Doklady Physics最新文献

Using the method of self-propagating high-temperature synthesis in the combustion mode, a product based on the α-Cu₂Se phase was obtained from a 2Cu + Se powder mixture. The effect of synthesis conditions on the composition of the combustion product was studied, and the unit cell parameters of the synthesized phases were determined. It has been established that, as a result of combustion of pressed 2Cu+Se mixtures at an Ar pressure of 0.5–1.5 MPa, a product is formed containing two modifications of Cu₂Se—the low-temperature monoclinic α-Cu₂Se and high-temperature cubic β-Cu_1.8Se phases. Combustion of a 2Cu+Se mixture of bulk density at an Ar pressure above 0.5 MPa yielded a single-phase product, the monoclinic α-Cu₂Se phase.

A geometrically nonlinear moment-membrane theory of elastic plates has been built as a continual theory of deformations of flexible graphene under the assumption of smallness of deformations, bending-torsional characteristics, and angles of rotation (including a free one) of plate elements on the basis of the 3D geometrically nonlinear moment theory of elasticity with preservation of only the nonlinear terms that come from normal displacement (deflection) and its derivatives. For this nonlinear theory of elastic plates, the resolving equations are presented also in mixed form by introducing stress functions: this is a system of equilibrium equations for the transverse bending deformation compiled in the deformed state of the plate and the deformation continuity equations expressed by the stress and deflection functions. The Lagrangian-type variational principle for the geometrically nonlinear moment-membrane theory of elastic plates has been established.

The 3D problem of elastic wave scattering by a crack in the form of a half-plane in an elastic medium is considered. It is noted that the previously published solutions do not fully cover the issues relevant to ultrasonic flaw detection, for example, detecting transverse cracks in welds. The proposed solution is made by the Wiener–Hopf technique. It is shown that scattered longitudinal and transverse waves can be generally written in closed form. Some features of the identification of the tip of transverse cracks are noted, including scattering features during sonication of cracks along their surface and the influence of critical angles.

Polyakov [Nuclear Phys. B, 396, 1993] proposed a program for expanding the group of symmetries of hydrodynamic models to the conformal invariance of statistics in inverse cascades, where the conformal group is infinite-dimensional. This study presents group G of transformations of the equation for an (n)-point probability density function f_n (PDF) from an infinite chain of the Lundgren–Monin–Novikov equations (the statistical form of the Euler equations) for the field of a vortex of two-dimensional flow. The main result obtained is that the group G transforms conformally the characteristics of the zero-vorticity equation and, invariantly, the family of the f_n equations for the PDF along these lines. Along with other characteristics, the equation is not invariant. The action of G retains the PDF class. The results obtained can be used to study the invariance of the statistical properties in optical turbulence.

It is known that, in wall-bounded flows, along with the growth of instability modes, an important role is played by the nonmodal (algebraic) mechanism of linear growth. In unbounded flows, including submerged jets, the nonmodal growth mechanism has been theoretically studied only in the last decade; this mechanism has not yet been identified in experiments. This paper describes experiments on excitation of the nonmodal “lift-up” growth mechanism. Special wavy structures (deflectors) are introduced into a laminar submerged jet of circular cross section, which excites a roller-like transverse motion. The data obtained make it possible to identify unambiguously the nonmodal lift-up growth of the introduced perturbations. The development of perturbations in the experiment qualitatively agrees with the theoretically calculated optimal perturbations. The specificities of the transition to turbulence caused by nonmodal growth are considered.

The results of the first direct experiments on the passage of pulses of ultra-wideband radiation of subnanosecond duration in the Earth’s atmosphere at a distance of more than 10 km are presented. In contrast to the work calculated, the preservation of the amplitude–time shape of the pulses in the process of increasing the distance is shown. The establishment of this fact is of decisive importance in the practical application of ultra-wideband pulses in new technological developments.

In this paper, we consider the attraction field of a rigid body close to a dynamically symmetric one. The question of how to approximate this attractive field by the attractive field of three homogeneous balls so that the series expansion of the gravitational potential for these fields coincides up to terms of the fifth order of smallness is studied. As an example, the three-ball approximation as applied to asteroid (433) Eros is considered.

In this paper, the results of lidar monitoring of magmatic aerosol variation are presented. The lidar was installed in a hot dead-end tunnel at the end of the auxiliary adit of the Baksan Neutrino Observatory (BNO) of the Institute for Nuclear Research, Russian Academy of Sciences, which is located above the small magma chamber of the Elbrus Volcanic Center. A sharp (within ~30 min) threefold increase in the aerosol concentration in the hot tunnel was detected (October 28, 2019, 22:30 UTC). The analysis involved data on volumetric radon activity, air temperature, and humidity. A synchronous increase in temperature, humidity, and aerosol concentration in the tunnel, as well as their correlation with the deformation of the Earth’s crust by tidal waves, which is monitored at the BNO with a laser strainmeter, was revealed. However, the threefold jump did not appear in these signals. The totality of data confirms the existence of a hot magma chamber at the base of Mount Andyrchi that was previously discovered by a small-aperture seismometer group. The assumption that the release of gases from the magma chamber could provide a jump in the concentration of aerosols without disturbing other parameters is discussed.

A class of power tensor series (constitutive relations) with coefficients (material functions) that are functions of three independent invariants is considered in three-dimensional space. Based on the Hamilton–Cayley formula, the exact expressions in the form of matrix series are found for the coefficients of three-term representations of such power series. The relationship of the coefficients of direct and inverse three-term constitutive relations is derived. Cases of tensor linearity, or quasi-linearity, as well as the independence of material functions from invariants, are discussed.

On the basis of a geometrically nonlinear model of a Bernoulli-Euler beam, the influence of the amplitude and shape of the initial curvature on the nature and stability of the static equilibrium positions of the system under conditions of electrostatic and thermal effects has been studied. With the help of numerical methods of the theory of bifurcations, a complete parametric analysis of the diagrams of evolution of the equilibrium positions for beams with initial curvature in terms of symmetric and asymmetric lower modes of free vibrations is conducted. The regions in the parameter space of the values of the axial force of a mechanical or temperature nature and the strength of the electrostatic field, in which the elastic system has the property of bistability, are determined. The prospect of using the considered class of structures with projected geometry perturbations as sensitive elements of high-precision microelectromechanical sensors of various physical quantities is noted.