Quantification of Critical Shear Stress
Photo of angle of repose experiment
This is a lab activity designed to give students experience with the concept and quantification of critical shear stress.
This is one of 5 laboratory activities I use in an upper level undergraduate/beginning graduate course on sediment transport. It is designed to fit into a 75 minute period (there is no separate laboratory section for this course).
Skills and concepts that students must have mastered
The students have been introduced to the concept of critical shear stress and the basic physics of the forces required to move particles off the bed. They have been introduced to the Shields Curve and how to use it to calculate critical shear stresses for particles of different size and density.
How the activity is situated in the course
This is one of a sequence of exercises on sediment transport.
Content/concepts goals for this activity
The goals of the exercise include giving students experience with the subjectivity of determining critical shear stress in the laboratory, calculating critical shear stresses using the Shields relationship, and with the effects of mixed particle sizes on entrainment.
Higher order thinking skills goals for this activity
Students gain experience with collecting, replicating, and interpreting data.
Other skills goals for this activity
The students work in small groups for this activity.
Description and Teaching Materials
Photo of students conducting experiment with laboratory stirrer
In this activity students observe the conditions under which spherical glass beads will first begin to move in a beaker stirred with a mechanical stirrer. They "calibrate" the stirrer by calculating the predicted critical shear stress for the particles according to the Shields relationship, and then use the calibration curve to measure the critical shear stress for some natural sands. Finally, the students conduct a simple experiment to illustrate that the critical shear stress for particles sitting on beds of larger or smaller particles will differ from that for homogeneous beds.
Student handout for critical shear stress lab (Microsoft Word 2007 (.docx) 38kB Jun3 14)
Shields Curve (Acrobat (PDF) 14kB Jun3 14)
Teaching Notes and Tips
Students hand in the laboratory exercise at the beginning of the next class period for grading
References and Resources
The first part of this exercises uses 1 liter glass beakers, glass beads (available from scientific suppliers like Fisher for chromotography), and an overhead laboratory stirrer. I use a Heidolph mechanical overhead stirrer with a cross blade impeller mounted on a Caframo heavy duty stand (I purchased all of this from Fisher). The key requirements for the stirrer are that it have a high enough speed to suspend the particles and that it have a speed scale that the students can adjust.
The second part of the activity uses clip boards on which I have glued a "bed" of glass beads. I used Gorilla Glue and was careful to leave the "pockets" between the beads open and as uniform as possible. The students use protractors to measure the angle to which the clip boards must be rotated before a loose grain will roll out of the pockets.