Technical Physics Letters最新文献

The work is devoted to the study of the effects of instability of the relaxation process in linear polymer systems based on a dynamic model of deformation processes. A system of equations for perturbations of the process characteristics is obtained, and a spectrum of perturbations is shown depending on the model parameters calculated from empirical relaxation curves. Instability conditions are obtained. It is shown that relaxation processes are stable in weakly nonequilibrium linear systems; with increasing disequilibrium and nonlinearity, long-wave fluctuations in deformation characteristics occur, which, in particular, macroscopically manifest themselves as a vibration relaxation effect.

Using the CST MWS 2023 software package, the controllability of the characteristics (elements of the scattering matrix and radiation patterns) of reconfigurable plasmonic antennas based on rectangular graphene nanoribbons on metallized (PGA MS) and dielectric (PGA DS) substrates has been modeled and the results of frequency scanning have been obtained at the resonance frequencies of the fundamental and higher modes of surface plasmon-polaritons with a change in the chemical potential of graphene in the terahertz (THz) and far- and mid-IR ranges. The possibility of forming a multibeam PGA MS radiation patterns on plasmon resonances of the surface plasmon-polariton second-order mode in the far- and mid-IR ranges has been investigated. The possibility of radiation of the waves of two orthogonal polarizations by the PGA DS has been shown and methods for controlling the polarization of the THz and IR PGA radiation have been identified, which are based on the selection of the operating frequencies corresponding to the surface plasmon-polariton mode resonances at the p- and s-polarization of the exciting waves and metallization of the dielectric substrate.

Modelling of diffraction gratings is an urgent problem due to the need for their use in thermonuclear fusion. The possibility of modeling diffraction gratings with higher diffraction efficiency allows the radiation power of laser facilities to be increased. To achieve this goal, spectral beam combining is used. The diffraction efficiency is calculated by methods of mathematical physics and mathematical modelling, as well as by numerical methods. The diffraction efficiency of gratings of different configurations has been calculated by the original sign change method. Calculations of the diffraction efficiency of the diffraction gratings with one and three thresholds per period have been compared. The discussed numerical examples of modeling gratings with a more complex configuration demonstrate the advantages of the proposed model for calculating the diffraction efficiency over the available algorithms.

Mid-IR antennas can play a critical role in advancing discoveries and innovations for applications such as IR wireless communications, imaging and visualization, remote sensing including environmental and biological, remote detection, security scanning, biomedical applications and astronomy, the Internet of Things, and sensors. The aim of the work is to study using the CST MWS 2023 software package the electronic control of the directional properties of reconfigurable plasmonic graphene antenna arrays, i.e., the main beam of the radiation pattern (RP), by using electrical frequency tunability by changing the chemical potential of graphene (by applying an external electric field), scanning the RP and changing the shape and parameters of the RP in the mid-IR range. In the mid-IR range, graphene exhibits plasmonic-like complex surface conductivity with low losses, which provides great potential for the development of tunable antenna arrays in this range. Research on the application of surface plasmon polaritons (SPPs) in graphene is focused on the ranges from terahertz to mid-IR, since current technologies allow reducing the width of graphene nanoribbons only to such an extent that they can excite plasmonic oscillations in the mid-IR range. To solve the electrodynamic problem using the CST MWS 2023 program, designed for numerical modeling of high-frequency antennas and devices, the Perfect Boundary Approximation (PBA) method is used, complementing the Finite Integration Technique (FIT) method, which works in the time domain. The results of electrodynamic modeling of the controllability of the RPs of plasmonic graphene antenna arrays with different numbers of emitters and electronic frequency scanning at the resonant frequencies of the fundamental SPP mode in the mid-IR range were obtained with a change in the value of chemical potential μ_c (0.3–1 eV) and their dependence on the geometric dimensions of the graphene elements and the antenna array periods. As a result of electrodynamic modeling, the possibility of effective electronic control of RPs of the plasmonic graphene antenna arrays in the scanning regime in the mid-IR range with a change in the chemical potential of graphene (μ_с = 0.3–1 eV) is shown: a change in the direction of the main lobe of the RPs, while the achievable sector of the scanning angles confirms the efficiency of beam control; a decrease in the width of the RPs at the half-power level and the level of side lobes; an increase in the radiation efficiency and, as a consequence, an increase in the gain.

To develop new 6G and beyond terahertz (THz) wireless communication systems with the high-throughput and data rate, the direction of polarization of emitted THz waves should be effectively controlled, whereas most methods are technologically complex and expensive. The implementation of THz antennas and devices based on 2D materials, e.g., graphene, solves the problem of the effective control. The possibility is studied of controlling the polarization of THz and IR radiation of plasmonic antennas based on rectangular graphene nanoribbons by changing the chemical potential (applying an external electric field). This important scientific problem related to the design of THz antennas can largely be solved by modeling in the CST MWS 2023 electrodynamic modeling program. Plasmon THz antennas based on rectangular graphene nanoribbons have been chosen as objects of study and the possibility of emitting waves of two orthogonal polarizations has been shown. Methods for controlling the polarization of THz and IR radiation of such antennas have been identified, which are based on choosing the operating frequencies corresponding to the plasmon resonances of the surface plasmon-polariton modes and metallization of a dielectric substrate. The possibility of controlling the polarization of THz and IR radiation allows the creation of both new elements of plasmonic antenna arrays and new communication technologies, including future 6G networks.

A method for deriving the Navier–Stokes equation with allowance for inhomogeneities of any order on the Laplace operator using the Boltzmann kinetic equation is proposed. The solution method is based on the theory of nonequilibrium phenomena and the entropy growth principle. The calculation has been carried out to find additional inhomogeneous terms on the Laplace operator in the right-hand side of the Navier–Stokes equation. New-type fundamental solutions to the stationary parabolic equation have been predicted, which are significant for application in solving some problems of mathematical physics.

Using the constructed Lagrange function (L) and calculated dissipative function (dot {Q}), a general system of dynamic equations has been obtained to describe the motion of a cylindrical body completely immersed in a fluid. It is assumed that the body is hinged at its one end where the origin of coordinates is selected. The free end can make almost any motions and is elastically held by a spring at an arbitrary point. The problem is solved in a spherical coordinate system in which differential equations of motion are derived in terms of two independent angular variables (theta ) and (varphi ) taking into account the viscosity (eta ) of a continuum.

It was experimentally found that speeds of hot submerged jets appearing in case of laser induced nucleation boiling of water near the tip of optical fiber submerged in water decrease exponentially with increasing laser power (heat flux). This result was obtained for a closed cylindrical cuvette where hot jets collided with walls and slipped the cuvette boundary transferring heat to it. The obtained result is to be taken into account in performing precise laser–induced surface cleaning inside confined volumes, in developing medical technologies for laser therapy of pathologically changed vessels or cysts, and in other applications.

Abstract—The results of numerical simulation of unsteady free convection developing near a suddenly heated plate, on which protrusions in the form of adiabatic cylinders of double height with respect to the diameter are arranged in a staggered order, are presented. The calculations were performed according to the Reynolds equations using a differential model of turbulent stresses. The range of variation of the Grashof number (plotted according to the thickness of the free convective flow), in which a significant intensification of heat transfer can be achieved, has been determined. It is shown that the best conditions for intensification are created if the longitudinal pitch in the array of protrusions is approximately twenty times the diameter of the latter.