EFFECT OF FLEXIBLE FLAP LENGTH ON FLOW GENERATION BY AN AIRFOIL PITCHING IN QUIESCENT FLUID

Abstract

We experimentally investigate the effect of the length of a chordwise flexible surface (flap) attached to the trailing edge of a symmetric rigid airfoil purely pitching in quiescent fluid on flow, and thus thrust generation. We aim to identify the optimum length of the flexible appendage necessary for coherent, orderly jet generation in a situation relating to hovering. To this end, we consider a flexible flap of a 50 μm thick polyethylene sheet. We systematically vary the length of the flexible flap (<i>c<sub>f</sub></i>) over a wide range, from <i>c<sub>f</sub></i> = 0.5 c to 4 c, where c is a rigid foil chord. Moreover, we explore the effect of pitching amplitude and frequency for each flap length. Based on length, the flaps are categorized as short (<i>c<sub>f</sub></i> ≤ 1), moderate length (1 < <i>c<sub>f</sub></i> ≤ 2), and long (<i>c<sub>f</sub></i> > 2) flaps. Detailed particle visualization study reveals that flap length has the predominant effect onflow generation. We find that for small amplitude-frequency pitching, flaps with moderate length generate narrow and coherent jets, while short and long flaps generate spread out and deflected jets. For moderate amplitude-frequency pitching, flexible flaps of all lengths, except the short flaps, generate coherent and aligned jet. However, for large amplitude-frequency pitching, only a few long flaps produce a narrow jet and thrust along the centerline. Results suggest that optimum flap length is a strong function of pitching kinematics. Interestingly, we also find that more than one optimal flap length exists for moderate and large pitching parameters which generate the orderly jet with small/minimum width.