Estimation of unsteady aerodynamics in the wake of a freely flying European starling

TL;DR

This study investigates the unsteady aerodynamics in the wake of a freely flying European starling using high-speed PIV and kinematic analysis, revealing the importance of unsteady effects in flight efficiency and drag-thrust balance.

Contribution

It provides the first detailed characterization of the unsteady wake in free bird flight and demonstrates the necessity of unsteady aerodynamics in understanding bird propulsion.

Findings

Unsteady aerodynamics are crucial for drag/thrust balance.

Classical profile drag is positive during most of the wingbeat.

Unsteady effects help explain how birds minimize drag during flapping.

Abstract

Wing flapping is one of the most widespread propulsion methods found in nature; however, the current understanding of the aerodynamics in bird wakes is incomplete. The role of the unsteady motion in the flow and its contribution to the aerodynamics is still an open question. In the current study, the wake of a freely flying European starling has been investigated using long-duration high-speed Particle Image Velocimetry (PIV) in the near wake. Kinematic analysis of the wings and body of the bird has been performed using additional high-speed cameras that recorded the bird movement simultaneously with the PIV measurements. The wake evolution of four complete wingbeats has been characterized through reconstruction of the time resolved data, and the aerodynamics in the wake have been analyzed in terms of the streamwise forces acting on the bird. The profile drag from classical aerodynamics was found to be positive during most of the wingbeat cycle, yet kinematic images show that the bird does not decelerate. It is shown that unsteady aerodynamics are necessary to satisfy the drag/thrust balance by approximating the unsteady drag term. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag during flapping flight.