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

[Juliane Dannberg]

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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