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Poroelasticity module swaps stress directions around?!
Posted 12.07.2021, 08:03 GMT-4 Geomechanics, Porous Media Flow, Parameters, Variables, & Functions Version 5.5 2 Replies
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Hello :)
before starting a larger model with COMSOL Multiphysics I wanted to test all smaller parts to know exactly how they work. So i started with a very easy 2D-axissymmetric model of porous rock with a Darcy line source injecting water in the middle. The model is closed of on all sides allowing no water to escape, but was build so large that the water injection would take decades to reach the surfaces. The general idea is to replicate the research of J.Altmann (2010, dx.doi.org/10.1016/j.ijrmms.2010.08.001), who created the same model with another software, to see if COMSOL does what its supposed to do.
After creating the model with all the same parameters I now have the exact same results as J.Altmann, BUT the stresses are swapped. If I go in radial (r) direction from the source, my tangential stresses solid.sz = solid.sphi are larger than my solid.sr and if i go in z-Direction from the source, its the other way around. I also created the same model as a 3D Model, but then its the same thing, just with solid.sy and sz being larger than sx in x-direction and so on.
I already checked that r and z are assigned right and plotting them in the results works perfectly. So either I am missing some basic understandig in COMSOL or something ain't working right regarding the formulas. If anyone has any ideas, or similar experiences, please let me know :)
Best regards, Lukas
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Without having looked into your example or the reference: It is common in geosciences that stresses are defined so that a compressive stress has a positive sign. In COMSOL Multiphysics, stresses are always defined using the 'standard' convention that a positive stress is tensile. Can this be a source of the confusion?
-------------------Henrik Sönnerlind
COMSOL
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Thank you for your answer Mr. Sönnerlind, yes, the stress convention has lead to some confusion in the past, but this was not the case here.
We have figured out the "issue" by now, in that COMSOL presents results as effective stresses and not total stresses. So all we need to do is substract the pore pressure from our result to get our total stresses.