FYFD

Many species of kelp change their blade shape depending on the current they experience. In fast-moving waters, the kelp grows flat blades, but when the water around them is slower, the same plant will grow ruffled edges on its blades. In a slow current, the ruffled version’s extra drag causes it to flutter up and down with a large amplitude. That helps spread the blades out to catch more sunlight and increase photosynthesis, but it comes at the cost of higher drag, which could tear the plant from its holdfast.

In contrast, the flat-bladed kelp collapses into a more hydrodynamic shape. This clumps the flat blades together, making photosynthesis harder, but it streamlines the kelp, making it easier to resist getting ripped out by fast-moving tides. (Image credit: J. Hildering; research credit: M. Koehl et al.; submission by Marc A.)

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.)

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)

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)

Air dancers--those long fabric tubes with fans blowing into the bottom--are a popular way for shops to draw attention. They bend and flutter, shake and kink, all due to the interaction of airflow in and around them with the fabric. When the interior flow is smooth and laminar, the tube will stand upright, with very little motion. As the air inside transitions, some fluttering of the tube can be observed. Ultimately, it is when the air flow becomes turbulent that the cloth really dances. Variations in the flow are strong enough at this point that the tube will occasionally buckle. Behind this constriction, the flow pressure increases until its force is enough to overcome the weight of the tube and lift it once more. (Video credit: A. Varsano)

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)

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.)

One of the interesting challenges in fluid dynamics is the coupling of aerodynamic forces with structural forces. This could be the result of external flow, as with aeroelastic flutter on aircraft or architecture, or internal flow, as with the video above. Here researchers blow air through compliant cylindrical shells--think of a straw made of an elastic solid like latex--and observe the vibrations that result. Depending on the flow rate and material properties, different vibrational modes can be activated. The first mode behaves much like a garden hose that's not being held; it vibrates wildly back-and-forth. The second mode wobbles the mouth of the shell open and closed, whereas the third mode forms three "flaps" that vibrate inward and outward. Each of these modes behaves very differently, and, for practical applications, it's important for engineers to be able to predict, control, and account for these kinds of structural behaviors under aerodynamic loading. (Video credit: P. Zimoch et al.)

We've talked about aeroelastic flutter and the demise of the Tacoma Narrows Bridge before, but this explanation from Minute Physics does a nice job of outlining the process simply. As noted in the video, the common explanation of resonance is inaccurate because the wind was constant, so there was no driving frequency for the system.  (In contrast, consider vibrating a fluid where the response of the fluid depends on the frequency of the vibrations. This is resonance.) Instead the constant wind supplied energy that fed the natural frequencies of the structure such that an uncontrolled excitation built up. (Video credit: Minute Physics)

Aeroelasticity is the study of the interaction of structural and aerodynamic forces on an object, and its most famous example is flutter, which occurs when the aerodynamic forces on an object couple with its natural structural frequencies in such a way that a violent self-excited oscillation builds. What does that mean? Take a look at the video above. This compilation shows examples of flutter on wind tunnel models, road signs, airplanes, and the Tacoma Narrows Bridge--one of the most famous examples of all time. When air moves over and around an object, like a stop sign, it exerts forces that cause the structure to twist or vibrate. Those vibrations then alter the airflow around the object, which changes the aerodynamic forces on the object.  If the motion of the object increases the aerodynamic forces which then increase the oscillation, then a potentially destructive flutter cycle has been created. Flutter is very difficult to simulate computationally, so tests are usually performed experimentally to ensure that any vibrations in the system will damp out rather than grow to the point of structural failure like many of the examples in the film.

Flutter is a rather innocuous term for a potentially dangerous phenomenon that can occur for any flexible structure in a moving flow. Aeroelastic flutter occurs when aerodynamic forces and a structure's natural modes of vibration get coupled: the surrounding flow causes the object to vibrate, which alters the nature of the aerodynamic forces on the object, which, in turn, feeds into the object's vibration. In some cases, damping will contain the motion to a limit cycle, but under other conditions, flutter results in an uncontrollable self-exciting oscillation that persists until destruction, as in the famous Tacoma Narrows Bridge collapse.

Sixty years ago yesterday the original Tacoma Narrows Bridge (a.k.a. Galloping Gertie) collapsed as a result of aeroelastic flutter during 42 mph winds. Flutter is a phenomenon in which the fluid dynamics and structural dynamics of a system are closely coupled, in this case resulting in a dramatic failure. The high sustained winds provided an energy source for self-excitation of one of the bridge's torsional modes; as the bridge contorted, the motion caused additional vortices to be shed from the bridge deck, causing further vibrational forces on the bridge. For an analysis of the bridge's collapse and its common misrepresentations, see Billah and Scanlan. The bridge's spectacular collapse prompted reconsideration and redesign of the decks of modern suspension bridges.

NASA Langley's Transonic Dynamics Tunnel (TDT) recently celebrated 50 years of operation. It's 16 x 16 ft test section has hosted models of many aircraft, including the Lockheed Electra, the C-141, the F-15, the F-16, and the FA-18 shown above. The tunnel is primarily utilized for aeroelastic studies of flutter, a potentially catastrophic phenomenon where aerodynamic forces couple to a structure's natural modes of vibration. (via JediOliver and NASA_Langley)