[CIG-LONG] Negative Pressures in Gale

[Walter Landry]

"Mark Fleharty" <mark.fleharty at gmail.com> wrote:
> Hello,
> 
> I have recently been working with Gale, and I'm having a difficult
> time understanding some of the output from the sample input files.
> After running:
> ./gale extension.xml
> 
> This produces an output.extension directory that has a lot of various
> data, 500 time steps in all.  Looking through the pressure*.vtk files
> I can verify that I have negative numbers, but I'm not sure what this
> means.  I also notice them when I view them with paraview.  I've
> attached an image file demonstrating the problem.

Negative pressure means that the element is undergoing an expansive,
rather than compressive stress.  It is expected for that setup. 
The hydrostatic pressure, especially near the top, is rather small.
Thus, any dynamic pressure easily overcomes the hydrostatic pressure.

> Also, does anyone have any tips on how to create St.Germain input XML
> files.  I've looked through the documentation as best I can, and I
> think I'm confused about some of the basics.  Is there any other
> documentation available on St. Germain outside of Gale that people
> could recommend?

There is the documentation for Underworld at 

  http://www.mcc.monash.edu.au/Software/UnderworldModels

Gale is mostly the same underlying software, although not entirely the
same.  So the models probably won't work out of the box, but they may
give you a better idea of how to create models.

Cheers,
Walter Landry
walter at geodynamics.org