Recent Advances in the Dynamics of the Transition Zone

This paper is a survey of certain recent advances in our understanding of the transition zone in a boundary layer. It is emphasized that the behaviour of turbulent spots and the distribution of intermittency hold the key to an improved description of the dynamics of the zone. Evidence from many different sources is presented to show that, in strong pressure gradients, the concept of subtransitions13; provides an essential ingredient of the picture. A linear combination model that incorporates the possibility of subtransitions and allows for the computation of the effects of highly favourable gradients on fully turbulent flow is shown to be in reasonable agreement with experimental data. Difficulties that remain are pointed out