The near and far wake of Pallas' long tongued bat (Glossophaga soricina)

Abstract

SUMMARYThe wake structures of a bat in flight have a number of characteristics not associated with any of the bird species studied to this point. Unique features include discrete vortex rings generating negative lift at the end of the upstroke at medium and high speeds, each wing generating its own vortex loop, and a systematic variation in the circulation of the start and stop vortices along the wingspan, with increasing strength towards the wing tips. Here we analyse in further detail some previously published data from quantitative measurements of the wake behind a small bat species flying at speeds ranging from 1.5 to 7 m s -1 in a wind tunnel. The data are extended to include both near-and far-wake measurements. The near-/far-wake comparisons show that although the measured peak vorticity of the start and stop vortices decreases with increasing downstream distance from the wing, the total circulation remains approximately constant. As the wake evolves, the diffuse stop vortex shed at the inner wing forms a more concentrated vortex in the far wake. Taken together, the results show that studying the far wake, which has been the standard procedure, nevertheless risks missing details of the wake. Although study of the far wake alone can lead to the misinterpretation of the wake topology, the net, overall circulation of the main wake vortices can be preserved so that approximate momentum balance calculations are not unreasonable within the inevitably large experimental uncertainties. Supplementary material available online at

Aerodynamics of gliding flight in common swifts

“…Rayner, 1979a;Rayner, 1979b;Spedding, 1987), and modern techniques of digital particle image velocimetry (DPIV) have revealed both qualitative and quantitative properties about bird and bat wakes (e.g. Spedding et al, 2003;Hedenström et al, 2006;Hedenström et al, 2007;Rosén et al, 2007;Johansson et al, 2008;Hubel et al, 2009). However, only one study to date has investigated the wake shed from a gliding bird (Spedding et al, 1987), showing a rather simple wake consisting mainly of two wingtip vortices.…”

Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure

“…Such two-dimensional visualizations, known as particle image velocimetry (PIV), are accomplished by seeding the air or water with small particles, and using a sheet of laser light to illuminate flow patterns that are then imaged at rates varying from 3 to 1000 Hz. These two-dimensional slices are then used to reconstruct an estimated three-dimensional flow model [1,2,[6][7][8]12,13]. However, building up a three-dimensional pattern of flow vorticity from separate two-dimensional slices is extremely challenging, owing to the need to estimate out-of-plane fluid motion and the requirement of phase-averaging intrinsically unsteady animal movements to derive an average flow pattern [14].…”

The vortex wake of blackcaps (Sylvia atricapillaL.) measured using high-speed digital particle image velocimetry (DPIV)

“…Circulation has been measured in the streamwise vertical plane at different positions along the span of the birds, but none of these species have been studied in the transverse plane [with the exception of hovering humming birds (Warrick et al, 2005)]. Recent findings from studies of bat wakes have pointed to the importance of transverse plane data for the reconstruction of the wake topology because important structures may be otherwise missed Johansson et al, 2008). The suggested vortex wake model for bats is more complex than the ones suggested for birds (Spedding et al, 2003;Henningsson et al, 2008) and includes streamwise structures (for example vortices shed at the wing root) not suggested by the bird data (Spedding et al, 2003).…”

“…Here we mainly focus on a different hypothesis for modulating the flow behind a bird; namely, that the function of the tail is to interact with wing root vortices, shed as a result of spanwise circulation gradients of flapping wings, and thereby reducing the cost of flight by extracting some of the energy shed into the wake by the wings. This hypothesis originates from the findings that bat wakes consist of vortices, of opposite sign to the wing tip vortices, near the wing root of two tailless bat species Johansson et al, 2008;Hedenström et al, 2009). Even though no studies of the wake in the transverse plane have been performed for forward flight in birds (where these vortices would be visible), the available data have not indicated any existence of wing root vortices in birds (Spedding et al, 2003;a reduced circulation behind the body relative to the wings occurs (Henningsson et al, 2008).…”