[CIG-SHORT] pylith error in model with big time step and low viscosity

[Charles Williams]

Brad is correct.  Using a high viscosity will not change the size of the stable time step.  If you really want to use a much larger time step, you can do as Brad suggested earlier — change the stability factor.  To insure that you are getting an accurate solution, you could run for a shorter time period with a smaller time step, and compare the results at that time.  That will at least make sure your solution to that point is accurate using the larger time step.

Cheers,
Charles


> On 11/06/2016, at 9:24 AM, Brad Aagaard <baagaard at usgs.gov> wrote:
> 
> David,
> 
> I am not sure what you mean by adding a second viscous layer with a high viscosity. The rate of the viscoelastic deformation is controlled by the material with the smallest viscosity, so as long as a low viscosity material is present, the appropriate time step to resolve the deformation will be small.
> 
> Regards,
> Brad
> 
> 
> On 06/10/2016 02:18 PM, David Rodrigo Mora Cofré wrote:
>> Mr.Brad Aagaard:
>> 
>> Thank you very much. I will check the manual. I think that adding a
>> second viscous layer (with high viscosity) will fix the problem.
>> 
>> best regards,
>> David.
>> 
>> 2016-06-10 12:28 GMT-03:00 Brad Aagaard <baagaard at usgs.gov
>> <mailto:baagaard at usgs.gov>>:
>> 
>>    David,
>> 
>>    The error message you are getting is associated with a built-in
>>    check that compares the Maxwell relaxation time with the time step.
>>    PyLith generates an error if the time step is greater than 1/5 of
>>    the Maxwell time. This insures that the simulation will be able to
>>    resolve the viscoelastic deformation with a reasonable accuracy.
>> 
>>    It is possible to circumvent this by using the TimeStepAdapt time
>>    stepper with a stability factor much less than 1.0, but you should
>>    be aware that your simulations will have significant errors in
>>    resolving the viscoelastic relaxation.
>> 
>>    Regards,
>>    Brad
>> 
>> 
>>    On 06/10/2016 08:07 AM, David Rodrigo Mora Cofré wrote:
>> 
>>        Mr.Brad Aagaard:
>> 
>>        I'm testing a surface load model with 2 elastic layers + 1
>>        viscoelastic
>>        layer  for study a postglacial rebound. When i use a viscosity
>>        1.0e21
>>        Pas (dt=100*year), pylith computing my model very well. But,
>>        when a use
>>        a viscosity =<1.0e20 Pas (dt=100*year , total_time=10000*year),
>>        pylith
>>        not computing my model because my dt > stable time step.  The stable
>>        time step suggested by pylith is < 10*year. As this dt (<
>>        10*year) is
>>        very small, pylith will take long time to computing the model.
>>        How i can
>>        force to pylith to computing the model with dt>=100*year and a low
>>        viscosity?
>> 
>>        The following is the error message:
>>        -------------------------------------------------------------------------------------------------
>> 
>>        File
>>        "/Users/rodrigo/fem/pylith-2.1.1-darwin-10.6.8/lib/python2.7/site-packages/pylith/problems/TimeStepUniform.py",
>>        line 87, in timeStep
>> 
>>              "step of %12.4e." % (self.dtN, dtStable))
>> 
>>        RuntimeError: Current nondimensionalized time step of 1.0000e+02
>>        exceeds
>>        the nondimensionalized stable time step of   9.2050e+00.
>> 
>>        application called MPI_Abort(MPI_COMM_WORLD, -1) - process 0
>> 
>>        /Users/rodrigo/fem/pylith-2.1.1-darwin-10.6.8/bin/nemesis:
>>        mpirun: exit 255
>>        ---------------------------------------------------------------------------------------------------
>> 
>>        best regards,
>> 
>>        PhD(c) David Mora
>>        Doctorado en Ciencias De La Tierra
>>        Universidad de Concepción
>>        Chile
>> 
>> 
>> 
> 
> _______________________________________________
> CIG-SHORT mailing list
> CIG-SHORT at geodynamics.org
> http://lists.geodynamics.org/cgi-bin/mailman/listinfo/cig-short

Charles A. Williams
Scientist
GNS Science
1 Fairway Drive, Avalon
PO Box 30368
Lower Hutt  5040
New Zealand
ph (office): 0064-4570-4566
fax (office): 0064-4570-4600
C.Williams at gns.cri.nz