| In recent years,the research and development of artificial structures for acoustic applications(including phononic crystals,acoustic metamaterials,and acoustic metasurfaces)have provided new ideas for the design of underwater acoustic wave modulation devices(filters,vibration isolators,acoustic diodes,etc.).A one-dimensional fluid-solid superlattice is an adjustable phononic crystal structure that can be easily implemented.With different bandgap characteristics compared to conventional phononic crystals,it shows characteristics of both zero-point transmission and Bragg scattering bandgaps.It is therefore of great significance to conduct in-depth research on the propagation characteristics of acoustic waves in such structures and their bandgap applications.In this study,the acoustic propagation characteristics of a one-dimensional fluid-solid superlattice was analyzed.Based on its bandgap characteristics,underwater wide-angle sound insulation devices and subwavelength asymmetric transmission devices were designed,and research on the structure optimization design of these devices was conducted.The content specifically includes the following four sections:1.Calculation method of acoustic band gap of fluid-solid superlattice and analysis of acoustic transmission characteristicsIn Section 2,the acoustic propagation theory of a one-dimensional fluid-solid superlattice is first introduced.The acoustic transfer matrix of the fluid-solid superlattice is derived in detail.The omnidirectional characteristics in the acoustic transmission of the fluid-solid superlattice are also calculated and analyzed.It was found that there exists a low-frequency transmission zeros forbidden bandgap in the first Bragg passband of the fluid-solid superlattice.Owing to the very small range of angles of this low-frequency transmission zeros forbidden bandgap,its practical application is severely limited.The variation characteristics of the angle range of this low-frequency transmission zeros forbidden bandgap are comprehensively investigated by using the method of control variables.The results show that the transverse acoustic wave speed,duty cycle,mass density,and lattice period number of the solid material have significant effects on the angle range of the forbidden bandgap.Based on these results,an effective strategy for designing a flow-solid superlattice with a wide-angle range for the low-frequency transmission zeros forbidden bandgap is provided.The above research results provide a theoretical basis and a research method for using the flow-solid superlattice bandgap mechanism to achieve underwater sound insulation and subwavelength wide-angle acoustic asymmetric transmission applications.2.Research on the design of sub-wavelength sound insulation structure under normal incidenceIn Section 3,the Gibbs oscillation phenomenon in the first Bragg passband of a finite-length fluid-solid superlattice under the condition of normal incidence is first analyzed using the transfer matrix.The influence of the solid material parameters as well as the lattice period number on the low-frequency Gibbs oscillation characteristics is investigated.It was found that increasing the solid mass density and the solid-layer duty cycle,and appropriately reducing the lattice period number can effectively intensify the strength of the strong Gibbs oscillation and broaden the bandwidth of the oscillation-formed transmission valley.Theoretical calculations and finite element simulations show that a one-dimensional fluid-solid superlattice with a thickness of only one-fourth of the incident wavelength can be employed to address broadband underwater sound insulation applications at vertical incidence with only1% of the incident energy.This study provides a new approach for the design of miniaturized underwater sound insulation materials or structures using Bragg bandgap phononic crystals.3.Research on Realization and Parameter Optimization of Sub-wavelength Wide-angle Acoustic Low TransmissionIn Section 4,to solve the problems of poor acoustic insulating performance(narrow frequency band,large size,and small incidence angle range)in existing underwater acoustic insulation devices or structures when incident acoustic waves are at an oblique angle,a fluid-solid superlattice structure with omnidirectional acoustic bandgap characteristics is proposed.Moreover,a genetic algorithm is proposed for the optimization of structural material parameters,and a wide-angle subwavelength acoustic insulation structure is designed.First,based on the results of the angle range for the low-frequency transmission zeros forbidden bandgap presented in Section 2and the modulation rules of the Gibbs oscillation phenomenon in a finite period structure described in Section 3,a “lead-water superlattice” structure that satisfies both the wide-angle low-frequency transmission zeros forbidden bandgap and the strong Gibbs oscillation phenomenon simultaneously is designed,and its transmission spectrum characteristics and energy band structure are studied.It was discovered that the transmission valley due to the strong Gibbs oscillation and low-frequency transmission zeros band are cleverly coupled to form an omnidirectional acoustic bandgap.In addition,a genetic algorithm is proposed to optimize the solid material of the omnidirectional bandgap fluid-solid superlattice,thereby avoiding the use of the environmentally unfriendly “lead.” The optimization results show that a wide-angle acoustic bandgap can also be obtained by using the 0-3 composite materials to construct a fluid-solid superlattice.This novel formation mechanism for acoustic bandgap has potential applications in the design of underwater subwavelength wide-angle acoustic insulation structures or devices.4.Research on the Design of Sub-wavelength Wide-angle Acoustic Asymmetric Transmission StructureIn Section 5,an acoustic asymmetric transmission structure based on an acoustic diffraction grating mechanism and wide-angle transmission zeros forbidden bandgap of a cascade fluid-solid superlattice is proposed in view of the limitations on the incident angle of current underwater acoustic asymmetric transmission devices.Although the low-frequency transmission zeros forbidden bandgap in a fluid-solid superlattice provides a new approach for controlling the propagation of low-frequency acoustic waves,the small angle range limits its practical application.Based on the rules for modulating a low-frequency transmission zeros forbidden bandgap revealed in Section 2,a cascade fluid-solid superlattice structure is proposed.The acoustic transmittance of the cascade fluid-solid superlattice is calculated using the transfer matrix and finite element methods.The results show that a cascade fluid-solid superlattice has a dual-angle low-frequency transmission zeros forbidden bandgap.Hence,the coupling effect of dual forbidden bandgaps that form a wide-angle zero-point transmission forbidden bandgap can be achieved by choosing a suitable solid material.Based on the study of a cascade fluid-solid superlattice,a subwavelength wide-angle acoustic unidirectional transmission structure coupled with a cascade fluid-solid superlattice is proposed.Numerical calculations and finite element simulations show that the acoustic waves are almost completely reflected when they are incident from multiple directions opposite to the incident directions,but the transmittance is high when they are incident in normal directions.The structure has a wide working bandwidth and can preserve the waveform in the subwavelength region.In addition,the forward acoustic transmittance is improved by optimizing the rotating angle of the acoustic grating.The superior acoustic unidirectional transmission performance of this structure comes from the wide-angle low-frequency transmission zeros forbidden bandgap of the cascade fluid-solid superlattice and the diffraction effect of the underwater acoustic grating structure.This new high-performance underwater subwavelength asymmetric transmission structure has potential applications in underwater communication and signal transmission. |
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