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vortices in Kundt's tube



Each year while going through the unit on waves and sound, my students
perform the standing wave resonance in a tube experiment. I usually have
them use a speaker driven by a function generator as a sound source and
have them locate the harmonics of the double open ended tube and a tube
with one end closed. While the students are busy taking data I set up a
demonstration of the standing wave resonance in a glass tube usually used
for the Kundt's tube experiment. By placing some cork dust inside of the
tube and exciting a resonant mode in the tube with speaker one can observe
the cork dust piling up in the nodes and moving away from the antinodes.
In the region where the displacement of the air in the tube is greatest I
have observed repeatedly the presence of a series of tiny vortices, made
visible by the cork dust, whose axis of rotation is perpendicular to the
length of the tube. As the vortices whirl they seem to sweep the dust out
of the antinode region and into the nodal region. The direction of
rotation of the vortices is opposite on either side of the location of the
antinode.

I have been unable to find any explanation of this phenomenon. Have others
also seen this, and can anyone point me in the right direction to find
something regarding this in the literature? Thanks

Brian




**************************************************************************
Brian J. Oliver
Instructor of Physics
Niagara County Community College
3111 Saunders Settlement Road
Sanborn, NY 14132
(716)731-3271 x375