This apparatus is going to demonstrate vortex shedding. Let me briefly explain what that is.
When an object moves through the air
vortices are formed in a certain range of velocities
in the wake of that movement.
Within that range of velocities
the shedding of these vortices happens that a very regular rate.
%um
so what we're going to do is move
an object--in this case a steel paino wire one millimeter in diameter,
which is under tension (there are actually two of them,
just to keep the apparatus symmetric).
We're going to move these wires through the air
at some constant rate.
And when these vortices are shed
that frequency at which it happens is going to be audible.
Now...
it's not very loud,
and it only becomes really audible
when the frequency of the vortex shedding
happens to be
coincident with one of the
frequencies of the higher harmonics of this wire.
Now, that doesn't happen instantaneously, because it
takes time for this
%uh wire to start
vibrating with appreciable amplitude
to make the sound audible.
%um
So as we move through
from one speed to the next
%um
it will take a little bit of time for
the volume up to build up,
and then we will hear a sound, which is the vibration of the wire but that %uh
frequency
that we hear is the same as the vortex shedding.
So let's see it in action!