PorousFlow mechanics with an empty pipe in a porous medium

[marinsiebert]

Hello everyone,

I have a functioning input file for a TH-problem where a heat-source is placed in a porous medium. The heat-source is a fluid filled pipe, the thermal processes in the pipe are currently calculated by simple HeatConduction Kernels outside of the PorousFlow module.
How would I proceed to incorporate mechanics in order to investigate consolidation of the porous medium surrounding the pipe?

[josebastiase]

Well, you just need to couple the mechanical part to your model

For a THM problem look at modules/porous_flow/examples/tutorial/04.i and for the consolidation problem here modules/porous_flow/test/tests/poro_elasticity/terzaghi_constM.i

[josebastiase]

With TransientMultiApps maybe? (https://mooseframework.inl.gov/syntax/MultiApps/index.html). With two input files one for your porous medium and one for your pipe. You need to transfer data from one to the other. But this is not fully coupled.

[GiudGiud]

Iterative coupling with multiapp s is "tight coupling" and it converges to the same answer as full coupling (single matrix approach)

[marinsiebert]

I've got the mechanics running so far, however the results seem a little counterintuitive.
The calculated stresses are positive, per convention this means they are expansive rather than compressive. Correspondingly, the resulting displacements are positive as well.

test.txt

I guess this comes from the pore pressure being calculated by porepressure = airpressure (1 atm) - capillary pressure (here roughly 15kPa)
This leads to a positive porepressure larger than the calculated stresses and thus expansion.
However, porepressure is normally given by rho_w * g * z and with the density of water smaller than of that soil should yield negative stresses. Am I missing something here?

[marinsiebert]

Giving this a push, as I am somewhat on a schedule.
Looking at #23541 and the other threads cited by @WilkAndy a porepressure of 0 corresponds to atmospheric pressure and negative porepressures to an unsaturated medium - this does not seem to be the case here. I've created a simplified model of the problem described above using a single phase with porepressure as main variable and with this I am now able to achieve the desired results. This is fine for me.
However, the question still remains if this is actually achievable with the NCG model?

[GiudGiud]

@cpgr