Imaging Shows How Bats Share Vital Flying Trait with Insects
According to conventional aerodynamic theory, the wings of bees and other insects should not generate enough lift, especially during hovering and slow flight. Nonetheless, insects do fly. Various researchers have determined that insects use an assortment of unsteady high-lift mechanisms, with the most important of these a leading-edge vortex.
Now a group with members from Lund University in Sweden and from the University of Southern California in Los Angeles has shown that bats use similar tricks when airborne. The results are not a complete surprise but, according to Anders Hedenström of Lund University, the study is groundbreaking because it represents the first demonstration in a noninsect animal.
In this image, the movement of air is mapped at different locations, with the formation of a vortex on the leading edge of the wing depicted. This vortex is responsible for much of the lift generated by the wing, allowing the bat to stay airborne at slow speeds. Courtesy of Anders Hedenström, Lund University, Sweden.
A leading-edge vortex, as the name implies, forms at the forward edge of a wing as it moves through the air. This effect generates up to two-thirds of the total lift of an insect’s wing, and previous studies had shown that bat wings also generate more lift than conventional aerodynamics would predict.
Uncovering the mechanism responsible for that additional lift presented a challenge. The researchers had to measure air flow over a moving wing on living animals with enough detail that relatively subtle effects would not be washed out.
Digital particle image velocimetry (DPIV) made the investigation practical. In this technique, a sheet of light generated by a laser illuminates air that is seeded with 1-μm fog particles, providing a snapshot of the positions of the particles at a specific point in time. The air flow is determined by measuring the particles’ changes in position over time.
“I think DPIV is the best method here. Smoke streaks would be hard to interpret,” Hedenström said.
They used a custom DPIV setup based on a dual-head Nd:YAG laser from Spectra-Physics of Mountain View, Calif., running at a repetition rate of 10 Hz. They also used a CCD camera from Redlake Inc. of Tallahassee, Fla., to capture particle images at 200-μs intervals.
Using various optics, they routed the beam and expanded it into a sheet that sliced across a wind tunnel. They trained bats to feed on honey-infused water that was dispensed from a tube suspended within the wind tunnel, with the location such that the laser sheet illuminated the bats. They set the wind tunnel air-speed to 1 m/s.
When the bats flew from their perches to get a drink, the researchers monitored their motions, recording data for later analysis. They acquired kinematics data of the bats in motion using a pair of synchronized Redlake cameras set to record at 250 Hz.
As reported in the Feb. 29 issue of Science, the investigators looked at the vortices spawned along the leading and trailing edges of the bats’ wings, tracking particle displacement as the wings traveled through their strokes. They found that the leading-edge vortex circulation increased toward the wing tip, a phenomenon consistent with the case for some insects. They also found that more than 40 percent of the total lift came from the leading-edge vortex — again consistent with insect research.
Hedenström noted that these findings demonstrate that bats may flap much more slowly than insects but that they can develop and control the stability of a leading-edge vortex. “It shows that the bat wing is a very delicate design, able to change the camber — curvature — of the wing appropriately for this to happen.”
The group is working with a new DPIV system that allows much higher sampling rates along with three-dimensional visualization and thus should provide more detailed information about vortex dynamics in the wake caused by beating wings, as well as about 3-D components of the leading-edge vortex.
Hedenström added that such information could have practical uses. For example, it might be possible to apply the knowledge to building a flying machine the size of a bat, which could have military, commercial and scientific applications.
Contact: Anders Hedenström, Lund University; e-mail: email@example.com.
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