The Theory of Unsteady Laminar Boundary Layers

In many problems of fluid flow past solid bodies, it can be assumed that at a general point in a fluid, viscous stresses may be neglected. This means that almost everywhere, the motion of the fluid is governed by Euler's equations of inviscid flow and, consequently, that the fluid has a velocity of slip past the body. Because the fluid in contact with the body must be at rest relative to it, there must be a thin layer of fluid adjacent to the body and called the boundary layer. In this, the viscous stresses cannot be neglected. In his classical paper, initiating its study, Prandtl assumed that changes in velocity occurred much more rapidly across the layer than along it. As a result, he was able to reduce the Navier–Stokes equations for viscous flow to a much simpler form by opening the way to the study of an important branch of fluid motion. Not only does its study provide information about local fluid properties such as skin friction, heat transfer between body and fluid, and the surface temperature of the body, but it also leads to a greater understanding of such large-scale phenomena as circulation, lift, and the drag of bluff bodies.