Design of Adjoint-Based Laws for Wing Flutter Control

An airplane, by its nature of being, is constructed so that it is as light as possible. The structural design is guided by static and dynamic factors. The more stringent constraints on the structural design are due to dynamic loads, caused by aero-elastic interactions. One of the most commonly encountered problems in aeroelasticity is flutter, a term that is used to recognize the transfer of energy from unsteady aerodynamics associated with the surrounding fluid to the wing structure, resulting in rapidly divergent behaviour. If flutter can be controlled at cruise speeds, we can design lighter wings and consequently more efficient airplanes. It is therefore, in the aircraft designer’s best interest to design innovative ways in which flutter can be controlled without making the resulting structure too heavy. There are three important choices to make while designing active control strategies for suppressing flutter. The first is the choice of actuator. In this paper, the actuators we use are jets in the walls through which there is a small mass flow, either by way of blowing or suction. The second is to define a clear control objective. Finally, we need to design a control law that will make suitable state measurements and drive the actuators so that the desired control objective is achieved. The concept of Active Flow Control is fast gaining popularity in Fluid Mechanics circles. Indeed, it is important to realize that adding or removing fluid at the wing surface is equivalent to effecting a shape modification. Flow control using surface jets should, in principle, have an effect very similar to that of morphing surfaces. The capability to directly alter the flow field offers a huge realm of possibilities. Seifert, Theofilis and Joslin categorize the problems that are amenable to using Active Flow Control: