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{{MODFLOW Links}}
{{MODFLOW Links}}
The GMG package is one of four [[GMS:MODFLOW Packages Supported in GMS#Solver Packages|solver packages]] for MODFLOW supported by GMS. The GMG solver is an iterative solver based on a multi-grid approach.
The GMG package is one of the [[GMS:MODFLOW Packages Supported in GMS#Solver Packages|solver packages]] for MODFLOW supported by GMS. The GMG solver is an iterative solver based on a multi-grid approach.
 
Parameters include:
* ''Inner convergence residual (RCLOSE)'' – The algorithm computes the 12-norm of the residual and compares it against RCLOSE.
* ''Maximum inner iterations (IITER)'' – For each linear solution. The default is 100. Specify a smaller number for nonlinear problems to prevent an excessive number of inner iterations.
* ''Outer convergence residual (HCLOSE)'' – For nonlinear problems. After each linear solve (inner iteration), the maximum norm of the head change is compared against HCLOSE. Can be set to a large number for linear problems. Ignored if ''Maximum outer iterations'' is 1.
* ''Maximum outer iterations (MAXITER)'' – Also referred to as MXITER. For linear problems, set to 1. For nonlinear problems, set to a higher number though it is usually unnecessary to go above 100.
* ''Damping parameter (DAMP)'' – A value of 1.0 should be used for linear problems. For nonlinear problems, a value less than 1.0 but greater than 0.0 may be necessary to achieve convergence.
* ''Damping option (IADAMP)'' – Flag that controls adaptive damping. Contains the following options:
<blockquote style="margin-top:0px; margin-bottom:0px;">
* "(0) Constant"
* "(1) Cooley adaptive"
* "(2) RRR adaptive"
</blockquote>
* ''Maximum damping (DUP)''
* ''Minimum damping (DLOW)''
* ''Maximum head change (CHGLIMIT)''
* ''Output flag (IOUTGMG)'' &ndash; Flag that controls the output of the GMG solver. Values can be 0&ndash;4.
<blockquote style="margin-top:0px; margin-bottom:0px;">
* 0 = Only the solver inputs are printed.
* 1 = For each linear solve, prints the number of PCG iterations, the value of the damping parameter, the 12-norm of the residual, and the maximum norm of the head change and its location (column, row, layer). At the end of a time/stress period, the total number of GMG calls, PCG iterations, and a running total of PCG iterations for all time/stress periods are printed.
* 2 = The convergence history of the PCG iteration is printed, showing the 12-norm of the residual and the convergence factor for each iteration.
* 3 = Same as 1 except the output is sent to the terminal instead of the output file.
* 4 = Same as 2 except the output is sent to the terminal instead of the output file.
</blockquote>
* ''Maximum head output (IUNITMHC)''
* ''Multi-grid preconditioner smoothing (ISM)'' &ndash; Flag that controls the type of smoother used in the multi-grid preconditioner.
<blockquote style="margin-top:0px; margin-bottom:0px;">
* "(0) ILU smoothing" &ndash; This smoothing requires an additional vector on each multigrid level to store the pivots in the ILU factorization.
* "(1) SGS smoothing" &ndash; Symmetric GaussSeidel (SGS) smoothing.
</blockquote>
* ''Multi-grid preconditioner coarsening (ISC)'' &ndash; Flag that controls semi-coarsening in the multi-grid preconditioner.
<blockquote style="margin-top:0px; margin-bottom:0px;">
* "(0) Row, Col., Layer" &ndash; Coarsens all the rows, columns and layers.
* "(1) Row, Col." &ndash; Coarsens all the rows and columns.
* "(2) Col., Layer" &ndash; Coarsens all columns and layers.
* "(3) Row, Layer" &ndash; Coarsens all rows and layers.
* "(4) None" &ndash; No coarsening.
</blockquote>
* ''Relocation parameter (RELAX)'' &ndash; Used to improve the spectral condition number of the ILU preconditioned system.


For more info on the GMG solver look at the following document:  
For more info on the GMG solver look at the following document:  


MODFLOW-2000, THE U.S. GEOLOGICAL SURVEY MODULAR GROUND-WATER MODEL -- GMG LINEAR EQUATION SOLVER PACKAGE DOCUMENTATION
:Wilson, John D., and Richard L. Naff. ''MODFLOW-2000, The US Geological Survey Modular Ground-Water Model&mdash;GMG Linear Equation Solver Package Documentation.'' No. 2004-1261. 2004. [http://pubs.usgs.gov/of/2004/1261/]
 
By: J. D. Wilson, NRC Post-Doc, and R. L. Naff, U.S. Geological Survey [http://pubs.usgs.gov/of/2004/1261/]


[[File:MODFLOW-GMG.jpg|thumb|none|center|300 px|The MODFLOW ''GMG Solver'' package dialog.]]
[[File:MODFLOW-GMG.jpg|thumb|none|center|300 px|The MODFLOW ''GMG Solver'' package dialog.]]
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[[Category:MODFLOW Dialogs]]
[[Category:MODFLOW Dialogs]]
[[Category:External Links]]
[[Category:External Links]]
{{stub}}
<!-- Needs more information. This article is giving the reader the run around. -->

Latest revision as of 19:38, 6 March 2023

MODFLOW
Pre-processing
MODFLOW Commands
Building a MODFLOW Model
Map to MODFLOW
Calibration
Packages Supported in GMS
Saving a MODFLOW Simulation
Importing MODFLOW Files
Unsupported MODFLOW Features
Run MODFLOW
Post-processing
MODFLOW Display Options
MODFLOW Post-Processing Viewing Options
Reading a MODFLOW Simulation
Tutorials
Packages
Flow: BCF6, HUF, LPF, UPW
Solvers:

DE4, GMG, NWT, PCG,

PCGN, LMG, SIP, SOR,

SMS
Other:

BAS6, BFH, CHD1, CLN,

DRN1, DRT1, EVT1, ETS1,

GAGE, GHB1, GNC, HFB1,

HUF, LAK3, MNW1, MNW2,

OUT1, RCH1, RIV1, SFR2,

STR1, SUB1, SWI2, WEL1,

UZF1
v • e

The GMG package is one of the solver packages for MODFLOW supported by GMS. The GMG solver is an iterative solver based on a multi-grid approach.

Parameters include:

  • Inner convergence residual (RCLOSE) – The algorithm computes the 12-norm of the residual and compares it against RCLOSE.
  • Maximum inner iterations (IITER) – For each linear solution. The default is 100. Specify a smaller number for nonlinear problems to prevent an excessive number of inner iterations.
  • Outer convergence residual (HCLOSE) – For nonlinear problems. After each linear solve (inner iteration), the maximum norm of the head change is compared against HCLOSE. Can be set to a large number for linear problems. Ignored if Maximum outer iterations is 1.
  • Maximum outer iterations (MAXITER) – Also referred to as MXITER. For linear problems, set to 1. For nonlinear problems, set to a higher number though it is usually unnecessary to go above 100.
  • Damping parameter (DAMP) – A value of 1.0 should be used for linear problems. For nonlinear problems, a value less than 1.0 but greater than 0.0 may be necessary to achieve convergence.
  • Damping option (IADAMP) – Flag that controls adaptive damping. Contains the following options:
  • "(0) Constant"
  • "(1) Cooley adaptive"
  • "(2) RRR adaptive"
  • Maximum damping (DUP)
  • Minimum damping (DLOW)
  • Maximum head change (CHGLIMIT)
  • Output flag (IOUTGMG) – Flag that controls the output of the GMG solver. Values can be 0–4.
  • 0 = Only the solver inputs are printed.
  • 1 = For each linear solve, prints the number of PCG iterations, the value of the damping parameter, the 12-norm of the residual, and the maximum norm of the head change and its location (column, row, layer). At the end of a time/stress period, the total number of GMG calls, PCG iterations, and a running total of PCG iterations for all time/stress periods are printed.
  • 2 = The convergence history of the PCG iteration is printed, showing the 12-norm of the residual and the convergence factor for each iteration.
  • 3 = Same as 1 except the output is sent to the terminal instead of the output file.
  • 4 = Same as 2 except the output is sent to the terminal instead of the output file.
  • Maximum head output (IUNITMHC)
  • Multi-grid preconditioner smoothing (ISM) – Flag that controls the type of smoother used in the multi-grid preconditioner.
  • "(0) ILU smoothing" – This smoothing requires an additional vector on each multigrid level to store the pivots in the ILU factorization.
  • "(1) SGS smoothing" – Symmetric GaussSeidel (SGS) smoothing.
  • Multi-grid preconditioner coarsening (ISC) – Flag that controls semi-coarsening in the multi-grid preconditioner.
  • "(0) Row, Col., Layer" – Coarsens all the rows, columns and layers.
  • "(1) Row, Col." – Coarsens all the rows and columns.
  • "(2) Col., Layer" – Coarsens all columns and layers.
  • "(3) Row, Layer" – Coarsens all rows and layers.
  • "(4) None" – No coarsening.
  • Relocation parameter (RELAX) – Used to improve the spectral condition number of the ILU preconditioned system.

For more info on the GMG solver look at the following document:

Wilson, John D., and Richard L. Naff. MODFLOW-2000, The US Geological Survey Modular Ground-Water Model—GMG Linear Equation Solver Package Documentation. No. 2004-1261. 2004. [1]
The MODFLOW GMG Solver package dialog.


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