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.
Tag: flutter
Aeroelastic Flutter
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.
Flutter and the Tacoma Narrows Bridge
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.
Langley’s Transonic Dynamics Tunnel
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)
