Schooling behavior of rigid and flexible heaving airfoils

The schooling behavior of rigid and flexible NACA0017 airfoils undergoing a heaving motion was experimentally explored using a merry-go-round configuration. Each airfoil was attached to the end of a horizontal support bar whose other end was connected to a freely rotating vertical axis. The axis was forced to undergo a sinusoidal motion in the vertical direction to generate a pure heaving motion of the airfoil in the frequency range of 0.4 to 4.8 Hz. The propulsion due to the heaving airfoil was expressed as the horizontal rotational speed of the support bar. This experimental setup simulates an infinite schooling of airfoils separated by a streamwise distance d undergoing in-phase heaving motions. The ratio of the distance to the chord length, d/c, was determined by the number of airfoils (2 ≤ n ≤ 8). The variation in rotational frequency F as a function of heaving frequency f was determined using different experimental parameters. The schooling number S = f /(nF), which represents the number of heaving oscillations between each pair of successive airfoils, was introduced to explain the schooling behavior of the airfoils. The effects of airfoil flexibility, d/c and f on the propulsive performance were examined in the context of the schooling behavior of the airfoils.