Experimental and Numerical Investigation of an Autonomous Flap for Load Alleviation

Abstract

This paper presents the design, a numerical aeroservoelastic investigation, and an experimental proof of concept of an autonomous flap system. Autonomous flaps are load-alleviation devices, intended for installation in the trailing edge of aerodynamic lifting surfaces (for example, wings or wind turbine blades) as plug-and-play units. An autonomous flap system, which is integrated into a free-floating flap actuated by a trailing-edge tab, consists of actuators, sensors, and a controller. Such a system is self-sufficient because it generates sufficient energy by harvesting the energy of mechanical vibrations. The energy-harvesting process is particularly efficient when the wing-flap system starts to flutter, which is then transformed into limit-cycle oscillations using the control system or structural delimiters. It has been shown experimentally and numerically that the autonomous flap system, when mounted in an aeroelastic apparatus, flutters at low wind speeds. During a wind-tunnel experiment, this flutter motion is converted into stable high-amplitude limit-cycle oscillations by limiting the amplitude of the flap deflections, whereas low-amplitude limit cycles could be reached by control activity. The autonomous flap system, when in controlled limit-cycle oscillation, has a positive energy balance. Sufficient energy could thus be generated to power sensors and actuators such that the flap system can be indeed used as an autonomous device.