Reflected acoustic wave manipulation by metasurfaces in a grazing flow

Acoustic metasurfaces (AMs) can manipulate acoustic waves in ways that are not reachable in natural materials, offering significant implications for engineering applications such as noise control. While previous studies have primarily been conducted in stationary mediums, this study delves into the sound reflections of wave-manipulation AMs subjected to a grazing flow. An analytical model is developed to predict the sound reflections of both periodic and non-periodic AMs under flow conditions based on the plane-wave expansion. The flow effects on the periodic and focusing AMs are analytically and numerically investigated. Experiments are also conducted in a newly designed aeroacoustic oblique plane wave (AOPW) facility at the Hong Kong University of Science and Technology (HKUST). Results show that the reflected sound pressure fields of wave-manipulation AMs under flow conditions can be predicted well by the analytical model. Good absorption of the periodic AMs can be achieved by adjusting the periodic length-to-wavelength ratio to below $(1-M_0^2)/2$ due to the surface wave conversion mechanism. The focal points of the AMs designed in the stationary air shift to the downstream direction due to the flow effects, which can be corrected by the proposed analytical model. The focusing AM design is also extended into a three-dimensional (3D) space and is validated analytically. This study extends the understanding of wave-manipulation AMs into flow conditions, which may help the AM design operating in non-stationary mediums, such as air and water flows.