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

[Laetitia Le Pourhiet]

sorry there was a mistake in my email
dt_diff = (min_length_of_a cell)^2/max_diffusivity_in_your_model...

Laetitia Le Pourhiet 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.
>
> 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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>>
>>
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