Structural Engineering & Statics
where the load goes
Generator

The vortex-lock-in generator

Rendered here at the parameters it defaults to, with every essay that calls it — which is the same list as the blast radius of changing it.
Lock-in: the frequency the wind sheds at, and what it does to the chimneyA 1.2 m cylinder at 0.9 Hz. Vortices leave it at St·U/D, so the critical speed at which they match the structure is 6 m/s — a breeze, met many times a year rather than once in fifty. The upper panel shows the shedding frequency locking on to the structure across a band from 5.1 to 7.8 m/s; the lower shows the amplitude that results. At a Scruton number of 11.2 the peak amplitude is 180.94 mm, which is 15% of the diameter. 024681012141618200123shedding frequency (Hz)1.2 m across · St 0.18 · 0.9 Hzlock-in6 m/s02468101214161820050100150200wind speed (m/s)cross-wind amplitude (mm)0.40% damping — Sc = 11.2, peak 180.94 mm

2 essays call vortex-lock-in. The drawing above is what it returns with no arguments at all; every call below passes it something, because a placement that passes nothing draws whichever member of the family the generator happens to default to rather than the one its essay argues about.

Where it is called

Changing this generator changes every one of these figures.

024681012141618200123shedding frequency (Hz)1.2 m across · St 0.18 · 0.9 Hzlock-in6 m/s02468101214161820050100150200wind speed (m/s)cross-wind amplitude (mm)0.40% damping — Sc = 11.2, peak 180.94 mm Dynamics

The wind that brings its own frequency

Every other load in this subject arrives at whatever rate it happens to arrive at. Vortex shedding arrives at a rate set by the wind speed — so for any chimney, mast or cable there is always a wind speed at which the shedding matches the structure exactly, and it is a breeze rather than a storm.

 024681012-0.01-0.0050.005wind speed (m/s)total damping ratio0.60% damping · 1.2 Hz4.14 m/s — nothing leftbelow: a disturbance dies awayabove: the structure drives itself Dynamics

The motion that feeds itself

A steady wind contains no frequency at all, and it can destroy a bridge. The force that does it is manufactured by the structure's own movement, so there is no excitation to resonate with — there is a wind speed above which the equilibrium is unstable, and below which nothing happens.

The library, page 4 of 4 — where vortex-lock-in sits