Phononic subsurfaces: Flow control by wave synchronization across the fluid-structure interface

Fluid-structure interaction is a dynamical process that is central to managing skin-friction drag, flow transition, and flow separation in air, sea, and land vehicles, as well as in other applications like turbomachinery. The magnitude of skin-friction drag, particularly for streamlined bodies, is a key factor in determining fuel efficiency—lower drag results in higher efficiency. A significant reduction in skin-friction drag can be achieved by delaying the transition from laminar to turbulent flow. Meanwhile, flow separation significantly influences aerodynamic and hydrodynamic stability, form drag, vehicle maneuverability, and applications requiring turbulent mixing. Minimizing or delaying these effects are therefore primary objectives in flow control. Both these fundamental flow transformations are tied directly with the behavior of unstable flow disturbances, or perturbations, that occur naturally in a flow. Among the most studied are Tollmien–Schlichting waves. The linear modal nature of these waves provides an opportunity to apply phased control to suppress or enhance their growth by wave coupling and interference.

Phononic subsurfaces (PSubs) is a technology that aims to passively and mechanistically intervene with these perturbations to enable either stabilization or destabilization as desired. A PSub comprises a synthetically designed architected material affixed beneath the surface exposed to the flow (e.g., of a wing or vehicle body). The function of the PSub is to manipulate small-amplitude vibrations on the surface, and by extension the flow perturbations near the wall. A PSub material and geometric architecture can be selected to function as a phononic crystal that exhibits Bragg scattering, or a locally resonant elastic metamaterial that exhibits resonance hybridizations. In either case, the phononic material is truncated and finite-size structural effects must be considered in the design alongside the unit-cell topology. In this talk, a series of latest developments in PSubs research will be overviewed, including utilization of PSubs for downstream control and their design to impede with perturbation waves over a broad frequency range.