Linear control applied to fluid systems near an equilibrium point has important applications for many flows of industrial or fundamental interest. In this article we give an exposition of tools and approaches for the design of control strategies for globally stable or unstable flows. For unstable oscillator flows a feedback configuration and a model-based approach is proposed, while for stable noise-amplifier flows a feedforward setup and an approach based on system identification is advocated. Model reduction and robustness issues are addressed for the oscillator case; statistical learning techniques are emphasized for the amplifier case. Effective suppression of global and convective instabilities could be demonstrated for either case, even though the system-identification approach results in a superior robustness to off-design conditions.

• Received 3 February 2016

DOI:https://doi.org/10.1103/PhysRevFluids.1.040501

• ^*This paper is based on an invited lecture given by Peter J. Schmid at the 65th Annual Meeting of the APS Division of Fluid Dynamics, which was held 18–20 November 2012 in San Diego (CA), USA.