Wake structure and wing kinematics: the flight of the lesser dog-faced fruit bat,Cynopterus brachyotis

Abstract

SUMMARYWe investigated the detailed kinematics and wake structure of lesser dog-faced fruit bats (Cynopterus brachyotis) flying in a wind tunnel. High speed recordings of the kinematics were conducted to obtain three-dimensional reconstructions of wing movements. Simultaneously, the flow structure in the spanwise plane perpendicular to the flow stream was visualized using time-resolved particle image velocimetry. The flight of four individuals was investigated to reveal patterns in kinematics and wake structure typical for lower and higher speeds. The wake structure identified as typical for both speed categories was a closed-loop ring vortex consisting of the tip vortex and the limited appearance of a counter-rotating vortex near the body, as well as a small distally located vortex system at the end of the upstroke that generated negative lift. We also investigated the degree of consistency within trials and looked at individual variation in flight parameters, and found distinct differences between individuals as well as within individuals. Supplementary material available online at

Hummingbirds generate bilateral vortex loops during hovering: evidence from flow visualization

“…Wing root vortices are generated through a similar mechanism as wingtip vortices, where streamwise vorticity of opposite spin rolls up into a distinct vortex. These vortices have been observed in insects, birds, and bats with varying strength and persistence (Hedenström et al 2007Muijres et al 2008;Muijres et al 2011Muijres et al , 2012Bomphrey et al 2009;Johansson and Hedenström 2009;Hubel et al 2010Hubel et al , 2012Henningsson et al 2011). The cause and function of these vortices has not been fully established.…”

“…Particle image velocimetry (PIV) methods have improved in temporal and spatial resolution over the last few years as have the coverage of species of different sizes and flight modes. One of the significant additions to the emerging model of the wakes of flying animals is that vortices can be shed from the wing roots in both birds and bats over a wide range of sizes and flight speeds, although the strength and persistence of these root vortices varies by species and speed Johansson and Hedenström 2009;Hubel et al 2010Hubel et al , 2012Henningsson et al 2011;Muijres et al 2011Muijres et al , 2012. The presence of root vortices indicates that if and when the body is included in producing vertical force, this requires some time to develop.…”

Biomechanical, Respiratory and Cardiovascular Adaptations of Bats and the Case of the Small Community of Bats in Chile

“…Recent studies using digital particle image velocimetry (DPIV) have allow us to study the wake left behind flying bats with much higher temporal and spatial resolution than those original studies. The emerging picture of the aerodynamic footprint left by bats is that the wake structures are more complex than expected, potentially because of the more complex wing kinematics than that of birds and insects as well as the compliant characteristics of the wing membrane, and currently it is an area of very active research (e.g., Hedenström et al, 2007;Johansson et al, 2008;Muijres et al, 2008;Hedenström et al, 2009;Hubel et al, 2009;Hubel et al, 2010;Wolf et al, 2010). Fig.…”

On Applicability of the Kutta-Joukowski Theorem to Low-Reynolds-Number Unsteady Flows

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“…In contrast, the Reynolds numbers for insects are typically less than 5000. To understand unsteady flow fields around flying birds and bats, particle image velocimetry measurements have been conducted in wind tunnels [3][4][5][6][7] . Since the aerodynamic forces of a flying animal cannot be directly measured using a force balance, the Kutta-Joukowski (K-J) theorem has been used in these studies to infer the lift after the circulation is estimated by integrating the vorticity field in a selected cross-section region near wings or in wakes generated in flapping flight.…”