Extraction of Flow Energy by a Wing Oscillating in Waves

This paper attempts to investigate the possibility of extracting energy from surrounding flow by a two-dimensional hydrofoil oscillating through a gravity wave in water, or by an airfoil in gust. The greatest possible rate of energy extraction is obtained by the optimum mode of heaving and pitching of the hydrofoil, which is determined by the method of variational calculation of parameters. It is found that energy extraction is impossible if the incident flow is uniform, and becomes feasible only when the primary flow contains a wave component having a transverse velocity distribution normal to both the mean free stream and the wing span. When such waves of sufficiently large amplitude are present, not only flow energy but also a net mechanical power can be extracted from the surrounding flow.