  The interactions between a solid surface and a fluid flow underlie dynamical
processes relevant to air, sea, and land vehicle performance and numerous other
technologies. Key among these processes are unstable flow disturbances that
contribute to fundamental transformations in the flow field. Precise control of
these disturbances is possible by introducing a phononic subsurface (PSub).
This comprises locally attaching a finite phononic structure perpendicular to
an elastic surface exposed to the flowing fluid. This structure experiences
ongoing excitation by an unstable flow mode traveling in conjunction with the
mean flow. The excitation generates small deformations at the surface that
trigger elastic wave propagation within the structure. By targeted tuning of
the unit-cell and finite-structure characteristics of the PSub, the returning
waves may be devised to resonate and reenter the flow out of phase, leading to
significant destructive interference of the incoming flow waves near the
surface and subsequently to their attenuation over the spatial extent of the
control region. This control mechanism is passive, responsive, and engineered
offline without needing coupled fluid-structure simulations; only the flow
instability's frequency, wavelength, and overall modal characteristics must be
known. We present a PSub in the form of a locally resonant elastic
metamaterial, designed to operate in the elastic subwavelength regime and hence
being significantly shorter in length compared to a phononic-crystal-based
PSub. Using direct numerical simulations (DNS) of channel flows, both types of
PSubs are investigated, and their controlled spatial and energetic influence on
the wall-bounded flow behavior is demonstrated and analyzed. We show that the
PSub's effect is spatially localized as intended, with a rapidly diminishing
streamwise influence away from its location in the subsurface.
