Phononic Subsurfaces: Phononic Crystals and Metamaterials for Passive Flow Control

Building on their success in controlling acousticAcoustic and elasticElastic wavesWaves, phononic materials may also be used to control unstable disturbances in a fluidFluid flow. This is enabled by attaching a finite phononic structure perpendicular to an elasticElastic surface exposed to a flowing fluid. This structure, called a phononic subsurface (PSub), experiences ongoing excitation by an unstable flow mode, or more than one mode, travelling in conjunction with the mean flow. The excitation generates elastic waveWaves propagation within the structure, travelling away from the flow and reflecting at the end of the structure to return to the fluid-structure interface and back into the flow. By targeted tuning of the unit-cell and finite-structure characteristics of the PSub, the returning wavesWaves may be devised to resonate and reenter the flow out of phase, leading to significant destructive interference of the incoming flow wavesWaves near the surface and subsequently to their attenuation over the spatial extent of the control region. This entire mechanism is passive and is engineered offline without the need for coupled fluid-structure simulations; only the frequencyFrequency and modal nature of the flow instability must be known. Disturbance stabilization in a transitional flow leads to delay in laminar-to-turbulent transition and reduction in skin-friction drag. Destabilization is also possible, which is beneficial for prevention of flow separation and enhanced combustion. In this chapter, we present the PSub concept and theory, and demonstrate its utilization using both a phononic crystal and a locally resonantResonant elasticElastic metamaterial.