Technical Physics最新文献

In this paper, we investigate the nonlinear scattering regime in water with different gas contents. It is found that the optical breakdown threshold at pump wavelength λ = 1064 nm is determined by the bulk number density of dissolved gas molecules. In degassed water samples, four-photon scattering occurs at frequency 2ω₀ – Ω₀, where ω₀ is the pump frequency and Ω₀ is a combination of the frequencies of symmetric and antisymmetric stretching vibrations of water molecules. We associate this scattering with the four-photon process that develops on the cubic nonlinearity of the medium. At the same time, in water saturated with dissolved air, the excitation threshold of optical breakdown is significantly lower than the excitation thresholds of four-photon scattering and optical breakdown in degassed water. At the same time, in samples of water saturated with dissolved air, nonlinear scattering near frequency 2ω₀ occurs simultaneously with the breakdown. We attribute this phenomenon to hyper-Raman scattering that occurs on a quadratic nonlinearity inside liquid shells of nanometer thickness surrounding gas nanobubbles. The possibility of recording hyper-Raman scattering near the second harmonic is due to the enhancement because of the excitation of plasmon resonance during optical breakdown inside gas nanobubbles. Plasmon resonance is excited as a result of beats of a wave at frequency 2ω₀ and Stokes/anti-Stokes waves at frequency 2ω₀ – Ω₀ during the interaction on a quadratic nonlinearity inside several liquid monolayers surrounding a gas nanobubble.

A channel bandpass filter of the satellite communication multiplexer has been developed around a design consisting of 12 coaxial resonators. To cut the group delay time and considerably improve the selectivity of the device, two additional inductive and two additional capacitive couplings between nonadjacent resonators have been applied. This makes it possible to provide two zero poles near the right and left edges of the passband. To reduce the spread of the passband’s transmission factor, a special method was applied that allows one to somewhat decrease reflection losses near the edges of the bandpass of the filter. This, in combination with the electrodynamic analysis of a 3D model developed for the given design, makes it possible to synthesize a small-size filter with the center frequency of the bandpass f₀ = 4 GHz and its width Δf = 45 MHz measured at a level of 0.8 dB from minimal losses. The measured characteristics of the filter prototype are in good agreement with calculation data.

An unsteady numerical investigation of thermosolutal natural convection is conducted within a square cavity, considering the effects of Dufour ((Du)) and Soret ((Sr)) for both aiding and opposing cases. The vertical walls of the cavity are maintained at constant but different temperatures and concentrations, while the other walls are adiabatic and impermeable. The finite volume method is used to solve the governing equations, and the SIMPLER algorithm is used in the solution process. The primary objective is to identify the flow regime for flows dominated by thermal and solutal effects, while also examining the impact of (Du) and Sr coefficients on this process. Results are presented for several parameters, such as the buoyancy ratio, thermal Rayleigh number ((R{{a}_{t}})), (Du), and Sr coefficients on flow pattern, and heat and mass transfer. The results show that the presence of (Du) and (Sr~) coefficients notably influences the structure flow. Furthermore, (Du~) can increase the heat transfer rate more than the mass transfer rate. Conversely, the (Sr~) coefficient increases the mass transfer rate more than the heat transfer rate. Additionally, the effects of (Du) and (Sr) on the oscillatory regime have been studied, revealing a significant impact on oscillation flow when the parameters are increased by delaying their appearance.

An approach to describing the thermal radiation of solid alloys, using the Ni–Cr system as an example, is proposed employing normalized entropy S/R. The emissivity is calculated using the Foote approximation based on data for the resistivity in the temperature range 400–1400 K. The provided heat flux values are normalized by density and a scale parameter (q_{1}^{*}). A universal exponential dependence between the logarithm of the normalized flux and the S/R value has been established, which is applicable for all considered Cr concentrations in the alloy. The high degree of correspondence (R² = 0.997) confirms the effectiveness of the entropy-based generalization and suggests its utility for estimating the radiative properties of alloys in the absence of direct experimental data.

This review describes the current state of the art, methods, and techniques of neutron generation, as well as physical processes occurring in neutron tubes and compact (small-sized) neutron sources (and others axis-symmetrical devices) during their operation. The results of experimental and theoretical studies of deuteron acceleration in small-sized Penning sources, plasma ion diodes with magnetic isolation, generators based on plasma focus and laser influence, and neutron acceleration tubes of increased efficiency are presented.

Processes in which solid or liquid cylindrical objects or spherical drops heated to high temperature fall into a cooling liquid are of great interest in developing safety methods for nuclear power facilities. Since the direct laboratory simulation of potential emergencies is impossible, relevant experiments described today in the international literature deal with processes taking place when high-temperature cylindrical or spherical liquid-metal objects fall into water. Hot metal drops, falling into a liquid, generate a vapor layer (film) between the surface of the high-temperature drop and liquid−vapor phase boundary. In this study, we have considered the properties of a spherical vapor−liquid interface in the presence of a heat flux from the drop. Based on experimental data currently available it has been shown that the interface consists of stationary and oscillationg parts. For the stationary part of the vapor film, a general relation with regard for molecular and radiation heat transfer components has been derived. It has been found that in many cases the radiation heat transfer can be estimated by thoroughly analyzing radiation processes inside the vapor space. Specifically, the optical thickness of the vapor film should be estimated. For the case of moderate temperatures, when transfer by radiation can be neglected, a simple estimating expression was obtained that gives vapor film thickness values don’t contradict with respective scarce data currently available (the dependences of the film thickness on the heat flux from the hot body and its temperature). A review of research articles (see the Introduction) shows that there exists an alternative approach based on describing the phase boundary dynamics using the modified Rayleigh‒Lamb equation.

This study analyzed the changes occurring on the surfaces of tantalum carbide and oxide under the influence of helium ions. A technique was employed to determine the composition and thickness of layers formed through the sputtering process of binary targets under the influence of light ions. The data obtained was utilized to calculate the thicknesses of the modified layers and their respective compositions. The results indicated that the surface layers contain a lower proportion of the lighter component compared to the original material, which aligns with experimental observations.

The paper considers the development of a mathematical model of the thermal drift of a fiber-optic gyroscope (FOG) due to the thermo-optic effect, which takes into account the features of quadrupole spooling of fiber on the spool. Such devices are widely used in stabilization, orientation, and motion control systems of aerospace and ground techniques. The main task in achieving this goal is to separate the nonstationary temperature function into a temporal component and a spatial one (the function characterizing the temperature distribution along the fiber filament for the quadrupole spooling of fiber at radial temperature gradient). When developing the model, the initial assumption is that the array of fiber filaments on the spool is considered as a periodic continuous structure (successive layers with the same thermophysical characteristics). This allows taking into account only radial temperature gradients and assuming that the temperature at each instant in the corresponding fiber layer on the spool is distributed uniformly. The study provides the justification of the correctness of the proposed approach to constructing the thermal drift model by simulating the temperature in each layer of the fiber spool using the method of elementary balances. Modeling is performed in specially developed software, in which the functions of graphical output of calculation results are implemented. Based on computational experiments, it is substantiated that in real conditions of FOG operation at a relatively low rate of change in ambient temperature, the law of temperature variation in the fiber spool in the radial direction can be assumed as linear. The function of the spatial distribution of the temperature field along the fiber filament is determined. Using this function, an algorithm of its application for plotting the temperature distribution in a fiber spool with given geometric parameters close to the real ones is implemented. An example of calculating the thermal drift of the device for specified parameters of the fiber and geometric parameters of the spool, which are close to the parameters of devices used in practice, is given. The proposed model for calculating the thermal drift of a fiber-optic gyroscope extends and complements the potentialities of the method of elementary balances, which makes it possible to implement a simple and effective algorithm for calculating non-stationary temperature fields and thermal drift of almost any fiber-optic gyroscope of typical design without engaging costly software. The proposed model will allow developers of automated object motion control systems to implement effective algorithms for calibration and correction of thermal drift of a fiber optic gyroscope.

The study is devoted to the development of models for predicting the refractive index of ceramic composite materials based on hydroxyapatite with the 0.1 and 0.5 wt % multi-walled carbon nanotubes additives by machine learning (ML) methods. The refraction prediction is based on experimental study results of the porous structure of ceramics in the frequency range from 0.2 to 1.6 THz. A new methodology which includes the construction of models based on methods such as linear and polynomial approximation, random decision forest and artificial neural networks, has been developed to quantitatively assess the influence of carbon nanotubes on the refractive index of the composite. The results of application of the neural networks showed significantly higher forecasting accuracy, the average absolute error of which is ~0.04%. Our findings underscore the effectiveness of using machine learning in non-invasive analysis of the porous structure of heterogeneous composites.