Synchronized frequency-dependent wave motion between a flowing fluid and a phononic subsurface

Phonon motion underneath a surface interacting with a flow may be tuned to cause the flow to stabilize, or destabilize, as desired, leading to delay of laminar-to-turbulent transition or delay of flow separation. The underlying control mechanism utilizes core concepts from crystal physics, primarily, the principle of destructive or constructive interferences and the notion of symmetry breaking. This is realized by installing a phononic subsurface (PSub), which is an architectured structure placed in the subsurface region and configured to extend all the way such that its edge is exposed to the flow, forming an elastic fluid-structure interface. The PSub may take the form of a phononic crystal or a subwavelength elastic metamaterial, with finite extent, and is typically oriented perpendicular to the fluid-structure interface. It is engineered to exhibit specific frequency-dependent amplitude and phase response characteristics at the edge exposed to the flow. We will demonstrate this mechanism, highlight its passive and responsive character, and present an analysis of the flow energetics as energy is exchanged between the PSub and unstable waves within the flow field, and between the instability flow field and the underlying mean flow.