[aspect-devel] nonlinear dependence on viscosity in instantaneous model

Magali Billen mibillen at ucdavis.edu
Thu Sep 10 16:25:33 PDT 2015

I would like to run an instantaneous model of subduction using a material model that includes viscosity that depends on the strain-rate (non-linear rheology).  However, I am unsure of the current state of Aspect in terms of dealing with non-linear rheology.

I have written my own material model (following the plugin from morency and doin) so I have the required lines that tell the simulator that my viscosity depends on strain-rate.

When I look in the manual  for information on how nonlinear dependence is handled, it says that the use of these is future work.  However, when I hunt around the code a bit I find that this information is passed to a function  called "stokes_matrix_depends_on_solution", which is used in core.cc within Simulator::solve_timestep. Within this part of the code I find four options for nonlinear solvers. 

I think that I should be using either NonlinearSolver::iterated_IMPES or NonlinearSolver::iterated_Stokes. These
both appear to iterate within the time-step by checking a residual value against a nonlinear_tolerance. Please note, I am on a steep learning curve with C++ and Aspect this summer, so I'm not completely confident that I'm following everything in the code.

Since I can't find anything in the manual about this - is there someone on the list that is familiar with these iterative solvers who can point me to which of these (if either) I should be using to make sure that even though I will only run for a single time-step many iterations will be done to make the solution consistent with the viscosity. It would also be helpful if someone could walk me through the "convergence" logic (I'm most familiar with how this was implemented in Citcom - leaves loop when difference between current and previous velocity solution is less than tolerance value), and share an input file that shows the input parameters that I need to include to specify the tolerance and any other values.

Thank you in advance,

Professor of Geophysics 
Chair, Geology Graduate Program
Earth & Planetary Sciences Dept., UC Davis
Davis, CA 95616
Room 2129 Earth & Physical Sciences Bldg.
Office Phone: (530) 752-4169

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