Blade physics
The physics of windmills

Assume that the wind, blowing in direction Ox is incident on an obstacle. As shown in figure  2 , the stream lines are deformed to circumvent the obstacle. At the same time, an area of increased pressure is created windward of the obstacle, that pressure pushing the stream lines outward. Behind the obstacle, however, a vacuum is created which is responsible for the reconvergence of lines. The difference in pressure between the windward and leeward faces involves the existence of a pressure force