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

[Laetitia Le Pourhiet]

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.

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

I think we should all include that in our simulation because for a 3km 
mesh and a diffusivity of 1e-6 it makes a timestep of 300ka which is 
about or a little bit less than what I get with dtfactor=0.5 for a 
1cm/year crustal extension problem.

hope this help
Laetitia



Charmaine Thomas wrote:
> Hi Walter,
>
> I did a series of experiments to get Gale to do pure thermal 
> conductivity problems without having a non-zero velocity boundary 
> applied to the right/left walls, ie velocity=0.
>
> Firstly I tried turning off the Stokes flow and the uzawa condition, 
> and although everything ran very smoothly and quickly, it took 
> ridiculously big timesteps, despite having an explicitly set 'dt' (I 
> assumed later that Stokes flow has to be turned on for this parameter 
> to kick in?). This predictably resulted in very high temperatures. I 
> had to do this experiment because I was dealing with non-newtonian 
> rheologies, and needed to  model a time of quiescence in my crust. 
>
> Finally I found the best option was to leave the Stokes/uzawa stuff 
> turned on and to apply either a very low strain-rate, or set the 
> right/left velocities as zero, but also have a right and left wall 
> stress boundary condition. This second option runs smoothly even with 
> non-newtonian rheologies, and takes more reasonably sized timesteps - 
> I could also try changing the dtfactor to speed things up. So does 
> this method sound more reasonable? Is there a better way?
>
> Cheers,
>
> Charmaine Thomas
> School of Geosciences | University of Sydney
>
>
> On Wed, Oct 20, 2010 at 10:21 AM, Walter Landry 
> <walter at geodynamics.org <mailto:walter at geodynamics.org>> wrote:
>
>     <Guillaume.Duclaux at csiro.au> wrote:
>     > Indeed.
>     >
>     > But, shouldn't it be possible to solve purely thermal problem with
>     > Gale?  (let's pretend the thermal expansion is null is Nicolas'
>     > problem).  ie a sill at a temperature of 1000 K has intruded a mass
>     > of rock at constant temperature (600 K) and I want to simulate the
>     > thermal evolution of the system as I change the thickness of the
>     > dyke or the radiogenic heat production of one or the other material.
>
>     It is possible to do pure thermal conductivity problems with Gale.
>     You have to turn off all of the Stokes flow stuff, but it does seem to
>     work.
>
>     > To ensure the solver timestepping is not missing the temperature
>     > perturbation timescale, how should the time be scaled?
>     > I guess viscosity doesn't matter if the problem is purely thermal,
>     > but as soon as the thermal expansion is on, some body forces act too
>     > creating some 'slow' displacement.
>
>     For this particular case, the displacement is so slow that it can be
>     neglected.  If you still want to solve the Stokes flow, then you can
>     set the timestep explicitly with 'dt' (see Appendix A.1.4).  Gale
>     should probably take the thermal diffusivity into account when
>     deciding upon a timestep, but it does not do that now.
>
>     Cheers,
>     Walter Landry
>     walter at geodynamics.org <mailto:walter at geodynamics.org>
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