Cs382/Diffusion Experiment

This is a detailed description of one experiment listed on the CS382 course page

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Goal

The goal of this experiment is to observe the diffusion of a dye plume in the groundwater simulator and note the plume's properties as it the dye travels through sand or gravel. The data gathered here will be used to verify a computer simulation of dye diffusion.

Equipment

• enVision sand and gravel groundwater simulator
• dark water soluble dye
• pipette
• water
• ruler
• kitchen timer or some other time measurement device

Procedure

1. Fill the groundwater simulator with fresh water This yields the greatest contrast between the dye plume and surrounding clean water.
2. Make sure the pump is off. If the pump was recently circulating water, make sure that the water table's level has had enough time to stabilize. This ensures that the dye's plume will only be affected by diffusion and not water movement.
3. Fill the pipette at least halfway full with dye.
4. Insert the pipette completely into well number 3 (sand).
5. Inject enough dye into the well to create a quarter sized plume.
6. Keep the pipette's bulb depressed until after it is removed from the well to avoid creating a vacuum and sucking dye from the plume back into the well. A little of this may occur as the water level in the well rises to fill the space formerly occupied by the pipette's barrel.
7. Measure the plume's diameter in the horizontal plane and start the timer. Describe the properties of the plume (shape, distribution of dye concentration, etc.)
8. Continue to measure the diameter of the plume at regular intervals until the plume has doubled it's original diameter. Make note of the plume's properties at each measurement.
9. Repeat this experiment in well number 5 (gravel)

Results

• Determine the dye's rate of diffusion for the two soil mediums in the simulator.
  • The rate of diffusion in this experiment is defined as the speed at which the dye at the edge of the plume travels from one time interval to another.
  • <math>r(t)</math> = radius of plume at time interval <math>t</math> (radius = diameter / 2)
  • rate of diffusion = <math>(r(t2)-r(t1))/(t2-t1)</math>
  • Does the rate of diffusion change over time?
  • Does the rate of diffusion differ between sand and gravel?
• Do the properties of the plume change over time?
  • Is the dye concentration consistent throughout the plume? Is there less of a concentration at the edges or center?
  • Does the plume's overall shape change over time?
  • Is there a difference in the plume's behavior in sand and gravel?

Considerations for a Computer Based Simulation

• If NetLogo is your computational tool of choice, how does the diffuse function (patch based) compare to the results you observed?
• Would it be better to simulate this with a cell or agent based simulation?