Movie of airflow in a whirly

movie of air motion in the fourth harmonic of a resonant tube

The motion of air in the fourth harmonic resonance of a tube open at both ends. Each drawing is 1/8 of a cycle after the one above it.

Look at the illustrations above:

The cyclical motion of the air begins with the top image.

In the second frame, an eighth of a cycle later, the dense regions of air are expanding into the less dense regions.

In the third frame, a quarter of a cycle after the first, every air parcel is back at its equilibrium position.

In the fourth frame the air has moved away from node 1 and toward node 2.

In the fifth and final frame


Note that the displacement at all of the nodes is always zero.

Note that the color change at the nodes is from dark orange to white, the greatest changes in density and pressure happen at the nodes of displacement.

Note that at the ends, the displacement arrows go from longest to the right, to zero, to longest to the left. Yet the color changes least. The ends are nodes of density or pressure change and antinodes of displacement change.

Sketch the sixth, seventh, and ninth frames yourself to test your understanding. (Answer below)

Scientific Explorations with Paul Doherty

© 2001

21 Feb 2001


The sixth frame is the same as the fourth, the seventh frame is the same as the third, and the ninth frame is the same as the first.