Attenuation of fluid perturbation energy using a subsurface acoustic diode

Subsurface structures inspired by elasto-acoustic metamaterials have been utilized to mitigate instabilities in boundary layer flows. By reducing the amplitude of perturbation energy associated with growing vortices such as Tollmien-Schlichting waves, a passive delayed transition to turbulence can be achieved which has major implications in drag reduction and energy savings due to reduced fuel consumption. In this work, we propose a novel approach for transition delay in a boundary layer flow using an energy trap mechanism which extracts the undesirable oscillations rather than interfere with it. We devise a subsurface acoustic diode which comprises a PnC with a wide Bragg bandgap augmented with a nonlinear material which disperses incoming waves into higher frequencies. By doing so, we demonstrate the ability to pluck out the bulk energy of a TS instability by smoothly guiding the waves into the impedance-matched subsurface PnC while impeding their return to the fluid via the nonreciprocal effect. The proposed approach provides a transition delay mechanism which harnesses the full width of the bandgap, thus enabling broadband control and an effect which is sustained downstream.