[CIG-LONG] Fwd: CIG-LONG Digest, Vol 47, Issue 2

[Walter Landry]

Laetitia Le Pourhiet <laetitia.le_pourhiet at upmc.fr> wrote:
> Hi all,
> 
> I was intrigued by all this problems with thermal diffusion...
> 
> I don't understand why this limitation for the time step is not
> included in gale, I guess it is historical since diffusion is
> negligible for convection problem and gale is based on underworld but
> as Charmaine showed, it is not negligible for post-rift simulation.

It turns out that there is some machinery in AdvectionDiffusionSLE to
use a maximum diffusivity to compute a step size.  However, the
maximum diffusivity is hard coded and can not be set from an input
file.  I do not know why that is.

> Anyway
> 
> I found that FiniteElementContext.c contains a parameter we can all
> use for the moment to limit the time step without using the very
> dangerous dt parameter...
> 
>  context->maxTimeStepSize = Dictionary_GetDouble_WithDefault(
>  self->dictionary, (Dictionary_Entry_Key)"maxTimeStepSize", 0.0 );
> 
> 
> So providing that function is really used in Gale, it means that
> everybody has to compute by hand its diffusion timestep i.e. :
> 
> dt_diff = sqrt(min_length_of_a cell)/max_diffusivity_in_your_model...
> 
> and  add
> <param name="maxTimeStepSize">dt_diff</param> in your xml ....
> to their xml

This parameter is used in Gale.  So that is probably the best way to
limit the timestep.

Cheers,
Walter Landry
walter at geodynamics.org