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

Noise — where it appears

Unwanted sound radiated by a flow, produced by unsteadiness rather than by any steady part of it. Its sources are the flow's own fluctuating momentum and the surfaces it passes, and what a listener hears now was emitted at a retarded time that depends on where the source was going.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

When the sound arriving now was made. The arrival time at a fixed observer against the emission time, for a source passing at Mach 0.8. The curve is monotone and its slope is not one: while the source approaches, a long stretch of emission arrives in a short stretch of time, and while it recedes the reverse.

The sound now is the source then

Every acoustic calculation is an exercise in bookkeeping about when. The pressure arriving at a listener was emitted at an earlier time, at a place the source has since left, and the Doppler shift is not a separate effect at all — it is the slope of the curve relating the two.

compressible · Aeroacoustics A blade passing the vortex it shed a passage ago. The lift the blade section feels as it passes a tip vortex at five per cent of the radius. The pulse is a doublet rather than a bump — upwash on one side and downwash on the other — so the blade is pushed one way and then the other in the width of a few chords.

A blade that flies through what it shed

A rotor blade meets the tip vortex the blade in front of it left, a fifth of a second earlier, at a few per cent of the radius. What it feels is a doublet — five and a half degrees of upwash and then five and a half of downwash within a few chords — and the peak goes as one over the miss distance.

circulation · Rotor What a downstream blade sees going past. The axial velocity a blade in the second row meets, over one revolution, as it passes through the wakes of thirty upstream blades. Each dip is one wake, and the blade meets all thirty of them every time it goes round.

A row that meets the row before it

A compressor blade is loaded and unloaded thirty times a revolution by geometry it does not have. The forcing sits at the blade count of the row in front of it and at multiples of that, and how far up the harmonics it reaches is decided by how far apart the two rows are.

circulation · Cascade

Named alongside it

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

MeasurementMemory kernelModel validityRegimeUnsteady liftWakeAeroacousticsBlade-elementCascadeCirculationConvolutionDoppler

All concepts