Study On Design And Properties Of Acoustic Metasurfaces

Acoustic metasurfaces are emerging concepts proposed by researchers based on the subwavelength structure of metamaterials and combined with the principle of phased array wave field control.The functional units under the subwavelength scale are periodically arranged in a specific way,and by changing the phase gradient on the surface of the structure,the metasurface can obtain extraordinary acoustic and mechanical properties which are not existed in traditional artificial periodic materials,providing a new technical approach for the modulation of acoustic waves and elastic waves.As a new type of artificial structure,acoustic metasurfaces have the characteristics of small size,low losses,more degree of freedom in wave manipulation,and easy processing and assembly.Thus,metasurfaces have a wide range of vibration and noise control applications in mechanical engineering,transportation,and human life.This paper addresses the phase shift phenomenon caused by the propagation of elastic waves in metasurfaces,the wave field modulations of groove-type metasurface and massoscillator metasurface are investigated using finite element simulation calculation and corresponding theoretical derivation.The main contents include the following aspects:Firstly,based on groove-type elastic metasurfaces,a method is proposed to modulate the vortex field by varying the angular momentum in the vortex formula.The branching of the vortex field is numerically simulated using the finite element software COMSOL Multiphysics for the angular momentum values 2,3,4,and 5,respectively,while the A0 mode wave excitation is applied to the center of the annular vortex source device.The simulation result shows that the proposed annular vortex device can achieve arbitrary vortex field modulation in ultrabroadband(5kHz-58kHz),which is consistent well with the theoretical value and has good robustness.Secondly,based on the genetic algorithm,using Livelink between COMSOL Multiphysics and the mathematical analysis software MATLAB to redesign the arrangement of functional units of the groove-type metasurface from the perspective of the topology optimization.The topology optimization design of the groove-type metasurface is achieved by encoding the groove position information of metasurfaces with mixed variables and proposing optimization objectives of anomalous refraction,beam focusing,and self-accelerating beams.Comparing with the unoptimized metasurface,the groove-type metasurface optimized by genetic algorithms is more consistent with the theoretical value and has a better wavefield modulation effect,which provides a novel and feasible idea for metasurface design work.Finally,based on the tunability of elastic metasurfaces,and balancing the processing and fabrication difficulties,the Modular Mass-oscillator Elastic Metasurfaces(MMEMs)are proposed to achieve the wavefield manipulation of flexural waves by retrofitting different sizes of mass-oscillators on the structure,which can arbitrarily modulate the incident flexural waves such as anomalous refraction,beam focusing,and self-accelerating beams with phase delay.Furthermore,the concept of Multiple Mass-oscillator Arraying Design(MMAD)is proposed to extend the operating frequency domain of mass-oscillator metasurfaces for continuous broadband(13kHz-41kHz)manipulation of flexural waves.