What is meant by the term "boundary layer" in aerodynamics?

The term "boundary layer" in aerodynamics refers to a thin layer of air that is in immediate contact with the surface of the aircraft. In this layer, the effects of viscosity are significant, and the velocity of the air changes from zero at the surface (due to no-slip condition) to nearly the free stream velocity just outside the boundary layer. This change in velocity is crucial because it affects drag and the overall aerodynamic characteristics of the aircraft.

Understanding the boundary layer is essential for grasping how flow behaves around the aircraft's surfaces, including how it influences lift and drag. The characteristics of the boundary layer can vary between laminar (smooth) and turbulent (irregular) flow, further affecting the performance and control of the aircraft. Thus, option B encapsulates the fundamental definition of the boundary layer by highlighting its proximity to the aircraft surface and its variable velocity profile.