The Near Wake of a European Starling

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

On the estimation of time dependent lift of a European Starling during flapping

We study the role of unsteady lift in the context of flapping wings in birds' flight. Both aerodynamicists and biologists attempt to address this subject, yet it seems that the contribution of the unsteady lift still holds many open questions. The current study deals with the estimation of unsteady aerodynamic forces on a freely flying bird through analysis of wingbeat kinematics and near wake flow measurements using time resolved particle image velocimetry. The aerodynamic forces are obtained through unsteady thin airfoil theory and lift calculation using the momentum equation for viscous flows. The unsteady lift is comprised of circulatory and non-circulatory components. Both are presented over wingbeat cycles. Using long sampling data, several wingbeat cycles have been analyzed in order to cover the downstroke and upstroke phases. It appears that the lift varies over the wingbeat cycle emphasizing its contribution to the total lift and its role in power estimations. It is suggested that the circulatory lift component cannot assumed to be negligible and should be considered when estimating lift or power of birds in flapping motion

Reconstruction of the wake velocity field from the time resolved PIV images from Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper (<i>Calidris mauri</i>), European starling (<i>Sturnus vulgaris</i>) and American robin (<i>Turdus migratorius</i>) using long-duration, time-resolved Particle Image Velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named as ‘double branch’). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag

Detailed description of the flow features characteristics for the individual birds from Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper (<i>Calidris mauri</i>), European starling (<i>Sturnus vulgaris</i>) and American robin (<i>Turdus migratorius</i>) using long-duration, time-resolved Particle Image Velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named as ‘double branch’). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag

Vorticity and circulation calculation from Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper (<i>Calidris mauri</i>), European starling (<i>Sturnus vulgaris</i>) and American robin (<i>Turdus migratorius</i>) using long-duration, time-resolved Particle Image Velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named as ‘double branch’). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag

Estimation of circulatory lift component based on wingbeat number 1.

<p>(a) The circulatory lift was estimation based on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0134582#pone.0134582.e038" target="_blank">Eq (33)</a>. The grey area indicates downstroke flapping phase. (b) Reconstruction of the starling’s wake vorticity as thought the bird flies from right to left.</p

Auxiliary diagram showing notation employed.

<p>Auxiliary diagram showing notation employed.</p

Variation of the average streamwise velocity in the wake with time.

<p>Variation of the average streamwise velocity in the wake with time.</p

An estimation of the wing’s maximum angle of attack from its instantaneous pitch and velocity relative to the air.

<p>In (a), the left wing of the bird points directly at the camera, and the pitch of the chord line at approximately (2/3)<i>b</i>_semi is represented by the blue line. In (b), the pitch of the chord line and the velocity of the wing section at (2/3)<i>b</i>_semi are used to estimate the angle of attack, <i>α</i>, as approximately 15°.</p

Different positions of the starling during the four wingbeats.

<p>The labels of the photographs correspond to the wingbeat number; ‘a’ marks the beginning of the downstroke, and ‘b’ marks the beginning of the upstroke. Each image center embedded with a red dashed cross lines for a visualization of the starling’s position during each wingbeat.</p