The boundary layer concept is attributed primarily to Ludwig Prandtl (1874-1953), a professor at the University of Gottingen. His 1904 paper on the subject formed the basis for future work on skin friction, heat transfer, and separation. He subsequently made fundamental contributions to finite wing theory and compressibility effects. (His name appears about 30 times in these notes.) Theodore von Karman and Max Munk were among his many famous students. R.T. Jones was a student of Max Munk and I have subsequently learned a great deal from R.T. Jones -- which makes readers of these notes great-great grandstudents of Prandtl.
The character of the boundary layer changes as it develops along the surface
of the airfoil. Generally starting out as a laminar flow, the boundary layer
thickens, undergoes transition to turbulent flow, and then continues to
develop along the surface of the body, possibly separating from the surface
under certain conditions.
In laminar flow, the fluid moves in smooth layers or lamina. There is relatively little mixing and consequently the velocity gradients are small and shear stresses are low. The thickness of the laminar boundary layer increases with distance from the start of the boundary layer and decreases with Reynolds number.
As the fluid is sheared across the surface of the body, instabilities develop and eventually the flow transitions into turbulent motion.
Turbulent boundary layer flow is characterized by unsteady mixing due to eddies at many scales. The result is higher shear stress at the wall, a "fuller" velocity profile,and a greater boundary layer thickness. The wall shear stress is higher because the velocity gradient near the wall is greater. This is because of the more effective mixing associated with turbulent flow. However, the lower velocity fluid is also transported outward with the result that the distance to the edge of the layer is larger.
Several fundamental effects are produced by viscosity:
Drag: Skin friction drag caused by shear stresses at the surface contribute a majority of the drag of most airplanes.
The pressure distribution is changed by the presence of a boundary layer, even when no significant separation is present. This changes CL and Cm.
Flow separation: Viscosity is responsible for flow separation which causes major changes to the flow patterns and pressures.
To compute these characteristics some basic boundary layer theory is described here with more detailed computational methods for laminar and turbulent boundary layers.