[aspect-devel] Far different velocity magnitudes & timestep sizes of the same Ra

[Juliane Dannberg]

Also, if you want to solve for only the dynamic pressure (and hence have 
only density
perturbations without the reference profile in the density in the 
material model) and
run a compressible model, you can use the ALA formulation to do that. If 
you want to
keep using ASPECT's "original" formulation, you can not just use a 
density that only
contains the perturbations, because in this case this same density would 
be used in the
temperature equations (so among other problems, the program would crash 
because
the density would be negative in some parts of the domain).

This means you basically have to decide if you want to use BA/ALA/TALA 
and then
you can also just solve for the dynamic pressure, or use our original 
formulation with
the full pressure.

On 04/26/2017 06:48 PM, Timo Heister wrote:
> Shganxin,
>
> if you use BA as the formulation, the density in the temperature comes
> from the "adiabatic conditions" module. The density from the material
> model is only used in the buoyancy term. See the section on
> "formulation" in the manual.
>
> On Wed, Apr 26, 2017 at 3:32 PM, Shangxin Liu <sxliu at vt.edu> wrote:
>> Hi Wolfgang and Juliane,
>>
>> Thanks for clarifying. I have one more question/concern to confirm with you.
>> If I use a material model in which the density has no constant term and only
>> contains perturbation term to make the full pressure equals the dynamic
>> pressure, what will the density term in the temperature equation be? If
>> Boussinesq approximation formulation is used, I can see that it should be
>> the input reference density of the material model, yes?  However, if the
>> compressible formulation is used, what will it be since the input density in
>> the material model is actually only the density perturbation?
>>
>> Best,
>> Shangxin
>>
>>
>>> On 04/24/2017 12:10 PM, Juliane Dannberg wrote:
>>>> As far as I know we always use the full pressure (unless one chooses the
>>>> density in the material model to only be the density deviations from the
>>>> reference profile, in which case the computed pressure would be the
>>>> dynamic
>>>> pressure), and looking at the manual, I agree that we could document
>>>> this better.
>>> Yes, we use the full pressure. The full pressure, however, equals the
>>> dynamic
>>> pressure if you use a material model in which the density has no constant
>>> term
>>> and only contains the thermal expansion term, i.e.,
>>>     rho(T) = -\alpha T
>>> There is a hydrostatic component of the pressure if the density is given
>>> as
>>>     rho(T) = rho_0 - \alpha T
>>>
>>> Which of the two you use is up to you.
>>>
>>> Best
>>>    W.
>>>
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