Underwater acoustic scattering from a radially layered cylindrical obstacle in a 3D ocean waveguide

A solution based on coupled mode expansions is presented for the 3D problem of acoustic scattering from a radially layered penetrable cylindrical obstacle in a shallow-water plane-horizontal waveguide. Each cylindrical ring is characterized by a general, vertical sound speed and density profile (ssdp), the ocean environment around the obstacle can be also considered horizontally stratified with a depth-arbitrary ssdp, and the bottom is assumed to be rigid. The total acoustic field generated by an harmonic point source is represented as a normal-mode series expansion. The expansion coefficients are calculated exploiting the matching conditions at the cylindrical interfaces, which results in an infinite linear system. The system is appropriately truncated and numerically solved by using a recursive relation, which involves the unknown coefficients of two successive rings. Results concerning the transmission loss outside and inside obstacles consisting of three cylindrical rings are given for a typical depth-dependent ocean sound-speed profile. The presented solution can serve as a benchmark solution to the general problem of 3D acoustic scattering from axisymmetric inhomogeneities in ocean waveguides at low frequencies.     

Acoustical scattering by arrays of cylinders in waveguides

Multiple scattering of acoustic waves in a planar horizontal waveguide by finite-length cylinders is considered. Cylinder height equals the waveguide depth, and both are vertically constrained by the pressure-release boundaries. An analytically exact solution is obtained via normal mode expansion method in conjunction with the concept of the T matrix. The problem is decomposed into an infinite number of two-dimensional multiple scattering problems, modulated by waveguide mode shapes. Examples are presented for an isovelocity waveguide in which the medium is uniform and the waveguide depth is constant. It is found that, in numerical computations, including one or two evanescent modes captures the essence of the evanescent modes. Multiple scattering in the waveguide is compared with the corresponding two-dimensional case. It is concluded that, in low frequencies, the wave patterns in the two cases are very similar, with a shift in the frequency. The similarity diminishes when there are multiple propagating modes. Despite the mode mixing, some key features in the scattering as observed in the two-dimensional problem remain observable in the waveguide case.     

Wave reflection and transmission due to defects in infinite structural waveguides at high frequencies

In waveguide structures, waves may be partially reflected by local non-uniformities such as cracks and other defects. The reflection and transmission characteristics associated with the presence of a discontinuity may be used, in principle, to give some indication of both the location and size of the defect. A combined spectral element and finite element (SE/FE) method has been used previously to investigate the effects of local non-uniformities at relatively low frequencies. However, for analysis at higher frequencies, where complex deformation of the waveguide occurs, it is necessary to extend this approach. Such high frequency analysis is necessary if small defects are to be located within the waveguide cross-section. In order to investigate wave propagation at higher frequencies, a combined spectral super element and finite element (SSE/FE) method is presented. This method allows the transmission, reflection and wave conversion at discontinuities to be determined for complex waveguides. As an example of the use of this method, wave reflection and transmission in rails are estimated at frequencies between 20 and 40 kHz for various notional sawcut-like defects of progressively increasing size. This shows the feasibility of the approach for realistic waveguides. However, from these simulations it is shown that defects have to be quite large before they can be detected using a single transducer position on the rail cross-section using train-induced vibration.     

Diffuse field transmission into infinite sandwich composite and laminate composite cylinders

This paper is concerned with the modelling of diffuse field transmission into composite laminate and sandwich composite infinite cylinders. Two models are presented and compared: Symmetrical Laminate composite and discrete thick laminate composite. The latter is shown to handle accurately, as a particular case, the first model, and the important case of sandwich composite shells. In both models, membrane, bending, transverse shearing as well as rotational inertia effects and orthotropic ply angle of the layers are considered. Starting from the dynamic equilibrium relations and stress–strain–displacement relations, a dispersion system is given in a wave approach context. Next, expressions for the matrix systems governing the structural impedance, critical frequencies and ring frequency are given. The developed equations are applied to the calculation of the diffuse field transmission of an infinite cylinder. Predictions with the presented models are compared to results presented in the literature for both laminate composite and sandwich composite configurations. They confirm the accuracy of both models and the general nature of the presented discrete thick laminate composite model.     

Formation of frequency pass and gap bands in an elastic waveguide with a system of obstacles

Using a semianalytical solution to the problem of wave excitation in an elastically supported one-dimensional waveguide with a system of point obstacles, we analyze the relation between the observed effects of blocking-transmission and the discrete spectrum point distribution in the complex frequency plane. We check the assumption, stemming from earlier numerical analysis for two-dimensional elastic waveguides with extended obstacles, on whether the occurrence of transmission frequencies in the blocking range is related to the presence of close-to-the-real-axis points of a discrete spectrum. The number of such spectral points increases in proportion to the number of obstacles, giving in the limit continuous transmission bands described by the Bloch-Floquet theory for infinite periodic structures.     

Acoustic phonons transport in a quantum waveguide embedded double defects

By using scattering matrix method, we investigate the acoustic phonons transport in a quantum waveguide embedded double defects at low temperatures. When acoustic phonons propagate through the waveguide, the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips, and acoustic waves interfere with each other in the waveguide to form standing wave with particular wavelengths. In the waveguide with void defects, acoustic phonons whose frequencies approach zero can transport without scattering. The acoustic phonons propagating in the waveguide with clamped material defects, the phonons frequencies must be larger than a threshold frequency. It is also found that the thermal conductance versus temperature is qualitatively different for different types of defects. At low temperatures, when the double defects are void, the universal quantum thermal conductance and a thermal conductance plateau can be clearly observed. However, when the double defects consist of clamped material, the quantized thermal conductance disappears but a threshold temperature where mode 0 can be excited emerges. The results can provide some references in controlling thermal conductance artificially and the design of phonon devices.     

电磁波在螺旋线上的传播

§1.引言 螺旋线最早是应用在高频电子管中,近年来在长距离波导通信中,又应用了螺旋波导管来抑制杂波,因此研究的兴趣并没有降低。在[2—4]中,他们应用了各种不同的方法,来研究它的传输特性。本文的目的是从螺旋坐标出发,进一步探讨它的性质;在§4,§5中,推导了一些特征方程,可以作为计算传输常数的参考。本文提供了一些方法,对于结果只作了一些定性的说明,如果有必要的话,定量的计算留在以后再作。希望本文对于了解螺旋线上的电磁波传播性质,从比较精细的一面,有所帮助。     

Acoustic phonons transport in a quantum waveguide embedded double defects

By using scattering matrix method, we investigate the acoustic phonons transport in a quantum waveguide embedded double defects at low temperatures. When acoustic phonons propagate through the waveguide, the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips, and acoustic waves interfere with each other in the waveguide to form standing wave with particular wavelengths. In the waveguide with void defects, acoustic phonons whose frequencies approach zero can transport without scattering. The acoustic phonons propagating in the waveguide with clamped material defects, the phonons frequencies must be larger than a threshold frequency. It is also found that the thermal conductance versus temperature is qualitatively different for different types of defects. At low temperatures, when the double defects are void, the universal quantum thermal conductance and a thermal conductance plateau can be clearly observed. However, when the double defects consist of clamped material, the quantized thermal conductance disappears but a threshold temperature where mode 0 can be excited emerges. The results can provide some references in controlling thermal conductance artificially and the design of phonon devices.     

A review of analytical methods, based on the wave approach, to compute partitions transmission loss

In this paper, various methods for calculating partition transparency are investigated. These methods are all based on the wave approach, yet they differ in the way of considering the incident field, partition leaf dimensions and the absorbing material, or absorbent, incorporated between the partition leaves. The method verification has been done through comparison with the experimental data available in the literature. Results are very convincing and the wave approach proves to be highly accurate. The basic wave approach is well suited for modeling of infinite single or double-leaf partitions, although the diffuse incident field requires spatial windowing to achieve agreement with experimental data. When porous material is incorporated inside double-leaf partitions, the model needs to be enhanced, based on the Biot theory, to ensure coincidence with experimental data. In the case of partitions, which cannot be considered infinite, spatial windowing is applied to the transparency to correct for the dimensional effect, especially in the low-frequency range. The final model turns out to be highly accurate, as long as spatial windowing is limited to the first coincidence frequency. The wave approach therefore proves to be a suitable method and calculation times are moreover acceptable.     

Directional properties of an array of sound receivers positioned in an impedance screen recess

Expressions for calculating the directional characteristics of an array of sound receivers positioned in a waveguide with impedance walls are obtained from the solution to the problem on the diffraction of a plane sound wave by the waveguide open end with impedance flanges. The waveguide can be of a finite length, and, in this case, it can be considered as an open cavity in an impedance screen. The solution of the integral equation for the sound pressure distribution over the opening area is reduced to the solution of an infinite system of algebraic equations for the coefficients of the field expansion in normal waveguide waves. Examples of calculated directional characteristics are presented for arrays with receivers positioned at different distances from the opening and for different values of the impedances of the waveguide walls and flanges.     

Radiation properties of ICRF waveguide couplers

The radiation spectra of a dielectric-filled waveguide (DWG) and a folded waveguide (FWG) antenna in the ion cyclotron range of frequencies are evaluated for tokamaks with the waveguide codes, which model the coupling of the waveguide modes to the plasma waves with outward radiating boundary conditions. The codes provide a self-consistent calculation of the fields across the waveguide apertures for various antenna phasings taking into account a complete set of waveguide eigenmodes including the reflected modes and, in the case of DWG, an arbitrary orientation of the antenna to the plasma magnetic field. In the studied cases, the spectra are found to be well described by a model which is based on a simplified wave field pattern determined by the antenna aperture geometry. Comparison to the corresponding loop antenna is carried out     

Eigenwaves in one-dimensional photonic crystals with gain

The dispersion characteristics of a one-dimensional photonic crystal are analyzed in detail on the basis of the exactly solvable Kronig-Penney model for the particular case of a periodic system consisting of an infinite number of insulator-air layers arranged in the direction of propagation of radiation. An analytical solution is obtained, and the behavior of eigenwaves near the edges of the photonic band is thoroughly studied. The behavior of roots on a complex plane is qualitatively analyzed with the use of approximate dispersion characteristics obtained via expanding the solution of the wave equation in series in terms of the harmonics of the lattice spacing. It is shown that, despite the amplifying properties of the material, no gain in the photonic band gap is present, because of the interference quenching of waves.     

Natural Electromagnetic Waves in a Circular Waveguide with a Chiral and a Dielectric Layer

Based on particular solutions of a system of wave equations which describe electromagnetic processes in an unbounded biisotropic medium and are represented in the form of generalized power series, a dispersion equation is obtained for a two-layer circular waveguide with a chiral and a dielectric layer. The solution of the dispersion equation is analyzed for some types of natural waves.     

单轴各向异性异向介质平板波导中的导模特性

 推导了介电常数张量和磁导率张量中各分量带有不同符号的单轴各向异性异向介质平板波导的导行条件。根据分量符号的正负组合，分情况讨论了导行TE波的偶模特性，画出了平板波导中为实横波数时的布里渊图。结果表明，在特定条件下此平板波导中有无穷多导模，这与传统介质波导及各向同性异向介质平板中只有有限数量模式存在有很大不同。此外，还发现当分量参数满足一定条件时，出现了多个虚横波数的传输模式解，并且该模式存在高频截止。     

Unidimensional models of acoustic propagation in axisymmetric waveguides

This paper presents a model of the linear acoustic propagation in axisymmetric waveguides, the pressure depending on a single space variable. The approach consists of writing the wave equation and the boundary conditions for a coordinate system rectifying the isobaric map at each time. The two-dimensional dependence of the problem is thus transferred from the pressure to the coefficients of the wave equation. From this result, an exclusively geometrical necessary condition is deduced for the admissibility of isobaric maps. However, the knowledge of the waveguide geometry is not sufficient to separate the pressure and the isobaric map solutions. In order to develop a unidimensional wave equation, a geometrical hypothesis is discussed. For lossless and motionless rigid waveguides, the deduced equation leads to exact results for tubes and cones. It may be interpreted as a Webster equation for a particular coordinate system so that the particular profiles for which analytical solutions of the pressure exist are redefined. The wave equation is also established for pipes with visco-thermal losses and, more generally, for mobile walls having a small admittance. The compatibility of the geometrical hypothesis with the exact model is specified for this general case.     

基于局域共振单元实现声子晶体低频多通道滤波

从理论上提出一种由局域共振单元组成的声子晶体低频多通道滤波模型.在二维三组元局域共振声子晶体中引入不同填充率的共振单元构成波导结构,通过有限元法计算出其能带结构、透射曲线和透射场图.结果显示:这种设计能够在低频带隙范围内不同填充率散射体的共振频率附近产生新的分立模,且这些分立模能够使相应的声波在声子晶体中沿波导方向传播;这些分立模只与相应的共振单元相关,抗干扰能力强.所得结果为低频多通道滤波器的设计提供了一种新的理论依据.     

Finite element prediction of wave motion in structural waveguides

A method is presented by which the wavenumbers for a one-dimensional waveguide can be predicted from a finite element (FE) model. The method involves postprocessing a conventional, but low order, FE model, the mass and stiffness matrices of which are typically found using a conventional FE package. This is in contrast to the most popular previous waveguide/FE approach, sometimes termed the spectral finite element approach, which requires new spectral element matrices to be developed. In the approach described here, a section of the waveguide is modeled using conventional FE software and the dynamic stiffness matrix formed. A periodicity condition is applied, the wavenumbers following from the eigensolution of the resulting transfer matrix. The method is described, estimation of wavenumbers, energy, and group velocity discussed, and numerical examples presented. These concern wave propagation in a beam and a simply supported plate strip, for which analytical solutions exist, and the more complex case of a viscoelastic laminate, which involves postprocessing an ANSYS FE model. The method is seen to yield accurate results for the wavenumbers and group velocities of both propagating and evanescent waves.     

Dispersive elastodynamics of 1D banded materials and structures: analysis

The elastodynamics of 1D periodic materials and finite structures comprising these materials are studied with particular emphasis on correlating their frequency-dependent characteristics and on elucidating their pass-band and stop-band behaviors. Dispersion relations are derived for periodic materials and are employed in a novel manner for computing both pass-band and stop-band complex mode shapes. Through simulations of harmonically induced wave motion within a finite number of unit cells, conformity of the frequency band structure between infinite and finite periodic systems is shown. In particular, only one or two unit cells of a periodic material could be sufficient for “frequency bandedness” to carry over from the infinite periodic case, and only three to four unit cells are necessary for the decay in normalized transmission within a stop band to practically saturate with an increase in the number of cells. Dominant speeds in the scattered wave field within the same finite set of unit cells are observed to match those of phase and group velocities of the infinite periodic material within the most active pass band. Dynamic response due to impulse excitation also is shown to capture the infinite periodic material dynamical characteristics. Finally, steady-state vibration analyses are conducted on a finite fully periodic structure revealing a conformity in the natural frequency spread to the frequency band layout of the infinite periodic material. The steady-state forced response is observed to exhibit mode localization patterns that resemble those of the infinite periodic medium, and it is shown that the maximum localized response under stop-band conditions could be significantly less than in an equivalent homogenous structure and the converse is true for pass-band conditions.