Liang Yu Wu

Acoustic energy harvesting using resonant cavity of a sonic crystal

This paper presents the development of an acoustic energy harvester using the sonic crystal and the piezoelectric material. A point defect is created by removing a rod from a perfect sonic crystal. The point defect in the sonic crystal acts as a resonant cavity, and the acoustic waves at the resonant frequency of the cavity can be localized in the cavity. The power generation from acoustic energy is based on the effect of the wave localization in the cavity of the sonic crystal and the direct piezoelectric effect of the piezoelectric material.

Acoustic energy harvesting by piezoelectric curved beams in the cavity of a sonic crystal

Acoustic energy harvesting by piezoelectric curved beams in the cavity of a sonic crystal is investigated. A resonant cavity of the sonic crystal is used to localize the acoustic wave as the acoustic waves are incident into the sonic crystal at the resonant frequency. The piezoelectric curved beam is placed in the resonant cavity and vibrated by the acoustic wave. The energy harvesting can be achieved as the acoustic waves are incident at the resonant frequency. A model for energy harvesting of the piezoelectric curved beam is also developed to predict the output voltage and power of the energy harvesting. The experimental results are compared with the theoretical.

Elastic wave band gaps of one-dimensional phononic crystals with functionally graded materials

The propagation of elastic waves in one-dimensional (1D) phononic crystals (PCs) with functionally graded materials (FGMs) is studied using the spectral finite elements and transfer matrix methods. FGMs typically treat the graded interlayer as a system of discrete layers, and the material properties are varied according to a well-known rule, such as the power law. The 1D PCs are composed of both FGMs and isotropic materials, and their band gaps can be changed with different FGM compositions and geometry parameters. By selecting the appropriate parameters, the desired filters can be designed.

The narrow pass band filter of tunable 1D phononic crystals with a dielectric elastomer layer

In this paper, we study the defect bands of a 1D phononic crystal consisting of aluminum (Al) and polymethyl methacrylate (PMMA) layers with a dielectric elastomer (DE) defect layer. The plane wave expansion (PWE) method and supercell calculation are used to calculate the band structure and the defect bands. The transmission spectra are obtained using the finite element method (FEM). Since the thickness of the dielectric elastomer defect layer is controlled by applying an electric voltage, the frequencies of the defect bands can be tuned. A narrow pass band filter can be developed and designed by using the dielectric elastomer.

An acoustic bending waveguide designed by graded sonic crystals

The acoustic bending waveguide can be designed by engineering the index distribution in a bending structure. We employ a two-dimensional graded sonic crystal (GSC) to realize a graded index medium. The refractive indices of a graded index medium can be discretized and the discrete refractive indices are approximated by using GSCs. The effective refractive indices of the sonic crystals are obtained by using the plane wave expansion method. Simulations and transmission spectra of the bending waveguides with the continuous index variation and the GSC approximation are calculated by using the finite-element-method. The results show that the acoustic bending waveguide based on the GSC can be achieved in the low frequency limit and its operating frequency is broadband.

Refractive and focusing behaviours of tunable sonic crystals with dielectric elastomer cylindrical actuators

The refractive and focusing behaviours of a tunable sonic crystal (SC) consisting of air and dielectric elastomer (DE) cylindrical actuators are investigated. The refractive direction of the acoustic wave in SCs is predicted from the equivalent frequency surface obtained by the plane-wave expansion method. The inner and outer radii of the cylindrical actuators are increased with the electric field which is applied between the electrodes on the internal and external lateral surfaces. Therefore, the dispersion relation is varied with the geometric adjustment of the DE tubes and the refractions of acoustic wave in SCs are changed from positive to negative with the increase in the applied electric field. Based on the geometry adjustment of the cylindrical actuator, a tunable acoustic superlens could be proposed by a SC slab with DE tubes. The distance from the surface of the slab to the centre of the image spot can be changed with different electric fields. Thus, extensive applications of such a phenomenon to acoustic devices are anticipated.

Wave propagation in a 2D sonic crystal with a Helmholtz resonant defect

In this paper, we study the wave propagation of a 2D local resonant sonic crystal consisting of polymethyl methacrylate cylinders in air background. The Helmholtz resonator is placed at the point defect to be a local resonant defect. Band structures are calculated by using the finite element method with a periodic boundary condition. Band structures of the sonic crystal with a circular and Helmholtz resonant (HR) defect are discussed and compared. The frequencies of the defect band depend strongly on the geometric size of the defect. The transmission spectra are measured experimentally. The experimental results are in good agreement with the analytical band structures. The defect mode characteristics of the sonic crystal with a HR defect can be used in the implementation of new acoustic devices.

The nondiffractive wave propagation in the sonic crystal consisting of rectangular rods with a slit

This study investigates the nondiffractive propagation of sound waves in two-dimensional sonic crystals consisting of rectangular rods with a slit. The plane wave expansion method is used to calculate the equifrequency surfaces of sonic crystals with a square lattice. At certain frequencies, straight contour lines in the equifrequency surfaces are found. That frequency is strongly dependent on the geometric sizes of the rectangular rods. Further, the nondiffractive propagation of the sound wave can be realized for omnidirectional incident angles, and the properties can be applied to design novel acoustic devices.

Acoustic planar hyperlens via transformation acoustics

An acoustic planar hyperlens is designed using transformation acoustics. Magnification effects are added to a planar channelling lens to obtain the planar hyperlens. A layered system approach is employed to achieve an effective transformation medium. The proposed planar hyperlens is capable of magnifying sub-diffraction-limit objects on one flat surface and forming magnified images which are above the diffraction limit on another flat surface.

An Acoustic Metamaterial Bending Waveguide Design Using Transformation Acoustics

An acoustic bending waveguide is designed by transformation acoustics. A two-dimensional square area with anisotropic and homogeneous material properties is transformed into a fan-shaped area with anisotropic and inhomogeneous material properties to rotate the direction of beam propagation. The transformation medium can be realized by alternating layered structure consisting of water and fluid with negative mass density. We propose that an acoustic metamaterial composed of three layers in water background can be designed to replace negative mass density fluid and achieve the acoustic bending waveguide.Copyright