The wake of a freely flying European starling (Sturnus vulgaris) was measured using high speed, time-resolved, particle image velocimetry, simultaneously with high speed cameras which imaged the bird. These measurements have been used to generate vector maps in the near wake that can be associated with the bird’s location and wing configuration. A kinematic analysis has been performed on select sequences of measurements to characterize the motion of the bird, as well as provide a point of comparison between the bird of the present study and other birds or flapping wings. Time series of measurements have been expressed as composite wake plots which relate to segments of the wing beat cycle for various spanwise locations in the wake. The wake composites invoke Taylor’s Frozen Flow Hypothesis. The applicability of Taylor’s Frozen Flow Hypothesis to the starling wake is discussed and evaluated. Measurements of the wake indicate that downwash is not produced during the upstroke, suggesting that the upstroke does not generate lift. Additional characteristics of the wake are discussed which imply the presence of (secondary) streamwise vortical structures, in addition to the wing tip vortices. The lack of downwash during the upstroke and the suggestion of secondary streamwise vortical structures constitute a deviation from a wake model which has been developed and supported by other bird species. Furthermore, these flow features indicate similarities between the wakes of birds and bats. In light of recent studies reported in the literature, the presence of secondary streamwise vortical structures may not only be a feature shared by birds and bats, but a general feature of flapping wings. Measurements also show spanwise vortical structures a short distance downstream of the bird. Based on existence of these spanwise vortical structures at such a close proximity to the bird, it is speculated that the wings of a starling may undergo dynamic stall during flight. This is also implied by the results of the kinematic analysis of the bird’s wing motion and comparison to other flapping wing studies. Dynamic stall, thought to be limited to hovering and slow flight, would enable high efficiency and high force coefficient generation.
Bird flight, European starling, flapping wing, particle image velocimetry (PIV), near wake, vortex wake, aerodynamics
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