Nonlinear stage of instability evolution in a monostable active medium

A theoretical analysis is made of the nonlinear stage of instability evolution in a monostable active medium described by a “reaction-diffusion” type equation. The boundary of that region of the medium in which instability develops propagates at constant velocity. Group-theoretical analysis of the problem yields an analytic expression for the propagation velocity of this boundary. The results may be important for the analysis of instability evolution in a wide range of active media. Pis’ma Zh. Tekh. Fiz. 24, 10–15 (July 26, 1998)     

Irregular dynamics of a chain of circle maps with quasiperiodic excitation

An analysis is made of the spatial evolution of quasiperiodic regimes in a chain of coupled circle maps. Mechanisms for the appearance of strange nonchaotic dynamics and the properties of irregular attractors are analyzed. Pis’ma Zh. Tekh. Fiz. 23, 73–79 (April 12, 1997)     

Characteristic time for the evolution of instability of a droplet charged to the Rayleigh limit

The nonlinear problem of the capillary oscillations of a charged droplet is solved to estimate the characteristic time for the evolution of instability of a droplet carrying critical charge. It is observed that breakup is delayed because of the hydrodynamic inertia of the droplet. Pis’ma Zh. Tekh. Fiz. 25, 41–45 (August 12, 1999)     

Indication of the presence or absence of convective motion in water by nuclear magnetic resonance

Nuclear magnetic resonance was used to determine the characteristic parameters for the evolution of an ensemble of gas bubbles in a liquid exposed to microwave irradiation. These parameters can be used to determine whether convective motion takes place in the liquid. It is observed that a liquid may be “strengthened” by low-intensity radio waves. Pis’ma Zh. Tekh. Fiz. 24, 87–90 (September 26, 1998)     

Basins of attraction for chaotic attractors in coupled systems with period doubling

This article studies basins of attraction for chaotic attractors and their evolution when the attractors are restructured through variations in the control parameters. Numerical calculations show the evolution of the boundaries of the attractors, the changes within simply-connected regions, etc. Some of the regularities could be identified in a physical experiment. Pis’ma Zh. Tekh. Fiz. 23, 40–46 (February 26, 1997)     

Nonequilibrium structural transitions as a mechanism of turbulence

A possible mechanism for turbulence is proposed and substantiated for the first time, whereby turbulence is considered as a nonequilibrium transition in ensembles of defects of the microscopic shear type, which are treated as real defects in the molecular structure of liquids. A statistical foundation is given for the evolution equations for the tensor order parameter, which characterizes an ensemble of such defects and has the meaning of nonequilibrium fluctuations of the strain rate. The types of macroscopic fluctuation modes of the strain rate are determined as self-similar solutions of the evolution equations for this tensor order parameter under conditions of nonequilibrium transitions, and qualitative correspondences are established between these solutions and real scenarios for the transition to turbulence. The Kolmogorov scaling laws (the natures of the “viscous” and “ inertial” intervals) are explained for fully developed turbulence. Pis’ma Zh. Tekh. Fiz. 23, 81–88 (July 12, 1997)     

Evolution of the morphological structure of a two-dimensional cluster obtained with a linear microparticle trajectory in the diffusion-limited aggregation regime

Two-dimensional diffusion-limited aggregation of spherical microparticles was achieved experimentally as a result of their rectilinear motion (“ballistic aggregation”) at a “water-air” interface. A percolation cluster was obtained and the evolution of its morphological parameters was studied under compression. A change in the orientation of the cluster pores and a nonlinear coalescence and breakup process were observed. A fabrication technology is proposed for monolayer films similar to the Langmuir method. Pis’ma Zh. Tekh. Fiz. 25, 80–85 (December 12, 1999)     

Vortex structures in a ceramic under high-velocity impact

A finite-element method in a two-dimensional axisymmetric formulation is used to analyze the characteristics of shock-wave processes in a ceramic plate under the impact of a high-speed cylinder. It is established that a vortex structure is formed and the evolution of the vortices is investigated. Pis’ma Zh. Tekh. Fiz. 23, 86–90 (December 26, 1997)     

Multistable generation of monochromatic radiation in solid-state lasers

It is shown that a solid-state laser with unpumped sections of action medium which act as a saturable absorber, may operate in a stable, single-mode cw lasing regime. The establishment of lasing in a particular mode under steady-state conditions depends on the initial conditions of transient evolution. Pis’ma Zh. Tekh. Fiz. 24, 89–92 (February 26, 1998)     

Modulation of capillary oscillations of a charged drop of low-viscosity liquid by its elastic oscillations

A second-order linear differential equation describing the temporal evolution of the amplitudes of capillary waves in a drop of low-viscosity liquid with elastic properties is given with a complex coefficient multiplying the first derivative. It is shown that the capillary oscillations of the drop are modulated by its oscillations associated with the relaxation of the viscosity of the liquid. Pis’ma Zh. Tekh. Fiz. 24, 83–87 (April 12, 1998)     

Near-wall electron instability of a plasma flux

Experimental data are obtained which indicate some characteristic features of the plasma-surface interaction leading to the emission of electrons from the walls of the accelerating channel and the evolution of a specific beam instability of the plasma flux. Pis’ma Zh. Tekh. Fiz. 23, 47–52 (May 26, 1997)     

Dynamic recrystallization in a near-eutectic Pb-Sn-Ag alloy under thermal cycling

An analysis is made of the behavior of solders under thermal cycling. Dynamic recrystallization can be used to explain the evolution of the microstructure of soldered joints during operation. Pis’ma Zh. Tekh. Fiz. 24, 28–31 (May 12, 1998)     

Characteristics of microwave beam breakdown in nitrogen and oxygen

The time evolution of the electron density in microwave discharges in nitrogen and oxygen has been investigated. It is shown that the kinetic processes of electron detachment from negative ions or associative ionization of electronically excited gas molecules play a significant role in the interaction between above-threshold microwave radiation and a discharge plasma. Pis’ma Zh. Tekh. Fiz. 23, 74–80 (May 26, 1997)     

Synergic model of the superplastic deformation of materials

A study is made of a synergic model of superplastic deformation of ultrafine-grained materials at high temperatures as a manifestation of collective modes of motion and self-organization in a system of stimulated grain-boundary slip. The evolution of grain-boundary slip is accompanied by the formation of a wavefront which separates two steady states of the system and leads to the appearance of the experimentally observed “running neck” at the surface of the sample. The synergic model is used to explain the scaling effect of superplasticity. Pis’ma Zh. Tekh. Fiz. 24, 43–47 (July 12, 1998)     

Distribution laws for microplastic deformations

Calculations and construction of bispectra are used to show that the deviation of the probability density of microplastic deformations from a normal distribution law is a measure of the evolution of synchronization effects and a measure of the completeness of the relaxation processes determining the plasticity of the material. Pis’ma Zh. Tekh. Fiz. 24, 28–30 (July 26, 1998)     

Dynamic characteristics of the plastic deformation kinetics of polycrystals

Methods of spectral analysis of local plastic deformations in polycrystalline copper on three structural levels are used to show that the tendency to synchronization is a general dynamic characteristic of the plastic deformation kinetics, and is a measure of the self-organization, evolution, stability, and decay of the dissipative structures in deformable polycrystals. The synchronization effect is directly related to the strength and plasticity characteristics. Pis’ma Zh. Tekh. Fiz. 24, 50–53 (August 26, 1998)     

Application of statistical methods to solve global reconstruction problems

A solution of the problem of reconstructing a mathematical model using statistical methods to specify the evolution operator is optimized. It is shown that this procedure can be used for modeling with short time series, so that the metric and dynamic characteristics of an attractor can be determined with a knowledge of the reconstructed model equations. Direct methods of analyzing scalar time series are less effective in this situation. Pis’ma Zh. Tekh. Fiz. 23, 7–13 (April 26, 1997)     

Alternation of nonequilibrium carrier states in n-GaAs

Numerical methods are used to demonstrate the alternation between stable and chaotic states of a time-dependent system of nonequilibrium carriers in a semiconductor with parameters similar to n-GaAs, under conditions suitable for the Gunn effect to occur. Changes in the quantitative characteristics of evolution of the phase variables (maximum Lyapunov exponent and Hausdorff dimension) are also studied as a function of the external field. Pis’ma Zh. Tekh. Fiz. 24, 32–37 (May 12, 1998)     

Deformations and overlap of instability zones in the Mathieu-Hill equation

A numerical analysis of the Mathieu-Hill equation describing the time evolution of the amplitudes of capillary waves at the interface between two liquids, the upper moving relative to the denser lower liquid at a time-dependent velocity, is used to show that for certain values of the characteristic physical parameters, the zones of unstable amplitude growth become deformed and overlap to form a single, singly connected instability zone. Pis’ma Zh. Tekh. Fiz. 25, 13–18 (October 26, 1999)     

Critical perturbations in a monostable active medium

An analysis is made of critical perturbations which initiate the evolution of instability in a monostable active medium described by a reaction-diffusion equation. A group-theoretical analysis of the problem yields an analytic expression for the energy of the critical perturbations. The results may be important for analyses of stability with respect to external perturbations of a wide range of active media. Pis’ma Zh. Tekh. Fiz. 24, 76–80 (November 26, 1998)