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

Abstract

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 ( Calidris mauri ), European starling ( Sturnus vulgaris ) and American robin ( Turdus migratorius ) 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 ‘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.

Flow Features of the Near Wake of the Australian Boobook Owl (Ninox boobook) During Flapping Flight Suggest an Aerodynamic Mechanism of Sound Suppression for Stealthy Flight

“…5a for the owl and in Figs. 4 and 5 in Gurka et al. (2017) for the sandpiper and starling, respectively.…”

“…We use a similar approach as Gurka et al. (2017 ), which followed guidelines suggested by Wies-Fogh (1973), where the wingbeat cycle was divided into four distinct phases: upstroke (US), transition from US to downstroke (USDS), DS, and transition from DS to US (DSUS).…”

“…An error analysis based on the root sum of squares method was applied to the velocity data and the wing kinematics, following Gurka et al. (2017 ).…”

“…2012 ; Kirchhefer et al. 2013 ; Gurka et al. 2017 ), and measurements have been made in both the far wake ( Spedding et al.…”

Coevolving advances in animal flight and aerial robotics

“…By reconstructing the air-flow patterns in the wakes of three species of wild birds, Gurka et al [11] discovered unsteady aerodynamic effects may play a common role in their lift generation during forward flight. One key challenge these and other birds face is that they need to fly despite moulting wings.…”