The History of Potential Flow and Boundary Layer

Prandtl cal ed the region where the viscous forces cannot be ignored the boundary layer. He arbitrarily suggested that it be considered that region in which the x component of the velocity is less than 0.99 times the free-stream velocity. Then, to obtain a complete solution to a flow problem in two or three 

This division does not necessarily correspond to any physically measurable boundary. The edge of the boundary layer does not correspond to any sudden change in the flow but rather corresponds to an arbitrary mathematical definition. Even with this simplification, the calculations are very difficult, and in general only approximate mathematical solutions are possible. Nonetheless, this idea clarified numerous unexplained phenomena and provided a much better intellectual basis for discussing complicated flows than had existed previously. As a result, the boundary layer has become a standard idea in the minds of fluid mechanicians. Once it became accepted in fluid mechanics, an analogous idea was tried in heat transfer and in mass transfer, generally with useful results. 

---