Influence of stroke deviation on the power extraction performance of a fully-active flapping foil

The influence of stroke deviation on the power extraction performance of a fully-active flapping foil is numerically investigated in this work. A NACA0015 airfoil placed in a two-dimensional laminar flow is employed to extract power from the flow. It synchronously executes a rotational motion and a translational motion. In the traditional flapping foil based power extraction system, the foil only translates in vertical direction (heaving motion). In the current study, however, the foil can translate in both horizontal direction (surging motion) and vertical direction, which is attributed to the stroke deviation of a flapping wing. At a Reynolds number of 1100 and the position of the rotating axis at one-third chord, the effects of the amplitude of horizontal motion, the phase difference between the horizontal motion and the vertical motion as well as the frequency of horizontal motion on the power extraction performance are examined in detail. It is shown that compared with the traditional flapping foil, the efficiency improvement of power extraction for the flapping foil with additional horizontal motion can be achieved. Based on the numerical analysis, it is found that the enhanced power extraction from the vertical motion, which is induced by the increased lift force under appropriate horizontal motion, directly benefits the efficiency enhancement.