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PFV compressibility in undrained triaxial test

Asked by Christian Jakob on 2014-08-25

Hi,

I want to get the pore pressure of a saturated soil sample in an undrained triaxial test when the sample volume is decreasing.

...
TriaxialStressController(thickness=0,stressMask=7,internalCompaction=False,label='triax')
...

triax.goal1 = 35000
triax.goal2 = 35000
triax.goal3 = 70000
triax.wall_back_activated = False
...
flow.viscosity = 1.3e-3
flow.fluidBulkModulus = 2e9
flow.bndCondValue=[0,0,0,0,0,0]
flow.bndCondIsPressure=[0,0,0,0,0,0]
...

For one of my simulations a decrease of ca. 1,8 % in porosity (=-dV/V) is obtained. When I calculate corresponding pressure increase I get

dp = K*(-dV/V) = 2e9*0.018 = 36000000 Pa

But the output from PFV shows maximum average pressures around of 16 Pa (not MPa!). As long as I use SI units the output of flow.averagePressure() should be Pa, right?

So is there a problem in the code or did I miss something in the compressibility option of PFV?

regards,

christian

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Language:: English Edit question

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For:: Yade Edit question

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Last query:: 2014-08-25

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Yade Guide (yade-guide) said on 2023-08-28:

Hey jakob-ifgt, check out some related threads. This is an automated reply.

Title: "PFV compressibility, not truly compressible?"
Question by rcaulk on 02 Jun 2017:
The user appreciates Yade's PFV implementation for solving poroelastic problems and acknowledges the developers. They question whether the method fully accounts for fluid compressibility due to assumptions made in Poiseuille's law and parallel plate approximation. The user suggests considering the coefficient of linearity as a function of absolute pressure to account for changes in viscosity and conductivity with pressure. Bruno agrees that this model is acceptable for most geomechanical fluids, but acknowledges that it would need a more sophisticated solver for non-constant viscosity problems. The user suggests discretizing the pore throat and linearizing the pressure gradient at each point along the discretization as an option to incorporate compressibility into the pore throat without going full 3D Navier-Stokes. Ah, I think I understand now. You are saying that the only approximation is the constant viscosity. I was just saying the change of viscosity of geofluids as a function of pressure is small enough in the liquid phase at a constant temperature that it is OK to neglect it. This is why I say we are assuming constant temperature with our constant viscosity assumption. But this is likely what you were getting at when you mentioned that this model is inappropriate for gases earlier. I also now realize that the change of fluid density at low mach numbers is negligible too, so it seems the scheme is actually accurately considering all possible compressibility given the aforementioned assumptions. I guess it would be futile to try and gain accuracy by considering these small changes to viscosity within the pore throats without considering full convective and conductive heat transfer in the system. summary
https://answers.launchpad.net/yade/+question/640093

Title: "Derived strain field in triaxial compression test"
Question by maquankun on 17 Oct 2018:
The user is experiencing difficulties with their script and seeks assistance from others. They are attempting to generate a strain field by using TW.setState(1) and tracking changes in the strain field over time under compression. The user has since resolved their issue, thanking Robert Caulk for his help.
https://answers.launchpad.net/yade/+question/675267

Title: "Triaxial compression test displacement field"
Question by maquankun on 29 Aug 2018:
Jan was having an issue with their program where it only generated a vtk file at the beginning and didn't continue generating files during the simulation. The user suggested using PyRunner in O.engines to call an export function, such as doExport(), or running the simulation 'in cycle' for better output. This solution resolved the problem.
https://answers.launchpad.net/yade/+question/673124

Hey jakob-ifgt, check out some related threads. This is an automated reply.

Title: "PFV compressibility, not truly compressible?"
Question by rcaulk on 02 Jun 2017:
 The user appreciates Yade's PFV implementation for solving poroelastic problems and acknowledges the developers. They question whether the method fully accounts for fluid compressibility due to assumptions made in Poiseuille's law and parallel plate approximation. The user suggests considering the coefficient of linearity as a function of absolute pressure to account for changes in viscosity and conductivity with pressure. Bruno agrees that this model is acceptable for most geomechanical fluids, but acknowledges that it would need a more sophisticated solver for non-constant viscosity problems. The user suggests discretizing the pore throat and linearizing the pressure gradient at each point along the discretization as an option to incorporate compressibility into the pore throat without going full 3D Navier-Stokes. Ah, I think I understand now. You are saying that the only approximation is the constant viscosity. I was just saying the change of viscosity of geofluids as a function of pressure is small enough in the liquid phase at a constant temperature that it is OK to neglect it. This is why I say we are assuming constant temperature with our constant viscosity assumption. But this is likely what you were getting at when you mentioned that this model is inappropriate for gases earlier. I also now realize that the change of fluid density at low mach numbers is negligible too, so it seems the scheme is actually accurately considering all possible compressibility given the aforementioned assumptions. I guess it would be futile to try and gain accuracy by considering these small changes to viscosity within the pore throats without considering full convective and conductive heat transfer in the system.    summary
https://answers.launchpad.net/yade/+question/640093

Title: "Derived strain field in triaxial compression test"
Question by maquankun on 17 Oct 2018:
 The user is experiencing difficulties with their script and seeks assistance from others. They are attempting to generate a strain field by using TW.setState(1) and tracking changes in the strain field over time under compression. The user has since resolved their issue, thanking Robert Caulk for his help.    
https://answers.launchpad.net/yade/+question/675267

Title: "Triaxial compression test displacement field"
Question by maquankun on 29 Aug 2018:
 Jan was having an issue with their program where it only generated a vtk file at the beginning and didn't continue generating files during the simulation. The user suggested using PyRunner in O.engines to call an export function, such as doExport(), or running the simulation 'in cycle' for better output. This solution resolved the problem.    
https://answers.launchpad.net/yade/+question/673124

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PFV compressibility in undrained triaxial test

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