High-speed stereo DPIV measurement of wakes of two bat species flying freely in a wind tunnel |
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Authors: | Anders Hedenström F T Muijres R von Busse L C Johansson Y Winter G R Spedding |
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Institution: | (1) Department of Theoretical Ecology, Lund University, Ecology Building, 223 62 Lund, Sweden;(2) Department of Biology, Bielefeld University, 33501 Bielefeld, Germany;(3) Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1191, USA;(4) Present address: Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria, 0002, South Africa |
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Abstract: | Previous studies on wake flow visualization of live animals using DPIV have typically used low repetition rate lasers and
2D imaging. Repetition rates of around 10 Hz allow ~1 image per wingbeat in small birds and bats, and even fewer in insects.
To accumulate data representing an entire wingbeat therefore requires the stitching-together of images captured from different
wingbeats, and at different locations along the wing span for 3D-construction of wake topologies. A 200 Hz stereo DPIV system
has recently been installed in the Lund University wind tunnel facility and the high-frame rate can be used to calculate all
three velocity components in a cube, whose third dimension is constructed using the Taylor hypothesis. We studied two bat
species differing in body size, Glossophaga soricina and Leptonycteris curasoa. Both species shed a tip vortex during the downstroke that was present well into the upstroke, and a vortex of opposite sign
to the tip vortex was shed from the wing root. At the transition between upstroke/downstroke, a vortex loop was shed from
each wing, inducing an upwash. Vorticity iso-surfaces confirmed the overall wake topology derived in a previous study. The
measured dimensionless circulation, Γ/Uc, which is proportional to a wing section lift coefficient, suggests that unsteady
phenomena play a role in the aerodynamics of both species. |
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