The Singing Wire
When wind blows past a telephone wire, power line, or flagpole, you often hear a pure tone — the aeolian tone. Named after Aeolus, the Greek god of wind, this sound arises from the periodic shedding of vortices alternating between the top and bottom of the cylindrical surface. Each shed vortex creates a transverse pressure impulse, and the regular alternation produces a nearly sinusoidal fluctuating lift force that radiates sound as an acoustic dipole.
Strouhal Scaling
The remarkable simplicity of aeolian tones lies in the Strouhal number: f·d/U ≈ 0.2 for circular cylinders across an enormous range of Reynolds numbers (300 to 200,000). This means the pitch is directly proportional to wind speed and inversely proportional to wire diameter. A 5 mm wire in a 10 m/s breeze sings at 400 Hz; double the wind speed and the pitch doubles to 800 Hz. This linear relationship made aeolian tones one of the earliest quantitative observations in fluid dynamics.
Von Kármán Vortex Street
Behind the cylinder, the shed vortices organize into a staggered double row — the von Kármán vortex street. This pattern is stable for moderate Reynolds numbers and produces the coherent pressure fluctuations responsible for both the acoustic tone and the structural vibration forces. At very high Reynolds numbers (above about 200,000), the boundary layer transitions to turbulent before separation, temporarily disrupting the regular shedding pattern in what is known as the drag crisis.
Engineering Applications
While aeolian tones are a charming natural phenomenon, the associated vortex-induced vibrations (VIV) cause serious engineering problems. Power line conductors can fatigue and break, marine risers and submarine cables can oscillate destructively, and tall chimneys can sway dangerously. Engineers use helical strakes, Stockbridge dampers, and aerodynamic fairings to disrupt shedding coherence and protect structures from VIV-induced fatigue.