Footprints of a flapping wing

Abstract

Birds have to flap their wings to generate the needed thrust force, which powers them through the air. But how exactly do flapping wings create such force, and at what amplitude and frequency should they operate? These questions have been asked by many researchers. It turns out that much of the secret is hidden in the wake left behind the flapping wing. Exemplified by the study of Andersen et al. (J. Fluid Mech., vol. 812, 2017, R4), close examination of the flow pattern behind a flapping wing will inform us whether the wing is towed by an external force or able to generate a net thrust force by itself. Such studies are much like looking at the footprints of terrestrial animals as we infer their size and weight, figuring out their walking and running gaits. A map that displays the collection of flow patterns after a flapping wing, using flapping frequency and amplitude as the coordinates, offers a full picture of its flying 'gaits'.

Undulatory and oscillatory swimming

“…We will show here that we should treat the wake structure and the swimming performance (thrust, power and efficiency), as two separate but inter-related topics, in that there is no obvious one-to-one connection between them. Zhang (2017), in the context of a flapping wing, cautioned that examining the flow pattern is much like looking at the footprints of terrestrial animals; although some information can be gleaned from the wake, it does not tell the whole story. Similarly, Taylor (2018) writes that an explanation of propulsive efficiency given only in terms of the wake feels a little incomplete, and notes that Eloy (2012) suggested that efficient development of the wake may be a consequence, rather than a cause, of efficient propulsion.…”

Influence of three-dimensionality on propulsive flapping

“…Koochesfahani (1989) indicated that the flow at certain range of St A over a high AR foil, which experiences periodical 2D motions such as pure heaving and pure pitching, is mostly assumed to be 2D. Researchers found that soap film experiments agreed with 2D simulations (Andersen et al 2016;Schnipper et al 2009;Zhang 2017), and high AR foil experiments agreed with 2D simulations (Moriche et al 2016;Muscutt et al 2017). These studies, however, did not explain why the flapping foil being subjected to periodic high angles of attack remains two-dimensional while a stationary foil at the same (or similar) angles of attack experiences strong separation (Uddin et al 2017) where 3D effects are not negligible.…”

Swimmers’ wake structures are not reliable indicators of swimming performance

“…When swimmers propel themselves through a fluid, they leave a distinct pattern of fluid flow in their wakes analogous to the footprints of terrestrial animals [1]. The flow structures in the wake carry much information with them; for example, a proper control volume analysis can translate the velocity and stress fields in the fluid to forces on the swimmer.…”