The invisible power of 'flutter'—from plane crashes to snoring to free energy

March 30, 2018 by Justin Webster, The Conversation
Flapping flags flutter. Credit: withGod/shutterstock.com

With the car windows down on the first warm day of spring, the urge is unshakable. You extend your arm into the wind, tracing the city skyline in a natural motion somewhere between swimming and waving. As you move your hand, you alter the flow of the air. The redirected air in turn exerts a force on your hand.

Interactions like this – between a , like water or air, and a flexible structure – are ubiquitous in nature. You can see them in a flapping flag, a garden hose spraying wildly or even the mild annoyance of a snoring significant other.

Such interactions are carefully considered in the design of buildings, bridges and aircraft. The principal reason? A structure can become fundamentally unstable when immersed in a fluid , like that of air or water.

This type of instability is known as flutter, and it can cause catastrophic failure. A harrowing example, sadly involving loss of one canine life, is the collapse of the Tacoma Narrows Bridge ("Galloping Gertie") in 1940.

As an applied mathematician, my goal is to understand flutter – why it happens, when it happens and how to help engineers stop it (or bring it about, depending on the situation).

Flutter 101

Whether or not you've ever used the word flutter, you've encountered the phenomenon. Preventing flutter in aircraft components, for example, constitutes a key challenge for a multi-billion-dollar industry.

Another pertinent example is the flutter of the human soft palate. Intense snoring correlates with the serious medical condition of , plaguing one in 15 adults in the U.S.

To an , the flutter phenomenon is known as a self-excitation. In other words, in the right conditions, an inherently stable structure can become unstable. Think back to the hand waving outside the car window: As the hand moves slightly, the is altered, pushing back on the hand. If the hand responds to this force, it changes the air flow again, and on and on.

For a flexible object under just the right circumstances, this cycle may persist, resulting in a potentially violent periodic motion. It's like the movement of a tuning fork or guitar string, but at the scale of building, airplane wing or bridge.

For contrast, consider the resonance phenomenon – like a child pushed on a swing or soldiers marching on a bridge. In these instances, a periodic application of force, acting with the right frequency, amplifies the scale of the existing oscillations. Flutter is fundamentally different and somehow more disconcerting, requiring only a surrounding flow and no cyclic application of force.

Closer study

In the early days of flight, with little academic knowledge of flutter, pilots could encounter wing and tail flutter simply by flying into a sustained headwind at the wrong altitude. Engineers now believe that many early aircraft crashes were the result of flutter events.

Some of the first academic studies of flutter occurred in the Cold War era, when countries maintained an interest in delivering rockets to one another. At extreme velocities at or above the speed of sound, the rocket paneling could flutter, potentially destabilizing the flight trajectory. Preventing panel flutter – or at least minimizing its effect – ensured that a projectile found its intended destination.

Today, engineers and scientists aim to produce sophisticated mathematical models that accurately capture flutter. This can mean a variety of things, but, most importantly, it means the model makes predictions that can be verified in a controlled experimental setting. If this is the case, and the is deemed viable, engineers and scientists can produce better designs with it.

Predicting if flutter occurs, for a given flexible object in a given fluid flow, is typically not the problem; simple mathematical models can often accomplish this. However, it's even more difficult to mathematically capture precisely what happens after the object becomes unstable and flutter begins. New, more complex models have been proposed, but are not yet completely understood.

For example, state-of-the-art models still struggle to capture the flutter from large flapping motions at the end of a long beam – like gusting wind along the length of a diving board. Engineers and mathematicians agree that many existing models are deficient, providing an active area of research.

New promise

However, the study of flutter isn't just about preventing catastrophes or more effectively delivering rockets. In the last decade, engineers and scientists have figured out how to harvest energy from certain types of flutter.

A small metallic strip just centimeters long can be easily excited by a flow along its length, in a manner analogous to a flapping flag. This motion can generate a small amount of electrical power. A viable mathematical model could capture the complex interactions at play and help engineers more efficiently harvest this energy from everyday sources like wind or a moving car.

If little clips like this could flutter, then it might generate enough power to, say, charge an iPhone. One day, such flutter technology could help power remote areas and cut down on battery-related waste.

Explore further: Study highlights readmit factors post atrial flutter ablation

Related Stories

Years of endurance exercise may raise A-fib/Flutter risk

October 18, 2014

(HealthDay)—Cumulative years of regular endurance exercise are associated with an increased risk for atrial fibrillation and atrial flutter, according to a study published in the Oct. 15 issue of The American Journal of ...

Optimizing flutter shutter to minimize camera blur

March 23, 2016

Whether taking photos recreationally or professionally, photographers understandably want their snapshots to appear sharp and clear. Image clarity is dependent on exposure time, or the amount of time that a camera's sensor ...

Recommended for you

Fish-inspired material changes color using nanocolumns

March 20, 2019

Inspired by the flashing colors of the neon tetra fish, researchers have developed a technique for changing the color of a material by manipulating the orientation of nanostructured columns in the material.

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

Macrocompassion
not rated yet Apr 01, 2018
Aircraft flutter is due to the combination of 3 significant properties: structural stiffness, mass and aerodynamic force. As the structural center of twist of a loaded wing or flat surface is moved backwards, the critical speed at which flutter commences will reduce. So a good design has a wing box at its leading edge.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.