Boundary layers, Prandtl’s and others

Anderson replies: The letters from Alan Faller, Barry Klinger, and Byron Sakiadis are a welcome addition to my article on the boundary-layer concept. Faller correctly points out the work of Vagn Ekman on three-dimensional boundary layers in rotating fluids. Ekman’s discovery, and his subsequent experimental and analytical work, was contemporary with Ludwig Prandtl’s and is an interesting example of an idea or concept whose time had come.

The history of science and technology is replete with such examples. The invention of the first successful heavier-than-air, pilot-controlled flying machine was an idea whose time had come at the beginning of the 20th century. The Wright brothers were simply the first to make it happen.

Both Faller and Klinger point out the role of the boundary-layer concept on the study of the motion of oceanic flows and circulation, a dimension of the concept not addressed in my article, written by an aerodynamicist from an aerodynamicist’s point of view. Sakiadis discusses an even more general application to flows over continuous surfaces.