The hydrodynamic performance of duck feet for submerged swimming resembles oars rather than delta-wings

Waterfowl use webbed feet to swim underwater. It has been suggested that the triangular shape of the webbed foot functions as a lift-generating delta wing rather than a drag-generating oar. To test this idea, we studied the hydrodynamic characteristics of a diving duck's (Aythya nyroca) foot. The foot's time varying angles-of-attack (AoAs) during paddling were extracted from movies of ducks diving vertically in a water tank. Lift and drag coefficients of 3D-printed duck-foot models were measured as a function of AoA in a wind-tunnel; and the near-wake flow dynamics behind the foot model was characterized using particle image velocimetry (PIV) in a flume. Drag provided forward thrust during the first 80% of the power phase, whereas lift dominated thrust production at the end of the power stroke. In steady flow, the transfer of momentum from foot to water peaked at 45(degrees) < AoA < 60(degrees), due to an organized wake flow pattern (vortex street), whereas at AoAs > 60(degrees) the flow behind the foot was fully separated, generating high drag levels. The flow characteristics do not constitute the vortex lift typical of delta wings. Rather, duck feet seem to be an adaptation for propulsion at a wide range of AoAs, on and below the water surface.

Automated summary

Waterfowl, such as ducks, use their webbed feet not only for swimming underwater, but also for generating lift and drag. This study investigated the hydrodynamic characteristics of a diving duck's foot and found that drag provided forward thrust during most of the power phase, while lift dominated thrust production at the end of the power stroke. The transfer of momentum from the foot to the water peaked at specific angles of attack.