The structure of radiation forces in a viscous fluid layer on an elastic half-space and the acoustic flows generated by them
The structure of radiation forces in a viscous fluid layer on an elastic half-space and the acoustic flows generated by them
D. A. Zharkov, V. A. Gusev
The field of a surface acoustic wave in the system "viscous fluid layer - elastic substrate" is calculated taking into account the shear components in the fluid. The dispersion equation was used to calculate the amplitudes of the surface wave. The radiation pressure arising in a viscous fluid from the side of a standing surface wave and acting on an element of its volume due to the nonlinearity of the equations of motion, and the acoustic flows created by it are calculated. It is shown that taking viscosity into account changes the spatial distribution of radiation pressure. Attenuation of the wave causes an additional tendency for the collection of suspended particles in the center of the system. Shear components lead to significant gradients of radiation pressure near the interface between the media. They play a decisive role in the formation of ordered ensembles of suspended particles at the last stage of the self-organization process.
Show AbstractAnalysis of the stability of the properties of a linear antenna array under parabolic bending
Analysis of the stability of the properties of a linear antenna array under parabolic bending
B. E. Nazarenko, T. K. Artyomova
The influence of the symmetrical bending of the profile of a linear antenna array along the arc of a parabola on its radiation pattern, efficiency, and consistency with the feeder is studied. The effects are demonstrated on the example of an 11-element model of an equidistant antenna array with different inter-element distances, for which an electrodynamic problem with boundary conditions for the electric and magnetic field vectors is numerically solved. The lattice was bent at a constant length along the arc of the parabola. Half of the angle of the aperture relative to the focus of the parabola is selected as the parameter that characterizes the profile. The study was carried out with in-phase equal-amplitude power supply of the grid elements. Estimates of the values of the best matching frequency and the best matching frequency offset when changing the bending profile are obtained, the dependence of the efficiency coefficient on the bending parameter, the horizontal and vertical cross-sections of the radiation patterns are constructed. The results show that with a decrease in the focal length, the efficiency is stable, while it deteriorates by no more than 0.1 dB, and the nature of the radiation changes from unidirectional to conical. The frequency parameters and the consistency with the feeder show a stable behavior when the profile is bent, but they change significantly when the inter-element distance changes.
Show AbstractEffect of object position on evoked potentials during long-term cognitive tests
Effect of object position on evoked potentials during long-term cognitive tests
A. О. Selskii$^{1,2}$, M. О. Zhuravlev$^{1,2}$, A. E. Runnova$^2$
This article examines the effect of object position on cognitive evoked potentials observed in subjects with chronic pain. Chronic pain often causes cognitive depression. The experiment consisted of passing small cognitive tests, where the subject assessed the number of squares presented on the left or right side of the screen, or in the center. A simple method is proposed for automatic identification of the main components of the cognitive evoked potential, based on the identification of extrema and the assessment of the amplitudes of the main components of the cognitive evoked potential. Examples of analysis of individual characteristics for several subjects are shown. The maximum difference between the adjacent main components of the evoked potential lies, as a rule, on the central axis, but can be located in the subjects in the frontal, parietal, or occipital lobes. For two subjects, the individual characteristics of the main components of the cognitive evoked potential were shown at different positions of the object on the screen during the performance of the cognitive test. For one, the evoked potentials change when the stimulus is located on the left side of the screen. For the second, large changes are observed when the stimulus is located on the right side of the screen. It is extremely important to know these characteristics of the subjects when setting up automated brain-computer interfaces.
Show AbstractDevelopment of a method for control of dose distribution in radiation therapy on photon beams
Development of a method for control of dose distribution in radiation therapy on photon beams
A. G. Sinelnikov, A. P. Chernyaev
Today, radiation therapy has a significant place in the treatment of cancer. In recent years, new radiotherapy methods of treatment have begun to be introduced, which, for example, make it possible to control tumors directly during the irradiation process, shortening the procedure time and minimizing the negative impact on the patient. An example of combining diagnostics and radiation therapy is the "Tomotherapy" device, with the help of which a beam of ionizing radiation is directed to the tumor with high accuracy, the built-in computer form determines the size and position of the tumor in a matter of seconds before the start of the session. We propose a method for assessing the distribution of doses in the body during irradiation with medical electron accelerators. The method uses positron emission tomography based on the registration of annihilation photons. It is based on the use of the electron-positron pair production mechanism. The positrons formed in the course of irradiation with photons undergo ionization deceleration and then annihilation with the formation of two photons with an energy of 0.511 MeV. By registering such photons for coincidence, it is possible to obtain the distribution of annihilation events in the irradiated tissue region. In the work, a mathematical model has been created that allows one to estimate the absorbed dose in a given tissue site. Within the framework of the computer experiment performed using the GEANT4 package, the correlation between the distribution of the absorbed dose of the primary photon flux and the distribution of the annihilation density of secondary positrons was estimated. The depth distributions of the absorbed dose and the number of annihilations are obtained, and an empirical recalculation function from the distribution of the number of annihilations in the depth estimate of the absorbed dose is selected.
Show AbstractInvestigation of the processes of charged particles acceleration in intermittent turbulent cosmic plasma
Investigation of the processes of charged particles acceleration in intermittent turbulent cosmic plasma
N. N. Levashov$^1$, V. Yu. Popov$^{1,2,3}$, H. V. Malova$^{3,4}$, L. M. Zeleny$^3$
In this article, we present the result of a numerical experiment on the acceleration of charged particles in a turbulent field with different levels of intermittency. Our main task was to investigate the effect of intermittency on particle acceleration in turbulent cosmic plasma. For this, a three-dimensional model of a turbulent field with a controlled intermittency level was developed. An intermittent electromagnetic field is modeled as a superposition of a turbulent electromagnetic field with a power-law spectrum, obtained using the sum of the Fourier harmonics and the electromagnetic field created by small plasmoids, the amplitudes of which are specified using a special distribution. Using this model, the role of intermittency in the acceleration of charged particles in a turbulent field in the tail of the magnetosphere was investigated. It is shown that the higher the level of intermittency, the higher the energy values are able to reach individual particles.
Show AbstractExperience in teaching courses «General issues of methods of teaching physical and mathematical disciplines» and «Fundamentals of teaching physics at a university» in a remote format
Experience in teaching courses «General issues of methods of teaching physical and mathematical disciplines» and «Fundamentals of teaching physics at a university» in a remote format
T. A. Bushina, A. I. Slepkov
The experience of teaching the courses "General issues of methods of teaching physics and mathematics in higher education" for master's students and "Fundamentals of teaching physics in higher education" for postgraduate students in a distance format at the Faculty of Physics of Moscow State University in the autumn semester of the academic year 2020-2021 is discussed. In the context of the coronavirus pandemic, these courses, like other lecture courses taught at the Faculty of Physics, had to be transferred to a remote format. As a result, in order to maintain interest in the courses and not lose feedback from the listeners, the principles of working with students and postgraduates were reworked. This paper discusses new approaches to working with students and postgraduates, provides examples of tasks that were offered to be performed in a remote format.
Show AbstractProblems of dynamical theory in statistical physics of liquid
Problems of dynamical theory in statistical physics of liquid
P. N. Nikolaev
In the paper it is investigated the process of formation of the modern dynamic theory liquid. The activity of N.N.Bogoliubov on the creation of a dynamic theory in statistical physics and the influence of his work on the formation of a dynamic theory of liquid are presented.
Show AbstractStudy of atmospheric optical phenomena with Physics faculty students
Study of atmospheric optical phenomena with Physics faculty students
S. B. Ryzhikov$^1$, Yu. V. Ryzhikova$^2$
The authors draw attention to the fact that the description of the mechanism of the rainbow and halo formation is absent in both school and university textbooks and teaching aids. An explanation of these phenomena is given in the popular science literature, but there is no indication of the fact of the polarization of the rainbow and the reasons for this polarization. At the same time, these phenomena can be studied within the framework of a general course in physics, spending about half an hour in seminars. The article provides a methodology for studying rainbows and halos with students. In addition, the authors draw attention to a common mistake associated with the history of Descartes' study of the rainbow.
Show AbstractEquations of state of a system of hard spheres in a reduced form
Equations of state of a system of hard spheres in a reduced form
P. N. Nikolaev
The equations of state are investigated for a system of hard spheres in the general quantum-mechanical case. The equations are presented in the reduced form. A comparison is made between the classical approximation, the semiclassical approximation with allowance for the first correction, and the cell-cluster expansion with allowance for three-particle clusters, inclusive. The cell-cluster expansion, as calculations show, is more efficient than the semiclassical approximation at high densities. It is shown that the density dependence in the caloric equation of state is significant, which is also preserved at high temperatures. In the general case, Joule's law does not hold for a system of hard spheres.
Show AbstractMulticircuit algorithm for the numerical solution of dynamic equations of the Vlasov-Darwin model
Multicircuit algorithm for the numerical solution of dynamic equations of the Vlasov-Darwin model
L. V. Borodachev, S. S. Annenkov
An approach to the optimization of the procedure for solving the equations of motion of particles of a discrete self-consistent model of a rarefied plasma based on the conjugation of dynamic schemes of different nature and computational properties is developed. The concept was algorithmically implemented and tested in mathematical modeling (using the PIC method) of the low-frequency kinetic Weibel instability. A significant reduction in the simulation time is shown when using the specified optimization technique in comparison with traditional computation based on only one (explicit or implicit) dynamic scheme. During computer experiments, the features of the behavior of the Weibel process at the stage of its saturation were revealed: the presence of damped low-frequency oscillations of the energy density of the magnetic field, the key parameter of instability.
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