[aspect-devel] Negative pressure

Tue Mar 15 10:42:45 PDT 2016

Hi Lev,

I had a look at your input file, but couldn't reproduce the problem you 
saw because you used your own material model and your own velocity 
boundary plugin.
The only thing that I saw that might cause problems is the non-linear 
solver scheme: It looks like your material model is non-linear, and if 
that's the case, you should use the "iterated IMPES" scheme (the "IMPES" 
scheme will only do one non-linear iteration, no matter what you set the 
"Max nonlinear iterations" to).

If you want to look at the dynamic pressure, there is the "nonadiabatic 
pressure" option in Postprocess/Visualization, which subtracts the 
adiabatic pressure from the full pressure and then visualizes the result.

Best,
Juliane

On 03/11/2016 03:28 AM, Lev Karatun wrote:
> 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 
> <mailto: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 <mailto: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 <mailto: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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