The portion of a fluid flowing past a body that is in the immediate vicinity of the body and that has a reduced flow due to the forces of adhesion and viscosity.
Friction between the body and the surrounding air holds back the flow nearest the surfaces, whilst the air further from the body in the mainstream flows past at unabated speed.
The layer of air next to the Earth′s surface, typically a few hundred metres thick. Ordinarily, only the boundary layer is appreciably affected by the properties of the surface. The region above the boundary layer is called the ′free atmosphere′.
Turbulent Boundary Layer
When the Reynolds number is high, there is a turbulent layer adjacent to the laminar boundary layer. Within this layer the Reynolds stresses are much larger than the viscous stresses.
See also: Atmospheric Boundary Layer.