Fluid Volume Concentration Threshold
Figure 1: Module 4B conceptual model
This post describes how to use the fluid volume concentration threshold command, introduced in the January 2022 release (revision 2342), to calculate fluid volumes throughout the model domain that exceed or fall below a user-defined concentration threshold. Conceptually, we can think of this command as a way to separate water into two categories based on solute concentration—those above the threshold (contaminated water) and those below (uncontaminated water). This command is active when the concentration value reaches the specified threshold, removing the need for post-processing in external software like Tecplot.
Figure 2: Command description
The command is called fluid volume concentration threshold and it's designed to calculate fluid volume throughout the model domain which falls above and below a user specified concentration threshold. This command can be extremely helpful in situations where the total volume of contaminated water is required, and removes the need for post-processing using Tecplot or other programs.
A simple example is available, based on Module 4B of the 'Introductory Modules' (the introductory modules are a series of increasingly complex box models which demonstrate contaminant transport through fractured media under steady-state vs transient conditions, homogeneous vs heterogeneous, etc.). The introductory modules are available for download on the Guide for New Users page.
(Click here to download an example problem (Module 4B) which demonstrates the fluid volume concentration threshold command.
Figure 3: Example of fluid volume concentration threshold command in .grok
Please note that this version of the example has been slightly modified to include a much higher specified concentration BC for the 'salt' solute. The specified concentration at the inlet has been set to 35 kg/m^3, i.e. to the approximate salinity of seawater.
First let's review the command itself. As you can see from Figure 2, it's quite simple to implement only requiring you to specify a label for the output file and a concentration threshold for each species.
If you scroll to the bottom of the Module4b .grok file we can see the application of the command. In this case the model calculate the total volume of 'brackish' and 'fresh' water throughout the model domain, based on a 1.0 kg/m^3 (i.e. 1,000 mg/L) threshold.
Figure 4: Resulting output file
Note: you MUST specify a threshold for ALL solutes defined in problem. Threshold values should be defined in the same order that solutes are created/defined in the .grok file. Associated output files will be generated for each solute.
After running phgs.exe we should see the associated file appear in the project folder (module4bo.fluid_volume_conc.threshold_Volume.salt.dat, see Figure 4). This file contains the total volumes of water greater than the specified threshold ("PM > Thresh"), less than or equal to the specified threshold ("PM <= Thresh") and the total volume of water in the porous medium domain ("PM Total").
By the end of the simulation (3.1536E+08 seconds = 10 years) we can see that approximately 25% of the total fluid volume would be considered brackish. Figure 5 illustrates the distribution of this volume using the isosurface feature in Tecplot, and we can see that the solute concentrations are centered around the location of fractures:
We hope you find this new command very useful, please let us know what you think!
Figure 5: Resulting salt concentration distribution (isosurface displayed at conc. = 1.0 kg/m^3)