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

Energy — where it appears

The quantity a flow conserves when nothing dissipates, made of kinetic energy, internal energy and the work of pressure. Following it through a device says what is recoverable and what has been turned into heat, which pressure alone does not.

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

Two roots walking towards each other, and one of them crosses. The roots of the characteristic quartic in the complex plane as the airspeed is raised from nothing to 105 metres per second — growth rate across, frequency up. At rest the two sit on the imaginary axis at the uncoupled frequencies. As the speed rises, the aerodynamic coupling drags them towards each other in frequency while pushing one left and the other right, and at 80.8 metres per second the right-hand one crosses the axis. Everything about the failure is in this picture: the coalescence, the crossing, and the fact that the flutter frequency is neither of the two the structure started with.

The shake that is not resonance

A steady airstream contains no oscillation at any frequency, so nothing is driving anything. What happens instead is that the aerodynamic forces couple two structural motions that were independent, drag their frequencies together, and turn one damping negative — and the wing takes the energy out of the air itself.

circulation · Flutter Six ways of reaching one speed. Six velocity histories, all starting from rest and all reaching exactly one at the same moment. Two are ramps, two are eased, one overshoots and comes back, and one goes backwards before it goes forwards.

Everything about the start, except one vector

Six ways of accelerating a body from rest to the same speed produce six force histories with nothing in common — peaks spanning a factor of thirty-nine, two of them negative for part of the journey. The impulse left in the fluid is the same ten-figure number in every case, and so is the energy.

inviscid · Impulse

Named alongside it

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

Added massAeroelasticityConserved quantityDivergenceEigenvalueFlutterForceImpulseMass balanceMeasurementMemory kernelModel limit

All concepts