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

Strouhal number — where it appears

A frequency made dimensionless by a length and a speed, fL/U. It is the number of times something happens while the flow travels one length, and for an oscillation set by a round trip it is the reciprocal of the trip measured in those units.

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

The street, as two rows of point vortices. The exact velocity field of a staggered double row of point vortices at the stable spacing ratio, with the row spacing and the circulation of one core printed from a line integral of the field rather than from the number that built it. This is a model of a wake. It contains no body, no viscosity and no mechanism that would shed anything, and the viscous stepper used here does not produce a street at any Reynolds number.

The street a steady calculation cannot draw

The alternating wake behind a cylinder is the most photographed structure in fluid mechanics, and a steady solution does not produce one. What can honestly be drawn instead is a model of it — and the model settles one thing exactly, which is the spacing.

turbulence · Wake A frequency set almost entirely by how nearly sonic the flow behind the shock is. The Strouhal number of the loop against the local Mach number behind the shock. The band drawn across it is what is measured on aerofoils in buffet, 0.06 to 0.08, and the arithmetic meets it at a post-shock Mach number of 0.971. Two per cent either side of that number changes the frequency by a third, which is why buffet onset is predicted by correlation rather than by calculation.

A shock that will not stand still

A converged transonic solution puts the shock in a definite place. Over a band of conditions the real one sweeps back and forth across a fifth of the chord at a fixed frequency, and the period is set almost entirely by how nearly sonic the flow just behind it is — a quantity that is small, hard to compute, and multiplies everything.

compressible · Transonic

Named alongside it

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

Model limitSeparationBuffetCirculationFeedbackLinear stabilityMeasurementPoint vortexShock waveSignal speedTransonicUnsteady

All concepts