Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2025 number 2

This paper presents the results of experiments to strengthen the surface of a fluoroplastic composite by creating a protective layer. The protective layer was formed by detonation spraying of VSNGN-85 powder consisting mainly of WC (81.56 wt.%) and Ni (10.6 wt.%). The granulometric and morphological properties of VSNGN-85 powder were studied, and its X-ray diffraction analysis was carried out. Optimal spraying modes for obtaining a dense coating 100÷115 mu thick were selected. X-ray diffraction analysis showed the presence of WO₃ tungsten oxide and NiWO₄(II) nickel tungstate in the coatings. The microhardness of the composite material with the protective coating was found to be 48 times higher than the microhardness of the composite without coating. The applied coating increases the life of the fluoroplastic composite, improves its strength, and slows down the processes of its oxidation, corrosion, and thermal destruction.

Fracture of the electrodes of a three-electrode pulsed X-ray tube with a rechargeable insulated electrode operating in explosive emission mode at a voltage of 600 kV and pulsed current up to 1000 A was investigated. The fracture was produced by the action of high pulsed electric fields and pulsed flows of electrons and plasma during vacuum breakdown in X-ray tubes. X-ray fluorescence microanalysis using a scanning microscope revealed the presence of numerous metal globules of size 10÷40 mu formed from both the tungsten anode material and the material of the third insulated Kovar electrode. Several stages of separation of the globules by the electric field were detected on the insulated electrode. Quantitative estimates for the critical values of Coulomb forces at which droplets arise were obtained in accordance with a dynamic model (taking into account the kinetic energy accumulated by the mass during the exposure to the electric field and surface energy), and the electric field pressures during the operation of the tube were estimated, allowing an explanation of the observed phenomena.

Results of experimental and numerical simulations of interaction of plane shock waves with gas-permeable cellular-porous targets are presented. Some of these targets are homogeneous over the thickness, while others consist of material layers with pores of different diameters. The experiments are performed in a shock tunnel in the range of the shock wave Mach numbers M=1,2÷1,8. High-porosity cellular nickel is chosen as a gas-permeable material. In numerical simulations, such cellular-porous materials are described by a toroidal model of a porous medium. The mechanism of formation of reflected shock waves is revealed. The intensity of waves reflected directly from the structural elements of the material and of waves reflected from the rear end face of the wind tunnel is shown to be lower in the presence of targets made of a gas-permeable high-porosity cellular material. The most effectively reflected waves are suppressed by combined targets composed of material layers with pores of different diameters.

Interaction of a shock wave with the boundary layer on a half-airfoil model is studied. The experiments are performed in a wind tunnel with the free-stream Mach number M ≈ 0.75 and stagnation pressure P₀ = 10⁵ Pa. The half-airfoil model is mounted on the wall of the test section of the wind tunnel. Pressure distributions over the model surface are obtained by a method with luminescent pressure transducers and by a method with pressure taps. The limiting streamlines on the model are visualized, and thermographic visualization is also performed. For experimental parameters, numerical simulations are performed with an approach based on using the Reynolds-averaged Navier--Stokes equations. The three-dimensional structure of the flow is analyzed, and significant differences in the measured and simulated results for the flow in corner separation regions are revealed.

Direct numerical simulations of propagation of disturbances generated by a heat source in a supersonic boundary layer interacting with an oblique shock wave are performed by the HyCFS-R hybrid code. The processes of excitation and evolution of unstable disturbances in the boundary layer, the influence of the incident shock wave on evolution of disturbances, and also the effect of disturbances on boundary layer separation and flow development in the separation region and in the laminar-turbulent transition region are studied. The influence of the duration of the thermal pulse on excitation and development of unstable waves in the boundary layer and interaction region is investigated. A case of disturbance generation by a pair of sources located at a certain distance from each other is considered. It is shown that reduction of the heat pulse duration leads to enhancement of the amplitudes of the first and second harmonics of the main unstable mode. For this reason, the disturbance spectrum in the interaction region changes, and flow turbulization is accelerated, which leads to reduction of the separation region size.

The behavior of calcite at pressures up to 1000 GPa is simulated taking into account the high-pressure phase transition. The shock-wave loading of calcite was calculated using a thermodynamically equilibrium model. A few-parameter equation of state of the Mie--Gruneisen type is used to describe the behavior of condensed phases. In the phase-transition region, the components of the sample under study are considered as a mixture of low- and high-pressure phases. The shock Hugoniots of single and double compression are constructed in the pressure range from 1 to 1000 GPa, and the heat capacity along the normal isobar, entropy as a function of temperature, and the thermodynamic potential along the shock Hugoniots were calculated. The simulation results were verified against experimental data and available calculation results

A two-dimensional problem of stationary nonlinear waves on the surface of a layer of finite-thickness ideal fluid is considered. The solution to the problem using the proposed technique includes the following steps. Firstly, the stream function trace is used to change the kinematic condition on the free surface. Secondly, the Bernoulli---Cauchy integral is applied to present the dynamic condition in a new form. Thirdly, an integral operator of the convolution type is introduced, which allows one to simplify the nonlinear boundary value problem of determining four functions of one variable, the main ones of which are a wave profile shape and a stream function trace at the zero horizon. This technique allows reducing the two-dimensional problem to a one-dimensional one. Two forms of the nonlinear dispersion relation are obtained: the dependence of the wave velocity on the amplitude of the fundamental harmonic of the wave and the dependence of the wave velocity on the wave amplitude. The cases of short and long waves are considered

In this paper, we have approximated the solutions of the Cahn-Hilliard equation (CH) with the logarithmic potential function which arises in the modeling of phase separation of binary alloys. The CH equation is a high-order nonlinear equation,consequently, utilizing a common difference scheme on the CH equation causes long stencil schemes. To resolve the faults of long stencil schemes, we split the CH equation to a second-order system under the Neumann boundary condition and we applied a second-order scheme based on the 2D Crank-Nicolson method to discrete it. The uniqueness and error estimation of the approximated solution is proved. Also, preserving the conservation of mass and decreasing the total energy are investigated. Finally, to confirm the theoretical results, three examples with various initial conditions are presented.

Perturbation propagation in a supersonic boundary layer on a straight parabolic wing is numerically simulated. Near the leading edge, a mass flow rate perturbation is introduced into the boundary layer, resulting in the formation of a first-mode wave packet, and the amplitude of the first mode increases downstream. It is shown that, upon reaching a critical amplitude, the perturbation begins developing nonlinearly and longitudinal structures appear. The nonlinear interaction of the boundary layer modes leads to the formation of a turbulent spot. The characteristics of the resulting spot with known data for gradient-free flows are compared

The properties of honey are studied experimentally.It is shown that the viscosity of honey decreases with an increase in its moisture content or temperature. Close results are obtained when measuring the surface tension coefficient of white honey using several methods: the drop weight method, the capillary ascent, and the detachment by an air bubble in a liquid. A formula is derivedthat allows one to estimate the dynamic viscosity of the liquid usingthe known surface tension coefficient and the known rate of recovery of an air bubble shape in a liquid after the bubble was stretched twice its size at the same measurement temperature

The paper describes an experimental study of the process in which a cluster of monodisperse droplets of isoparaffin oil ascends to the surface in distilled water in a range of Reynolds numbers Re = 200÷800. It is shown that the initial volume concentration of droplets in the cluster is one of the main parameters determining its dynamics. The study also touches upon the motion of two types of clusters, characterized by the presence or absence of collisions of the droplets contained in it. Experimental dependences of the rate of ascent of the droplet cluster on the Reynolds number are obtained for different values of the initial volume concentration of droplets. The limiting value of the initial volume concentration of droplets is determined, corresponding to the onset of cluster motion with a velocity exceeding the velocity of a single droplet

This paper touches upon the formation of thermomagnetic convection in a stratified magnetic colloid filling a Hele---Shaw cell placed in an external nonuniform magnetic field and heated from the side of a narrow edge in such a way that the constant field strength gradient is co-directed with the temperature gradient. The Galerkin method is applied to obtain critical magnetic Rayleigh numbers for the cases of monotonic convection and oscillatory convection arising due to the nonuniformity of the concentration of magnetic nanoparticles caused by thermal diffusion and magnetophoresis. In the case of oscillatory instability, the dependence of the neutral oscillation frequency on the mixture separation parameter and the concentration Rayleigh number is determined. For supercritical monotonic and oscillatory convection conditions, the current function, temperature, and concentration distributions are presented, and the behavior of the amplitudes of various spatial harmonics is investigated

The main goal of the present study is to investigate the mechanism of forced turbulization of the laminar boundary layer on a swept wing with domination of crossflow instability by means of using spanwise-periodic cylindrical trip devices. The experiments are performed with hot-wire anemometry in a low-turbulence wind tunnel based at the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences (Novosibirsk) at low subsonic free-stream velocities; a 25-degree swept wing is chosen as a model. Results of detailed precision measurements of the mean and fluctuating structure of the flow are obtained in a broad space range in 12 different regimes of measurements for five types of trip devices (and also without them) for two flow velocities. The range of the Reynolds numbers based on the trip height is from 565 and 3613. The present paper describes the first part of the study, i.e., the main scenarios of the transition to turbulence induced by trip devices. They are demonstrated to be highly effective. Subsequent studies will be described in Part 2 of the paper.

An incompressible gas flow past a two-dimensional backward-facing step on the flat plate surface is experimentally studied under the conditions of a controlled low-frequency action on the separated flow. It is found that flow oscillations whose frequency is smaller by an order of magnitude than the frequency typical for convective instability of the separated boundary layer induce significant changes of the time-averaged and fluctuating characteristics of the flow in the separation region.

A history of development of various cycloidal rotors used as aircraft propellers is presented. The operating principle of a cycloidal rotor with a swinging blade is described. Differentpapers on this issue are reviewed, quantitative data are summarized and analyzed, and the most important parameters affecting the characteristics of a cycloidal rotor with a swinging blade are identified. Optimal values of the main parameters for improving the characteristics of a CR with a swinging blade are obtained. A method for the engineering evaluation of the thrustof a cycloidal rotor with a swinging blade is proposed

Functionally graded B₄C-CrB₂ material with a CrB₂ concentration gradient along the thickness equal to 5, 15, and 25 % was produced by reactive hot pressing of a mixture of boron carbide, chromium oxide, and a carbon material with high specific surface area. A general view of the microstructure and the size distribution of chromium diboride particles in each layer are presented. The change in mechanical properties with an increase in the CrB₂ concentration is shown to be non-monotonic due to the features of boron carbide reduction.

This paper presents an algorithm for the developed overdetermined method of constructing the asymptotics of the stress field near a nanocrack tip in an anisotropic linear elastic body with cubic symmetry. For the problem of combined loading (normal separation and transverse shear) of a nanoplate of monocrystalline copper and aluminum with a central cut, a molecular dynamics solution was obtained and used to derive an asymptotic representation for the stress tensor components. The atomistic stresses in the vicinity of the crack tip were calculated, and the coefficients of the series representing the stress field were determined using the values of the atomistic stress tensor components. The molecular dynamics calculations were performed for a temperature of~10~K to exclude the appearance of a zone of nonlinear elastic deformation near the crack tip. The stress fields in the atomistic and analytical solutions were compared. The stresses at different distances from the nanocrack tip are in good agreement with the solutions of problems of macroscopic classical theory of anisotropic elasticity and can be described using a series that extends the Williams series to anisotropic media. In the series extended to anisotropic materials, the regular (non-singular) terms were taken into account and the series coefficients of these terms were determined. It is shown that the developed algorithm can be used to effectively determine the coefficients of the higher-order terms of the series

An approach to the simulation and evaluation of the parameters of thin elastic open shells and plates under postcritical deformation is proposed. A finite element model of the longitudinal bending of a plate with a trough-shaped profile is considered. A numerical simulation of the behavior of a plate with quasi-zero stiffness was performed, and the effectiveness of such a plate as an elastic element of a vibration isolation mechanism was studied. Finite element simulation results were compared with the results of calculations using analytical models and with experimental data. The possibilities of adjusting the quasi-zero stiffness of plates were analyzed. The results obtained indicate that such elastic elements can be widely used in vibration isolation systems for ground and onboard equipment

The assessment of material toughness is governed by codes based largely on rigorous experimental results. However, the problem of transferability from the laboratory specimen to field-scale structures limits the extent to which these results can be used. The present work is a developmental contribution to a new approach for assessing the toughness of pipeline steels. The procedure concerns the interaction between material fracture curve based on the three-parameter fracture criterion (K-T-A₃) and the surface longitudinal notch driving force of a pipe under internal pressure. This could be applied as an important engineering parameter for assessing the structural integrity of pipelines during long-term operation.

An unsteady problem of heat conduction for a rod is considered. The classical heat conduction equation based on the assumption of temperature differentiability with respect to time and coordinate is derived. A solution of a model problem with boundary conditions of the second kind is obtained, which determines the temperature distribution in the thermally insulated rod over its length and in time. For the classical formulation of the problem, the temperature change rate at the initial time is found to be singular, and the condition of temperature differentiability with respect to time is not satisfied. A modified form of the heat conduction equation is proposed, which is based on nonlocal determination of temperature as a time-dependent function. In contrast to the traditional definition of temperature, this function is not the temperature value at a fixed time; instead, it is the mean value on a finite time interval called the nonlocal temperature. As a result of using such an approach, the heat conduction equation retains the classical form, but contains the nonlocal temperature rather than the traditionally used temperature. Traditionally, the temperature is determined by means of solving the Helmholtz equation including an unknown time interval over which the temperature is averaged and which is determined experimentally. The classical and nonlocal solutions are compared with experimental data. The nonclassical Maxwell--Cattaneo equation of heat conduction is discussed, which implies a finite rate of temperature propagation in time.

A number of new formulas are obtained for vector fields and vector analysis used in geometry and the vector field characteristics used in differential geometry: curvature vector, associated vector field, degree of nonholonomy, and the Laplacian value. Non-classical characteristics such as the vector potential of the curvature vector field of a vector field and the sum of three curvature vectors of vector lines of the Frenet unit vector fields of a family of curves are also studied. It is shown that all the listed quantities are related to Aminov's divergent representations for the Gaussian curvature or for the total curvature of the second kind. The obtained formulas can be considered as properties of the family of curves. Some formulas have divergence form, which makes it possible to derive differential conservation laws for the family of curves as well as for the eikonal equation and Euler's hydrodynamic equations