Pore Fluid Contact Properties

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.

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 ( $q_{across}$ ) is associated with flow across the interface. A positive value of $q_{across}$ corresponds to flow out from the main surface and into the secondary surface.

• The other contribution ( $q_{gapS}$ for the secondary surface and $q_{gapM}$ 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 $q_{gapS}$ and $q_{gapM}$ are positive when these contributions flow into the respective surfaces (while the gap width decreases). The sum of $q_{gapS}$ and $q_{gapM}$ (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 $q_{gapS}$ and $q_{gapM}$ 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.

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 ($q_{across}$) and the contribution of fluid flow into the interface ($q_{gap}$).

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:

where $p_A$ and $p_B$ 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 ($q_{across}$, discussed above in Including Pore Fluid Properties in a Contact Property Definition) is proportional to the difference in pore pressure magnitudes across the interface:

When defining k directly, define it as

where

$p_{contact}$

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

${\overline{p}}_{pore}=\frac{1}{2}\left(p_{poreA}+p_{poreB}\right)$

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

$\overline{\theta }=\frac{1}{2}\left({\theta }_A+{\theta }_B\right)$

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

${\overline{f}}_{\gamma }=\frac{1}{2}\left(f_{\gamma }^A+f_{\gamma }^B\right)$

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.

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.

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.