From the vibrations of the rear-view mirror just as a car reaches 70 miles per hour to a building that collapses when, in an earthquake, it begins to vibrate at a specific frequency, there is untapped energy that could be harnessed for propulsion. Researchers found relationships between frequencies and the passive dynamics at play when vehicles move in air or water that help them understand how to use these forces to enhance performance.
Fish swim and birds fly very efficiently. These observations can be used to inform paradigm shifts in locomotion strategies; for example, the wing of a bird and the tail of a fish are flexible and when these animals fly or swim, the air and water around them induce passive motion. Understanding this fluid-structure interaction or fluid-structure coupling at a very basic level could be used to design aircraft and submarines with a very different kind of locomotion.
The speed of the air or water flow around the vehicle and the density of the materials they are made from play a role, both in the resonance and in the passively induced motion. One sticking point in this research was that the standard definition of resonant frequency assumed that the structure was in a vacuum; however, it is in fluid and the fluid affects the resonance frequency. Consequently, step one was to define a notion of resonance that incorporates the effect of the fluid and then it had to be confirmed that over a wide range of different parameters, performance benefits near this resonant frequency occurred.
If the structure flaps or moves at a certain frequency within this flow, it leads to an improvement in thrust. Depending on the regime, the peak thrust is near this resonant frequency associated with small amplitude. If this passive motion can be useful in locomotion, it can reduce the amount of energy put into the system.
A next phase of the research will be to look at modern active materials that can be tuned to have the right resonant frequency to induce passive dynamics with the desired output.