The Origins of Turbulence

The process of transition from a laminar to a turbulent boundary layer flow is discussed. From detailed experimental measurements and dye filament observations it appears that transition often occurs in a very localised way involving the formation of distinct coherent structures. Mostly experiments as well as analysis are carried out on purely periodic disturbances and these features are not apparent. It turns out that naturally occurring waves, excited by a random turbulence field, for example, behave in a quite different manner from periodic excitation. This aspect of the transition process is explored.