Physics Photo of the Week
October
19, 2007
Leaf litter fronts
During
a heavy rain on August 26, 2007, the pine needles and leaf litter on
the road
had bunched into these parallel windrows about 20-30 cm apart.
Wilbur the basset serves as a good scale for the picture.
The heavy rain had flowed in a steady sheet down the road in the same
direction earlier in the day. Of course the rain washed the
leaves, pine needles, and small sticks down the road as well.
These windrows are the result of shear flow. However, the "fluid"
doing the flow is not the rain water but the layer of leaves and sticks
as the litter is washed along the road. This is analogous to the
wind gusts and water flow bunches shown in the PPOW
for October 5, 2007. In each of these cases, the fluid (air,
thin sheet of water, or leaf litter) exhibits a shear flow. The
upper layer flows faster than the bottom layer due to friction with a
fixed surface. As a result material builds up into the "fronts"
as the top layer piles up material above the slower moving bottom
layer. The fronts were "frozen" in position after the rainwater
subsided.
Another
example of water "flow fronts" can be seen in water flowing down the
spillway of Beetree Dam near Warren Wilson College in the photograph at
right. Here the upper layer of water falls faster than the water
in contact with the concrete sluiceway and bunches into the fairly
uniformly spaced "fronts".
Similar effects are manifest in cloud formations (another photo
someday) weather storm patterns, and astrophysics where there is a
velocity shear in the flow of stellar material.
The next Physics Photo of the Week will be published in two weeks -
November 2, 2007, due to the fall break at Warren Wilson College.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to dcollins@warren-wilson.edu.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
