Jiu-jiu Chen

Lamb wave band gaps in a homogenous plate with periodic tapered surface

In this paper, we present the numerical investigation of Lamb wave propagation in a homogenous plate with periodic tapered surface, which gradually increases the width from the lower base to the upper base. The dispersion relations, the power transmission spectra, and the displacement fields of the eigenmodes are studied by using the finite-element method. We investigate the effects of the geometrical parameters (including the ratio of the lower base width to the upper base width, and the ratio of the upper base width, the thickness of the tapered surface, and the thickness of the homogenous plate, respectively, to the lower base width) on the band gaps. Numerical results show that the band gaps can be effectively shifted by changing the geometrical parameters. Especially, the width of the first band gap changes approximately linearly by changing the ratio of the upper base width to the lower base width and in return. The transmission bands of the structure with the tapered surface are more flat than thos...

Lamb waves in phononic crystal slabs: Truncated plane parallels to the axis of periodicity

A theoretical study is presented on the propagation properties of Lamb wave modes in phononic crystal slabs consisting of a row or more of parallel square cylinders placed periodically in the host material. The surfaces of the slabs are parallel to the axis of periodicity. The dispersion curves of Lamb wave modes are calculated based on the supercell method. The finite element method is employed to calculate the band structures and the transmission power spectra, which are in good agreement with the results by the supercell method. We also have found that the dispersion curves of Lamb waves are strongly dependent on the crystal termination, which is the position of the cut plane through the square cylinders. There exist complete or incomplete (truncated) layers of square cylinders with the change of the crystal termination. The influence of the crystal termination on the band gaps of Lamb wave modes is analyzed by numerical simulations. The variation of the crystal termination leads to obvious changes in the dispersion curves of the Lamb waves and the widths of the band gaps.

ONE-DIMENSIONAL PHONONIC CRYSTAL SLABS BORDERED WITH ASYMMETRIC UNIFORM LAYERS

We investigate theoretically the propagation of Lamb waves in a one-dimensional phononic crystal (PC) slabs bordered with asymmetric uniform layers based on the supercell plane wave expansion (SC-PWE) method. The validity is proved by using the finite-element (FE) method with Comsol Multiphysics 3.5a. The effect of changing the thickness of substrate and superstrate on the variation of the band gap width and frequency are studied when the total thickness of the loading layers is fixed. We also investigate the property of band gap maps by changing the total thickness or the material of the loading layers. The results show that the band structure can be tuned by changing the material or the thickness of the substrate or the superstrate. These structures may be used as filters and acoustic sensors.

Local Resonance Broadband Gap in a Homogeneous Plate with Periodic Truncated Cones

We investigate the band structures in a homogeneous plate with periodic truncated cones for the square lattice based on finite element method (FEM). The radius of the truncated cone is gradually reduced from the lower base to the upper base. Compared to the classical stubbed phononic plates, a considerable enlargement of the bandwidth by a factor of 2.04 is obtained by varying the semiangle of the truncated cone, while the truncated cones weight is only 65.19% of that of the cylinder of the stubbed case. Moreover the relative bandwidth is also enlarged by a factor of 1.68. We show that this band gap enlargement is due to the acoustic waves of different wavelengths is localized at the different parts of the truncated cone. These characteristics of elastic or acoustic waves suggest potential applications in aerospace.

Tunable Lamb wave band gaps in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field

This paper theoretically investigates the band gaps of Lamb mode waves in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field. With the assumption of uniformly oriented magnetization, an equivalent piezomagnetic material model is used. The effects of magnetostatic field on phononic crystals are considered carefully in this model. The numerical results indicate that the width of the first band gap is significantly changed by applying the external magnetic field with different amplitude, and the ratio between the maximum and minimum gap widths reaches 228%. Further calculations demonstrate that the orientation of the magnetic field obviously affects the width and location of the first band gap. The contactless tunability of the proposed phononic crystal slabs shows many potential applications of vibration isolation in engineering.

Asymmetric propagation of the first order antisymmetric lamb wave in a tapered plate based on time domain analysis

The asymmetric propagation of the first order antisymmetric (A1) Lamb wave in a tapered plate respectively carved with sharp bottom corner and round bottom corner is theoretically investigated. Through numerical simulation of A1 Lamb wave in time domain, we find that when the thickness of the waveguide abruptly decreases to below the cut-off thickness, about half of the A1 mode is converted into the fundamental symmetrical S0 and antisymmetrical A0 modes to pass through the defected region. Furthermore, the transmitted modes A0 and S0 are completely apart from each other and can be quantitatively evaluated. Conversely, when the thickness change is very smooth, most of the energy of A1 Lamb wave is reflected back. It is the unique mode conversion behavior that leads to great transmission difference value of A1 Lamb wave along the opposite directions. Finally, the influence of geometrical parameters on the transmission coefficient is also studied. The higher efficiency and proper working frequency range can be realized by adjusting the slope angle θ, height h1 and h2. The simple asymmetric systems will be potentially significant in applications of ultrasound diagnosis and therapy.

Thermally tunable topological edge states for in-plane bulk waves in solid phononic crystals

HighlightsA thermal field is first utilized for tuning topological band gaps and edge states.We realize the actively frequency tunability of topologically protected edge states.The active frequency controllability has special advantage in practical applications. ABSTRACT The remarkable properties of topological insulators have inspired numerous studies on topological transport for bulk waves, but the demonstrations of topological edge states with tunable frequency are few attempts. Here, we report on the active frequency tunability of topologically protected edge states for in‐plane bulk waves by applying a thermal field. We find that the center frequency of topological band gap is shifted down and the band width is enlarged as the temperature increases. Meanwhile, the frequency range of topologically protected edge states is also shifted to low frequency region with the higher temperature. Furthermore, the robust propagation of in‐plane bulk waves along a desired path is demonstrated within different frequency bands. The tunable frequency for both topological band gaps and topologically protected edge states achieves the active control of the transport for in‐plane bulk waves, which may dramatically facilitate practical applications of novel phononic devices.

Multi-objective optimization of asymmetric acoustic transmission with periodical structure

HIGHLIGHTSMulti‐objective optimization of asymmetric acoustic transmission is first proposed.RBF model is employed to obtain the relationship between design variables and objectives.NSGA‐II optimization algorithm combined with RBF model is performed to form the Pareto front.Numerical validation experiments of optimal solutions show asymmetric propagation properties as expected. ABSTRACT Asymmetric acoustic wave propagation is important for control and manipulation of the acoustic wave signals in various devices. However, previous approach to find optimal asymmetric acoustic transmission (AAT) is through repeatedly adjusting the geometrical parameters, thus causing time‐consuming. Here we propose a study on the multi‐objective optimization of the AAT, aiming to achieve the widest working frequency range (fr) and the highest transmittance peak (&eegr;) with regard to the design variables. For this purpose, the Radial Basis Function (RBF) neural work and finite element (FE) method are applied to obtain the valuable data in the procedure. Furthermore, local sensitivity analysis of design parameters on the fr and &eegr; are analyzed. Ultimately, the Non‐Dominated Sorting Genetic Algorithm II (NSGA‐II) is adapted for getting the Pareto‐optimal solutions. The optimization results show great improvement for the overall performance of the AAT, which could be potentially significant in designing various sound devices.

Unidirectional and tunable acoustic diode made by asymmetric double layer metallic grating with periodical structure

This paper theoretically investigates unidirectional and tunable acoustic transmission in a structure of double layer metallic grating immersed in water. Though numerical simulations, we find that the asymmetric acoustic transmission can be achieved by changing the width and the thickness of the double layer metallic gratings. We also find that the contrast transmission ratio of unidirectional transmission is highly influenced by the ratio of the thickness h1/h2. And further study indicates that the working frequency range of the asymmetric transmission can be easily controlled by adjusting the location of the double layer metallic gratings. This constructed acoustic system shows many potential applications of medical ultrasound diagnosis and related fields.

Lamb wave band gaps in one-dimensional magnetoelastic phononic crystal plates

In this paper, we present the theoretical calculations of the dispersion curves of Lamb mode waves in one-dimensional magnetoelastic phononic crystal (PC) plates by using the finite element method (FEM) and the plane wave expansion (PWE) method. The effects of filling fraction and plate thickness to lattice pitch ratio on the first band gap (FBG) are analyzed under different magnetic boundary conditions. The numerical results show that the FBG characteristics are significantly influenced by different magnetic boundary conditions. Further calculations demonstrate that the filling fraction and the plate thickness to lattice pitch ratio are the most important parameters that affect the width and location of the FBG.