# [aspect-devel] Negative pressure

Lev Karatun lev.karatun at gmail.com
Fri Mar 11 01:25:20 PST 2016

```Hi Juliane,

>>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.
yes, exactly

>>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
Do you mean prescribing velocity to the side walls (including the edges of
the top surface), or the top surface itself? For the model that I was
talking about, I only prescribed the velocities to the side and bottom
boundaries, top was free slip.

>>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?
It is, if I set pressure normalization to "surface". If it's set to "no", I
get a low-pressure anomaly relatively deep. I guess in the former case I
could use the "surface parameter", but it's still very strange to see
pressure at the surface being 20 orders of magnitude higher than presssure
30km deeper. I don't think it's correct.

>>Another point to think about is the inflow: is the sum of your in- and
outflow zero?
yes

Best regards,
Lev Karatun.

2016-03-10 12:26 GMT-05:00 Juliane Dannberg <dannberg at gfz-potsdam.de>:

> 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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