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

[Max Rudolph]

Juliane,
Thanks very much for pointing this out. I'm not sure where it would be
appropriate to point this out in the manual, but yes, it might have helped
us avoid a small amount of troubleshooting.

Max

On Fri, Oct 7, 2016 at 4:40 PM, Juliane Dannberg <dannberg at gfz-potsdam.de>
wrote:

> Hi Max,
>
> I think we already have a somewhat similar functionality in Aspect.
> If you expect a conduction-dominated problem, you can set
>
> Use conduction timestep = true
>
> in the input file, and then the time step size will be computed as the
> minimum of the convection time step and CFL*d^2/kappa (where d is the cell
> size).
> Is that along the lines of what you had in mind? Should we document this
> better so that it's easier to find that the option is there?
>
> Cheers
> Juliane
>
> On 10/07/2016 04:36 PM, Max Rudolph wrote:
>
> Dear All,
> 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.
>
> Cheers
> Max
>
>
>
>
>
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>
>
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