Comparative aerodynamic performance of flapping flight in two bat species using time-resolved wake visualization

Abstract

Bats are unique among extant actively flying animals in having very flexible wings, controlled by multi-jointed fingers. This gives the potential for fine-tuned active control to optimize aerodynamic performance throughout the wingbeat and thus a more efficient flight. But how bat wing performance scales with size, morphology and ecology is not yet known.Here, we present time-resolved fluid wake data of two species of bats flying freely across a range of flight speeds using stereoscopic digital particle image velocimetry in a wind tunnel. From these data, we construct an average wake for each bat species and speed combination, which is used to estimate the flight forces throughout the wingbeat and resulting flight performance properties such as lift-to-drag ratio (L/D). The results show that the wake dynamics and flight performance of both bat species are similar, as was expected since both species operate at similar Reynolds numbers (Re) and Strouhal numbers (St). However, maximum L/D is achieved at a significant higher flight speed for the larger, highly mobile and migratory bat species than for the smaller non-migratory species. Although the flight performance of these bats may depend on a range of morphological and ecological factors, the differences in optimal flight speeds between the species could at least partly be explained by differences in their movement ecology.

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.…”

Bat flight: aerodynamics, kinematics and flight morphology

“…These vortices are shed from the intersection between the wing and body (wing root) because of a steep gradient in lift force (hence circulation) between the wing and body (usually referred to as 'root vortices'). The root vortices are mainly present during the downstroke (Hedenström et al, 2007;Muijres et al, 2011a;Hubel et al, 2009Hubel et al, , 2010Hubel et al, , 2012 and in some species also into the upstroke (Muijres et al, 2011a) (Fig. 2A).…”

“…1). This technique is quite laborious and therefore only a limited number of species have been studied so far (Hedenström et al 2007(Hedenström et al , 2009Johansson et al 2008;Hubel et al 2009Hubel et al , 2010Hubel et al , 2012aMuijres et al, 2011a;Spedding and Hedenström, 2009). Even though PIV studies are restricted to wind tunnels, where the animals may have a more controlled flight than in the open, these studies have significantly improved our understanding of the aerodynamics of bat flight.…”

Speed-dependent modulation of wing muscle recruitment intensity and kinematics in two bat species

“…A deeper understanding of the relationship between mechanical and aerodynamic power, force production and flight speed in bats might emerge from simultaneous measurements of muscle recruitment intensity for the full downstroke group [pectoralis major, serratus anterior, subscapularis, latissimus dorsi, biceps and teres major (Vaughan, 1970;Hermanson and Altenbach, 1983)] combined with an improved understanding of aerodynamic force production, as can be achieved using, for example, particle image velocimetry of the wake (e.g. Muijres et al, 2011;Hubel et al, 2012) and measurement of metabolic power (von Busse et al, 2013).…”