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

Correction — where it appears

An adjustment applied to a measurement to remove a known effect of the instrument. A correction is trustworthy in proportion to how well the effect is understood, and a measurement whose correction is comparable with its answer is not a measurement.

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

The wall is a boundary condition, solved for rather than reflected. A model's trailing vortices in a closed working section, with the sheet of sources that makes the wall a wall drawn as a displacement of the wall itself. The interference is an upwash — the model looks better than it is, and the correction factor comes to 0.1242 against the exact value 0.1250 that the image at the inverse point gives for a circle. The arrows are the interference velocity alone, with the model's own downwash removed, drawn to a common scale set by the longest of them.

The walls are in the answer

A wind tunnel measures a wing in a box the aeroplane will never fly in, and the box is worth a fifteenth of the induced drag. The correction is exactly an eighth for a closed circular section and exactly minus an eighth for an open jet, and the sign is the whole argument.

circulation · Tunnel interference One coefficient, two errors, two places they vanish. The two wall-interference errors against the slot parameter, in units of the tunnel's half-height. Both are positive for an open jet at the left and negative for a solid wall at the right, so each passes through zero — the blockage at 1.184 and the streamline curvature at 1.590. A wall has one coefficient and the two zeros are a third of a tunnel height apart.

One coefficient, two errors

A closed tunnel's walls push a measurement one way and an open jet's push it the other, so a wall somewhere between them should push it neither. There is such a wall, its coefficient is a length, and it nulls the blockage at 1.184 tunnel half-heights — and the streamline curvature at 1.590.

circulation · Tunnel interference

Named alongside it

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

BlockageBoundary conditionImagesMeasurementPotential flowWind tunnelCirculationDownwashInduced dragInterferenceLift coefficientModel limit

All concepts