# zone fluid command

Syntax

zone fluid keyword

Primary keywords:

Set parameters for a groundwater fluid flow analysis.

These commands are only available after a model configure fluid command. These parameters apply to zones only, other model elements may have different values.

active b

Set the fluid process on or off. The process is on for zones, by default, when the model configure fluid command is given. Specify off to suppress the fluid calculation for zones.

biot b

Set use of the Biot coefficient. If on, the fluid-flow calculation uses the Biot coefficient, $$α$$, and the Biot modulus, $$M$$. If off, then the fluid modulus, $$K_f$$ , and porosity, $$n$$, are used and $$α$$ = 1. The default is off. See Biot Coefficient and Biot Modulus.

cmodel keyword

Associate a fluid constitutive model with specified zones in the grid. The command also allows querying of properties and states of constitutive models.

assign keyword <range>

Assign a fluid model to all zones in the range. This keyword may only be given if the model is configured for fluid calculations (i.e., the command model configure fluid has been issued). Fluid models are described in Grid Configured for Fluid Flow.

anisotropic

Assign the anisotropic flow model.

isotropic

Assign the isotropic flow model.

null

Assign the null fluid model.

list

List the available fluid constitutive models and their properties.

fastflow b

Turn the fastflow solution scheme on/off. See Fully Saturated Fast Flow.

fastflow-relaxation f

Set the relaxation parameters used in the fastflow solution scheme.

implicit b <keyword>

Turn implicit mode on/off. By default implicit mode is off. Implicit mode requires a manually specified timestep. If no keyword is specified, the implicit logic uses iterative Jacobi solver. If Jacobi solver fails to converge to the solution, the logic switches to preconditioned conjugate gradient solver (see below). The following keywords apply:

solver-jacobi

Use the iterative Jacobi solver based on the local matrix formulation (see Implicit Formulation). Note that specifying a timestep that is too large can cause the Jacobi solver to fail to converge. If this keyword is specified and the solver diverges, the system will exit with an error message. The benefit of using the Jacobi solver compared to other solvers is that it is usually faster for small implicit timesteps (<< the divergence timestep).

solver-direct <keyword>

Use the direct solver similar to matrix inversion (see Advanced implicit solvers). This highly efficient solver is able to handle small to medium size models (< 50-100K zones) and it is unconditionally stable for any timestep. Before this solver is called, the global right-hand-side (RHS) vector and global coefficient matrix (M) are formed for the whole model to incorporate contributions from all relevant gridpoints. Then the solver solves a system of linear equations for unknown global vector of pore pressure changes. The following keyword is available:

preserve

Specify to preserve the global pressure coefficients matrix and only update its diagonal containing timestep-dependent terms. In general, the global matrix M must be re-created each time the implicit solver is called as model parameters, such as geometry, physical properties, implicit timestep, etc., may change. However, if the model parameters do not change during the simulation but user wishes to specify a different implicit timestep (e.g., to gradually increase it every certain number of steps), only the diagonal terms of the global matrix must be updated, which is very efficient. To stop this procedure and form a new global pressure coefficients matrix, the command zone thermal implicit must be issued (this resets the implicit solvers settings).

solver-pcg <keyword>

Use the preconditioned conjugate gradient iterative solver (see Advanced implicit solvers). This highly efficient solver is able to handle very large models and it is unconditionally stable for any timestep due to its special technique of matching desired solution accuracy to the number of iterations required to reach this accuracy. Before the solver is called, the global right-hand-side (RHS) vector and global pressure coefficients matrix (M) are created for the whole model to incorporate contributions from all relevant gridpoints. Then the solver analyses matrix properties (eigenvalues, condition number) and determines the required number of iterations, after which the iterative solution procedure starts. The following keywords are available.

tolerance

Specify an error tolerance (solution accuracy) that will determine when to stop iterations. The default tolerance value is 1.e-6.

iterations-limit

Specify the limit on the number of iterations. By default, the number of iterations is determined based on the analysis of the global matrix properties, such as minimum/maximum eigenvalues and the the condition number. This analysis is usually done rather fast, and it assures that the system can be solved to reach desired solution accuracy. Specifying a limit on the number of iteration skips the global matrix analysis, which may decrease overall solution time, but the risk of this is that the number of iterations specified may be insufficient to reach desired accuracy. The pcg solver has a general limit of 1000 on the number of iterations.

preserve

Specify to preserve the global pressure coefficients matrix and only update its diagonal containing timestep-dependent terms. For detailed description see the equivalent usage above with the solver-direct keyword.

list
information

Output current configuration settings and status information for the fluid model.

property keyword <range>

Output the values of the specified fluid property in all zones in the range.

biot f

Biot coefficient (grain compressibility), $$\alpha$$. The default is 1.0.

hydraulic-conductivity f

List isotropic hydraulic conductivity, $$k$$. This is the conventional hydraulic conductivity in units of [Length/Time]. Non-zero gravity and water density should be set in advance. Hydraulic conductivity must be non-zero if the command zone fluid active on applies. Only one between hydraulic conductivity and permeability is needed.

porosity f

List porosity, $$n$$. The default is 0.5.

undrained-thermal-coefficient f

List undrained thermal coefficient, $$\beta$$.

permeability f

List isotropic permeability, $$k$$. The permeability used in FLAC is defined as the conventional hydraulic conductivity divided by the unit weight of water. Permeability must be non-zero if the command zone fluid active on applies. Only one between hydraulic conductivity and permeability is needed.

dip f (3D Only)

List the principal permeability plane dip angle (in degrees).

dip-direction f (3D Only)

List the principal permeability plane dip direction angle (in degrees).

permeability-1 f

List the principal permeability value.

permeability-2 f

List the principal permeability value.

permeability-3 f (3D Only)

List the principal permeability value.

permeability-xx f

List the xx-component of permeability.

permeability-xy f

List the xy-component of permeability.

permeability-xz f (3D Only)

List the xz-component of permeability.

permeability-yy f

List the yy-component of permeability.

permeability-yz f (3D Only)

List the yz-component of permeability.

permeability-zz f (3D Only)

List the zz-component of permeability.

rotation f

List the principal permeability plane rotation angle (in degrees).