A plate glass tank was used to create the gravity currents. It's
measurements, including the glass width, are 200cm x 20cm x 40cm. At
one end of the tank a plexiglass plate could be inserted to divide the
tank into two sections; the lock and the environment. Only two lock
lengths of 20 and 40cm were used in this experiment.
To study a gravity current in a two layer fluid we must first create a
layered environment. To create a layered environment the tank was filled with
fresh water to a depth of 15cm,then salt was added and dissolved, and the
density was measured. This created a dense lower layer. Using a special float
10cm of fresh water was then layered on top of the salt water giving a stably
stratified environment with total depth 25cm. As the fresh water is being
layered food coloring is added to dye the interface making it visible.
The density of the fresh water was also measured and recorded.
Next a plexiglass plate was inserted either 20 or 40cm from the left end
wall of the tank creating a lock. More salt was added to the water in
the lock making it's density greater than the salt water in the
environment. The density of the lock water was measured and the water
dyed with food coloring to make the gravity current visible. The depth
of the water in the lock is the same as that of the environment,
25cm.
A fluorescent light was placed behind the tank and a 5cm X 5cm grid was
drawn on the front of the tank. A video camera was set up to record the
resulting current when the lock is removed.
Results:
Four experiments were conducted. In each experiment the depth of layer 1
was 15cm and the depth of layer 2 was 10cm. The total depth for the
environment and lock was 25cm. The densities of layers 1 and 2 were the
same to 2 decimal places for all experiments. The only differences in the
experiments were the lock lengths, either 20 or 40cm, and the density of the
fluid in the lock. Each experiment was video taped and a movie
amd still images from some of the experiments are displayed below.
The recording of the experiments allowed quantitative results to be obtained
using the computer software "digimage". Using this program we could measure
the speed of the current and the amplitude of the interfacial waves. A sample
of these results will be listed below the images of the currents.
The qualatitive results are listed below:
1. The longer the lock length the larger the gravity current speed.
2. The longer lock lengths produce gravity currents with larger heads
since the current is composed of more fluid. For the 40cm lock a
head height of 10-15cm was observed and for the 20cm lock a 5-10cm
head was observed.
3. The gravity currents produced by locks of longer length generated
interfacial waves with larger amplitudes than the currents generated
by smaller lock lengths.
4. Currents produced from longer locks lasted longer since the longer locks
contian more fluid hence the fluid mass has more potential energy.
5. The greater the density difference betweent the lock and the environment
the larger the gravity current speed.
6. Currents produced by small density differences tended to produce interfacial
waves that travelled at the same speed as the current.
7. Currents produced from a small density difference tended to be cut in two
by the interfacial wave after they reflected from the end wall creating
two disturbances.
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