tutorial 9 - changing the global grid resolution · section 1 – changing the global grid...
TRANSCRIPT
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Tutorial 9
Changing the Global Grid Resolution
Table of Contents
Objective and Overview ………………………………………………………………… 1
Step-by-Step Procedure ……………………………………………………………….... 2
Section 1 – Changing the Global Grid Resolution ……………………………………….. 2
Step 1: Open Adaptive Groundwater Input (.agw) File …………………………………. 2
Step 2: Discussion ……………………………………………………………………….. 5
Section 2 – Example Simulation Results for Runs 1-4 …………………………………… 5
Objective and Overview
Demonstrate how easily the global grid resolution can be varied without changing the boundary
condition and other input data specifications. Four completed Adaptive Groundwater input files
(AGW Projects) for this tutorial, representing four different AMR grids, are included in the
Tutorial_9 subdirectory of the tutorials directory under the Adaptive_Groundwater program
folder:
Run 1: C:\Adaptive_Groundwater\Tutorials\Tutorial_9\Tutorial_9_NLEV5_IREF2.agw
Run 2: C:\Adaptive_Groundwater\Tutorials\Tutorial_9\Tutorial_9_NLEV4_IREF2.agw
Run 3: C:\Adaptive_Groundwater\Tutorials\Tutorial_9\Tutorial_9_NLEV3_IREF2.agw
Run 4: C:\Adaptive_Groundwater\Tutorials\Tutorial_9\Tutorial_9_NLEV3_IREF4.agw
where NLEV is the number of Adaptive Mesh Refinement (AMR) levels and IREF is the grid
refinement factor, IREFINE (either 2 or 4; see below). The Run 1 input data are the same as
Tutorial 6 except for two differences: (i) the riverbed permeability was reduced by a factor of
three, so that the plume is drawn into the extraction well; and (ii) the starting location for the
Gaussian plume is closer to the river (to reduce run times).
For Runs 2 and 3 the number of AMR levels is reduced to four and three, respectively, while
maintaining IREFINE = 2. As a result, the cell size on the highest level of refinement in Runs 2
and 3 is a factor of two and four larger (i.e., coarser grid resolution) compared to Run 1. In
addition, the total number of cells on the highest level of refinement decreases by factors of 8
(23) and 64 (4
3) in Runs 2 and 3, respectively (for the same domain volume).
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The grid for Run 4 increases the grid refinement by a factor of four (IREFINE = 4) in each
higher AMR level. Run 4 uses three AMR levels, and the cell size on Level 3 is the same as Run
1 (a factor of 16 smaller than the Base Grid).
This tutorial is divided into two sections. The first part covers changing the grid resolution by
varying the number of grid refinement levels and the grid refinement factor. Section 2 compares
the simulated plumes and numerical meshes for the different degrees of spatial resolution. The
effects of grid resolution on maximum plume concentration and computational (i.e., cpu) time
are also illustrated.
Step-by-Step Procedure
Section 1 – Changing the Global Grid Resolution
Step 1 - Open Adaptive Groundwater Input (.agw) File
Go to File > Open in the main menu to open the file Tutorial_9_NLEV5_IREF2.agw (Run 1)
that is stored in the following subdirectory:
C:\Adaptive_Groundwater\Tutorials\Tutorial_9
In the main menu select Simulation > Simulation Control Parameters and click on the “AMR”
tab in the Simulation Control Parameters dialog (Figure 1). The parameter values are the same
as those used in Tutorial 6. Figures 2-4 are the Simulation Control Parameters dialogs from
Runs 2-4, respectively.
In addition to the changes to the NLEVEL and IREFINE parameter values, small reductions of
the minimum horizontal (Nsubcell_hor) and vertical (Nsubcell_vert) subgrid dimensions were
made in Runs 2-4 so that the refined parts of the grid more closely bounded the river, extraction
well, and plume. Nsubcell_hor = 14, 10, 8, 8 in Runs 1-4, respectively. Similarly, Nsubcell_vert
= 10, 6, 6, 6 in Runs 1-4. Although these reductions in minimum subgrid size were not required,
they contributed to more efficient grid refinement (i.e., reduced number of grid cells) around the
river, extraction well, and plume. Generally, the discretization (i.e., cell delineation) in regions
of high hydraulic and/or concentration gradient(s) is more efficient as the number of AMR levels
increases.
We recommend that you read the “Help” discussion for this dialog for more information on this
subject.
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Figure 1
Figure 2
4
Figure 3
Figure 4
5
Step 2 - Discussion
The simple input changes outlined in Step 1 are the only ones that are necessary to change the
entire grid structure. However, whenever you increase NLEVEL you should re-define any
hydraulic head or solute concentration B.C.’s that were specified using the Aquifer Boundaries
option (e.g., Step 7 in Tutorial 1).
An example of this situation would be if you had set up the upgradient and downgradient
hydraulic head boundary conditions in Tutorial 1 with NLEVEL = 3 using the Aqufier
Boundaries option. Then, if you decided to run a simulation with a more refined grid (e.g.,
NLEVEL = 4 or 5) the width of the B.C. zone at the aquifer boundaries would incorporate more
than one column of cells on the highest level of refinement. However, only the boundary cells
should be specified as “constant head” in this case.
Section 2 – Example Simulation Results for Runs 1-4
In this section we present example results from Runs 1-4, which illustrate some of the effects of
grid resolution changes. A graph presented at the end of this section compares the maximum
plume concentrations as a function of the grid resolution. Since Tutorials 1-8 have shown how
to generate these plots, this section just presents the results.
Figures 5 a-d are flood contour plots of the simulated plume and groundwater pathline for Run 1.
Figures 6 a-c are concentration versus time plots for the seven monitoring points in Run 1.
Figure 6a shows the Monitoring Point dialog that is loaded when you click on the monitoring
locations while viewing output. Figure 6b is a screen image of the Graphics Printing or
Hardcopy Export child window that is loaded when you click on the “Print/Export” button in
Figure 6a. To generate the Windows metafile image for Figure 6c, (i) select Output Type in the
menu for the Graphics Printing child window; (ii) selected the “Enhanced Windows Metafile”
radio button in the Print/Export Options dialog (Figure 6b); and select File > Generate Output
in the menu for the Graphics Printing child window.
You can see the attenuation of the maximum plume concentration with travel distance and time
in Figure 6. Note that monitoring location OW-7 is located in a Level 5 cell that contains part of
the extraction well screen. As discussed in the User’s Manual, flux-averaged concentrations are
computed for extraction cells. Therefore, the OW-7 concentrations are affected by dilution from
all quadrants of the extraction well capture zone.
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Figure 5a
NLEVEL = 5, IREFINE = 2
x-z Plume Cross-Section at y = 848 m, t = 5,000 days
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Figure 5b
NLEVEL = 5, IREFINE = 2
x-z Plume Cross-Section at y = 989 m, t = 17,000 days
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Figure 5c
NLEVEL = 5, IREFINE = 2
x-z Plume Cross-Section at y = 942 m, t = 17,000 days
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Figure 5d
NLEVEL = 5, IREFINE = 2
x-y Plume Slice at z = 33 m, t = 17,000 days
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(a)
(b)
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0 10 20 30 40 50 60 700
.10
.20
.30
.40
.50
.60
.70
.80
Time (years)
C (
mg/L
)
Monitoring Point Concentration vs. Time
Figures 6 a-c
NLEVEL = 5, IREFINE = 2
Concentration vs. Time at Monitoring Points
Figures 7 a-c are flood contour plots of the simulated plume and groundwater pathline for Run 2
(NLEVEL = 4, IREFINE = 2). As discussed above, the cell size on the highest level of
refinement is a factor of two larger than the most refined cells in Run 1.
Figure 8 compares the maximum plume concentrations, CMAX, for Runs 1 and 2 and the
computational (cpu) time required to reach t = 46 years in each simulation. Coarsening the grid
by a factor of two (Run 2) results in an approximate 20 percent reduction in CMAX for t = 30-50
years, but much less during earlier and later parts of the simulation (e.g., ~ 5-7 percent reduction
for t > 50 years). However, the cpu time for Run 2 is about a factor of four smaller compared to
Run 1.
OW-1
OW-2
OW-3
OW-4
OW-5
OW-6 OW-7
(c)
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Note: The maximum plume concentration for any output time is shown under the “Contour
Options” tab in the Contour Parameters and Overlays dialog (Figure 9).
Figure 7a
NLEVEL = 4, IREFINE = 2
x-z Plume Cross-Section at y = 853 m, t = 5,000 days
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Figure 7b
NLEVEL = 4, IREFINE = 2
x-z Plume Cross-Section at y = 946 m, t = 17,000 days
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Figure 7c
NLEVEL = 4, IREFINE = 2
x-y Plume Slice at z = 33 m, t = 17,000 days
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Figure 8
Time (years)
CM
AX
(mg
/L)
Ru
nT
ime
(cp
um
ins)
0 20 40 60 8010
-4
10-3
10-2
10-1
100
0
10
20
30
40
50
60
70
80
90
100
5 24 23 2
3 45 24 23 23 4
Levels RefinementFactor
Maximum Plume Concentration vs. Grid Resolution
CPU
CMAX
Figure 9
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Figures 10 a-c are flood contour plots of the simulated plume and groundwater pathline for Run
3 (NLEVEL = 3, IREFINE = 2). As discussed above, the cell size on the highest level of
refinement is a factor of four larger than the most refined cells in Run 1.
As shown in Figure 8, coarsening the grid by a factor of four (Run 3) results in an approximate
50-60 percent reduction in CMAX for t = 30-50 years, but less during earlier and later parts of the
simulation (e.g., ~ 40-50 percent reduction for t > 50 years). However, the cpu time for Run 3 is
about a factor of 15 smaller compared to Run 1. This is an example of why screening runs with
coarser grids can be an efficient approach when using Adaptive Groundwater to run simulations
of a site as part of a conceptual model development.
Figure 10a
NLEVEL = 3, IREFINE = 2
x-z Plume Cross-Section at y = 843 m, t = 5,000 days
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Figure 10b
NLEVEL = 3, IREFINE = 2
x-z Plume Cross-Section at y = 956 m, t = 17,000 days
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Figure 10c
NLEVEL = 3, IREFINE = 2
x-y Plume Slice at z = 34 m, t = 17,000 days
Figures 11 a-c are flood contour plots of the simulated plume and groundwater pathline for Run
4 (NLEVEL = 3, IREFINE = 4). As discussed above, the cell size on the highest level of
refinement is the same for Runs 1 and 4, but the cell-size change from one AMR level to the next
is a factor of two greater in Run 4.
As shown in Figure 8, CMAX for Runs 1 and 4 are about the same. The cpu time for Run 4 is
almost 50 greater compared to Run 1 due to a larger number of total grid cells in Run 4.
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Figure 11a
NLEVEL = 3, IREFINE = 4
x-z Plume Cross-Section at y = 848 m, t = 5,000 days
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Figure 11b
NLEVEL = 3, IREFINE = 4
x-z Plume Cross-Section at y = 942 m, t = 17,000 days
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Figure 11c
NLEVEL = 3, IREFINE = 4
x-y Plume Slice at z = 33 m, t = 17,000 days