SMS-303: Integrative marine sciences III.
Lab 1, Waves. Pictures and movies from lab:
Class demo: ‘dead-water’ – famous oceanographic problem- boat in
stratified fluid can encounter large resistance. Many different explanations
(including biological ones)
until Ekman (1904) provided the correct one. See: Ekman V. W., 1904. On dead water. Sci.Results Norw. North Polar Expedi. 1893-1896, 5(15).
Short movie showing the tank with a 2-layer setup, salty (dense and blue) below and fresh above.
The 'boat' is released in the stratified system. It takes 19sec to reach the green tape. Note the generation of internal waves in its wake.
The fluid is then mixed and the 'boat' is released again:
This time it takes only 10sec for the 'boat' to reach the green tape.
Stations:
I. You are about to measure the period of wave sloshing back and forth in a small vessel. Q: Which do you expect to propagate faster: a wave in a tank with little water or that where the water is deeper?
Students observe waves sloshing back and forth in the small tanks. Because the velocity is proportional to the square root of (gH) the period of the sloshing in the vessel with 6cm of water was half that in the one where the water was 1.5cm deep.
II.
Buoyancy oscillations:
In a tall cylinder with salty water on the bottom and fresh water on top you have a floating object parked between the fluids.
Q: What will happen if you push the ping-pong ball down? Why?
How will the period of the oscillation change if the water is more/less stratified?
A ball with intermediate density is parked between two fluids of larger and small density. When pushed the ball oscillate. The frequency of oscillation is proportional to the square root of the difference in density between the two fluids.
III.
Large tank with paddle + current meter (ADV) attached to a computer.
A power supply is attached to the paddle allowing us to change the frequency of the forcing.
Students observe how the wave set in the tank affect particles within it as well as forcing a non-periodic circulation. Largest waves do not occur when the forcing applied is largest but rather when the forcing frequency matches the natural frequency of the tank. Current meter (ADV) provide real time 3-D velocity data demonstrating to the student how the velocity of real waves differ from a perfect sinusoid shape when multiple frequencies are present and even when a single wave dominates.
IV:
Slinky- use a slinky to make a transverse wave (where the wave motion is at 90 to that of the particles) and a longitudinal wave (where the wave and particles move in the same direction). Classify sound, light, and gravity waves as transverse of longitudinal. Use:
http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html displayed on the computer next to you to learn more about them.
As simple as a slinky is it is a great tool for demonstration of wave concepts.
V.
Internal waves:
You have a small tank with a partition in the middle. Fill one side of the partition with cold fresh water and the other with hot and fresh or cold and salty water. Q: What will happen when you raise the partition between the fluids?
Following the removal of the partition the heavy fluid flows below the light one once it reaches the wall an internal wave propagates back and forth along the density interface. The propagation speed is much slower than the surface gravity waves observed in station 1.
©Boss, 2006
This page was last edited on 11/19/2006