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

Curvature — where it appears

The rate at which a line or a surface turns, one over the radius of the circle that fits it. Streamline curvature costs a centripetal force, so it sets a pressure gradient across the flow and destroys the hydrostatic approximation where it is large.

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

How big before gravity shows. A drop's height over its width against the Bond number, which is the ratio of its weight to the force its own skin can supply. The number is one where those two are equal, and by then the drop is a bun: it is one per cent from a ball at Bo = 0.0079, five per cent at 0.054 and ten at 0.13. Every one of those is below one, and the first is below it by a factor of a hundred and twenty-six.

The size a drop is allowed

The Bond number sets a drop's weight against the force its own skin can supply, and it is one when they are equal. By then the drop is a bun — it is a per cent from being a ball at Bond number 0.0079, which is a water drop half a millimetre across.

regimes · Bond The current that runs across the river. The transverse velocity through the depth in a bend, beside the downstream velocity that causes it. The surface water is thrown outward and the bed water is driven inward, the two cross at about half the depth, and the whole profile carries no net discharge — which it cannot, because the cross-section has to pass what it is given. What it does carry is the bed load, inward.

The current that runs across the river

The standard channel calculation is one-dimensional, and a river is not. Put a bend in it and the surface tilts by a few centimetres, which is invisible — and a helix forms, a few per cent of the main flow, which decides where the river will be in a hundred years.

applied · Open-channel

Named alongside it

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

DimensionlessBond numberCapillary lengthConservationContact angleDropFree surfaceHydrostaticMeasurementModel limitOpen-channelSecondary flow

All concepts