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

[Julian Giordani (Sci)]

On 21 October 2010 05:59, Walter Landry <walter at geodynamics.org> wrote:

> 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.
>

just on this note and assuming the GALE implementation is the same as
Underworld...

Maximum diffusivity is initially hard-coded in AdvectionDiffusionSLE.c -
some legacy condition I suspect.
Then the value of 'self->maxDiffusivity' is dynamically calculated using the
all material diffusivity values or the value of 'defaultDiffusivity', this
is done in the file UpwindParameter.c

the diffusive timestep = self->courantFactor * minSeparation * minSeparation
/ self->maxDiffusivity

so max diffusivity should be a reasonable value.

cheers,
Julian


> > 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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