[CIG-LONG] Fwd: CIG-LONG Digest, Vol 47, Issue 2
Laetitia Le Pourhiet
laetitia.le_pourhiet at upmc.fr
Wed Oct 20 08:38:24 PDT 2010
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.
> 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
> 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?
>> 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
>> > 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
>> 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
>> > 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.
>> Walter Landry
>> walter at geodynamics.org <mailto:walter at geodynamics.org>
>> CIG-LONG mailing list
>> CIG-LONG at geodynamics.org <mailto:CIG-LONG at geodynamics.org>
>> CIG-LONG mailing list
>> CIG-LONG at geodynamics.org
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