FLAC3D Theory and Background • Fluid-Mechanical Interaction

# Calculation Modes for Fluid-Mechanical Interaction

The calculation mode and commands required for a pore-pressure analysis
depend on whether the grid has been configured for fluid flow
(i.e., whether the `model configure fluid-flow`

command has been specified).

The flow calculation can be performed independent of, or coupled to,
the mechanical deformation calculation when `model configure fluid-flow`

is specified.

It is also possible to conduct simplified, uncoupled, fluid-mechanical calculations with FLAC3D such as slope stability analyses, without configuring the grid for fluid flow. This option, when applicable, provides faster solution than the fluid flow configuration.

Grid Not Configured for Fluid Flow and Grid Configured for Fluid Flow reflect the two possibilities. For convenience, all commands described below are summarized in Input Instructions for Fluid-Flow Analysis, at the end of this section.

## Grid Not Configured for Fluid Flow

If the command `model configure fluid-flow`

has *not* been given,
then a fluid-flow analysis cannot be performed,
but it is still possible to assign pore pressure at gridpoints.
In this calculation mode, pore pressures do not change, but failure,
which is controlled by the effective-stress state,
may be induced when plastic constitutive models are used.

A pore-pressure distribution can be specified at gridpoints,
with the `zone gridpoint pore-pressure initialize`

command with a gradient,
with the `zone gridpoint pore-pressure plane`

command, or
with the `zone gridpoint pore-pressure geometry`

command.

If the `zone gridpoint pore-pressure plane`

command is used,
a hydrostatic pore-pressure distribution is calculated automatically by the code,
below the given water table level.
In this case, gravity (`model gravity`

) as well as the fluid density (either using the `fluid-density`

keyword or by having
previously specified a global `zone fluid-density`

).

In both cases, zone pore pressures are calculated by averaging from the gridpoint values,
and used to derive effective stresses for use in the constitutive models.
In this calculation mode, the fluid presence is not automatically accounted for in the calculation of body forces:
wet and dry medium densities must be assigned by the user, below and above the water level, accordingly.
The commands `zone gridpoint list fluid`

and `zone list pore-pressure`

print grid-point and zone pore pressures, respectively.

A simple example application of this calculation mode is given
in the topic The Effect of Water in the *Problem Solving* section.

## Grid Configured for Fluid Flow

If the command `model configure fluid-flow`

is given, a transient fluid-flow analysis can be performed,
and change in pore pressures,
as well as change in the phreatic surface, can occur.
Pore pressures are calculated at gridpoints, and zone values are derived using averaging.
Both effective-stress (static pore-pressure distribution)
and undrained calculations can be carried out in `model configure fluid-flow`

mode.
In addition, a fully coupled analysis can be performed,
in which changes in pore pressure generate deformation,
and volumetric strain causes the pore pressure to evolve.

If the grid is configured for fluid flow, *dry* densities must be assigned by the user (both below and above the water level),
because FLAC3D takes the fluid influence into account in the calculation of body forces.

A linear isotropic or anisotropic flow model is assigned to zones automatically.
An impermeable material can be assigned to zones with the `zone fluid cmodel assign inactive`

command.

Fluid properties are assigned to zones. Some of these properties area automatically averaged to zones in use.
Zone fluid properties include mobility-coefficient (isotropic or anisotropic), fluid modulus, porosity, Biot coefficient, Biot modulus, undrained thermal coefficient and fluid density.
Zone fluid properties are assigned with the `zone fluid property`

command.
(Note that fluid density can also be specified globally with the `zone fluid-density`

command.)

For isotropic flow, permeability is specified using the mobility-coefficient property keyword. For anisotropic flow, a full permeability tensor can be specifies using the mobility-coefficient-tensor property keyword.

If not specified, the Biot coefficient = 1 and porosity = 0.5, by default.

Note that fluid compressibility is defined in one of two ways in the `model configure fluid-flow`

mode:
1) Biot coefficient and Biot modulus are specified; or
2) fluid bulk modulus and porosity are specified.
The first case accounts for the compressibility of the solid grains
(Biot coefficient is set to 1 for incompressible grains).
In the second case, solid grains are assumed to be incompressible
(see Biot Coefficient and Biot Modulus).

The zone properties can be printed with the `zone fluid list property`

command,
The fluid-flow properties can be plotted as a contour plot.

An initial gridpoint pore-pressure distribution is assigned the same way
for the `model configure fluid-flow`

mode as for the non-`model configure fluid-flow`

mode
with one of the options in the `zone gridpoint pore-pressure`

command.
Fluid sources and sinks can be applied with the `zone apply well`

command.
See describes the various Fluid-Flow Boundary and Initial Conditions.

The fluid-flow solution can be controlled by the `model fluid active`

on and `model solve`

commands.
In practice it is often more straightforward to use the `model solve-fluid`

and `model solve-static`

commands to indicate if
only fluid or mechanical calculates are required.

The section in Solving Flow-Only and Coupled-Flow Problems describes the various coupling levels and the recommended solution procedure. Example applications, ranging from flow-only to coupled mechanical-flow calculations, are also described in this section.

Results from a fluid-flow analysis are provided in several forms.
The commands `zone gridpoint list fluid`

and `zone list pore-pressure`

print gridpoint and zone pore pressures, respectively.
Histories of gridpoint and zone pore pressures can be monitored
with the command `zone history pore-pressure`

.
And for a transient calculation, the pore pressure can be plotted versus real time
by monitoring the flow time with the `model history fluid time-total`

command.
General information on the `model configure fluid-flow`

calculation mode is printed with the `model fluid list`

and/or `zone fluid list`

commands.
Several fluid-flow variables can be accessed through FISH.
These are listed in Input Instructions for Fluid-Flow Analysis.
There is one grid-related variable, `gp.flow`

, that can only be accessed through a FISH function;
this corresponds to the net inflow or outflow at a gridpoint.
The summation of such flows along a boundary where the pore pressure is fixed is useful
because it can provide a value for the total outflow or inflow for a system.

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