[aspect-devel] mesh refinement strategy question

Thu Jun 7 14:28:58 PDT 2018

Hey Magali,

This is a problem I also struggled quite a bit with during my master thesis, and in the end I arrived at the solution which works really well for my applications (both in 2d and 3d) and I think it will also well for your application since it looks very similar.

The problem here is that most mesh refinement strategies in aspect are gradient based with an error indicator or location based. The trick here is to realise that you probably only want to refine in cold area's, because that is where the slab currently is. So what you want to do is refined based on absolute temperature or isotherms. I have written a plugin for this quite a while ago, but never got around to fully merging it into the main branch. You can find it at: https://github.com/MFraters/aspect/tree/MFraters-isoterms-plugin, or in the pull request https://github.com/geodynamics/aspect/pull/295. I have attached a figure which shows how the grid follows the evolution of the slab really nicely based on this approach.

If you have questions on this, or need help with it, please let me know.

Cheers,

Menno

________________________________
From: Aspect-devel <aspect-devel-bounces at geodynamics.org> on behalf of FELIPE ORELLANA ROVIROSA <f_orellana at berkeley.edu>
Sent: Thursday, June 7, 2018 9:20:00 PM
To: aspect-devel at geodynamics.org
Subject: Re: [aspect-devel] mesh refinement strategy question

 Hi Magali,

      Nice project.

      I am no expert on this, but I have some experience. I hope other modelers can speak out.

     Time steps between mesh refinement = 1  !!! watch out with this, it may be appropriate for some running time, but not always.

     For the strategy, if you use Temperature, that should help the appropriate solving of the energy equation. More so, if your slab is defined as  thermal lithosphere, a proper temp-based refinement should reveal the slab properly, with lots of mesh points defining its shape.

    The sole temp strategy may fail to properly solve the momentum equation, for which you should turn to the strain rate strategy.

    These days I am using:

       set Strategy = strain rate, temperature
       set Refinement criteria merge operation = plus

but in my experience it 's kind of an art..sometimes I do find problems, even with all of these precautions, and I need to play with refinement/coarsening fractions.

   I guess you should start with a purely temp-based refinement, maybe at the very start, or maybe even for some run-geologic time. Then, I would surely add an extra criteria, something that takes care of the velocity field (e.g. the strain rate strategy).

cheers,
Felipe

On Thu, Jun 7, 2018 at 8:48 AM, Magali Billen <mibillen at ucdavis.edu<mailto:mibillen at ucdavis.edu>> wrote:
Hello,

I am starting a project for subduction modeling in Aspect and I have a  question about how certain mesh refinement strategies work together. Specifically, I need to define a “proto-slab” based on temperature,
 which is needed to start the subduction process. However, once the slab starts sinking its location in the model domain depends on the dynamics and it can move 1000s of kilometers from its starting location.

So, I have first set this up using the  Strategy = minimum refinement function (see below for the
full subsection definition). And, I can get a nice looking initial slab (see below).  However, after the initialization,
I would like the mesh refinement to depend on things like the temperature, (and later composition, and strain-rate).  It does not work to just use Strategy = temperature, because this just refines on the temperature field defined on the coarsest mesh (see picture).

So, what is not clear to me is whether there is a way to use one Strategy during the initial adaptive refinement
and a different (set) of criteria after that point, or to combine them such that the minimum refinement function
is no longer have much impact after the first step.

Has anyone else had a problem like this?  Any ideas?
Instead of thinking about refining, should I start with a really fine mesh everywhere and then use the
Strategy = temperature to coarsen the mesh where refinement isn’t needed. It doesn’t seem like it set up
to work that way?

- Magali
Here’s the prm file for the nice looking slab using the function to refine based on location:

Here’s just the mesh  refinement section:

subsection Mesh refinement
  set Initial global refinement                = 4
  set Minimum refinement level                 = 4
  set Initial adaptive refinement              = 5
  set Time steps between mesh refinement       = 1
  set Strategy = minimum refinement function
  subsection Minimum refinement function
    set Coordinate system   = cartesian
    set Variable names      = x,y
    set Function constants = YMAX=1e6,LITH=1.5e5,SM=2.5e5,XTR=2.2e6,RC=4e5
    set Function expression = if( (YMAX-y<=LITH)||((YMAX-y<=SM)&&(x<=XTR+3*RC)&&(x>=XTR-0.5*RC)),9,0)
  end
end

And, here are some screen-shots

Full domain

Zoom-in on slab

This is what happens if I just use Strategy = temperature

And if I combine them  (this is just the 0th time-step), I need to set up some other things before
I can run forward in time.

____________________________________________________________
Professor of Geophysics
Earth & Planetary Sciences Dept., UC Davis
Davis, CA 95616
2129 Earth & Physical Sciences Bldg.
Office Phone: (530) 752-4169
http://magalibillen.faculty.ucdavis.edu

Currently on Sabbatical at Munich University (LMU)
Department of Geophysics (PST + 9 hr)

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