Acoustic metasurfaces and their application to turbulent flow

Abstract

The use of surface structures can have a significant impact on the pressure fluctuations that manifest above them. Acoustic metamaterials that comprise structured surfaces have been designed to influence the acoustic pressure field for a variety of applications such as acoustic filtering, focusing, and absorption, whilst in the aerodynamics of turbulent boundary layer flows, surface treatments can cause significant changes to the sound generated from, and pressure fluctuations present within, the flow. So, the question arises, can we apply acoustic metamaterial concepts as a strategy to influence a turbulent flow? In principle, metasurfaces used to control acoustic fields should also respond to evanescent pressure fluctuations generated by turbulence. However, interfacing a metasurface and boundary layer poses some challenges, not least in the large disparity between acoustic and flow scales at low Mach number, and the need to avoid metasurface configurations that generate drag or other undesirable flow effects. In this talk I will discuss our recent work on acoustic surface waves and their application to a turbulent fluid flow. First I will talk about the characteristics and structures that support acoustic surface wave propagation in a quiescent ‘wholly acoustics’ environment. I will then present the strategy developed with collaborators at Virginia Tech, that interfaces a cavity-type metasurface with a turbulent flow using a hydrodynamically smooth but acoustically transparent membrane. Our experiments demonstrate the excitation of a bound acoustic mode using the stochastic near-field pressure fluctuations of a turbulent flow. The tested metasurfaces demonstrate a route for acoustic surface mode dispersions with reduced phase velocities to match to the convective velocity of the turbulent structures.We view this as an important first step in interfacing flows and metamaterials, with potential applications to the control of flow-generated edge noise or energy harvesting.