[aspect-devel] Help needed with setting up an EBA benchmark

[Ian Rose]

Hi Wolfgang and all,

Excuse the overly verbose answer, I'm talking this out as much for me as
anyone.  So, thermodynamically, the adiabatic heating term is sensitive to
the total temperature rather than differences in temperature (as in, for
instance, the buoyancy).  I see that at the top of page three they have
defined T_0 to be the dimensional surface temperature over the temperature
scale, \Delta T.  In the anelastic case there would be an adiabatic profile
starting from T_0, but in this case, because it is incompressible, the
reference temperature is T_0 all the way down.

The way it shakes out in these equations, T_0 becomes a free parameter that
can increase or decrease the effect of adiabatic heating on the model.  For
the benchmark case, they use T_0 = 273/3000, like you said, but I could put
in anything, really.  All this is a long way of saying that in your
parameter file you have set the boundary conditions for temperature to go
from zero to one at the bottom.  To my eyes, it looks like you can
reproduce the conditions of the benchmark by simply changing the
temperature boundary conditions to 0.091 - 1.091.

I'm not sure if that made sense, but hopefully it did.
Best,
Ian



On Thu, Sep 4, 2014 at 5:18 PM, Wolfgang Bangerth <bangerth at tamu.edu> wrote:

> On 09/04/2014 07:09 PM, Katrina Arredondo wrote:
>
>> But why does it need to be nondimensionalized? Just to be consistent
>> with the benchmark?
>>
>
> It doesn't have to be. The benchmark is defined in terms of nondimensional
> quantities -- it is what it is. But ASPECT does not take Rayleigh and
> Dissipation numbers as inputs. I have to somehow translate them into the
> physical quantities.
>
>
>
> > I guess I was thinking that this could be a test for how "easy" it is
> > to translate the geodynamics nondimensional terms into dimensional
> > terms in ASPECT. Otherwise why have the dimensional terms at all, if
> > it's too hard.
>
> Because the real world is sufficiently complicated that
> nondimensionalization does not work. (For example, what exactly is the
> Rayleigh number when you have variable gravity, a geometry for which there
> is no obvious lengthscale, and thermal expansion and viscosity depend
> strongly on the temperature?) I think the better question is why we should
> have the nondimensional form if it only describes idealized but
> non-realistic situations :-)
>
> Best
>  Wolfgang
>
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth               email:            bangerth at math.tamu.edu
>                                 www: http://www.math.tamu.edu/~bangerth/
>
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