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

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

Control volume around a two-dimensional bird wing section in a uniform free stream.

<p>Control volume around a two-dimensional bird wing section in a uniform free stream.</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