# [aspect-devel] Negative pressure

Max Rudolph maxwellr at gmail.com
Thu Mar 10 12:34:10 PST 2016

```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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