Foil (fluid mechanics)

This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so for foils generating lift the resulting flowfield about the foil has a higher average velocity on one surface than on the other.

[1][2][3][4] A more detailed description of the flowfield is given by the simplified Navier–Stokes equations, applicable when the fluid is incompressible.

[5][6] For hydrofoils at high speeds, of the order of 50 knots (26 m/s) according to Faltinsen,[7] cavitation and ventilation – with air penetrating along the strut from the water surface to the foil – may occur.

[8] Since even a flat plate can generate lift, a significant factor in foil design is the minimization of drag.

[9] Other types of foils, both natural and man-made, seen both in air and water, have features that delay or control the onset of lift-induced drag, flow separation, and stall (see Bird flight, Fin, Airfoil, Placoid scale, Tubercle, Vortex generator, Canard (close-coupled), Blown flap, Leading edge slot, Leading edge slats), as well as Wingtip vortices (see Winglet).