# [aspect-devel] Negative pressure

Juliane Dannberg dannberg at gfz-potsdam.de
Thu Mar 10 09:26:43 PST 2016

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.
>