Whispering Gallery Waves Research Articles

Acoustic scattering from elastic target buried in water-sand sediment

Acoustic scattering from objects buried in water-sand sediment is the foundation of target detection and identification. It is also a research hotspot in areas of acoustic scattering while the domestic research on scattered field from buried targets is not deep. This paper deduces an approximate Green's function of acoustic scattering from targets buried in water-sand sediment, which describes clearly the whole physical process during the propagation of scattered waves. Next, on basis of geometric acoustics, the corresponding Helmholtz-Kirchhoff formula of integration is presented. Complicated integration of the full wave number spectral representation of the Green's function is avoided by employing approximate formula derived from the method of ray acoustics. As a result of neglecting the influence from lateral waves, the Helmholtz-Kirchhoff integral given applies to supercritical incidence case. The function of COMSOL Multiphysics software is expanded by writing this formula of integration into it. By means of finite-element method, numerical calculation models for two-dimensional axisymmetric targets are established on the software platform. The proposed model built in free field is verified through comparing numerical results obtained with the Rayleigh method which has been validated in previous research achievements of acoustics. The target strength of buried elastic solid sphere is calculated under different conditions in order to analyze the change regularity of buried scattered field. We provide a summary about the law of target strength of the elastic sphere varying with frequency, buried depth, and the attenuation of sand. Finally, we conduct acoustic scattering experiments in free space and shallow buried conditions and process the data with the method of isolation and identification of resonance to separate eliastic echoes from reverberation echo and specular echo. Results from the experiment of free field show that components of the scattered wave should include Rayleigh waves and whispering gallery waves. The processed data of objects buried inside layered fluid media indicate that characteristics of resonance spectra can be used to identify and detect the target effectively while echo signal is not available for identification of target. The proposed technique is verified through the comparison of data from total scattered field between experiment and theoretical prediction. This study has important guiding significance for detecting and identifying targets embedded within layered acoustic media in practical applications.

Acoustic scattering of a complex target with partially solid-filling immersed in water: numerical simulation and experiment

Resonance peaks of spectral function transformed from echoes are the most important characteristics for distinguishing the different targets. So in frequency domain, response function is usually calculated with small interval in a wider frequency band to satisfy the demand of fast and high precision prediction in practical engineering. According to axis-symmetric model, we use 2 dimensional finite element method to solve the acoustic scattering problem efficiently, even when the scattering target has a large size and complex structure. This article focuses on the explanation of scattering characteristics of a special target, namely, a partially solid-filling cylinder with hemispherical cap and thin-shell. Supposing that the receiver and transmitter are in monostatic arrangement, we calculate scattering strength in far field in a frequency range of 50 Hz-10 kHz, and give pseudo-color image represented by frequency-angle to describe influences of shell, filling and the orientation of the incident wave on scattering properties. According to the numerical results, the following conclusions are given: when the transmitter is facing the hemispherical cap (the cap has a vacuum inside, and the incident angle θ is equal to 0°), the main contribution of scattered wave comes from the shell of target. When θ = 180°, the internal filling inhibits the elastic resonance of the shell, and plays an important role in the total scattering field. Because the acoustic impedance of the shell is much larger than that of the water, elastic resonance of the shell is more difficult to excite than that of the solid filling. While the material property of the solid filling is not significantly different from that of the water, so the elastic resonance of the filling fluctuates relatively fast, and the scattering function vibrates approximately with equal amplitude in a wider frequency band. When θ= 90°, the sound wave is perpendicular to the axis of the cylinder, the shell and the filling work together on scattered waves. Once the incident angle deviates from 90° and the sound wave obliquely illuminates target with respective to the axis of the cylinder, the echo of the filling material plays a predominant role in the total scattering field. The frequency-angle spectrum of the solid filling model presents the “bowl” type resonance curve. In order to validate which physical and geometrical structure must be considered in solution of scattered far field, the acoustic scattering experiments are performed in tank with a target suspending in water, which is in monostatic arrangement and satisfies the free field condition. Frequency of incident wave is in a frequency range of 10-40 kHz. For obtaining pseudo-color image of distance-angle, echoes are received and measured when the target is rotated from 0°-360°. The scattered waves are divided into mirror reflection and various components of elastic wave, and the mechanisms of these echoes are explained based on circumferential wave around the surface. Whispering gallery waves are also considered and clearly seen in the experiment. Due to the coupling interaction between the filling and elastic shell, the resonance curve of frequency-angle spectrum splays “bowl” curve outward the sides of normally direction. Experimental and numerical results are in good agreement, which is indicated by comparing the resonance peaks characteristic in spectral domain. The results of this article will be helpful in studying underwater target with more complicated structure.

Narrow Magnonic Waveguides Based on Domain Walls.

The channeling of spin waves with domain walls in ultrathin ferromagnetic films is demonstrated theoretically and through micromagnetics simulations. It is shown that propagating excitations localized to the wall, which appear in the frequency gap of bulk spin wave modes, can be guided in curved geometries and propagate in close proximity to other channels. For Néel-type walls arising from a Dzyaloshinskii-Moriya interaction, the channeling is strongly nonreciprocal and group velocities can exceed 1 km/s in the long wavelength limit for certain propagation directions. The channeled modes represent an unusual analogy of whispering gallery waves that are one dimensional and nonreciprocal with this interaction. Moreover, a sufficiently strong Dzyaloshinskii-Moriya interaction can create a degeneracy of channeled and propagating modes at a critical wave vector.

Horizontal refraction and whispering gallery sound waves in area of curvilinear coastal wedge in shallow water

Horizontal refraction of sound signals in area of coastal wedge determines specific structure of the sound field in horizontal plane, including multipath areas and caustics (in the framework of approximation of horizontal rays and vertical modes). This structure depends on mode number and frequency. In the paper sound field of signal, radiated by the point source is studied in the shallow water area which can be considered as a wedge with curvilinear coastal line (lake, gulf, cape, etc.). It is shown that for some distances from the source to the beach whispering gallery waves (whispering gallery horizontal rays) exist, propagating along the coast, where energy is concentrated in narrow area in horizontal plane. Structure and characteristic of these rays depend on mode number, frequency and waveguide parameters (in particular, steepness of sea floor and radius of curvature). Intensity fluctuations as well as variation of pulse shape and frequency spectrum are studied; analytical estimations are presented as well as discussion of possible experimental observations. [Work was supported by RFBR and BSF.]

A Short-Wave Point Source Near an Inhomogenous Half-Plane

The diffraction of waves produced by a point source by an inhomogenous half-plane is considered. The square of the wave number inside the inhomogenous region is assumed to decrease linearly with distance from the interface. The main aim of the article is to obtain the asymptotics of whispering gallery waves as k→ ∞, where k is the wave number in a homogeneous region. Bibliography: 7 titles.

Diffraction of plane and cylindrical waves by a cylindrical metamaterial shell with a negative refractive index

The problems of scattering of plane and cylindrical waves by a cylindrical metamaterial shell are solved rigorously. The influence of the geometric dimension of the shell, the value of the negative refractive index of the metamaterial medium, and the location of the cylindrical wave source on the near (far) field structure is investigated. It is shown that, in the quasi-opticl region, a caustic with one cusp is formed inside an electrically thick shell and whispering-gallery waves or standing waves are formed in between the facets of an electrically thin shell. It is found that, in the resonance region, reactive (surface) fields with substantial amplitudes are observed near the boundaries of a thin shell.

The green’s function of sh-polarized surface waves

In this paper, the Green’s function corresponding to surface SH-waves is found. Whispering gallery waves, creeping waves, and waves propagating with a constant transverse velocity in an elastic medium are investigated. Bibliography: 5 titles.

Study of Circumferential Waves on a Poroelastic Cylinder

The dispersion relations of acoustic modes in poroelastic cylinders with and without elastic shell coating are determined and solved. The influence of elastic frame parameters and the Biot parameters on the dispersion curves is studied in the configuration with and without coating. The dispersive Rayleigh and whispering gallery waves are highly sensitive to the density and the shear modulus of the skeleton, opening a perspective for the evaluation of the mechanical parameters of poroelastic materials confined in cylindrical tubes during the manufacturing process. The predicted dispersion curves are validated with experimental results obtained by use of different experimental set-up in the case of a porous circular cylinder.

Point source waves near the interface between elastic and liquid media

Combined surface waves are under consideration; they can be represented as a combination of whispering gallery waves (concentrated near the boundary in a layer of width O(ω −2/3) as ω → ∞, where ω is the frequency) and standard surface waves (exponentially decaying with distance from the interface with exponent proportional to ω), or waves oscillating with distance from the boundary. Those waves are obtained near the boundary z = 0 of an inhomogeneous elastic medium z > 0 (propagation velocities a(z) and b(z)) and an inhomogeneous liquid (the velocity in the liquid is a 0(z)). In the latter case, there are wave fields propagating with phase velocity close to the velocities of Stonely and Rayleigh waves, and also close to the velocities a 0, b, and a on the interface. Bibliography: 10 titles.

The whispering gallery effect in neutron scattering

We present the first experimental evidence for neutron localization (whispering gallery wave) in the quasistationary quantum states near a cylindrical mirror surface. The boundary effective well is formed by the centrifugal effective potential and the mirror neutron–matter optical potential. We present a formalism that describes quantitatively the neutron scattering at a cylindrical mirror surface and compare the experimental results to this model. We discuss further prospects based on this study.

Acoustic resonance scattering from a multilayered cylindrical shell with imperfect bonding

The method of wave function expansion is adopted to study the three dimensional scattering of a time-harmonic plane progressive sound field obliquely incident upon a multi-layered hollow cylinder with interlaminar bonding imperfection. For the generality of solution, each layer is assumed to be cylindrically orthotropic. An approximate laminate model in the context of the modal state equations with variable coefficients along with the classical T-matrix solution technique is set up for each layer to solve for the unknown modal scattering and transmission coefficients. A linear spring model is used to describe the interlaminar adhesive bonding whose effects are incorporated into the global transfer matrix by introduction of proper interfacial transfer matrices. Following the classic acoustic resonance scattering theory (RST), the scattered field and response to surface waves are determined by constructing the partial waves and obtaining the non-resonance (backgrounds) and resonance components. The solution is first used to investigate the effect of interlayer imperfection of an air-filled and water submerged bilaminate aluminium cylindrical shell on the resonances associated with various modes of wave propagation (i.e., symmetric/asymmetric Lamb waves, fluid-borne A-type waves, Rayleigh and Whispering Gallery waves) appearing in the backscattered spectrum, according to their polarization and state of stress. An illustrative numerical example is also given for a multi-layered (five-layered) cylindrical shell for which the stiffness of the adhesive interlayers is artificially varied. The sensitivity of resonance frequencies associated with higher mode numbers to the stiffness coefficients is demonstrated to be a good measure of the bonding strength. Limiting cases are considered and fair agreements with solutions available in the literature are established.

Boundary layer approach to describe an interference head wave

The process of diffraction by an interface l between two homogeneous media is discussed in the paper. The process of generation and propagation of an whispering gallery wave in a boundary layer close to the interface and a head wave is considered.

Reflection and transmission of the surface P-waves on the line of jump in elastic parameters

Problems of diffraction of elastic surface waves of horizontal polarization (P-waves) on a line of jump in elastic parameters are considered. The corresponding coefficients of reflection, refraction, and transmission are obtained by means of the parabolic equation method. A comparison of the Rayleigh waves, SV-waves, and P-waves is carried out. Numerical values of the transformation matrix are found in the case where the whispering gallery wave transforms to a sum of whispering gallery waves and a homogeneous wave transforms to the first five modes of whispering gallery waves. Bibliography: 6 titles.

Acoustic scattering characteristics of a thick-walled orthotropic cylindrical shell at oblique incidence

The method of wave function expansion is adopted to study the scattering of a plane harmonic acoustic wave incident at an arbitrary angle upon an arbitrarily thick cylindrically orthotropic homogeneous cylindrical shell submerged in and filled with compressible ideal fluids. A laminate approximate model and the so-called state space formulation in conjunction with the classical transfer matrix (T-matrix) approach are employed to present an analytical solution based on the three-dimensional exact equations of anisotropic elasticity. The solution is used to correlate the perturbation in the material elastic constants of an air-filled and water-submerged aluminium cylindrical shell to the sensitivity of resonances associated with various modes of wave propagation appearing in the backscattered amplitude spectrum (i.e., axially guided, Lamb, Rayleigh and Whispering Gallery waves). The effects of shell wall thickness as well as inner fluid loading on the frequency response of the shell are also examined. A limiting case is considered and good agreement with the solution available in the literature is obtained.

The whispering-gallery waves in a disk dielectric resonator located on a dielectric substrate

The paper is devoted to investigation of peculiarities of natural azimuthal electromagnetic oscillations in a dielectric resonator located on a substrate, whose permittivity is lower than that of the material of the main resonator. The impact of the dielectric substrate thickness on the spectrum of natural frequencies of the resonant structure is considered. Also, the behavior of electrodynamic characteristics of the resonator as a function of frequency and dielectric substrate’s width is analyzed.

Edge-excited rays on convex and concave structures: a review

In this paper, a review is presented on edge-excited rays on convex and concave perfectly conducting structures with sharp edges, curvature discontinuities, or inflection lines. The incident wave is either a space wave, a creeping wave, or a whispering-gallery wave. The observation point may be at large distance from the structure or located on its surface. In addition to more classical formulas, the paper also contains some new solutions, and gives an updated state of the art on this subject.

On the negative value of dielectric permittivity of the water surface layer

The present letter reports on experimental measurements of a resonant frequency shift of a radially two-layer quasioptical dielectric resonator with whispering gallery wave modes when the internal layer is water or benzine. The sign of the shift indicates that the real part of dielectric permittivity of the water surface layer adjacent to a solid-state dielectric becomes negative (ε′<0). The measurements were carried out at room temperature in microwave K band.

Optical solitons appearing during propagation of whispering-gallery waves

The properties of solitons appearing during the propagation of whispering-gallery waves in a homogeneous glass cylinder are considered. It is shown that such solitons can be used for the light frequency conversion.

Whispering-gallery waves in optical fibres

The process of excitation of whispering-gallery waves (WGWs) in optical fibres (microcavities) with the help of a bitapered fibre is analysed. It is shown that useful information on the WGW modes can be obtained from the spectrograms recorded by scanning the exciting-radiation frequency. Based on the geometrical-optic approximation, the longitudinal sizes of the WGW modes are estimated and it is shown that the ultimate diameter of the fibre exists for optical fibres (microcavities) where a mode can be still excited with the help of a bitapered fibre.

Interaction of whispering-gallery electromagnetic waves with acoustic waves in tapered quartz rods

The propagation of whispering-gallery waves in a dynamic cavity formed by a tapered quartz rod and the plane interface between two regions of the rod with different refractive indices moving along the axis of the rod is analysed. It is shown that the limiting frequency shift of light in such a cavity is determined by its Q factor and the attainable refractive index discontinuity. The possibility of using acoustic waves for obtaining a dynamic cavity is considered.