Turbulent Boundary Layers

Abstract

In this lecture I hope to bring together the two branches of turbulence studies, (1) the fundamental study of the physics and mathematics of turbulent flow and (2) the methods of calculating turbulent flow development which are used by aircraft designers and other fluid dynamicists. While it is entirely reasonable that aircraft designers should wish to predict the behaviour of turbulent boundary layers without going deeply into the physical processes which govern their development, it is by no means reasonable that those who develop calculation methods in the first place should ignore the physics of the flow, or represent it by simple-minded formulae chosen for mathematical convenience rather than physical plausibility. Turbulence is not a simple phenomenon! The more naive integral methods that were in vogue until a few years ago have been disposed of once and for all by Thompson. The only method that emerged from Thompson’s review with any credit was Head’s entrainment method, which was distinguished from most of the other integral methods by being based on a clear physical hypothesis about turbulent flow. Head himself regarded this hypothesis as a very crude attempt to represent the entrainment properties of the turbulence, and indeed it is easy to see that this sort of hypothesis must fail in some of the more difficult situations.