The acoustic metasurfaces (AMs) can achieve the broadband sound absorption with subwavelength thicknesses, if they are properly designed. However, most previous studies on AMs are conducted in stationary acoustic environments, limiting the applications in more realistic conditions. This thesis investigates noise control by AMs under various aeroacoustic conditions, such as background flow and in the nonlinear acoustic regime, which can extend our understanding of AMs to practical applications.

The sound reflections of wave-manipulation AMs under flow conditions are analytically and experimentally investigated. An analytical model is developed based on plane-wave expansion to predict the reflected sound fields of periodic and non-periodic AMs in a flow environment. Numerical analysis ex...[ Read more ]

The acoustic metasurfaces (AMs) can achieve the broadband sound absorption with subwavelength thicknesses, if they are properly designed. However, most previous studies on AMs are conducted in stationary acoustic environments, limiting the applications in more realistic conditions. This thesis investigates noise control by AMs under various aeroacoustic conditions, such as background flow and in the nonlinear acoustic regime, which can extend our understanding of AMs to practical applications.

The sound reflections of wave-manipulation AMs under flow conditions are analytically and experimentally investigated. An analytical model is developed based on plane-wave expansion to predict the reflected sound fields of periodic and non-periodic AMs in a flow environment. Numerical analysis examines the flow effects on the reflection behavior of periodic and focusing AMs, with experiments conducted in a newly developed aeroacoustic oblique plane wave test rig at HKUST. By considering the mean flow profile, the thesis also explores the behavior of periodic AMs in grazing sheared flows. The shear flow effects are considered in the periodic AM design by a convective generalized Snell’s law (CGSL) and an impedance transformation method. The presence of a sheared layer introduces refraction effects which could weaken the wave-manipulation capability of the AM, and should be considered in the AM design process. Based on the CGSL, a broadband porous metasurface is developed to achieve wave manipulation and sound absorption across a wide frequency range in the stationary air. The nonlinear properties of the sound-absorbing AM composed of coupled Helmholtz resonators with extended necks (HREN) are also systematically investigated. An analytical prediction model is derived to estimate the sound absorption performance of the AM, considering the nonlinear effects induced by high-amplitude excitation. The acoustic characteristics of two coupled HREN units are analyzed both analytically and numerically, with experimental validation conducted across different sound pressure levels (SPL). The flow and impedance characteristics of the AM reveal that the superior absorption performance of the AM at low excitation amplitude stems from the coupling effects between the adjacent HREN units, which generates extra energy loss for the acoustic system. Instead, at high SPL, strong vortices are shed from the necks of the AM and induce stable vortex rings, exaggerating the energy loss and hence maintaining the quasi-perfect absorption performance. By further considering the duct mode behavior, a circular gradient impedance metasurface (GIM) is theoretically designed for effectively attenuating the acoustic modes and its performance is experimentally investigated. Both simulations and experiments show good attenuation of the higher-order azimuthal mode by the designed GIM subjected to a grazing flow with the Mach number up to 0.15. Notably, more than 10 dB IL on the acoustic modes (the azimuthal mode order up to 7) is achieved in the frequency range of 1500 to 3000 Hz with a maximum thickness of 24 mm and a length of 258 mm in both stationary and flow conditions. In summary, this thesis reveals the acoustic behavior of AM under complex aeroacoustic conditions, which could benefit AM-based acoustic treatment design in engineering applications.