When your predators use echolocation to locate you, it pays to have an ultrasonic deterrence. So, many species of ermine moths have structures on their wings known as tymbals. These areas have a band of ridges, and, when the moth’s wing lifts or falls, the ridges buckle one-by-one. A nearby bald patch on the wing acts as an amplifier, making these ultrasonic snaps louder. Altogether, the mechanism deters prowling bats anytime the moth flaps its wings — without any additional effort on the moth’s part. Since the moths have no ears, they presumably don’t even know that they’re making the sound! (Image credit: Wikimedia/entomart; research credit: H. Mendoza Nava et al.; via APS Physics)
Tag: aeroelasticity
Rattling Feathers for Attention
Peacocks are known for their colorful mating displays, but it turns out there’s more to them than meets the eye. To help them gain a penhen’s attention, peacocks will sometimes rattle their train-feathers. The sound this makes is mostly below the range of human hearing, but the rustle creates subtle vortices in the air that cause the feathers atop a peahen’s head to vibrate. Playing back the sound at peahens from typical train-rattling distances also gets the females’ attention. Researchers found the playback makes peahens’ crests vibrate at a resonant frequency, suggesting that these feathers are for more than display; they’re used for communication as well! (Image and research credit: S. A. Kane et al.; video credit: Science)
The Tacoma Narrows Bridge
One of the most dramatic and famous engineering failures of the twentieth century is also one of the most complicated: the collapse of the Tacoma Narrows Bridge. This early suspension bridge earned the name “Galloping Gurtie” from construction workers while it was still being built because its flexibility made it prone to moving up and down under even relatively light winds. That vertical motion was due to vortex-induced vibration. As the wind blew, it shed vortices off the downstream side of the bridge. These vortices alternated, coming off the top and then bottom of the bridge deck. The resulting forces made the bridge shift up and down.
That wasn’t the bridge’s ultimate downfall, though. Shortly before it collapsed, the bridge stopped flexing up and down and instead twisted back and forth. This was a clear sign that the bridge had moved into aeroelastic flutter. In this situation, you get a feedback loop between the bridge’s aerodynamics and its structural dynamics. When the wind twists the bridge deck to a positive angle of attack, it will try to continue forcing the bridge to twist that direction. The internal forces of the bridge will try to twist it back, but when that happens, it can overshoot and end up at a negative angle of attack. At that point, the wind tries to push it further that direction and internal forces twist it back, overshooting the other way. This back-and-forth can create a dangerous feedback loop where the twisting of the bridge keeps getting worse and worse. In fact, that’s exactly what happened – right up until the bridge collapsed rather than twisting any more. (Video and image credit: Practical Engineering)
Pigeon Flutter
Birds are well-known for their vocalizations, but this isn’t their only way to produce noise. A new study on crested pigeons finds that the birds’ wings produce distinctive high and low notes during take-off. A low note takes place during each upstroke, and a high note is heard during the downstroke. A major source of the noise is the highly modified P8 feather. When airflow over the feather is fast enough, it sets off twisting and torsion in the feather through aeroelastic flutter. It’s this vibration that causes the noise. By playing back the notes at different speeds, researchers found that the crested pigeons use the notes’ timing as an alarm. When the cycle of high and low repeats in quick succession, they respond by taking off to escape the perceived danger.
Other bird species are also known to use aeroelastic flutter to make noise. Check out these hummingbirds, which use flutter in their mating displays. (Video credit: Science; research credit: T. Murray et al.)
Flag Flapping
Everyone has watched a flag flutter in the breeze, but you may not have given much thought to it. One of the earliest scientists to consider the problem was Lord Rayleigh, who wrote an aside on the mathematics of an infinite flag flapping in a paper on jets (pdf). Today researchers consider the problem in terms of fluid-solid interaction; in other words, to study a fluttering flag, you must consider both the properties of the flag – its flexibility, length, elasticity, and so on – and the properties of the fluid – air speed, viscosity, etc. The combination of these factors governs the complicated shapes taken on by a flag. The image above is a composite of several photos of a string (a 1-d flag) flapping in a flow that moves from left to right. By combining photos, the image highlights the envelope of shapes the flag takes and demonstrates at a glance just how far the flag flutters in either direction along its length. (Image credit: C. Eloy)
Inside Singing
These are the vocal folds of a woman singing. Human speech (and song) results from interactions between elastic muscles and aerodynamics. As we exhale, the vocal folds are initially pushed apart, then the flow of air moving past creates low pressure (via the Bernoulli effect) that helps pull the folds together. As the folds close, high pressure again forms to force them open. This sets a cycle of oscillation or vibration that produces sound. To change the pitch of the sounds we create, we can lengthen or shorten the vocal folds or change their tension. In this respect, they behave somewhat similarly to the strings of a musical instrument. If you’d like to admire more vocal folds in action, check out this endoscopic video for four singers performing together. (Image credit: LinguaHealth, source)
Buzzing Straws
Many woodwind instruments owe their sound to the vibration caused when air moves past parts of them. As Nick Moore demonstrates in this video, you can create a simple version of this effect with a slit drinking straw. The buzzing the straw produces when air passes through is a sort of aeroelasticity – it’s a combination of aerodynamic and structural forces that drive the behavior. Low pressure created by the fast-moving flow tends to draw the straw together, but once flow is stopped, the elasticity of the straw makes it rebound open, allowing air to flow again. Even more elaborate vibrations are possible when the straw is elastic. (Video credit: N. Moore)
Fluttering Feathers
Birds do not always vocalize in order to make their songs. The male African broadbill, shown in the top video above, makes a very distinctive brreeeet in its flight displays, but as newly published research shows, the sound comes from its wings, not its voice. During the display, the broadbill spreads its primary feathers and sound is produced on the downstroke, when wingtip speeds reach about 16 m/s. By filming a broadbill wing with a high-speed camera in a wind tunnel at comparable air speeds, researchers could localize the sound production to the 6th and 7th primary feathers.
In the second video above, you can see these feathers twisting and fluttering in the breeze. This is an example of aeroelastic flutter, a phenomenon in which aerodynamic and structural forces couple to induce oscillations. The same phenomenon famously caused the collapse of the Tacoma Narrows Bridge in 1940. In the birds, however, the flutter is non-destructive and the vibration produces audible sound which the other feathers modulate into the calls we hear. Broadbills aren’t the only birds to use this trick; some species of hummingbirds use flutter in their tail feathers during mating displays. (Video, image, and research credits: C. Clark et al.; additional videos here)
Flapping Flags
Sometimes structural forces and aerodynamic forces combine to produce instabilities. One of the most common and familiar examples of this, a flag flapping in the breeze, remains extremely complex to analyze and describe. The flexibility of the flag, and its small but finite resistance to bending, combine with the variability of air flow around the flag to create a fascinating dance of effects. This same aeroelastic flutter can create disastrous results for structures and aircraft. For more on the flapping flag, see Argentina and Mahadevan (2004). (Video credit: S. Morris)
Hummingbirds Singing with their Tail Feathers
Aeroelastic flutter occurs when fluid mechanical forces and structural forces get coupled together, one feeding the other. Usually, we think of it as a destructive mechanism, but, for hummingbirds, it’s part of courtship. When a male hummingbird looks to attract a mate, he’ll climb and dive, flaring his tail feathers one or more times. As he does so, air flow over the feathers causes them to vibrate and produce noise. Researchers studied such tail feathers in a wind tunnel, finding a variety of vibrational behaviors, including a tendency for constructive interference–in other words two feathers vibrating in proximity is much louder than either individually. For more, check out the original Science article or the write-up at phys.org. (Video credit: C. Clark et al.)
