ABSTRACT
The flow of a real fluid (except at extremely low pressures) has two fundamental characteristics. One is that there is no discontinuity of velocity; the second is that, at a solid surface, the velocity of the fluid relative to the surface is zero, the so-called no-slip condition. As a result there is, close to the surface, a region in which the velocity increases rapidly from zero and approaches the velocity of the main stream. This region is known as the boundary layer. It is usually very thin, but may sometimes be observed with the naked eye: close to the sides of a ship, for example, is a narrow band of water with a velocity relative to the ship clearly less than that of water further away.
