Flow energy harvesting based on oscillating passively-deforming airfoils
The rising global trend to reduce dependence on fossil fuels has provided significant motivation toward the development of alternative energy conversion methods and new technologies to improve their efficiency. Recently, the idea of using oscillating airfoils has been gaining a wider scope of attention as a means of extracting kinetic energy from streams, rivers, tidal flows and wind (Xiao & Zhu 2014). A large contribution to the existing knowledge has come through the studies of animal flight and swimming (Drucker & Lauder 1999; Triantafyllou et al. 2000; Sane 2003; Ho et al. 2003), where the oscillatory/flapping motion of wings or fins are used to achieve high propulsion efficiency and maneuvering. The concept of flow energy harvesting using oscillating airfoils was first proposed by McKinney & DeLaurier (1981). The motion kinematics of the oscillating airfoil, which is typically modeled as combined heaving and pitching motion at very large angles of attack, results in ow separation and formation of leading edge vortices (LEVs). LEV structures are exploited by oscillating airfoils to attain high energy harvesting efficiency values. This is in contrast to the conventional rotary turbines, where the ow around the blades must remain fully attached to the surface to achieve high efficiency levels. Preliminary studies show that oscillating airfoil energy harvesters are capable of extracting energy with efficiency comparable to rotary devices Kinsey & Dumas (2008); Zhu (2011); Young et al. (2014). Furthermore, there are several prominent features of oscillating airfoil energy harvesters compared to the conventional turbines: (i) they are environmentally friendly in terms of noise generation due to their relatively low tip-speed, thus reducing impact on the navigation of flying/swimming animals; (ii) without the centrifugal stress associated with rotating blades, the oscillating devices are structurally robust; and (iii) oscillating devices sweep through a rectangular cross section of the ow, and therefore the swept area of a single airfoil can be wide and shallow, allowing large systems to be installed in shallow water (Zhu 2011). With the rapid development of such devices (industry is already involved in developing full-scale prototypes), the knowledge of their underlying fluid dynamics is required to improve the efficiency of existing devices. In particular, there is a need to thoroughly understand the spatio-temporal evolution of the ow around the oscillating airfoil, in order to develop mechanisms to control the LEV dynamics that will lead to improved energy harvesting performance.