Iso-surface plot of Q-criterion (2500) showing the vortices generated at the transition between upstroke and downstroke at 2 m/s, viewed obliquely from above and behind. Iso-surface plot of Q-criterion (2500) showing the vortices generated at the transition between upstroke and downstroke at 2 m/s, viewed obliquely from above and behind.
Long-eared bats are assisted in flight by their ears and body, according to a study by researchers at Lund University in Sweden. The recent findings improve understanding of the bats’ flying technique and could be significant for the future development of drones etc. Contrary to what researchers previously assumed, Christoffer Johansson Westheim et al show long-eared bats are helped in flight by their large ears.
“We show how the air behind the body of a long-eared bat accelerates downwards, which means that the body and ears provide lift. This distinguishes the long-eared bats from other species that have been studied and indicates that the large ears do not merely create strong resistance, but also assist the animal in staying aloft,” says Christoffer Johansson Westheim.
This is a photo of a flying bat. Credit: Anders Hedenstrom / Lund University
The findings entail a greater understanding of the flight technique of bats. They also highlight the evolutionary conflict between flying as efficiently as possible and eco-locating, i.e. discovering objects by sending out soundwaves and perceiving the resulting echoes.
Iso-surface plot of Q-criterion (2000) showing the body/wing root wake at 4 m/s during a fraction of the downstroke seen from above, colored by vertical speed (red upward, blue downward) (A) and streamwise flow relative to free stream (red backward, blue forward) (B). Multiple vortex ring structures form during a wing beat showing upward and forward induced flow, indicating negative weight support and drag. Flight direction is indicated by the arrow between A and B. Vector field, smoothed once, of the induced flow at mid downstroke of a bat flying at 4 m/s (C). Colors represent streamwise vorticity, with red being counter clockwise and blue clockwise rotation. Note the clear downwash behind the body and the upwash between the body and the wing root vortices.
Another discovery made during the experiments and never previously described in research is how the bats generate forward motion when flying slowly. The forward motion is generated when the wings are held high and away from the body at the end of each beat. “This specific way of generating power could lead to new aerodynamic control mechanisms for drones in the future, inspired by flying animals,” says Christoffer Johansson Westheim.
The experiments were conducted in a wind tunnel in which trained bats flew through thin smoke to reach a stick with food on it. Meanwhile the researchers aimed a laser beam at the smoke behind the bats and took pictures of the illuminated smoke particles. The researchers measured how the smoke moved to calculate the forces generated by each beat of the bats’ wings.
Video: https://www.youtube.com/watch?v=YhEkperhZMQ http://www.nature.com/articles/srep24886
http://www.eurekalert.org/pub_releases/2016-05/lu-bft050416.php
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