A Novel Hovering Mechanism from a Flapping Two-Dimensional Flexible Foil

Abstract

In the present experimental study we show that an airfoil, with a flexible flap attached to the trailing edge, which when pitched sinusoidally in still fluid creates a narrow, coherent, undulating jet composed of staggered vortices in the form of a 'reverse Benard-Karman vortex street'. This jet creates a corresponding thrust. We show that this is a simple and new mechanism of thrust generation in still fluid compared to the known mechanisms like normal, avian and vertical stroke-plane hovering present in insects and birds. This study is in part to understand the effect of flexibility, often observed in the wings of flying creatures, on flapping wing propulsion. We observe that in contrast, airfoil with rigid trailing edge produces a divergent, weak jet that meanders randomly. The substantial differences in the flows produced by the rigid and flexible models suggest that flexibility plays a crucial role in determining the flow structure.