Wind Harp

Frequency against wind speed for one string. The curve is its Strouhal frequency St·U/D; the horizontal lines are its harmonics. Where the curve crosses a harmonic, the dark band is where the linear analysis finds the string and its wake unstable together (lock-in); the pale extension is how far a string already locked holds on (hysteresis). The dot is the wind now, with its gusts trailing.

Above: the string from the side, its swing magnified, with the wake at each station (dots: bright when shedding in step with the string). Below: across the string at its liveliest point, the wind blowing left to right, vortices peeling off alternately into a von Kármán street while the string swings across the wind.

The last few seconds of sound, low notes at the bottom. Hops between harmonics show as steps; a gust that loses the lock leaves a gap with the faint aeolian tone sliding with the wind.

A singing string sounds the mode its wake has locked onto, often a harmonic rather than the fundamental. The green marker slides with the wind; the string sings while it sits near a yellow one.