2. Boundary layers

2.1 Definitions

The viscosity of a real flowing fluid always implies a greater or lesser velocity gradient in the vicinity of the walls. This is because fluid particles in contact with walls must have the same velocity as the wall, whereas the velocity is generally very different far from the wall. Thus, in certain types of flow, referred to as external flows, two zones can be distinguished in a section perpendicular to the wall: one in which the velocity varies significantly, the other in which the velocity is practically constant. The first zone is called the kinematic boundary layer. It is generally very limited in size compared to the flowing fluid as a whole. The second zone, in which velocity is constant, is often referred to as the potential flow zone, since flow here is...