Design Of Pentamode Metasurfaces And Their Applications For Underwater Acoustic Manipulation

High-performance underwater acoustic regulation materials are key technologies in underwater acoustic detection,cloaking,communication,and other fields.With the continuous expansion of sonar detection frequency band to low frequency,the development of low-frequency and broadband acoustic cloaking materials is of great significance to improve the survivability of submarines,unmanned underwater vehicle,and other underwater equipment.The acoustic cloaking idea of acoustic metasurface using single-layer gradient structure to direct acoustic waves to reduce and eliminate acoustic signal characteristics can overcome the problem of significantly lower low-frequency dissipation efficiency of acoustic cloaking materials from the principle,which provides a brand new idea for the development of underwater acoustic cloaking technology.The idea of using solid-based structure to simulate fluid medium is the most feasible way to achieve underwater acoustics control on the metasurface for engineering applications,so the use of pentamode metamaterials to design metasurfaces for efficient underwater acoustics control and acoustic cloaking is worth exploring in depth.The lack of systematic pentamode metasurface design theory,the narrow range of acoustic property parameter adjustment leading to only small angle underwater acoustic directional guidance,and the narrow bandwidth of underwater acoustic control are the first bottlenecks of applying pentamode metasurfaces to underwater acoustic cloaking.This paper focuses on the above problems,combining theoretical analysis,numerical simulation and experimental verification,and conducts research on pentamode metamaterials design and equivalent medium analysis,acoustic cloaking theory with synergistic effect of multiple underwater acoustic modulation mechanisms,and the design and optimization of integrated acoustic cloaking coverings of metasurfaces and acoustic absorbing materials to explore the feasibility and technical ways to realize low-frequency broadband underwater acoustic cloaking design based on pentamode metasurfaces.The main work is as follows:1.A large gradient pentamode acoustic metasurface design theory was established.A paradigm for describing the equivalent mechanical intrinsic parameters of pentamode metamaterials based on the Timuchenko beam theory,which takes into account the bending,shear,expansion deformation and additional counterweight mass blocks of the cell walls,is established.On basis of this,a pentamode metasurface design scheme with significantly increased underwater acoustic directional guidance capability is proposed.2.The feasibility and approach of integrating gradient metasurface and underwater acoustic absorbing materials to realize low-frequency and broadband underwater acoustic regulation are explored.An acoustic cloaking theory with the synergistic effect of multiple underwater acoustic modulation mechanisms is proposed.Based on the finite element model and multiple scattering model of underwater acoustic absorbing materials,the low-frequency acoustic energy dissipation mechanism,influencing factors,and laws of hollow underwater acoustic absorbing materials are systematically analyzed;Then,an integrated design scheme is proposed to realize the significant expansion of effective sound absorption frequency band to low frequency by combining sound-absorbing material with gradient metasurface;Based on the systematic analysis of the sound absorption characteristic curve and the variation law of sound absorption mode of the integrated model,the underwater acoustic regulation and energy dissipation enhancement mechanism of the gradient metasurface are deeply revealed,and the theory of underwater acoustic regulation with the synergistic effect of multiple mechanisms is proposed.The results show that,in addition to the far-field modulation effects such as directional guidance and focusing,the phase gradient of the metasurface also has an obvious modulation effect on the acoustic field mode in the near-field range far less than the wavelength.The introduction of the gradient metasurface can provide a new degree of design freedom for the low-frequency and broadband design of thin-layer underwater acoustic cloaking materials.3.The multi-parameter optimization of the integrated acoustic cloaking structures is studied depth,and the integrated functional-structural-material design method of the pentamode metasurface acoustic cloaking structure is established.Based on the global optimal algorithm(GASA)combining genetic algorithm and simulated annealing algorithm,an efficient multi-parameter optimization design theory for pentamode metasurface integrated acoustic cloaking structure is developed,and an integrated acoustic cloaking structure with both low-frequency and broadband characteristics is designed.The integrated functional-structural-material design method of the integrated acoustic cloaking structure including phase gradient,cavity resonance and interface contact effect was established.It is shown that the optimized integrated structure is able to achieve an efficient acoustic absorption efficiency of more than 0.8 in the frequency band from 500 Hz to 1500 Hz.In conclusion,this paper focuses on the demand for further development of underwater acoustic cloaking technology and the problems faced by pentamode metasurfaces applied to underwater acoustic modulation,and thoroughly investigates the design of pentamode metamaterials with extreme parameters and the theory of multi-mechanism cooperative acoustic cloaking based on pentamode metasurfaces.The acoustic cloaking design model guiding the integration of metasurface and sound-absorbing materials,and its integrated functional-structural-material design method are proposed,which can maintain broadband acoustic cloaking effect while significantly expanding the control band to low frequencies.The research work in this paper is an important reference for promoting theoretical research on underwater acoustic metasurfaces and exploring applications in the field of underwater acoustic cloaking.