A novel design scheme for acoustic cloaking of complex shape based on region partitioning and multi-origin coordinate transformation

Acoustic cloaking is an important application of acoustic metamaterials.This article proposes a novel design scheme for acoustic cloaking based on the region partitioning and multi-origin coordinate transformation.The cloaked region is partitioned into multiple narrow strips.For each strip,a local coordinate system is established with the local origin located at the strip center,and a coordinate transformation in the local coordinate system is conducted to squeeze the material along the strip length direction to form the cloaked region.To facilitate the implementation of the acoustic cloak,the multilayer effective medium is used to approximate the non-uniform anisotropic material parameters.The effectiveness of the proposed coordinate transformation method is verified by comparing the results from our method with those in the literature.Firstly,the results of a circular acoustic cloak in the literature are reproduced by using our finite element(FE)simulations for validation.Then,a comparison is made between the traditional coordinate transformation scheme and our new scheme for simulating an elliptical acoustic cloak.The results indicate that the proposed multi-origin coordinate transformation method has a better cloaking effect on the incident wave along the ellipse minor axis direction than the traditional method.This means that for the same object,an appropriate transformation scheme can be selected for different incident wave directions to achieve the optimal control effect.The validated scheme is further used to design an arch-shaped cloak composed of an upper semicircular area and a lower rectangular area,by combining the traditional single-centered coordinate transformation method for the semicircular area and the proposed multi-origin method for the rectangular area.The results show that the designed cloak can effectively control the wave propagation with significantly reduced acoustic pressure level.This work provides a flexible acoustic cloak design method applicable for arbitrary shapes and different wave incident directions,enriching the theory of acoustic cloaking based on coordinate transformation.

Improving the performance of acoustic invisibility with multilayer structure based on scattering analysis

In this paper, acoustic scattering from the system comprised of a cloaked object and the multilayer cloak with only one single pair of isotropic media is analyzed with a recursive numerical method. The designed acoustic parameters of the isotropic cloak media are assumed to be single-negative, and the resulting cloak can reduce acoustic scattering from an acoustic sensor while allowing it to receive external information. Several factors that may influence the performance of the cloak, including the number of layers and the acoustic dissipation of the medium are fully analyzed. Furthermore, the possibility of achieving acoustic invisibility with positive acoustic parameters is proposed by searching the optimum value in the parameter space and minimizing the scattering cross-section.

Design of elliptical underwater acoustic cloak with truss-latticed pentamode materials

Pentamode acoustic cloak is promising for underwater sound control due to its solid nature and broadband efficiency,however its realization is only limited to simple cylindrical shape.In this work,we established a set of techniques for the microstructure design of elliptical pentamode acoustic cloak based on truss lattice model,including the inverse design of unit cell and algorithms for latticed cloak assembly.The designed cloak was numerically validated by the well wave concealing performance.The work proves that more general pentamode acoustic wave devices beyond simple cylindrical geometry are theoretically feasible,and sheds light on more practical design for waterborne sound manipulation.

Manipulation of invisible cloaking in PT-symmetric thermoacoustic dimer

Making the propagation of sound waves immune to interference from obstacles with high transmission efficiency is a long-term pursuit in acoustic science and engineering.Recent proposal pointed out that perfect transmission through obstacles can be achieved by deploying a bulky gain-loss distribution in parity-time(PT)symmetry.Here we demonstrate a modified methodology to achieve the extraordinary physical property of acoustic cloaking accompanied by perfect transmission at the exceptional points(EPs).Systematically probing reveals two complementary solutions of EPs corresponding to acoustic cloaking,in the system composed of an equivalent medium slab sandwiched by a pair of PT-symmetric admittance metasurfaces.To model the crucial acoustic gains that are not present in nature,we employ actively controlled ultra-thin carbon nanotube dimer films to mimic admittance metasurfaces perfectly via thermoacoustic effect,and manipulate acoustic cloaking over a wide frequency band in experiments.This divergent strategy releases restrictions on the operating frequency,shape and size of the obstacle,which can be applied to acoustic sensing,directional imaging,and other related wave disciplines.

Acoustic cloak constructed with thin-plate metamaterials

We propose a strategy for designing the cylindrical acoustic cloak with thin-plate metamaterials.The inhomogeneous cloaking shell as derived by transformation acoustics is first discretized into a three-layer anisotropic metafluid,and their material parameters are optimized by minimizing the external scatterings.Then these metafluids are practically realized by thin-plate structures according to the metamaterial concept.As an example,an acoustic cloak is designed with nine layers of thin plate and totally 900 plate units.Numerical simulations are performed to assess the cloaking performance of the designed structure.

Scattering Analysis and Optimization of Spherical Acoustic Cloak with Unideal Pentamode Material

The acoustic scattering is theoretically studied in this paper for three-dimensional spherical cloak composed of unideal pentamode material,for which small shear rigidity is always inevitable for a real designed microstructure.A theoretical formulation is developed to efficiently evaluate the cloaking performance.The generic scattering feature of the cloak and the efTects of material imperfectness and inner cloak boundary constraints are systematically examined.The preferable constraint type and the critical imperfectness parameter of the material are identified for possible broadband invisibility.In addition,a very practical lining shell scheme is proposed to tune the constraint strength on the inner boundary.By combining the theoretical model with optimization algorithm,it is further proved that the cloak can be reduced by several piecewiseuniform layers and optimized to achieve excellent invisibility in targeted frequency bands.The study will provide valuable guidance for the future microstructural design of cloaks.

Sound scattering from underwater elastic spherical shell covered with multilayered medium approximated acoustic cloak

The anechoic performance and mechanism of underwater elastic spherical shell covered with coating are studied at low frequencies.The acoustic cloak is anisotropic material,which can be designed with homogeneous isotropic materials on the basis of effective medium approximation theory.The analytic expression of scattering acoustic field from the shell covered with multilayered medium is formulated and the scattering form function,resonance mode,acoustic field distribution are computed,the scattering characteristics and mechanism of transmission are analyzed.The results show that the direction of sound transmission inside the multilayered medium is changed,the acoustic field is deflected gradually,and the acoustic energy flux is guided around the target,which reduces the scattering intensity at low frequencies,the acoustic intensity of target＇s surface is very weak.Excepting the first resonance peak in spectrum produced by the zero order partial wave,the other resonance modes of elastic spherical shell are not excitated and the multilayered medium can suppress the resonance of the spherical shell effectively.