Instability, transition to turbulence and predictability

A concise state of the art review on the phenomenon of transition which constituted the opening address at the Fluid Dynamics Panel Symposium on Laminar-Turbulent Transition (AGARD-CP-224) held in Lyngby, Denmark, 2-4 May 1977 is presented. Various instability mechanisms leading to transition are proposed and discussed. A variable insight is provided based on existing experimental evidence and postulated flow structures. Critical questions are asked relating to the conceptual foundation on which much of the transition effort is based. To enhance understanding of the basic mechanisms and processes, detailed microscopic experiments are encouraged to increase the data base.