You can use surface-to-surface contact elements and the node-to-surface contact element in combination with the coupled pore-pressure mechanical solid elements (CPT212, CPT213, CPT215, CPT216, CPT217) to model pore fluid flow normal to the contact surface. To activate both the structural and pore-pressure degrees of freedom, set KEYOPT(1) = 8. To activate the structural, pore-pressure, and temperature degrees of freedom, set KEYOPT(1) = 9 for coupled thermal-pore-fluid-flow-structural analyses.
The following pore fluid flow features are supported:
Note: The reported contact pressure and the friction computations do not include the pore fluid pressure contribution directly. The program assumes that the pore fluid flows only in the contact normal direction. There is no fluid flowing tangentially to the contact surface.
To take into account the pore fluid permeability across two surfaces that are in contact or separated by a small gap distance (near-field), you need to specify the pore-pressure permeability coefficient, which is input as real constant PCC of the contact element.
The pore fluid flux density from the contact surface to the target surface is defined as:
where
 = the pore fluid flux density per unit area |
 = the pore-pressure permeability coefficient, having
units of length5/(time * force) for
force-based node-to-surface contact, or units of
length3/(time * force) for the
traction-based model. |
 and  = pore pressures of the contact points on the target
and contact surfaces. |
A small value of 
yields a measured amount of imperfect contact and a
pore-pressure discontinuity across the interface. For large values of
, the resulting pore-pressure discontinuity tends to vanish and
perfect permeable contact is approached.
The pore-pressure permeability coefficient is input via the PCC real constant
on the contact element. By using tabular input (see Defining Real Constants in Tabular Format), you can define PCC as a function of
temperature (
), pressure (positive PRESSURE index values indicate
compression, negative PRESSURE index values indicate tension), time, and initial
contact detection point location (at the beginning of solution).
For example, you could use tabular input to specify PCC as a function of GAP such that different fluid permeability coefficients are applied based on the contact status, whether it is closed contact (positive GAP) or near-field contact (negative GAP). You could also specify a cutoff gap distance beyond which no fluid flow occurs (PCC = 0).
The USERCNPROP user subroutine is also available for defining PCC. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine.
When modeling pore fluid permeability, a pore pressure for both the contact and target surfaces is required.
For a deformable target surface, the pore pressure varies along the surface. In this case, the temperature at the intersection between the target surface and the normal from the contact detection point represents the target pore pressure.
For a rigid target, the pore pressure on the pilot node (if a pilot node is defined) represents the pore pressure for the entire rigid target surface.
To model pore fluid volume flow, which is associated with adding or removing fluid volume for near-field contact while the gap distance is changing, you would typically perform a coupled transient pore-fluid-flow analysis. (In a steady-state analysis, the rate of the opening gap distance is zero, so the contribution from this kind of gap-related fluid volume flow is zero.)
Two contact element real constants are required to model pore fluid volume flow:
FPFT is the participation factor of the gap-related pore fluid flow.
FPWT is the weight factor for the distribution of gap-related pore fluid flow between contact and target surfaces.
In a transient pore fluid flow analysis, the pore fluid volume flows into the contact surface due to the gap distance change:
and the pore fluid volume flows into the target surface:
where
 = the rate of contact gap distance |
 = the participation factor of the gap-related pore
fluid flow (input as a contact element real constant). The FPFT value is
in the range of 0-1 and defaults to 0 (no contribution from gap-related
fluid flow) |
 = the weight factor for the distribution of pore fluid
volume flow between contact and target surfaces (input as a contact
element real constant). FPWT defaults to 0.5 |
If you input a value of 0 for real constant FPWT, the program interprets that
as the default value. In order to have a true zero weight factor, enter a very
small, non-zero value, such as 1 x 10-8. In general,
you can specify the weight factor as follows: 
, where 
and 
are the material permeabilities of underlying elements from
the contact and target sides.
The FPFT real constant can be made a function of temperature (
), gap distance (negative GAP index values indicate an open
gap), time, and initial contact detection point location (at the beginning of
solution) by using tabular input (see Defining Real Constants in Tabular Format). For
example, you could specify a cutoff gap distance (via a function of GAP) beyond
which no gap-related fluid flow occurs (FPFT = 0).
The USERCNPROP user subroutine is also available for defining FPFT. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine.
To model pore fluid flow from far-field contact to the environment, you need to specify the following contact element real constants:
PSEE is the pore fluid seepage coefficient.
ABPP is the ambient pore pressure.
The pore fluid flow for open contact is defined as:
where
 = pore pressure on the contact surface |
There are three pore fluid flow conditions for open contact:
Impermeable (PSEE = 0)
Semipermeable (PSEE > 0)
Drainage-only flow
The drainage-only flow condition indicates that normal pore fluid flow occurs only from the far-field contact to the environment when the pore pressure at the contact surface is positive:
Use the real constant ABPP to define the ambient pore pressure, which has units of force/length2. The default value of ABPP is 0.
The pore seepage coefficient, PSEE, defaults to 0, which defines the impermeable condition for far-field contact. Specify a positive PSEE value to define the semi-permeable condition. PSEE has units of length5/(time * force) for the force-based node-to-surface contact model, or units of length3/(time * force) for the traction-based model.
As a special definition, you can specify ABPP to the value of -1.E20 to activate the drainage-only flow condition. In this case, the program sets ABPP = 0 internally. To achieve near-zero pore pressure at the contact surface (free-draining flow), you need to specify the value of PSEE to be much larger than:
Large values of PSEE can lead to poor conditions in the global stiffness
matrix and convergence difficulties. Choosing a value of 
would obtain near-zero pore pressure (free-draining) on the
far-field contact surface, in most cases.
The PSEE and ABPP real constants can be made a function of temperature
(
), time, and initial contact detection point location (at the
beginning of solution) by using tabular input (see Defining Real Constants in Tabular Format). For example, you might define the ambient pore
pressure, ABPP, to be a function of position. You can also define a non-uniform
seepage coefficient ABPP for consolidation analysis
The user subroutine USERCNPROP is also available for defining PSEE and ABPP. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine.
To model heat transfer across the contact interface in conjunction with the pore fluid flow, set KEYOPT(1) = 9. In this case, you can define the pore-pressure permeability coefficient (PCC), the pore fluid seepage coefficient (PSEE), and the ambient pore pressure (ABPP) to be a function of temperature. See Modeling Thermal Contact for details on defining other heat transfer-related properties.