[aspect-devel] mesh refinement strategy question

[FELIPE ORELLANA ROVIROSA]

 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> 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)
>
> Avoid implicit bias - check before you submit:
> http://www.tomforth.co.uk/genderbias/
> ___________________________________________________________
>
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