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

Max Rudolph maxwellr at gmail.com
Fri Oct 7 19:26:43 PDT 2016

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.


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

> 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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