[aspect-devel] Negative pressure

Lev Karatun lev.karatun at gmail.com
Fri Mar 11 01:28:42 PST 2016

```Hi Max,

the viscosity plot is attached. The box dimensions are 512*1024km, the rest
is in the .prm file attached to the previous email.

Best regards,
Lev Karatun.

2016-03-10 15:34 GMT-05:00 Max Rudolph <maxwellr at gmail.com>:

> Lev,
> Can you post plots that show viscosity? It would also be useful since this
> is a dimensional calculation to show us the dimensions of the box.
>
> Max
>
> On Thu, Mar 10, 2016 at 10:00 AM, John Naliboff <jbnaliboff at ucdavis.edu>
> wrote:
>
>> Hi Lev,
>>
>> Following from Julianne’s points below, it would be quite helpful if you
>> could make separate plots of hydrostatic vs dynamic pressure and provide a
>> bit more detail about the model (bc, material model, etc).
>>
>> Cheers,
>> John
>>
>> > On Mar 10, 2016, at 9:26 AM, Juliane Dannberg <dannberg at gfz-potsdam.de>
>> wrote:
>> >
>> > Hi Lev,
>> >
>> > I see your point that the pressure is positive at the top of the model
>> and then decreases with depth, which normally shouldn't be the case.
>> > But just from seeing the pictures it is difficult for us to find out
>> what the problem is.
>> >
>> > If your gravity is positive (which I assume it is), other reasons for
>> negative pressures I sometimes see in my models are prescribing velocities
>> at the boundaries. For example, if you prescribe convergent velocities at
>> the top boundaries, there is a point somewhere in the middle of the top of
>> the domain, where velocities point inwards from both sides, and so you get
>> a very high spike in dynamic pressure in this place. It looks like this
>> could be the case in your model. If you then normalize your pressure with
>> the values at the surface, they might become negative in a layer below.
>> >
>> > How does your pressure gradient look like? Is that basically density *
>> gravity once you are a few cells away from the top, or is it different? If
>> you find that the problem is only because of prescribed velocities at the
>> surface, you can just use a different value for the surface pressure, one
>> that you think is reasonable for your model.
>> >
>> > Another point to think about is the inflow: is the sum of your in- and
>> outflow zero?
>> >
>> > Best,
>> > Juliane
>> >
>> >
>> > On 03/10/2016 09:14 AM, Wolfgang Bangerth wrote:
>> >> On 03/09/2016 11:45 PM, Lev Karatun wrote:
>> >>>
>> >>> thank you for the quick reply. The pressure normalization was actually
>> >>> set to "no". I tried changing it to "surface", but it made made it so
>> >>> that the pressure across the entire model domain except for the very
>> lop
>> >>> layer became negative =(
>> >>
>> >> But the point remains true: the Stokes equations only determine the
>> pressure up to a constant. If you want to add 100 GPa to the pressure
>> everywhere, it will still solve the equations. In other words, whether the
>> pressure is negative or positive matters from a physical perspective, but
>> has no mathematical meaning in the context of the equations you are solving
>> because you can make the pressure positive everywhere or negative
>> everywhere by just adding a constant. Mathematically, what matters are only
>> pressure *differences*, not the overall pressure.
>> >>
>> >> Best
>> >> W.
>> >>
>> >
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>>
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>
>
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