Pore Fluid Contact Properties

The pore fluid contact property models:

  • are often used in geotechnical applications, where pore pressure continuity between material on opposite sides of an interface must be maintained;

  • govern pore fluid flow across a contact interface and into a gap region for nearby contact surfaces;

  • are applicable when pore pressure degrees of freedom are present on both sides of a contact interface (if pore pressure degrees of freedom are present on only one side of a contact interface, the surfaces are treated as impermeable);

  • affect the pore fluid flow normal to the contact surfaces;

  • can apply to small- and finite-sliding contact formulations; and

  • assume that there is no fluid flowing tangentially to the surface.

Contact in coupled pore fluid diffusion/stress analysis involves displacement constraints to resist penetrations and pore fluid contact properties that influence the fluid flow. See Coupled Pore Fluid Diffusion and Stress Analysis for details on coupled pore fluid diffusion/stress analyses. See Defining the Constitutive Response of Fluid within the Cohesive Element Gap for details on the use of pore pressure cohesive elements as an alternative to using contact models and pore fluid contact properties.

This page discusses:

Contact Pressure in Pore Fluid Interactions

The pore fluid contact properties discussed in this section apply when pore pressure degrees of freedom exist on both sides of a contact interface. In such cases the calculated contact pressure is effective; it does not include the pore fluid pressure contribution.

If only one side of a contact interface includes pore pressure degrees of freedom, no fluid flow into or across the contact interface occurs. In this case the reported contact pressure represents the total pressure, including the effective structural and pore fluid pressure contributions; but only the effective contact pressure is used for the computation of friction.

Including Pore Fluid Properties in a Contact Property Definition

Abaqus/Standard assumes that pore fluid flows in the normal direction at a contact interface and does not flow tangentially along the interface. Two contributions to the fluid flow into each surface at a contact interface are generally present, as shown in Figure 1. The fluid flow into the main and secondary surface at corresponding points on the interface are q S and q M , respectively.

  • One contribution ( q a c r o s s ) is associated with flow across the interface. A positive value of q a c r o s s corresponds to flow out from the main surface and into the secondary surface.

  • The other contribution ( q g a p S for the secondary surface and q g a p M for the main surface) is associated with removing or adding fluid from the region between the surfaces while the gap distance is changing. The sign convention is such that q g a p S and q g a p M are positive when these contributions flow into the respective surfaces (while the gap width decreases). The sum of q g a p S and q g a p M (which is the same as the sum of q S and q M ) is equal to negative one times the rate of change of the gap width up to the threshold distance discussed in Controlling the Distance within Which Pore Fluid Contact Properties Are Active.

In steady-state analyses the rate of separation of the surfaces is zero, so the fluid flow contributions qgapS and qgapM are zero; all fluid flowing out of one surface flows into the other in steady-state analyses.

Flow patterns in the interface contact element.

Pore fluid flow at a contact interface typically occurs even if contact permeability characteristics are not explicitly specified in the contact property definition. Alternatively, you can directly specify contact permeability characteristics for enhanced control over the flow of fluid across a contact interface and the type of constraint enforcement method.

Controlling the Distance within Which Pore Fluid Contact Properties Are Active

The models governing fluid flow across a contact interface are most appropriate for two surfaces in contact or separated by a relatively small gap distance. By default, Abaqus assumes no fluid flow occurs once the surfaces have separated by a distance larger than the characteristic element length of the underlying surfaces. Alternatively, you can directly specify a cutoff gap distance beyond which no fluid flow occurs. Separate controls are provided for the contribution of fluid flow across the interface (qacross) and the contribution of fluid flow into the interface (qgap).

Controlling Contact Permeability Associated with Fluid Flow across a Contact Interface

If you do not specify contact permeability characteristics, the implied physical model is continuity of the pore pressures on opposite sides of a contact interface (although the condition will be approximated if penalty enforcement is used—see Controlling the Constraint Enforcement Method) while the contact separation is less than the threshold distance discussed in Controlling the Distance within Which Pore Fluid Contact Properties Are Active:

pA-pB=0,

where pA and pB are pore pressures at points on opposite sides of the interface. This relationship implies that contact permeability across the interface is infinite.

Alternatively, you can specify a contact permeability, k, such that fluid flow across a contact interface (qacross, discussed above in Including Pore Fluid Properties in a Contact Property Definition) is proportional to the difference in pore pressure magnitudes across the interface:

qacross=k(pA-pB).

When defining k directly, define it as

k=k(pcontact,p¯pore,θ¯,f¯γ),

where

pcontact

is the contact pressure transmitted across the interface between A and B,

p¯pore=12(pporeA+pporeB)

is the average of the pore pressures at A and B,

θ¯=12(θA+θB)

is the average of the surface temperatures at A and B, and

f¯γ=12(fγA+fγB)

is the average of any predefined field variables at A and B.

Figure 2 shows an example of k depending on the contact pressure. Use tabular data to specify the value of k at one or more contact pressures as p increases. The value of k remains constant for contact pressures outside of the interval defined by the data points. Once the surfaces have separated, k remains at a constant value until the separation between the surfaces exceeds the specified flow cutoff distance (see Controlling the Distance within Which Pore Fluid Contact Properties Are Active), at which point k drops to zero.

Contact-pressure-dependent contact permeability.

Defining Gap Permeability to Be a Function of Predefined Field Variables

In addition to the dependencies mentioned previously, the gap permeability can be dependent on any number of predefined field variables, f¯γ. To make the gap permeability depend on field variables, at least two data points are required for each field variable value.

Controlling the Constraint Enforcement Method

The default enforcement method depends on whether the contact permeability is specified. If contact permeability characteristics are not explicitly specified, the continuity of pore pressure across the interface is approximated with a penalty method (large permeability) for general contact and directly enforced for contact pairs. You can optionally specify the penalty method for contact pairs.

If contact permeability is specified, fluid flow consistent with the specific permeability is directly enforced for both contact pairs and general contact. If contact permeability is specified, the penalty method is not applicable and not allowed.

Coupled Heat Transfer–Pore Fluid Contact Properties

Heat transfer can be considered simultaneously with pore fluid flow, in which case heat flow across the contact interface can occur in conjunction with fluid flow. These various contact property aspects are defined with separate options as part of a single contact property definition that you assign to the contact interaction; see Thermal Contact Properties for details on defining heat transfer properties.

Output

You can write the contact surface variables associated with the interaction of contact pairs to the Abaqus/Standard data (.dat), results (.fil), and output database (.odb) files. In addition to the surface variables associated with the mechanical contact analysis (shear stresses, contact pressures, etc.) several pore fluid-related variables (such as pore fluid volume flux per unit area) on the contact interface can be reported. A detailed discussion of these output requests can be found in Surface Output from Abaqus/Standard and Writing Surface Output to the Output Database.

Abaqus/Standard provides the following output variables related to the pore fluid interaction of surfaces:

PFL

Pore volume flux per unit area leaving the secondary surface.

PFLA

PFL multiplied by the area associated with the secondary node.

PTL

Time integrated PFL.

PTLA

Time integrated PFLA.

TPFL

Total pore volume flux leaving the secondary surface.

TPTL

Time integrated TPFL.