卷 64, 编号 6 (2018)

Wave propagation that occurs in a prestressed cylindrical waveguide with inhomogeneity along the radial coordinate under a periodic radial load concentrated in a ring-shaped region is investigated. With the use of Fourier integral transform, the problem is reduced to analyzing an operator sheaf depending on two parameters. Structural features of the dispersion set are investigated, and displacement components are determined based on the theory of residues and the analysis of an auxiliary spectral problem.

Sonic crystals are the periodic arrangements of scatterers embedded in a homogeneous material. Their ability to prevent sound wave to propagate in a particular range of frequency demonstrates their use as potential noise barriers. The sonic crystal considered in this work is an array of PVC cylinders (5 × 5) in air bounded by acrylic sheets. This paper studies the sound transmission loss in the sonic crystal by changing the location of the sidewalls. The optimized location of sidewalls of the sonic crystal to get wide band gap and high sound transmission loss has been investigated. To increase the transmission loss, a periodic structure having bi-periodicity, i.e., periodicity in two perpendicular directions is introduced. Both computational (Finite Element simulation) and experimental work has been performed to study the sound transmission loss and the band gaps. To bridge the gap between the two results, an improved finite element model has been proposed with an aim to replicate the experimental situation more closely. Generally, in experiments, insertion loss is calculated while numerically transmission loss is computed, and the two are compared. In this paper, a comparison between insertion loss and transmission loss has also been made numerically, which is compared with the experimental results.

The features of propagation of coupled hybrid transverse elastic waves in a nonuniform dense micropolar medium with spatial dispersion are studied. It is shown that in the region of the medium corresponding to the intersection point of unperturbed dispersion curves of elastic waves of different types, efficient transformation of a shear wave into a rotational wave or vice versa may occur.

An analytical inverse method to design lenses of isotropic inhomogeneous refractive index (RI) distribution is presented, where the wave ray propagation is described by the eikonal equation. We show that some particular RI distributions can be obtained by the angles of incidence and emergence when the rays pass through the surfaces of the lenses. This method is applied to design lenses that perfectly focus rays or bend them to arbitrary angles. In addition, gradient refractive index (GRIN) devices are proposed, able to generate self-bending acoustic beams and obtain illusion shadows of arbitrary objects. The ray tracing and finite elements method simulation results indicate the validity of the method. The method may have potential applications in designing acoustic and optic GRIN devices for controlling energy flux, such as medical imaging, therapeutic ultrasound, acoustic levitation, energy isolation, acoustic and optic camouflaging, etc.

Model experiments are used to obtain the transfer functions of an Arctic-type shallow-water acoustic waveguide in the spring–summer period, including in the presence of surface waves. The calculations were performed for frequencies from 150 to 1000 Hz using the theory of coupling modes. From the numerical simulation results, the usable frequency range is determined for stable underwater communications suitable for waveguides with both acoustically soft and acoustically hard bottoms. It is demonstrated that for certain distances between the sound source and receiver and their location depths, errorless information transmission can be achieved even without special processing methods. Channel compensation with passive time reversal leads to increased transmission reliability predominantly in a waveguide with an acoustically hard bottom.

It is well known that a sound source related to nonstationary motion of vortices at small Mach numbers can be obtained in the incompressible inviscid fluid approximation. In this study, it is proposed to describe the perturbation dynamics of an incompressible ideal fluid using the Lagrangian and Hamiltonian formalism with the displacement field and momentum density perturbation as canonical variables. Based on Noether’s theorem, the conditions of quadrupole moment conservation in the evolution of small perturbations of stationary flows have been formulated. It has been shown that these conditions are always satisfied for perturbations of uniform jet flows. The obtained results not only yield a solution to the general mechanics problem on motion integrals; they are also significant in aeroacoustics because the quadrupole moment of a vortex flow is the principal term of sound source expansion in the Mach number.

The computation of a compressible flow for aeroacoustic prediction is a challengeable work insofar as the fluctuation is usually very small in a sound field compared with the flow field. For the low Mach number considered in this study, a discrete vortex method in conjunction with fast multipole time-domain boundary element method is developed and applied to predict far-field sound resulting from a 2D vortex dominated flow. The flow field is simulated employing the classical discrete vortex method. The sound field scattered by solid bodies is determined by using a time-domain boundary element method combined with the convolution quadrature approach, by means of which the convolution integral is approximated by a quadrature formula utilizing a Laplace-domain fundamental solution. In addition, the fast multipole method is applied to improve the computational efficiency. Finally, several examples are presented to check the applicability and accuracy of the method. Numerical results indicate that the noise predicted by the present method agrees well with the experimental results, and the sound pressure levels of the cylinder models have a dipole-like directivity at vortex shedding frequency.

To study the directional properties of geoacoustic emission, the paper proposesthe use of a combined point receiver system moored at the bottom of a natural waterbody in Kamchatka. This system was used to analyze geoacoustic radiation in seismically quiescent periods and prior to earthquakes for the period of 2008–2016. For 111 cases of geoacoustic emission anomalies recorded in a three-day interval prior to earthquakes, the directions of maximum radiation were considered. These maxima were used to assess the orientation of the axis of maximum compression of rocks at the recording point.

Comprehensive studies have been carried out on determining the acoustic and vibroacoustic characteristics of multilayer polymer composite panels of variable thickness installed in the opening between reverberation chambers. The acoustic insulation and acoustic excitability of the panels excited by a diffuse acoustic field, as well as their modal density, radiation loss factor, and total loss factor, have been determined for excitation with a vibration exciter in a wide frequency range. It has been shown that the panel characteristics have a universal character. Simple empirical relations have been derived to describe these characteristics. The measured vibroacoustic characteristics of the panels made of a polymer composite material have been compared to those of a traditional stiffened fuselage panel.

The paper considers the problem of sound absorption by a Helmholtz resonator in a room with perfectly hard walls. The resonator parameters are determined that yield the maximum absorption in the vicinity of the lowest eigenfrequency of the room: the coefficient of friction, elasticity, and mass.

The paper presents a novel numerical method for solving the problem of noise emission by a body moving in a fluid. The method represents noise emission as a process by which turbulence-generated short-circuited pseudosound waves are scattered by inhomogeneities of a surface in a flow. An example of solving the model problem of edge noise generated by a thin plate is used to demonstrate the mesh independence of the method and determine the requirements for the parameters of the numerical calculation domain. The method is partially verified by third-party analytical models.