Search

Wind-energy converters with rotating blades are most common for energy harvesting. In special cases a system of flapping wings may be of interest where the wings are oscillating in combined pitching and flapping (plunging) modes respectively. Then the ratio of pitching and plunging amplitudes is of main concern: If the amplitude of the pitching motion exceeds the amplitude of the induced angle of the plunging motion, energy transfer from the oncoming flow is present. To accomplish maximum energy transfers and maximum efficiencies a considerable number of parameters have to be varied. Most of the published investigations assume harmonic time variations of both pitching and plunging motion. In the present paper the time variation will be modified such that the stroke reversal times are favorably be influenced, i.e., the fraction of the period where the airfoil changes the angle of attack from its maximum to its minimum or vice versa. In the present paper this variation is represented by an analytical function. Further, the phase between pitch and plunge motion and the time dependent deformation of the airfoil-leading edge will be investigated. It will be shown that the different variations and modifications will lead to considerable increases of power transfer and efficiency.

Aerodynamic forces on flapping wings create forward thrust. Natural flyers like birds and insects apply these forces effectively. Extensive studies have shown that the efficiency of flapping wings can be improved by various means. Of importance is the development and control of leading edge vortices (LEV’s). Thrust production means that energy is transferred from the flapping system into the fluid. In a similar way it can be achieved that energy flows from the fluid into the flapping system, i.e. fluid energy may be harvested. Responsible for the direction of energy flow is the ratio of pitching amplitude versus amplitude of the induced incidence of the plunging motion. If this ratio is smaller than unity thrust energy is produced; if it is larger than unity energy is transferred into the flapping system. In the present paper, emphasis is placed on the detailed study of the aerodynamic effects and on some ideas of optimization of energy harvesting of a flapping system. It will be shown that similar to the thrust production mode also in the energy harvesting mode, the influence of LEV’s is of major concern. The control of these vortices by airfoil deformation is shown to be beneficial for optimizing the efficiency of energy harvesting.