Poroelasticity Calculations

NairnMPM can do poroelasticity calculations coupled with stresses and strains through tracking of pore pressure of saturated media. This feature is only available in OSParticulas.

Introduction

Poroelasticity is modeled by extension of method first described by Biot. In brief, each particle tracks a pore pressure that models pressure within a liquid for a material saturated in the material. As the material volume changes, the pore pressure increases or decreases. The pore pressure can transport between particles based on Darcy tensor for permittivity of the material for the given liquid. Some new poroelasticity material properties control coupling between stresses and strains and the pore pressure.

The extension from Biot are to model poroelasticity in anisotropic materials and to allow the pore pressure to drop below zero.

Activating Poroelasticity

In scripted files, diffusion is activated with the command

Poroelasticity (yesorno),<(p0)>,<(eta)>

In XML input files, diffusion is activated with the <Poroelasticity> command, which must be within the <<MPMHeader> element:

<Poroelasticity reference="(p0)" viscosity="(eta)">

where

• (YesOrNo) must be "Yes" or "No" to activate or not activate poroelasticity calculations. In XML input files, the presence of a <Poroelasticity> command activates poroelasticity methods. The default is "No".
• (p0)is reference pore pressure (in pressure units). The default (p0) is 0.
• (eta) is fluid viscosity for fluid in the pores (and same for all materials). Default is 1 cP in Legacy units or 1 viscosity unit in consistent units (note that Legacy units default to water viscosity, but consistent units default to 1 viscosity unit which may not be water viscosity - it is therefore best to always enter this viscosity and not rely on default setting being for water).

Note that poroelasticity models fluid transport through materials by transport methods nearly identical to those used to model diffusion. Because they share same methods, a simulation can activate poroelasticity (with above commands) or diffusion (with comparable Diffusion commands), but cannot activate them both. Any simulation, however, can combine poroelasticity or diffusion with thermal calculations and conduction.

Poroelasticity Material Properties

The constitutive law for an isotropic, poroelastic material is

      [math]\displaystyle{ \tens\sigma \tens C\tens \varepsilon - \alpha dp }[/math]

where [math]\displaystyle{ \tens C\lt math\gt is the compliance tensor, \lt math\gt \alpha\lt math\gt is the poroelasticity Biot coefficient (dimensionless from 0 to 1) and \lt math\gt dp }[/math] is increment of pore pressure in the current time step.

Concencentration changes are coupled to stress and strains through concentration expansion coefficients defined for the materials. By default, all moisture expansion coefficients are zero which decouples diffusion and strains. By entering non-zero values, the coupling will occur. Isotropic materials have a single solvent expansion coefficient while anisotropic will have two or three solvent expansion coefficients.

The rate of diffusion is controlled by the solvent diffusion constant defined for each material. Isotropic materials have a single solvent diffusion constant while anisotropic will have two or three solvent diffusion constants.

To be able to model diffusion in composite materials where different phases may absorb different amounts of solvent, all diffusion calculations are done in terms of a chemical potential for the solvent in the material, where chemical or concentration potential, μ, is approximate by

      [math]\displaystyle{ \mu= {c\over c_{sat}} }[/math]

where c is the weight fraction of solvent absorbed in the material and c_sat is the saturation solvent weight fraction for that material (which is specified in the material definition). This concentration potential varies from 0 to 1 and equilibrium conditions corresponds to all particles being at the same concentration potential.

Archived Concentrations

When calculated concentrations and concentration gradients are archived, they are converted to actual concentration in weight fraction using the material's saturation concentration setting:

      [math]\displaystyle{ c = c_{sat}\mu }[/math]

This conversion applies both to particle archives and to global archiving.

Poroelasticity Boundary Conditions

When diffusion is activated, you can set, the possible concentration boundary conditions are:

• You can set concentration on the grid.
• Rigid particles can provide moving concentration boundary conditions.
• You can set a concentration flux on particle surfaces.

Note that setting initial particle concentrations different than the reference concentration will cause strains to immediately evolve toward the changed state. The net effect will be an instantaneous "impact" of concentration change that might cause undesirable dynamic effects.