Coiled Phononic Structure Super-Resonates
As air flows over an airplane wing, the boundary layer near the leading edge is laminar and low drag, but farther downstream it can become turbulent and high drag. The transition to turbulence typically proceeds via the growth of instabilities such as Tollmien-Schlichting waves. Stifling the waves would forestall turbulence and save energy. Ways to do that using elaborate feedback methods have been devised, but they are limited to single instabilities. Now Mahmoud Hussein of the University of Colorado Boulder and his collaborators have conceived and computationally tested a device that suppresses multiple instabilities both passively and across a broad range of frequencies [1].
The device’s basic unit—a phononic subsurface (PSub)—consists of a long, thin resonating structure made of aluminum, a tough plastic called ABS, and air cavities. Ten of these units form a sheet whose top edge lies flush with the solid-fluid boundary and perpendicular to the flow. If the sheet were left to dangle from the boundary, only the topmost segment would intercept and trap instabilities. However, the researchers realized that if they coiled the sheet so that successive segments within the structure also lie flush with the boundary and converge within a small region, the instabilities have more ways to interact with the device. Specifically, coiling the structure effectively widens the frequency range, creating what the researchers describe as a super-resonance.
In their numerical experiments, the researchers studied a fluid flow that simultaneously developed four Tollmien-Schlichting waves at different frequencies. Across the four instability modes, the super-resonant coiled PSub locally reduced the perturbations’ kinetic energy by nearly 1%. Although modest, this number is expected to increase to produce a significant effect when more modes are present within the broad frequency window covered.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- A. R. Harris et al., “Super-resonance: Breaking the bandwidth limit of resonant modes and its application to flow control,” Phys. Rev. X 16, 021045 (2026).