Reversed flapping flight and inverted hydrodynamical drafting

I will discuss two recent laboratory experiments on interactions between moving boundaries and a surrounding fluid. Both experiments have shown surprising results. In the first experiment, we study the flapping flight of a rigid wing that is allowed to pitch (rotate) passively in response to fluid forces. We demonstrate a new state in which the flapping wing flies backward as the flapping frequency exceeds a certain threshold. This counter-intuitive transition occurs as the wing's leading edge in forward flight becomes the trailing edge in reversed motion. A dimensionless number -- the "passive pitching" -- demarcates the transition. In the second experiment, we investigate the schooling of fish or the flocking of birds by studying the group dynamics of interacting flapping flags in a moving fluid. We have found that, unlike the well-known hydrodynamic drafting of rigid objects placed in tandem, the leading flapping flag enjoys a reduced drag while the follower suffers a drag increase. If this result from passive, flexible objects can be applied to self-propelled locomoting bodies, it implies that it is easier to lead than to follow in a group.