Download - Tutorial 9 - Changing the Global Grid Resolution · Section 1 – Changing the Global Grid Resolution Step 1 - Open Adaptive Groundwater Input (.agw) File Go to File > Open in the

1

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




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).

2

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.

3

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.

6

Figure 5a

NLEVEL = 5, IREFINE = 2

x-z Plume Cross-Section at y = 848 m, t = 5,000 days

7

Figure 5b



8

Figure 5c



9

Figure 5d


x-y Plume Slice at z = 33 m, t = 17,000 days

10

(a)

(b)

11

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


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)

12

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



13

Figure 7b



14

Figure 7c



15

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

16

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



17

Figure 10b



18

Figure 10c



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.

19

Figure 11a



20

Figure 11b



21

Figure 11c

