Issues in Active Flow Control: Theory, Control, Simulation, and Experiment ∗ S. Scott Collis † Sandia National Laboratories ‡ , P.O. Box 5800, Albuquerque, NM 87185-0370 USA Ronald D. Joslin § Office of Naval Research, 800 N. Quincy Street, Arlington, VA 22217, USA Avi Seifert ¶ Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel Vassilis Theofilis  Escuela Tecnica Superior, Universidad Polit´ ecnica de Madrid, Plaza Cardenal Cisneros, 3, E-28040 Madrid, Spain. May 26, 2004 Abstract The goal of this paper is to provide a perspective on the current status and future directions for active flow-control technology with particular emphasis on oscillatory control. This is not a comprehensive review of the literature; rather, certain issues that are often neglected in studies are highlighted showing their importance or impact on the reported observations and targeted outcomes. Feasible routes using flow instability as an efficiency enhancement tool are discussed as an emerging means to explain the physical phenomena of active flow- control and as a tool for control law design and development. Traditional and more recent theoretical approaches to control design are discussed and recommendations are made relevant to numerical complications on the route to design oscillatory flow-control systems. A generic flow control process is put forward and illustrated using experimental examples. ∗ Distribution Statement A: Approved for public release; distribution is unlimited. † Senior Member of Technical Staff, Optimization and Uncertainty Estimation ‡ Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000 § Program Officer, Undersea Weapons & Countermeasures ¶ Senior Lecturer, Department of Fluid Mechanics and Heat Transfer, school of Mechanical Engineering, also visiting scientist, National Institute of Aeronautics, Hampton, VA, USA, Tel. +972 (3) 6405310; fax: +972 (3) 6407334; Email: seifert@eng.tau.ac.il  Ram´ on y Cajal Research Fellow; also visiting Professor, College of Aeronautics, Cranfield University, UK. 1