Density-modified flow

Jim Trexler
University of Nevada, Reno
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Students study and experiment with the effects of slope and denisty differences on density-modified currents, flowing in fresh water.

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This lab is used in a required undergraduate course in sedimentary processes and deposits.

Skills and concepts that students must have mastered

This lab activity is scheduled after lectures on density-modified flow, turbidites, submarine landslides, and deep marine/lacustrine sedimentation.

How the activity is situated in the course

This is a stand-alone lab exercise. The experimental runs can be completed easily in a three-hour lab session.


Content/concepts goals for this activity

The fundamental concepts of flow of dense fluids within and under less-dense fluids is critical to understanding sediment transport and deposition in lakes and the ocean. This lab exercise enables students to see the real thing, experiment with some parameter changes, and calculate the effects.

Higher order thinking skills goals for this activity

The students are asked to predict what effect the changes in density and slope parameters will have on flow, and then observe the results, confirming or refuting their hypothesis.

Other skills goals for this activity

The lab exercise involves successfully mixing fluids to accurate densities, manipulating the test tank, recording time, angle, and distance data, plotting the results, and writing a coherent report.

Description of the activity/assignment

River and wind processes can be readily studied in the field, and we have devised a series of lab exercises in western Nevada that take advantage of our rivers and deserts. But for density-contrast flows, there was no easy way to get the students beyond pictures and formulae. With the assistance of Tripp Plastics, we designed acrylic tanks that fit on a lab bench. They have a ramp with screw-adjustable slope up to 20º. Students mix a solution of Epsom salt (MgSO4) to several experimental densities. They add a dye to make the dense fluid visible. The dyed fluid is released at the top of the slope. The grid allows the flow to be accurately timed and described. The students determine how density changes and how slope affect the flow velocity and structure.

Determining whether students have met the goals

A complete and formal lab report is required, and is the best assessment of mastery. Students are also closely observed in the lab, and nudged in the right direction when needed.

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