[CIG-LONG] Gale stress tensor verse pressure output in .dat and vtk files

[John Naliboff]

Hi Walter,

I have a number of questions regarding the Gale stress tensor and pressure output values in the .dat and vtk files.

Specifically, I've been doing tests with side stress boundary conditions and in doing so I noticed that there seems to be
a discrepancy between the stress tensor values and the magnitude/value of the pressure.

My interpretation of the pressure is (sigma_xx + sigma_yy)/2 in 2D or (sigma_xx + sigma_yy + sigma_zz)/3 in 3D.

However, I've found that when I calculate the pressure from the stress tensor values it does not match the pressure value
output in the vtk or .dat files.

As an example, I attached the output from running the cookbook model multi_material.xml using version 1.4.1.  The .dat and vtk files for time step 9 are attached.  

Based on the tensor output format described in the manual,   sigma_xx and sigma_yy from the first line (x=0, y=0) in StressField.00009.dat are, respectively, 1.04375459900302 -1.05894084102394.  The predicted pressure would then be ~ -0.007593.  The pressure value (again, first line, x=0 and y=0 node) in PressureField.00009.dat is -0.715401855544872.

In the VTK file (fields.0.00009.vts) sigma_xx and sigma_yy at x=0 and y=0 (first nodal point value listed for each variable) are, respectively, 1.04399321765544 and -1.04778331139934.  The predicted pressure ([sigma_xx+sigma_yy]/2 is -0.001895 while the listed pressure field value is  -0.714266837882578.

I am completely misinterpreting what the pressure output represents or is there something else I'm not taking into account in terms of the stress tensor/pressure representation or output format?  Is the pressure derived from the principal stresses (although this really shouldn't make a difference as the shear stresses are small in this case)?

I looked at the VTK output source code (VTKOutput.c) and the pressure is calculated (lines 652-685) from a different variable (stressValues[]) than the variable used to output (lines 286-295 in VTKOutput.c) the stress tensor (stress[]).  I'm not sure what the difference is, if any, between these two variables.

I have some additional questions, but it will probably be easiest just to start with what I have so far, as the answers to these questions may clear up everything else.

Thanks in advance for the input and apologies in advance if this is just something very basic I've overlooked.

Cheers,
John

John Naliboff
Postdoctoral Scholar, Department of Geology
One Shields Avenue, University of California, Davis, CA 95616





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