卷 65, 编号 3 (2019)

In this paper, the acoustic field excited by a single force with arbitrary direction in a semi-infinite elastic space is studied and its mathematical expressions are obtained. It shows that there are many complex behaviors when the elastic wave reaches the free boundary. The numerical simulation shows that there are several kinds of waves in the semi-infinite elastic space: direct P wave, direct SV wave, SP wave propagating along the free surface which can generate Head wave and Rayleigh wave. The forming mechanism of the SP wave and Rayleigh wave is specially studied. The waveforms at the observation point on the free surface of the semi-infinite space contain only direct P wave and direct SV wave when the SV wave incident angle is within the critical reflection angle. However, if the incident angle from the source to the observation point is exceeding to the critical reflection angle, not only direct P and direct SV wave but also the SP wave and Rayleigh wave are all be generated. It is focused on the relationships of the direction of single force to the excitation intensity of each wave. The relationship of each wave packet to the single force and observation direction is obtained and analyzed.

The propagation of detonation waves in a channel filled with a bubbly liquid that suddenly widens is investigated. Possible scenarios for the dynamics of detonation waves after their transition to the widening part of the channel are analyzed. The influence of the volume content of a combustible gas and the geometric parameters of the channel on the propagation and breakdown of the detonation wave has been established.

Numerical simulation is applied to analyze the possibilities of using vertical receiving arrays covering the entire waveguide for underwater acoustic communication on the shallow Arctic shelf. Binary phase shift keying with a carrier frequency of 750 Hz is selected for data transfer. It is shown that an algorithm for selecting acoustic signals corresponding to the first waveguide mode or an algorithm based on using passive-phase conjugation eliminates intersymbol interference. In case of spatial signal processing and waveguide depth of 30 m the error-free information transmission is possible when the signal-to-noise ratio is ≈15 dB less than that for a single receiver. It has been demonstrated these advantages to be present in different conditions on the Arctic shelf: with an acoustically soft or acoustically hard bottom, with surface waves, or with ice cover.

The aeroacoustic characteristics of vortex rings of different sizes have been studied using multimicrophone methods. The experiment recorded the acoustic emission upon initiation of a vortex ring and its movement along a trajectory. In order to apply the self-similarity theory of vortex motion on the entire trajectory, the formation of a vortex ring was numerically simulated in the starting area during ring initiation from nozzles of different sizes, taking into account the experimentally determined law of piston motion for each initiation. Computations are performed to obtain the geometrical dimensions of the vortices used in the self-similarity theory. Thus, the required vortex ring parameters were found throughout the entire trajectory, which made it possible to compare the characteristic frequency of noise emission from freely flying turbulent vortex rings of different sizes with the theoretical value.

An experimental study of noise reduction methods has been carried out for realistic small-scale (1 : 10) models of aircraft landing gear based on two ideas: (1) the useful interference of turbulent wake sources and reflected sources induced by them, effected by changing the geometry of the legs/struts, and (2) direct reduction of source intensity in a turbulent wake by mounting a fairing on the wheel assembly. The mutual complementarity of these methods in different frequency ranges is demonstrated. The influence of the yaw angle on the effectiveness of these noise reduction methods is also studied. Their applicability to large-scale models is analyzed.

The paper discusses an acoustoelastic study of residual stresses and related structural changes in thin-walled austenitic steel pipes. A probing ultrasonic (US) beam is formed owing to the thermoelastic effect by absorption of a laser pulse in an optoacoustic transducer. Normal and oblique incidence (at an angle close to critical) of the US beam was used on the studied. Distribution maps of US velocity variations over the object’s surface and the volume structural inhomogeneities inside the metal were plotted. Estimates are presented for the residual stresses in the sample under nonstationary thermal loading. The coincidence between the residual stress distribution, structural inhomogeneities of the metal, and distribution of thermal loading sources is established. The possibility of estimating the residual life and finding of macrocrack nucleation centers is discussed.

Low-frequency vibration-bandgaps in elastic metamaterials open new possibilities to minimize low-frequency vibration and noise. Unfortunately, fabricating a complete vibration bandgap for low frequencies still represents a challenging engineering task. In this paper, a new type of a low-frequency complete vibration bandgap in a thin non-metal elastic metamaterial plate is introduced and investigated numerically. The proposed elastic metamaterial plate consists of decoupling-resonators, which are deposited on a 2D, locally resonant phononic-crystal plate, made of an array of rubber fillers, which are embedded in a nonmetallic plate. The dispersion relationship, the power-transmission spectrum, and the displacement fields for the eigenmode are calculated using the finite element method. It is shown that coupling between the local resonance mode of the decoupling-resonators and the Lamb-wave mode of the epoxy plate, consistent with the modal superposition principle, is responsible for the formation of vibration bandgaps. Moreover, the equivalent spring-mass system for the coupling-resonators can be decoupled by introducing a rubber filler. In addition, both longitudinal and the transverse elastic wave bandgaps can be tuned to the same low-frequency range. As a result, a novel kind of low-frequency complete vibration bandgap, which can damp a low-frequency elastic wave, is produced. Furthermore, the effects of the decoupling-resonators on the vibration bandgap are investigated. It is now possible that an elastic metamaterial plate can be dampen with complete low-frequency vibration bandgaps, which can potentially be used for commercial noise and vibration reduction.