Defining Pore Fluid Flow as a Function of the Current Pore Pressure in Consolidation Analysis
In consolidation analysis you can provide seepage coefficients and sink pore
pressures on element faces or surfaces to control normal pore fluid flow from
the interior of the region modeled to the exterior of the region.
The surface condition assumes that the pore fluid flows in proportion to the
difference between the current pore pressure on the surface,
,
and some reference value of pore pressure, :
where
is the component of the pore fluid velocity in the direction of the outward
normal to the surface;
is the seepage coefficient;
is the current pore pressure at this point on the surface; and
is a reference pore pressure value.
Specifying Element-Based Pore Fluid Flow
To define element-based pore fluid flow, specify the element or element set
name; the distributed load type; the reference pore pressure,
; and
the reference seepage coefficient, .
The face of the elements upon which the normal flow is enforced is identified
by a seepage distributed load type. The seepage types available depend on the
element type (see
About the Element Library).
Specifying Surface-Based Pore Fluid Flow
To define surface-based pore fluid flow, specify a surface name, the seepage
flow type, the reference pore pressure, and the reference seepage coefficient.
The element-based surface (see
Element-Based Surface Definition)
contains the element and face information.
Defining Drainage-Only Flow
Drainage-only flow types can be specified for element-based or surface-based
pore fluid flow to indicate that normal pore fluid flow occurs only from the
interior to the exterior region of the model. The drainage-only flow surface
condition assumes that the pore fluid flows in proportion to the magnitude of
the current pore pressure on the surface, ,
when that pressure is positive:
where
is the component of the pore fluid velocity in the direction of the outward
normal to the surface;
is the seepage coefficient; and
is the current pore pressure at this point on the surface.
When surface pore pressures are negative, the constraint will properly
enforce the condition that no fluid can enter the interior region. When surface
pore pressures are positive, the constraint will permit fluid flow from the
interior to the exterior region of the model. When the seepage coefficient
value, ,
is large, this flow will approximately enforce the requirement that the pore
pressure should be zero on a freely draining surface. To achieve this
condition, it is necessary to choose the value of
to be much larger than a characteristic seepage coefficient for the material in
the underlying elements:
where
k
is the permeability of the underlying material;
is the fluid specific weight; and
c
is a characteristic length of the underlying elements.
Values of
will be adequate for most analyses. Larger values of
could result in poor conditioning of the model. In all cases the freely
draining flow type represents discontinuously nonlinear behavior, and its use
may require appropriate solution controls (see
Commonly Used Control Parameters).
Modifying or Removing Seepage Coefficients and Reference Pore Pressures
Seepage coefficients and reference pore pressures can be added, modified, or
removed as described in
About Loads.
Specifying a Time-Dependent Reference Pore Pressure
The magnitude of the reference pore pressure, , can
be controlled by referring to an amplitude curve. If different variations are
needed for different portions of the flow, repeat the flow definition with each
referring to its own amplitude curve. See
About Loads
and
Amplitude Curves
for details.
Defining Nonuniform Flow in a User Subroutine
To define nonuniform flow, the variation of the reference pore pressure and
the seepage coefficient as functions of position, time, pore pressure, etc. can
be defined in user subroutine
FLOW.
Prescribing Seepage Flow Velocity and Seepage Flow Directly in Consolidation Analysis
You can directly prescribe an outward normal flow velocity,
,
across a surface or an outward normal flow at a node in consolidation analysis.
Prescribing Element-Based Seepage Flow Velocity
To prescribe an element-based seepage flow velocity, specify the element or
element set name, the seepage type, and the outward normal flow velocity. The
face of the element for which the seepage flow is being defined is identified
by the seepage type. The seepage types available depend on the element type
(see
About the Element Library).
Prescribing Surface-Based Seepage Flow Velocity
To prescribe a surface-based seepage flow velocity, specify a surface name,
the seepage flow type, and the pore fluid velocity. The element-based surface
(see
Element-Based Surface Definition)
contains the element and face information.
Prescribing Node-Based Seepage Flow
To prescribe node-based seepage flow, specify the node or node set name and
the magnitude of the flow per unit time.
Prescribing Seepage Flow at Phantom Nodes for Enriched Elements
Alternatively, you can specify the seepage flow at a phantom node located at
an element edge between two specified real corner nodes directly or indicate
that the pore pressure applied to a phantom node located at an element edge is
interpolated from the specified real corner nodes.
Modifying or Removing Seepage Flow Velocities and Seepage Flow
Seepage flow velocities can be added, modified, or removed as described in
About Loads.
Specifying Time-Dependent Flow Velocity and Flow
The magnitude of the seepage velocity, ,
can be controlled by referring to an amplitude curve. To specify different
variations for different flows, repeat the seepage flow velocity or seepage
flow definition with each referring to its own amplitude curve. See
About Loads
and
Amplitude Curves
for details.
Defining Nonuniform Flow Velocities in a User Subroutine
To define nonuniform element-based or surface-based flow, the variation of
the seepage magnitude as a function of position, time, pore pressure, etc. can
be defined in user subroutine
DFLOW. If the optional seepage velocity,
,
is specified directly, this value is passed into user subroutine
DFLOW in the variable used to define the seepage magnitude.