[CIG-LONG] Fwd: CIG-LONG Digest, Vol 47, Issue 2
Walter Landry
walter at geodynamics.org
Wed Oct 20 11:59:20 PDT 2010
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
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