Aeroacoustic characteristics of metasurfaces with stair-stepping honeycomb structures covered by perforated panels

The aeroacoustic properties of a kind of metasurface with periodic structures, which is mainly used for wave manipulation, are investigated under a grazing flow condition. The metasurface consisting of a series of different units in one period is designed first. Each unit is composed of honeycomb cavities with a specified thickness and a perforated panel. The choices of thickness are made to achieve the expected phase responses of reflected waves to cover 2p in one period and also consider the effects of the grazing flow on the impedance. The Goodrich impedance model is used to characterize the impedances at different flow speeds of individual units. In the case setup, the metasurface is lined on one of the side walls of a rectangular duct to evaluate its noise attenuation performance. In the study, the metasurfaces are studied numerically using the finite element method. The numerical simulation firstly establishes the flow field using the SST turbulence model. The acoustics is then determined by solving the Linearized Navier-Stokes equation. The metasurfaces with different periodic lengths are studied and the results show that better noise attenuation compared with a surface made of uniform units can be achieved through carefully designing the periodic length of the metasurface.