Excitation of airborne acoustic surface modes driven by a turbulent flow

Abstract

This experiment demonstrated the generation of trapped acoustic surface waves excited by a turbulent flow source through the coupling of pressure fluctuations at the interface between an acoustic metamaterial and a flow environment. The turbulent flow, which behaves as a stochastic pressure source, was produced using a fully developed turbulent wall jet. The plate in the wall jet was perforated with a single cavity. On the flow-side it was capped by a Kevlar weave to ensure the cavity did not significantly disturb the flow, whilst on the adjacent side the cavity was open to the quiescent (static) environment. The through-cavity opening, on the quiescent side, was flush with an acoustic metasurface waveguide, which, through evanescent diffractive coupling of the pressure field, produced an acoustic surface mode. This acoustic mode was trapped at the plate surface, with its mode dispersion determined by the surface geometry. The results of two different metasurface geometries are discussed; (1) a slotted cavity array, and (2) a meander connected cavity array, each demonstrating a different trapped surface wave dispersion behavior. Fourier transform and correlation analyses of spatially-resolved temporal acoustic signals, measured close to the metamaterial surface, were used to construct the frequency and wave vector-dependent acoustic mode dispersion. Results demonstrated the flow can indeed be used to excite these acoustic modes and that their mode dispersion can be tailored towards realizing novel control of turbulent flow through acoustic-flow interactions