Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2025 number 4

The cavitation process in a pulsed focused ultrasonic field is investigated. For the first time, an abnormally long delay (up to several minutes) in the onset of non-stationary cavitation relative to the moment the ultrasound is switched on has been recorded. A methodology for selecting field parameters that enables control of the dynamics of cavitation region development is proposed.

The characteristics of pressure pulsations on a flat semi-infinite barrier impinged by a supersonic underexpanded jet are investigated. The Mach number at the nozzle exit is M _а = 1, the degree of underexpansion is n = 2,1, and the distance from the nozzle exit to the barrier is h/D_а = 2 ÷ 15 ( D_а is the nozzle exit diameter). It is shown that tangential injection of six microjets can significantly reduce pressure pulsations on the barrier. The mass flow through all microjets is 0,3 % of the mass flow through the main jet. It is established that when h/D_a < 3, a self-oscillatory mass-flow regime of jet-barrier interaction occurs; when 3 < h/D_a < 8, a self-oscillatory regime with acoustic feedback takes place; and when h/D_a > 8, the turbulent aperiodic interaction regime is realized.

The concept of plasma-assisted combustion, which offers several advantages (reduced ignition delay, improved mixing, and stabilization of the flame front), is considered. The paper presents the results of a study of ignition by means of a longitudinal direct-current discharge of a fuel-air mixture injected at supersonic speed into the core of a supersonic airflow. To eliminate the effect of mixing, the fuel (ethylene) was premixed with the oxidizer (air). The design of the ethylene-air mixing system and the injection system of the resulting mixture into the test-section channel is described. For this configuration, results of gas-flow modeling using the FlowVision software package and experimental results on fuel ignition in a supersonic flow are presented.

The processes of wear and failure of VT6 alloy with a ZrN protective coating in the initial coarse-grained and ultrafine-grained states under high-velocity dynamic erosion by corundum particles with average sizes of 109, 58, and 23 μm in an air flow at velocities of 50 ÷ 250 m/s and exposure times of 30, 60, and 300 s are experimentally studied. The experiments determined the fraction of viscous failure, the depth of the damaged layer, changes in the alloy microstructure near the eroded surface with and without coating, as well as weight loss and surface roughness. It is shown that under high-velocity erosion, wear and failure processes of the alloy strongly depend on exposure time, erosion velocity (particle velocity), particle size, and substrate structure.

Using a fiber laser and post-weld heat treatment, high-strength laser-welded joints of an Al-Cu-Li alloy were produced. Synchrotron transmission studies were used to analyze the structural-phase state of the weld seam before and after treatment. Data on the structural and phase composition of the weld seam as a three-dimensional volume were obtained. The influence of heat treatment on the fatigue resistance of Al-Cu-Li alloy laser-welded joints was studied. Static mechanical tests of the laser-welded joints were carried out at normal, elevated, and reduced temperatures to determine the strength and deformation characteristics of the Al-Cu-Li system. Dynamic tests of welded joints of the Al-Cu-Li system under impact bending were performed using the Charpy method.

The mechanism of aluminum (Al) coating formation on a titanium (Ti) substrate during cold gas dynamic spraying was investigated. Structural studies of the coatings obtained by this method showed that they contain a rough transition layer in which Al and Ti atoms are intermixed. It is assumed that the transition layer forms during particle impact with the rough substrate due to a convective mechanism. Numerical modeling of mixing using the smoothed-particle method was performed for the problem of Al microparticle impact on a Ti substrate containing a conical cavity. It was shown that in this case, a cumulative jet of Al is formed, penetrating into the Ti substrate, cooling, crystallizing, and remaining in it as an aluminum inclusion. As a result of numerous microparticle impacts, a transition layer is formed on the substrate with reduced activation energy for bond formation between Al microparticle atoms and Ti atoms of the substrate. The impact of Al microparticles on the transition layer can lead to their adhesion to the surface and the formation of an aluminum coating.

A one-dimensional evolutionary system of equations is proposed, which describes in the Boussinesq approximation the motion of a thin bottom layer in a flooded domain of a lighter fluid, taking into account the development of shear instability and the formation of an intermediate mixing layer. For hydrostatic flows, the propagation velocities of perturbations are determined, and the concept of subcritical (supercritical) flow is formulated. The stationary problem of the mixing layer is considered. It is shown that, depending on the Froude number of the incoming flow, either a monotonic or a wave-type mixing layer is formed. In the first case, a regime of maximum entrainment is achieved, and the stationary solution is determined over a finite interval. When accounting for non-hydrostatic pressure in the lower layer, stationary solutions are constructed in the form of second-mode solitary waves adjacent to a given steady flow. Unsteady calculations of the formation and propagation of large-amplitude bottom waves were performed.

An experimental study was carried out on the interaction of a longitudinal vortex generated by a jet vortex generator with a turbulent boundary layer developing over a flat plate. The main measurements were performed using the PIV method. Based on these data, Reynolds stresses were obtained, and their contribution to the Navier-Stokes equations was investigated. Data analysis allowed the derivation of integral relations characterizing the influence of longitudinal vortex intensity on the turbulent boundary layer. It was found that the action of the jet vortex generator can lead to a reduction of energy dissipation in the turbulent boundary layer.

Results of the implementation and application of adiabatic and isothermal wall boundary conditions with slip and temperature jump in the computational code HyCFS-R for modeling near-continuum flows are presented. Validation was performed on problems with external and internal flows. As an external flow, the flow around a T2-97 cylinder with a skirt was selected; as an internal flow, the propagation of a shock wave in a long tube and the flow in a nozzle at low Reynolds numbers were considered. It was found that, in the case of flow around a cylinder with a skirt, the implementation of the slip boundary condition provides better agreement with experimental data on separation and reattachment points than the no-slip condition. It was shown that, in the calculation with the slip boundary condition, the shock wave propagates along the long tube faster than in the calculation with the no-slip condition. The calculated shock wave propagation velocities are in satisfactory agreement with experimental data. In the case of gas ejection from a nozzle, the use of the slip boundary condition leads to better agreement between the calculated and experimental temperature distributions along the surface.

The structure of upward bubbly flow in an inclined circular tube was studied experimentally. Local gas content profiles were obtained using a point conductivity sensor. Wall shear stress was determined using the electrodiffusion method. It was shown that the orientation of the tube has a significant effect on the local characteristics of gas-liquid flow. In the upper part of the inclined tube, a significant increase in gas phase concentration occurs, leading to higher maximum values of local gas content near the wall and increased wall shear stress compared to single-phase flow. The most significant increase in shear stress occurs at inclination angles from 40° to 60°.

Mathematical models in the long-wave approximation for a three-layer fluid, taking into account mixing and entrainment at the layer interfaces, are considered. These nonlinear models are used to describe changes in the characteristics of heterogeneous flows when passing over localized seabed elevations. Comparison of numerical results with field measurements shows that the models adequately reproduce the structure of shelf and deep-water currents, in which flow splitting occurs with subsequent thickening of the passive intermediate layer and the formation of an intense downslope jet downstream of the obstacle.

Based on a formulated mathematical model, numerical experiments were carried out to study the degradation of permafrost containing ice, accumulations of metastable preserved gas hydrates, and free gas under thermal and saline effects of solutions, taking into account the osmotic effect. Multivariate calculations were used to investigate the patterns and rates of permafrost degradation and methane release. A comparison of some model results with corresponding experimental data showed good agreement.

Based on filtration equations, the problem of CO₂ injection into a viscoelastic porous medium is considered. A two-dimensional problem of deformation of the porous skeleton, taking into account changes in porosity, was studied. For the model system of equations in a thin layer, an exact solution of the initial-boundary value problem was constructed.

The initiation of an inclined edge crack of mixed type (modes I and II failure) in a thin strip of sheet steel under tensile loading is considered. During rolling, an initially isotropic sheet metal typically develops significant anisotropy, characterized by differences in plastic properties along the rolling and transverse directions. The quadratic Hill yield criterion is used to describe plastic anisotropy. The failure process of such materials is described using the modified Leonov-Panasyuk-Dagdale model. Under complex loading, the crack path curves, so the trajectory deflection angle is determined using the force-based integral strength criterion. In the asymptotic representation, stress components near the crack tip account for non-singular terms (T-stresses). To obtain critical failure parameters of a strip with an inclined edge crack, a two-parameter strength criterion is proposed. The parameters in the analytical model are analyzed. Dimensionless geometric parameters of the structure are determined numerically using the finite element method. A system of two nonlinear equations is obtained for the critical length of the pre-failure zone and the critical load under complex stress conditions. Using Hill’s yield criterion, the shape and size of the plastic zone near the crack tip in a plastically anisotropic material are determined.

Results of computations of continuity and stress fields (in the Lagrangian formulation) near the crack front under creep conditions are presented and discussed. Calculations were performed using the finite element method to determine asymptotic distributions of stresses and continuity (damage) near the edge of the cut. For various material constant values, the shape of the damaged material zone under creep developing ahead of the crack tip was determined. Analysis of radial stress distributions obtained from finite element computations shows that the damage accumulation process alters the asymptotic behavior of stresses near the crack tip in a material described by a power-type governing equation. It was shown that, in the absence of damage accumulation, the numerical solution approaches the Hutchinson-Rice-Rosengren asymptotics, whereas accounting for the damage accumulation process affects the stress field near the cut or crack. Using finite-element radial stress and continuity distributions, characteristic sizes of zones dominated by different asymptotics near the crack tip can be determined.

Physical modeling of the forming process of a panel made of AK4-1 (Al-Cu-Mg) alloy under creep conditions at an annealing temperature of T = 420 °C was carried out. The forming process was simulated by tensile creep of cylindrical specimens to various strain values not exceeding 6 %. The tests yielded parameters of the classical Boyle-Norton model for steady-state creep. In accordance with the technological forming process under creep conditions, heat treatment (quenching and aging) was performed to restore the panel’s strength properties. Stress levels for fatigue tests were determined using stepwise cyclic loading with increasing stress amplitude. Tests were stopped upon the occurrence of plastic deformations. A comparison of fatigue resistance characteristics was performed for as-received specimens, specimens stretched under creep, specimens after heat treatment, and specimens with zero creep strain after heat treatment. It was shown that accumulated creep strain exceeding 2 % can reduce the fatigue performance compared to as-received specimens.

The problem of forced bending vibrations of a strip-bar with two cantilevers and a fixed finite-length section on one of the lateral surfaces is addressed. The classical Kirchhoff-Love model is used to describe deformation of the cantilevers, and the fixed section is described by a refined Timoshenko shear model accounting for transverse compression, modified to consider the prescribed displacements of the support element. Kinematic coupling conditions for the fixed section and cantilevers are formulated, and using Hamilton-Ostrogradsky’s principle, equations of motion, boundary conditions, and force coupling conditions for the bar sections are derived. An exact analytical solution is obtained for harmonic forced vibrations under the action of a harmonic transverse force at the end of one cantilever. Numerical experiments were conducted to study forced bending vibrations of a strip-bar made of D16AT duralumin. It was shown that vibrations of the unloaded cantilever are primarily determined by the prescribed displacements of the support element.

Results of studies on the reliability of mathematical modeling of metal plastic flow using the finite element method implemented in various software products are presented. The limitations of the finite element method and their influence on the description of deformation processes occurring during intensive shaping of a workpiece are analyzed. Special attention is paid to the study of regions with complex metal flow. A comparison of mathematical modeling results obtained using MSC Simufact.Forming, Transvalor Forge, SFTC DeForm, and QuantorForm QForm with parameters of actual products is carried out. It is shown that the results of mathematical modeling performed with the mentioned applied software products for complex metal flow studies are sufficiently reliable.

The problem of group classification is solved for equations describing vibrations of a nonlinear elastic plate in a gas flow. Exact solutions of these equations are presented, which can be used as benchmarks for numerical solutions.