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

[Charmaine Thomas]

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>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
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