Dancing Wires: Part Two

In his recent article on dancing wires , Neil Davis noted that one way for wires to receive the energy to cause their dancing is directly from moving air. The mechanism is one involving the formation of vortices behind a wire when the wind blows perpendicular to the wire.

Air moving past the wire is impeded by the wire; the air closest to the wire moves the slowest. The result is a curling up of the air behind the wire to form vortices--rotating spirals within which the air spins faster the closer it is to the center of the vortex.

One by one, the vortices break away from the wire. The breaking away alternates between those formed behind one side of the wire and those formed on the other side. As each vortex leaves, it gives the wire a sideways kick. If the rate of breaking away is somewhere near the natural frequency at which the wire wants to vibrate, large, continuing oscillations will build up the dancing wires.

Wires on which snow has built up are major obstructions to the wind so that vortex shedding can take place at barely perceptible wind speeds.

Next time you see a wire dancing on a cold winter day, try measuring the direction and speed of the wind by blowing soap bubbles into the air. Aside from proving that the air is moving, it is fun to see how the soap bubbles freeze and roll along the ground.