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Том 61, № 7 (2018)
- Год: 2018
- Статей: 6
- URL: https://journal-vniispk.ru/0033-8443/issue/view/15173
Article
The Role of the Effect of Shadowing of Some Wavy Regions by Other Wavy Regions in the Formation of an Image of Snell’s Window
Аннотация
The shadowing of some regions of a wavy water surface by other regions can significantly influence the backscattered signal not only in the problem of remote monitoring of a water reservoir at grazing angles (e.g., in radar), but also in the problems of underwater vision. In this work, we present the results of a theoretical study of this phenomenon with respect to the model of underwater imaging of the sky (Snell’s window) near the image boundary. The numerical-simulation method is used to develop the statistically mean image of Snell’s window, which allows for the shadowing effects, and the corrected formula of the estimate of a wave parameter, namely, the water-surface slope variance, is obtained using the value of blurring of the boundary of Snell’s window.
467-477
Evolution of Narrow-Band Noise Beams for Large Acoustic Reynolds Numbers
Аннотация
We consider propagation of intense acoustic beams having a noise temporal structure at the initial aperture. The evolution of the probability distribution and the wave spectrum at a discontinuous stage of propagation is studied experimentally when the field on the radiator axis represents a sequence of discontinuities with universal behavior between them. It has been shown, both theoretically and experimentally, that in this case the field spectrum retains its shape determined by the probability distribution of the frequency of the initial wave.
478-490
Detection of a Crack and Determination of Its Position in a Plate by the Nonlinear Modulation Method Using Lamb Waves
Аннотация
Features of detection of a single crack in a plate are experimentally studied by the nonlinear acoustic modulation method provided that single defects (cracks and cavities) are located between a low-frequency (tens of kilohertz) ultrasonic transducer of Lamb waves and a high-frequency (units of megahertz) ultrasonic transducer. It is shown that using modulation of the defect characteristics by a sequence of phase-inverted Lamb waves, by the level of modulation of the high-frequency wave reflected from the defect one can determine its type (a crack or a cavity). Changing step by step the interaction region of low-frequency and high-frequency waves by varying a delay between their radiation, it is possible to study selectively the nonlinear acoustic characteristics of the defects arranged in series on the location path. Based on the obtained results, we created a combined transducer which scans a defect and modulates its characteristics simultaneously in the preset area and which is suitable for moving it by hand on the surface of a plate for seeking hidden cracks.
491-499
Nonlinear Acoustic Effects in Rod Resonators with Rigid Boundaries
Аннотация
We theoretically study nonlinear acoustic effects (amplitude-dependent loss, resonant-frequency shift, and the second-harmonic generation) and determine the thresholds of parametric generation of subharmonic oscillations at fractional frequencies in the rod resonators with rigid boundaries and various nonlinearity types (elastic, hysteretic, different-modulus, etc.).
500-515
Scattering of a Plane Electromagnetic Wave by a Multilayer Spherical Lens
Аннотация
We propose an analytical solution of the problem of diffraction of a plane electromagnetic wave by a multilayer dielectric (including plasmon) sphere. The solution is obtained using the method of separation of variables. New efficient recurrence relationships are obtained for calculations of the fields in layers, as well as formulas for the fields in the near and far diffraction zones. The novelty of the proposed solution is connected with the way of representing its radial part in the form of normalized functions. It is shown that as the number of the lens layers, which approximate the smooth profile of dielectric permittivity, grows, the electric field at the focusing point increases and reaches the maximum value. This allows one to determine the minimum required number of layers in practical problems. Resonance properties of metal-dielectric nanoparticles are studied in the optical band.
516-527
Estimation of Loss When Detecting Signals by a Receiver with Adaptive Threshold on the Basis of the Method of Ordered Statistics
Аннотация
We consider the method of formation of adaptive threshold of signal detection against the background of receiver inherent noise using the nonparametric algorithms on the basis of ordered statistics. The simulation results and the efficiency of using this method on the basis of estimating the quantiles of the statistical distribution of the process compared with the classical methods of “moving average” in the case of complicated signal-interference environment (weak-signal masking by intense interference, mutual masking of several signals simultaneously staying in the sliding data window, and the useful-signal presence in the region of the jump-like variation of interference) are shown. A mathematical model of estimating the loss introduced when detecting a useful signal by the threshold device based on the method of ordered statistics is developed.
528-535