergence should be
extremely fast.

Let us know if you have any further questions.

In fact I have a question for you. We are constantly looking to improve
the available options in Aspect and include new models. Could you ask
Alessandro, whether he would be willing to share the viscosity profile
you used with the general Aspect users? In that case we could include it
in the official version as an alternative to the existing one by
Steinberger & Calderwood.

Best,
Rene



On 07/25/2014 01:14 PM, Wolfgang Bangerth wrote:
> 
> Petar,
> let me pull this discussion to the mailing list since the authors of the
> Steinberger model are there.
> 
> 
>> I have experienced a problem running ASPECT. It seems that the iterative
>> Stokes solver did not converge after three days of the job running on 48
>> CPUs - please see attached log files (/aspect.e2477653, aspect.o2477653
>> & log.txt/).
> 
> This is almost certainly an unrelated observation, but with as few
> unknowns as you have, you should first run small models using only a
> single processor when you try things out.
> 
> My rule of thumb is that you can run models with a few 100,000 unknowns
> on a single processor. For parallel computations, you typically don't
> gain anything if you have fewer than 50,000 unknowns per processor --
> i.e., in your case, 4 or 5 processors is about the optimum.
> 
>> However, my goal is to use the depth-dependent (i.e., radial) viscosity
>> profile (Mitrovica & Forte, 2004; please see attached file:
>> /mitro-forte-2004-radial-visc-new.txt/) jointly with lateral variations
>> in viscosity (see below /shell_simple_3d_new.prm/). For example, to use
>> something similar to the Steinberger model:
>> subsection Material model
>>      set Model name = Steinberger
>>      subsection Steinberger model
>>          set Bilinear interpolation = true
>>          set Compressible = true
> 
> What happens if you set this to false?
> 
> I don't know the parameters to the Steinberger model, so can't comment
> on their validity. I hope someone else on the mailing list may be able
> to provide more feedback.
> 
> Best
>   Wolfgang