Vol 63, No 1 (2017)

Flexural wave propagation along a bar whose thickness smoothly decreases down to zero within its end piece is considered. The propagation velocity tends to zero as the tapered end of the bar is approached, and the time of wave propagation to the tapered end is infinite. As a consequence, waves propagating along the bar are not reflected from the end. Previous quantitative study of the effect in the WKB approximation shows that, in the case of parabolic tapering, the WKB approximation yields a uniform asymptotics, which is valid (or invalid) for any of the bar’s cross sections. In the case of a bar with parabolic tapering, the equation of flexural vibrations of the bar has exact analytic solutions in the form of power functions. Based on these solutions, a modified WKB approximation is proposed to solve equations for bars with nonparabolic thickness variation laws. The input impedance of a bar with a parabolic tapering is calculated and analyzed.

We present a theory of resonance oscillations at doubled and tripled frequencies in a pipe open at one end. The boundary condition at the open end is obtained with allowance for the subharmonicity of speed fluctuations at the open end; it does not contain empirical parameters. Quite good qualitative and quantitative coincidence of the theoretical and experiment results is achieved.

Jet–flap interaction noise for a small-scale swept wing model of a trendsetter plane has been studied experimentally, with a double-stream nozzle installed nearby. The effect of the flap angle on the noise of jet–wing interaction has been analyzed. It has been discovered that the flap angle considerably affects the interaction noise in a wide frequency range in such a way that the noise intensity in each frequency band grows exponentially as the flap edge approaches the shear-layer boundary (on a logarithmic scale, this corresponds to linear scaling of the spectrum depending on the flap angle). The exponential noise reduction as the flap angle decreases agrees with the known theoretical simplified-configuration models that are based on the effect of interaction of the near field of instability wave with the edge. Not only does this agreement show that noise increase/reduction mechanism may be associated with similar processes, it also provides an effective tool for controlling the interaction noise.

Hydrodynamic and acoustic processes associated with a drop impact on a water surface were studied experimentally. Acoustic signals were detected underwater (with a hydrophone) and in air (with a microphone), the flow pattern was recorded with a high-speed camera, and the surface perturbation was monitored with a laser detector. The dimensionless parameters of flows (Reynolds, Froude, and Weber numbers) induced by the impact varied with fall height within the ranges of 5000 < Re < 20000, 20 < Fr < 350, and 70 < We < 1000. The sequence of acoustic signals incorporated an impact pulse at the moment of contact between a drop and the surface and a series of acoustic packets attributable to the resonance emission of gas cavities. The top of the impact pulse, which was detected clearly in the entire fall height range, had a complex structure with short high-frequency and longer low-frequency oscillations. The total number and the parameters of emitted acoustic packets depended to a considerable extent on the fall height. The cases of lacking, one-time, and repeated emission of packets were noted in a series of experiments performed at a constant fall height. The analysis of video data showed that the signal variability was induced by considerable differences in the scenarios of water entry of a drop, which assumed an ovoid shape at the end trajectory segment, in the mentioned experiments.

An analytic representation is constructed for a nonaxially symmetric sound field to simulate a hydroacoustic waveguide the bottom of which is hard and has an axially symmetric relief. A numerical analytic method for finding the velocity potential is proposed, for which undetermined coefficients for normal modes are determined from a corresponding infinite system of algebraic equations. The sound fields are studied with for variations of the problem parameters.

We consider separation of acoustic modes in an experiment carried out in the Florida Straits. The features of the approach are separation of modes using data from single hydrophones, not vertical mode arrays, and a passive scheme of noise interferometry in which the source consists of ocean noise. Processing made it possible to reliably separate the first four modes of the acoustic field. The results allow a conclusion on the possible use of this method for shallow-water monitoring under complex hydrological conditions.

The paper demonstrates that during triaxial loading of a core sample of dry sandstone, genetically related frequency ranges form in the waveguide, which are significantly spatially separated, but have a similar dynamics. This is explained by the authors' previously suggested model for the generation of a low-frequency branch of seismic emission as a result of amplitude instability of the seismic envelopes of acoustic oscillations in a structurally inhomogeneous medium.

The frequency and transfer characteristics of dual element ultrasonic transducers are theoretically and experimentally investigated in the long pulse radiation mode for the case where one of the piezoelectric elements is connected to a control circuit in the form of an inductance coil or a resistor. For the controlled damper and controlled layer cases, the characteristic features of radiation as functions of the control circuit parameters are determined, as well as the conditions for an increase in ultrasonic wave radiation power. With certain conditions being satisfied, we demonstrate the possibility of amplitude modulation of the emitted ultrasonic wave by periodic switching of control circuit elements.

Near field acoustic traveling wave is one of the most popular principles in noncontact manipulations and transportations. The stability behavior is a key factor in the industrial applications of acoustical noncontact transportation. We present here an in-depth analysis of the transportation stability of a planar object levitated in near field acoustic traveling waves. To more accurately describe the pressure distributions on the radiation surface, a 3D nonlinear traveling wave model is presented. A closed form solution is derived based on the pressure potential to quantitatively calculate the restoring forces and moments under small disturbances. The physical explanations of the effects of fluid inertia and the effects of non-uniform pressure distributions are provided in detail. It is found that a vibration rail with tapered cross section provides more stable transportation than a rail with rectangular cross section. The present study sheds light on the issue of quantitative evaluation of stability in acoustic traveling waves and proposes three main factors that influence the stability: (a) vibration shape, (b) pressure distribution and (c) restoring force/moment. It helps to provide a better understanding of the physics behind the near field acoustic transportation and provide useful design and optimization tools for industrial applications.

The paper considers the derivation of the wave equation and Helmholtz equation for solving the tomographic problem of reconstruction combined scalar-vector inhomogeneities describing perturbations of the sound velocity and absorption, the vector field of flows, and perturbations of the density of the medium. Restrictive conditions under which the obtained equations are meaningful are analyzed. Results of numerical simulation of the two-dimensional functional-analytical Novikov–Agaltsov algorithm for reconstructing the flow velocity using the the obtained Helmholtz equation are presented.