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

Spinning cylinder — where it appears

A rotating cylinder in a stream, which carries circulation and therefore lift with no aerofoil section anywhere. It is the cleanest demonstration that lift is circulation, and its stagnation points merge and leave the surface at a computable rotation rate.

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

Ideal flow past a cylinder with circulation -3.4. A uniform stream past a circular cylinder in a fluid with no viscosity. The solution is exact and closed-form: streamlines part at a stagnation point, run round the surface and close up perfectly behind, and the pressure recovers to exactly what it was in front.

Lift with no wing at all

A spinning cylinder has no camber, no aerofoil section and no trailing edge, and it lifts exactly as hard as its circulation says it should. Which settles what lift is caused by.

circulation · Lift Where the flow stops, at Γ = -9. A cylinder with circulation, with the points where the flow is at rest marked. As the circulation grows the two points slide round the surface towards each other, meet at the bottom, and then leave the body — after which there is nowhere on the surface where the air is at rest at all.

How much circulation is too much

Spin a cylinder faster and it lifts harder, with no limit in the equations. What does have a limit is the flow's willingness to stop anywhere on the surface — the two points where the air is at rest slide round towards each other, meet at the bottom, and leave the body altogether.

circulation · Lift

Named alongside it

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

CirculationKutta–Joukowski theoremLiftMagnus effectIdeal flowPotential flowStagnation pointSuctionSurface speed

All concepts