[aspect-devel] Onset of convection, time step choice

[Max Rudolph]

Wolfgang,
I am actually happy to make this the default, but I was hoping that others
might weigh in if they think that this is a bad idea. I do not see a reason
why you wouldn't want enable this by default for most convection problems,
but it's difficult to anticipate the needs of other users. I would be happy
to contribute a cookbook on this topic, and I may follow up about this off
the mailing list.

Max

On Mon, Oct 10, 2016 at 8:50 AM, Wolfgang Bangerth <bangerth at tamu.edu>
wrote:

>
> Max,
>
> I am teaching geodynamic modeling this term, and asked my students as an
>> introductory assignment to try to use ASPECT to determine experimentally
>> the
>> critical Rayleigh number for the onset of convection for an isoviscous
>> fluid,
>> heated from below with free-slip boundary conditions. The students did
>> this by
>> modifying cookbooks/convection-box.prm. One student found that convection
>> began at Ra less then half the critical Ra for the chosen aspect ratio of
>> 4:3.
>> Of course, it turned out that too large a timestep was allowed. If the
>> timestep size is limited to, say 100 Myr, we recover the correct value of
>> Ra_{cr} within ~5% even for relatively coarse meshes (refinement level
>> 4). I
>> was a bit surprised, however, that the default timestep taken by ASPECT
>> was so
>> large. In fact, we had convergence problems for higher refinement levels
>> for
>> the default maximum timestep. For a problem like the one that we were
>> running,
>> a reasonable maximum timestep seems to be L^2/kappa (thermal diffusion
>> timescale), where L is a characteristic length scale for the domain. Does
>> it
>> seem reasonable to change the way that ASPECT computes the maximum time
>> step
>> based on this idea rather than using 10^300 (the current default value,
>> which
>> seems entirely arbitrary)? I am happy to submit a pull request to do
>> this, but
>> don't want to waste my time if it's unlikely to be accepted.
>>
>
> As Juliane already pointed out, (i) there is already an option to do that,
> but (ii) as you mentioned you don't want to make it the default.
>
> I don't actually have much of an opinion (I'm going to blame it on jetlag)
> but if your opinion is that this shouldn't be the default, the making
> L^2/kappa the maximum time step also seems like the wrong solution. Or do I
> misunderstand?
>
> One option I see is to make the work you and your students did into a
> cookbook. I think that would be great. That would also provide a platform
> where we could talk about the issue in a highlighted comment.
>
> Best
>  Wolfgang
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth          email:                 bangerth at colostate.edu
>                            www: http://www.math.colostate.edu/~bangerth/
>
>
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