134 C. Fernandez-Escudero et al. Wing Fuselage Flap Flap-NES Aileron Typical Section Torque NES h α+ β+ β Fig. 17.1 Configuration of wing with NES with zero added mass Fig. 17.2 CAD of 3DOF mechanism [6] a purely nonlinear stiffness that presents high vibration absorption characteristics for a broadband frequency range and a low added mass. With the objective of controlling passively flutter of a wing for a minimum added mass, the idea of this work is to design one of the wing’s flaps as a NES (Fig. 17.1). In this respect, the work continues on the fundamental observations of Amar [6]. For a better understanding of the system, an experimental campaign was carried on a 2 degrees of freedom (DOF) and 3DOF aileron/airfoil typical section with a strongly nonlinear stiffness in the control surface. Similarly, pertinent numerical analysis is carried out. The aerodynamic forces acting on the airfoil are computed and these forces together with the equations of motion enable the computation of the fluid-structure interaction and, therefore, the resolution of the aeroelastic system. Different numerical methods are used for this purpose: Theodorsen theory, UVLM, unsteady Euler and URANS. The experimental setup was designed to enable the number of degrees of freedom to be varied to enable, firstly, the stiffness of each DOF to be calculated and, secondly, the 2DOF and 3DOF cases to be tested. In order to enable the movement of the wing, a mechanism is placed above and below the wind tunnel test section so that it has no influence on the airflow. The mechanical system can be divided into three subsystems each responsible for one of the DOF (see Fig. 17.2). Also, the stiffness of each DOF can be modified by changing the springs so nonlinearies can easily be introduced. The inlet velocity of
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