Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2023 number 3

Based on the volume-of-fluid (VOF) multiphase model and the fluid-structure interaction (FSI) model, combined with the overset grid technology of the STAR-CCM+ software, a computational model for the oblique entry of truncated projectiles into water is established. The hydrodynamic characteristics and structural response characteristics of the projectile for various water entry angles are calculated and analyzed, and the evolution law of cavitation is determined. The numerical results are found to be in good agreement with experimental data

The parameters of the plasma generated by an arc discharge in gaseous helium at pressures of 3, 25, and 50 torr are studied experimentally and theoretically. The properties and orphology of synthesized soot are investigated by methods of the X-ray diffraction analysis, thermogravimetry, and transmission electron microscopy. A theoretical model is used to predict the radial distribution of the gas temperature, which is consistent with the results of thermocouple measurements. It is demonstrated that a change in the pressure in the arc discharge alters the residence time of carbon vapor in various temperature regions. This fact ensures different conditions of formation of carbon nanostructures and allows obtaining soot with essentially different structural and physical properties.

The influence of sinusoidal pulsations of the flow rate of the dispersed phase on the flow characteristics of immiscible high-viscosity liquids in a T-shaped microchannel has been studied. The flow regimes in the unperturbed flow and in the flow subjected to external perturbations with different frequencies and amplitudes were visualized. A dimensionless complex is proposed that describes the transition from parallel to plug flow due to external pulsations for a fixed capillarity number of the carrier phase. The application of perturbations to the plug flow regime is found to lead to flow stabilization and to a reduction in the spread of values of the generated plugs at a frequency equal to the natural frequency of plug separation in the unperturbed flow. The average length of the plugs and the spread of its values are shown to increase with decreasing perturbation frequency.

The atomization of metastable superheated water injected into the atmosphere from a convergent-divergent nozzle at a temperature of 240-260 °С was studied experimentally. The dispersion composition of the spray jet has a bimodal character with a predominance of submicron droplets, whose proportion increases with increasing temperature and reaches 80% at the nozzle outlet at a water temperature of 260 °С. The influence of droplet coagulation on the distribution of the proportion of large droplets along the length of the spray jet was estimated.

A numerical and experimental study of the influence of viscous and capillary forces on the characteristics of multiphase flow in the pore doublet model, which is one of the most well-known elementary models of the pore space, has been carried out. The OpenFOAM platform was used for numerical simulation. A multiparametric analysis of the process of oil displacement by various agents in the pore doublet model was carried out with varying values of the wettability of the pore surface, pressure drop, surface tension coefficient, and the ratio of the sizes of the channels of the pore doublet. It is shown that the obtained results of numerical simulation are in good agreement with the experimental data for the pore doublet model in the case of a hydrophobic surface at various values of the capillary number. The physical model of the pore doublet is implemented in a microfluidic chip fabricated using the soft lithography method. The proposed numerical-experimental microfluidic approach makes it possible to carry out a numerical study of two-phase filtration in models of a porous medium corresponding to laboratory studies, as well as to scale the results obtained by the characteristic core sizes.

The paper presents the results of studying the dynamics of a wave signal as it passes through gas-vapor bubble "screens" in a liquid, taking into account heat and mass transfer at the interface in the acoustic approximation. On the basis of numerical calculations using the fast Fourier transform method, wave patterns for pressure impulses are obtained and the influence of various parameters of the state of a liquid with vapor-gas bubbles on the reflection and transmission of acoustic waves through the “curtain” is studied.

Interface dynamics between immiscible liquids with a large difference in viscosities is experimentally studied by varying the frequency and amplitude of oscillations, the relative initial position of the liquids, and the thickness of the working liquid layer. It is shown that with an increase in the amplitude of the interface oscillations, a finger-like instability, which has a local character, manifests itself in a threshold manner on its surface. The found instability of the oscillating boundary is similar to the Suffman-Taylor instability, which develops when a viscous fluid is uniformly displaced from a slot channel (porous medium).

The classical two-dimensional Cauchy-Poisson problem for an ocean modelled as an incompressible fluid with an elastic bottom is considered here. In accordance with the linear theory, the problem is formulated as an initial-value problem for the velocity potential in the fluid region, dilation potential, and rotational potential in the elastic medium below the fluid region. The Laplace transform in time and the Hankel transform in space are used in the mathematical analysis to obtain the form of the free surface depression and ocean bed vertical displacement component in terms of multiple infinite integrals. These integrals are evaluated asymptotically by the method of steepest descent. Variation of the ratio of the ocean bed amplitude to the free surface amplitude for different forms of the prescribed initial axially symmetric surface depression or the impulse for different values of elasticity parameters is investigated. The results obtained in the study are compared to the analytical solution of the problem in the case with a rigid bottom.

DNS and RANS computation results for flows in two-dimensional channels with bumps are processed to generate input and output data for a machine learning method aimed to enhance the Reynolds stress anisotropy model and, thus, improve the RANS approach accuracy. The tensor basis random forest method is chosen as a machine learning tool. The prediction of the new model for the Reynolds stress anisotropy tensor is in better agreement with DNS data for two channel flow geometries than those obtained by the conventional linear eddy viscosity model.

The two-dimensional problem of potential flow around two circular cylinders is considered for given velocity at infinity, circulations around cylinders, and the radii and relative position of the cylinders. Exact analytical formulas for the complex potential and the complex conjugate velocities in terms of the Jacobi theta functions are derived. The circulations around the cylinders are uniquely determined using the Gol'dshtik's minimax principle: the circulations should be selected so that the maximum fluid velocity in the stream is minimal.

The propagation and destruction of nonlinear internal waves in the summer-autumn period at the hydrophysical test site of the Pacific Oceanological Institute, FEB RAS in the shelf zone of the Sea of Japan have been studied for a number of years. By continuous measurement of vertical temperature and velocity distribution at depths of 20-60 m using bottom stations and by a regular study of the distributions of the main hydrodynamic characteristics along selected paths, mechanisms have been determined for the generation and propagation of nonlinear packets of internal waves due to internal tide decay and other short-period sources of thermocline deformation have been detected that can be attributed to the propagation of benthic and surface vortex structures. High spatiotemporal resolution of the temperature field in the neighborhood of bottom stations made it possible to reveal the fine structure of wave perturbations of various types. Multilayer shallow-water models for wave-packet propagation have been developed and verified using the data of full-scale and laboratory experiments.

This paper presents a three-dimensional direct numerical simulation of the chaotic dynamics of the free surface of a dielectric liquid placed in an external tangential electric field. The physical model includes the effects of energy pumping (external force), energy dissipation (viscosity), and surface tension. As the external electric field strength increases, a transition from the turbulence of dispersive capillary waves (at zero field) to anisotropic electrohydrodynamic wave turbulence is observed. In the strong field limit, where the fluid motion becomes highly anisotropic, a cascade of small-scale capillary waves is formed that propagates perpendicular to the external field direction. In this regime of motion, a new turbulence spectrum occurs, which differs from the classical spectrum of capillary turbulence.

The possibility of changing the morphology of the silicon surface at certain parameters of the glow discharge is shown. It has been established that oxidation is the main process influencing the surface morphology during glow discharge plasma treatment. As a result of processing in the investigated range of parameters, various stages of the surface oxidation process are observed: the formation of a uniform oxide layer, the formation of nano- and microstructures of silicon oxide. It is shown that these processes lead to surface modification, which acquires stable hydrophilic and superhydrophilic properties.

Predicted numerical distributions of the vibrational temperature of molecular oxygen behind a reflected shock wave are compared with experimental measurements in a shock tube. The computations are performed with the use of five two-temperature models (those developed by Park and Kuznetsov, β-model, and models of Marrone and Treanor and by Macheret and Fridman), five dissociation constants, and three variants of the source term that describes the rate of change of the vibrational energy due to chemical reactions. The Landau-Teller model is used to calculate the rate of translational-vibrational energy transfer, and the time of vibrational relaxation is calculated by the Millikan-White formula with allowance for Park's high-temperature correction. The numerical and experimental results are found to be in reasonable agreement. The greatest difference between the numerical and experimental data is observed in the region of relaxation of the shock wave incident onto the wall.

To study the effect of laser surface remelting (LSR) on the organization and properties of a 304 stainless steel surface layer, the microscopic morphology, hardness, roughness, adhesion, and corrosion resistance of the remelted layer (RL) are examined by changing the laser scanning speed (LSS). The experimental results show that the LSR technique hardens the 304 stainless steel substrate surface with a substrate hardness of 185 HV, and the maximum hardness after remelting is 248.9 HV. With an increase in the LSS, the surface roughness gradually decreases, while the bonding force first increases and then decreases, with the maximum bonding force being 26.1 N. At the LSS of 20 mm/s, the phase distribution in the RL is more uniform. The maximum self-corrosion potential of the RL reaches -0.718V, and the self-corrosion current density is 3.872 A/cm². The surface properties of 304 stainless steel are improved by using LSR.

This paper presents the results of numerical analysis of the filtration equations for highly miscible liquids obtained by two-scale homogenization of the Navier-Stokes and Cahn-Hilliard equations for two-dimensional flows. It is shown that the permeability tensor is generally anisotropic. For one-dimensional flows, the miscibility dynamics is investigated, and it is shown that the displacement of one phase by injection of another phase can occur even in the absence of a pressure gradient in the sample.

Taking into account the degradation of the properties of materials, an analysis of the nonlinear processes of deformation and fracture in a preloaded three-dimensional body with a sharp concentrator in the zone of a dissimilar joint under impact was performed. A generalized mathematical model of nonlinear interrelated deformation and destruction of damaged polycrystalline media subjected to time-varying thermomechanical influences is presented. The strong nonlinearity of the model is due to large (finite) strains and the strain-rate dependent behavior of media with a variable microstructure. Taking into account the anisotropic hardening of media and the Bauschinger effect, the corresponding nonlinear boundary value problems are formulated and their solutions are obtained using efficient numerical methods. The viscosity of the medium and second-order gradients from the internal variables of the system were used as regulators of the correctness of the problem statement. Experimental data were used to test the model. Simulation results presented.

The strength of a rectangular plate with two opening-mode (mode I) edge cracks was studied using the Neuber-Novozhilov approach and a modified Leonov-Panasyuk-Dugdale model with nonzero prefracture zone. A coupled discrete-integral fracture criterion is used since the stress field in the vicinity of the crack tip has a singularity. At the tip of a real crack, the fracture criterion for ultimate strain is satisfied, and at the tip of the model crack, the criterion for normal stresses is satisfied. The constitutive equations for the analytical model are analyzed. Simple formulas for fracture load in quasi-brittle and quasi-ductile fracture are derived. Plate fracture curves are constructed for a plane stressed state.

Equations are obtained that describe the process of propagation of elastic waves in a collapsing solid. It is shown that the solution of the equation for the velocity potential of acoustic emission longitudinal waves can be represented as a superposition of the fields of an ensemble of radiating cavities - monopoles. The parameters of the ensemble of monopoles are selected in such a way that the structural characteristics of a medium with cavities differ slightly from those of a solid body.

The microstructure, surface quality, and thermal shielding properties of the coatings prepared by the methods of magnetron sputtering and electroplating are studied. The results show that the Cr-Ag coating prepared by means of magnetron sputtering is smoother, its porosity and emissivity are lower, and the hardness is higher than that of the coating prepared by means of electroplating.

An algorithm for solving an inverse problem of shaping of a ribbed panel made of the AK4-1 alloy (analog of the AA2018 and Al-Cu₂-Mg₂-Ni₁ alloys) at a temperature of 200 °С is proposed. The problem is reduced to solving auxiliary direct problems on the stress-strain states of the elements of the panel structure. The structural elements are taken to be T-beams subjected to pure bending. The resultant moment for obtaining a desired shape is calculated by the Nelder-Mead optimization method. Two methods of obtaining target-shaped articles are considered: deformation under the plastic flow condition and under the creep condition. The material models used in the study take into account the difference in the mechanical properties under tension and compression, as well as the presence of accumulated damage in the material. The calculations are performed with allowance for elastic spring unloading of the article after load removal.