Modeling of a Lifting Surface 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 a former work by the authors, the aeroservoelastic behavior of a two dimensional (2D) wind turbine typical section with an active smart flexible flap was studied. In that work, the potential vibration control properties of an active flexible flap were exposed. In the present work, the study is extended to the three dimensional (3D) space. The flap is modeled as a flexible trailing edge, excited by a piezoelectric actuator, which allows the active morphing of the aerodynamic profile. Structurally, the flap is modeled as a continuum plate, with fixed-free boundary conditions and a piezoelectric actuator at its surface. The flap deflection, relative to the blade surface, is described by the assumed modes method. The flap bending modes are excited actively by means of a commercial piezoelectric actuator. Aerodynamically, the blade-flap system is modeled using an unsteady version of the vortex lattice method. In this model it is assumed that the viscous effects are confined at the boundary layer attached to the surface and the wake shed by the surface. The wake is modeled with vortex rings and it is allowed to move force-free. 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 axis wind energy turbines.