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The first bat wake

Our first study focused on the wake properties of a small nectar feeding bat, Glossophaga soricina. This is a well studied species which have been held and bred in captivity for decades and therefore ideal for aerodynamic studies. It also proved very convenient as a wind tunnel animal because these bats will feed honey water from a thin tube suspended in the test section, and hence we could count on repeatable flight behavior as the bats would return to the feeder to feed.

Frontal view of a bat flying superimposed on the vector field of its wake
An illustration of the bat flying in front of a transverse flow field as measured with PIV.

We pointed high speed cameras directly to the flight position at the feeder, and the PIV camera and light sheet just behind. The wakes were sampled at the speed interval 1-7.5 m/s, using both a light sheet aligned with the flow and one transverse with the flow. In this way we could more easily interpret the 3D wake structure.

Bat wakes appeared significantly different from those of birds, especially during the wing upstroke. The bats push air upwards during the upstroke, which was never observed in birds. The bat wake also exhibits other novel features, such that each wing generates its own isolated vortex loop, while in birds there seem to be one vortex loop shed from the whole animal.

Composite vector fields showing the wake of a bat
Example of a composite velocity and vorticity plot of longitudinal wake measurements behind a Pallas' long tongued bat flying at 5 m/s.
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Cartoons of wake models
Comparison between suggested wake models of birds (A) and of bats (B) in cruising flight. The wake of bats appears to be much more complex.