ABSTRACT
The majority of research related to the development or applications of flow control techniques has beendone under steady-state conditions.Even though synthetic jet actuators are unsteady in the sense that they produce an oscillating exit velocity, the actuator frequency
and external flow conditions are usually fixed, which makes it a stationary problem. However, in many practical situations, it becomes necessary to adjust the actuator to the changing flow or flight conditions, which make problems inherently unsteady. Just as conventional ailerons must be proportionally deflected on an aircraft wing during a turning maneuver, so must the modern active flow control (AFC) actuators provide variable levels of control at some rate determined by the maneuver. A key question in the application of AFC becomes how do we achieve time-varying flow control with modern AFC actuators? Following that we want to know how fast the AFC can be applied, that is, what is the achievable bandwidth with AFC? This chapter examines some of the issues related to these questions by considering the “lessons learned” from recent attempts to control lift on wings in an unsteady freestream.
