Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "flutter"

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)