Aerofoil

Aerofoil is primary flow element used to measure flow in rectangular
duct. It works on the principle of the relationship between flow
velocity and the pressure fields in friction less flow. Since the air
particles follow the curved streamlines above the upper surface there
must be a centripetal force across the streamlines which is
accelerating the flow towards the centre of the curvature. That force
must be associated with a pressure gradient across the streamlines
i.e. ambient atmospheric pressure at some distance from the surface
grading to a lower pressure on the upper wing surface.