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

Strouhal — the series

2 essays on one idea, from the one that introduces it to the one that assumes the rest.
  1. Three bluff bodies, whose Strouhal numbers agree once the wake's width is used. The measured Strouhal number of each body, and Roshko's universal number formed with the wake's width and the speed on the free streamline that bounds it. The raw numbers span a factor of 1.462; the collapsed ones span 1.0011, with a mean of 0.16281. The shedding was never body-dependent — the length in the number was.

    The frequency a wake chooses

    Bluff bodies shed at Strouhal numbers from 0.145 to 0.212, and the spread is not a fact about shedding — it is a fact about which length went into the number. Change the length to the wake's own width and three bodies agree to a tenth of a per cent. Then let the body move, and the number stops deciding anything at all.

    part 1 · regimes
  2. 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.

    part 2 · regimes

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