Active and Passive Turbulent Boundary-Layer Drag Reduction

A 50 year journey through flow control strategies that seek to achieve viscous drag reduction in turbulent boundary layers is presented. These are shown to focus on different mechanisms underlying coherent motions that have been observed in the early period of turbulent boundary-layer research. The approaches have generally focused either on modifying the large-scale outer motions or the small-scale motions near the wall that are particularly associated with low-speed wall streaks. The degree of success of these approaches is highlighted.