Fluid Flow & Aerodynamics
why things fly, and why the usual explanation is wrong
Concept

Vortex shedding — where it appears

The periodic release of vortices from alternate sides of a bluff body into its wake. It sets an oscillating force on the body at a frequency fixed by the Strouhal number, and it is what makes cables sing, chimneys sway and falling drops shake.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Captured, and not captured. The amplitude of the wake's oscillation over time, for a forcing inside the capture band and one outside it. Inside, the amplitude settles; outside, it beats at the difference between the two frequencies, because the wake is keeping its own time and the forcing is keeping its.

A wake told what to do

Force a shedding wake near its own frequency and it abandons its own and adopts the forcing's. The band over which it will do so is proportional to how hard it is pushed — and near the edge of the band it takes six times as long to make up its mind.

regimes · Strouhal The first three ways a drop can ring. A drop's first three modes of oscillation, each drawn at the two ends of its swing: the shaded outline and the red one are half a period apart. The fundamental, l = 2, alternates between a lemon and a lentil. The next two ring at 1.94 and 3.00 times its pitch, with three and four lobes. The swings are drawn at a fifth of the radius so they can be seen; the frequencies belong to swings much smaller than that.

A drop rings like a bell

Disturb a drop and it swings between a lemon and a lentil at a pitch set by its surface tension and its size, and it keeps swinging for dozens of cycles because water is nearly frictionless at that scale. The usual story has a falling raindrop's wake ringing it. The wake strikes several times too fast for the note the drop plays.

regimes · Drop shape

Named alongside it

The objects these essays reach for when they reach for this one.

ResonanceDampingDropKinetic energyLegendreLock-inMeasurementMemory kernelModel limitModel validityOscillationOscillator

All concepts