Aeroservoelastic Behavior of a Wind Turbine Typical Section with an Active Smart Flexible Flap

Abstract

In the past years, the consumption of energy produced by wind turbines had an exponential growth. This requirement gave momentum to the development of larger turbines with the goal of producing more energy at the same site, reducing the initial investment, and the operation and maintenance costs. In order to achieve this objective, longer, lighter, maintenance-free blades are required so that smaller loads are transferred to the other, more expensive, wind turbine components.
The resulting larger flexibility, imposes new challenges to the blade and controller designs; henceforth, new concepts are being developed to add more intelligence into these systems. During the last few years, the electronics industry had invested resources into the research and development of practical applications for piezoelectric ceramic materials. The result of this effort was the development of high precision piezoelectric actuators and sensors, which achieve forces and deformations that are compatible with the ones needed for the control of aerodynamic surfaces. In this work, the aeroservoelastic behavior of a wind turbine blade typical section equipped with an active smart flap is numerically simulated. The bending and torsion stiffness of the blade are modeled by means of two springs placed at the shear center of the blade's section. The displacements associated to these two deformation modes are described by means of two discrete generalized coordinates. Structurally, the flap is modeled as a continuous beam, with fixed-free boundary conditions, and an embedded piezoelectric actuator. The bending mode of the flap is actively excited through the use of a commercially available piezoelectric actuator. The model response was compared to the data published by the actuator manufacturer. Aerodynamically, the blade-flap system is modeled assuming the hypotheses of thin airfoil theory. The aerodynamic loads are determined by replacing the vortex sheet with a two dimensional (2D) version of the non-linear, unsteady, vortex lattice method. To capture the physical aspects from the control-fluid-structure interaction, the models are combined using a strong coupling technique. The equations of motion of the system are integrated numerically and interactively in the time domain. In addition, the stability and sensitivity of the system for input perturbations are analyzed. The results show the feasibility of using this type of system in large horizontal wind energy turbines.