Wing flapping is one of the most widespread propulsion methods found innature; however, the current understanding of the aerodynamics in bird wakes isincomplete. The role of the unsteady motion in the flow and its contribution tothe aerodynamics is still an open question. In the current study, the wake of afreely flying European starling has been investigated using long-durationhigh-speed Particle Image Velocimetry (PIV) in the near wake. Kinematicanalysis of the wings and body of the bird has been performed using additionalhigh-speed cameras that recorded the bird movement simultaneously with the PIVmeasurements. The wake evolution of four complete wingbeats has beencharacterized through reconstruction of the time resolved data, and theaerodynamics in the wake have been analyzed in terms of the streamwise forcesacting on the bird. The profile drag from classical aerodynamics was found tobe positive during most of the wingbeat cycle, yet kinematic images show thatthe bird does not decelerate. It is shown that unsteady aerodynamics arenecessary to satisfy the drag/thrust balance by approximating the unsteady dragterm. These findings may shed light on the flight efficiency of birds byproviding a partial answer to how they minimize drag during flapping flight.
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