Control of turbulent boundary layers

The objective of this paper is to give an overview of recent progress on boundary layer control made by the author’s research group at University of California, Los Angeles. A primary theme is to highlight the importance of a certain linear mechanism and its contribution to skin-friction drag in turbulent boundary layers—and the implication that significant drag reduction can be achieved by altering this linear mechanism. Examples that first led to this realization are presented, followed by applications of linear optimal control theory to boundary-layer control. Results from these applications, in which the linear mechanism in turbulent channel flow was targeted, indirectly confirm the importance of linear mechanisms in turbulent—and hence, nonlinear—flows. Although this new approach has thus far been based solely on numerical experiments which are yet to be verified in the laboratory, they show great promise and represent a fundamentally new approach for flow control. The success and limitations of various controllers and their implications are also discussed.