Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2023 number 6

The physical processes of crystallization of melt droplets in water have been studied using the droplet granulation and melt centrifugation methods. A mathematical model has been developed that makes it possible to determine the cooling and crystallization rates and dendritic structural parameter of aluminum alloy granules from the initial data of the process, the diameter of melt droplets, and cooling conditions. Predicting the dendritic parameter of the microstructure of granules allows one to predict the level of microstructure dispersion and hence the strength properties of the granulate material. The model parameters take into account the speed of droplet motion, features of heat removal processes, and the temperature dependence of the thermophysical parameters of the media. An application program implementing the developed mathematical model has been developed. Implementation of the developed mathematical model has been carried out using the Microsoft Visual C++ programming language. The mathematical model has been tested in the granulation of highly alloyed aluminum alloys (D1 and D16 alloys of the Al-Cu-Mg system, and B95 and B96Ts alloys of the Al-Zn-Mg-Cu system) obtained by centrifugal melt spraying and the droplet methods with cooling in water. Crystallization rate in full-scale samples has been measured based on an analysis of the dendritic structural parameter of the material. Analysis of the calculated values of the dendritic parameter and its measurements for real granule samples has shown that there is good agreement between the simulation and measurement results.

The deposition of a nickel-based cermet coating containing 60 % tungsten carbide has been studied. The effect of substrate preheating up to 500 °С on the intensity of the in-situ synthesis of secondary ceramic particles obtained by dissolving the initial particles in a metallic matrix has been investigated. It has been shown that preheating contributes to the prevention of cracks and pores in the coating. Using synchrotron radiation, it has been found that additional heating of the sample during surfacing to a temperature of 500 °С does not cause significant changes in the phase composition of the resulting composite.

The influence of substrate preheating on the structure of a cermet-coating obtained by direct metal deposition from Ti64 alloy reinforced with B₄C particles. Coatings containing 15 %. B₄C were obtained for the first time. It has been shown that the use of substrate preheating makes it possible to produce cermet coatings without cracks. It has been found that in tribological tests, that the scratch volume of the cermet coating decreases by a factor of eight compared to the coating with the content of B₄C 10 %.

Results on a nanoparticle collision with a target calculated by the molecular dynamics method are presented. The first problem being solved is a nanoparticle collision with a target under the conditions of cold gas-dynamic spraying. The second problem deals with nanoparticle extension, which adheres to the target due to the collision. It is shown that a chemical bond between the nanoparticle and target due to the collision. The bond in the case of titanium nanoparticle impingement onto an aluminum target is found to be stronger than that in the case of aluminum nanoparticle impingement onto a titanium target. The reason is that the titanium nanoparticle penetrates into the aluminum target to a greater depth.

The centrally symmetric problem of determining the velocity field in a continuous elastoplastic medium under a camouflage explosion has been solved using the assumptions of the unoscillatory nature of the motion and the incompressibility of the medium in the regions of plasticity and elasticity. The solution was found using a camouflage equation-the relation for determining the pressure on the contact surface of an expanding explosion cavity. The solution allows one to estimate the dimensions of the expansion and plastic deformation regions and the impact of explosive disturbances on objects

A nitrogen jet expanding during its exhaustion from a nozzle into a vacuum chamber is numerically simulated by a hybrid approach. The flow parameters in the nozzle and in the near field of the jet are determined by solving the Navier-Stokes equations by using the ANSYS Fluent software. The gas flow at large distances from the nozzle exit is modeled by the Direct Simulation Monte Carlo method by using the SMILE software system. Such an approach makes it possible to perform sufficiently accurate simulations in the near field of the jet; moreover, the temperature nonequilibrium of the expanding gas jet and other effects of rarefaction in the far field of the jet are taken into account. The approach is verified by using various approximate analytical models of gas exhaustion into vacuum. A comparison of the numerical results with available experimental data shows that they are in good agreement.

The flow in a far plane momentumless turbulent wake is described using a mathematical model obtained from the algebraic Rodi model of Reynolds stresses. The model is similar to the two-parameter ( k ̶ ε ) turbulence model in the far-wake approximation with a modified empirical constant in the diffusion terms of the equations. A group-theoretical analysis of the mathematical model of the wake was carried out and and a self-similar reduction of the equations of models to a system of ordinary differential equations was performed. An approximate solution of the corresponding boundary value problem is constructed using an asymptotic expansion of the solution in the neighborhood of the singular point.

A passive method for reducing the drag of cylinder by installing a flat plate behind it at a Reynolds number Re=10⁵ has been studied. The paper presents the results of an ANSYS Fluent simulation of the flow around the cylinder-plate system, including the velocity and pressure fields, streamlines, and the dependences of the drag coefficient and the position of the separation point on the surface of the cylinder on the relative length of the plate. It has been found that the drag coefficient of the cylinder-plate system can be reduced by approximately 40% compared to the case of an isolated cylinder.

The full Navier-Stokes equations are used to study the linear stability of plane Poiseuille flow in a channel with the lower wall corrugated along the flow, due to which the flow has two velocity components. The generalized eigenvalue problem is solved numerically. Three types of disturbances are considered: flat periodic (the Floquet parameter is zero), flat doubly periodic (finite values of the Floquet parameter), and spatial. Neutral curves are analyzed in a wide range of the corrugation parameter and Reynolds number. It is found that the critical Reynolds number above which disturbances that increase over time appear depends in a complex way on the dimensionless amplitude and period of corrugation. It is shown that in the case of flow in a channel with corrugated wall, three-dimensional disturbances are usually more dangerous. The exception is the small amplitude of corrugation, at which plane disturbances are more dangerous.

This paper presents the results of a statistical analysis of the turbulent structure in a free bubbly jet at a Reynolds number of 12500 based on PIV measurements of the carrier-phase velocity. The distributions of higher-order statistical moments for velocity fluctuations (skewness and excess coefficients) and the energy spectra of turbulence for single-phase and gas-saturated jets were obtained after applying the procedure of statistical filtering to instantaneous velocity fields. The influence of the dispersed phase (bubbles with an average diameter of 0.8 mm) with a volume gas fraction of 0, 1, 2, and 3% for the specified characteristics of the continuous phase was analyzed.

A description of a small climatic wind tunnel designed for studying the icing processes at the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences is presented, The use of such a wind tunnel offers a possibility not only of studying the physics of the icing process, but also of testing methods of anti-icing control, validating numerical methods used for calculating the icing processes, etc.

The longitudinal structures generated by external vortical and thermal waves in subsonic and supersonic boundary layers have been studied. Particular attention is paid to setting boundary conditions on the external boundary of the boundary layer. It has been found that disturbances of the longitudinal velocities inside the boundary layer can be several times higher than the amplitude of the external vortical wave. The generation efficiency decreases with increasing Mach number. It has been shown that the influence of thermal external waves on the flow structure in the boundary layer is much weaker than the influence of the vortex structure of the flow

The influence of various physical mechanisms at the stage of collapse of a vapor bubble and the subsequent formation of a cumulative jet in the process of laser-induced boiling near the end of a thin waveguide immersed in a cold liquid is studied numerically. Depending on the intensity of evaporation, three process modes are identified and described.

Three-dimensional numerical simulations of the air flow in the full human bronchial tree in situations of obstructive and chronical pulmonary diseases are performed. Based on the previously developed three-dimensional analytical model of the lower respiratory airways, the air distributions in the lungs (from the trachea to alveoli) in situations with lung injuries and bronchial asthma are calculated. Breathing modeling is based on a numerical technique of stage-by-stage computations, which allows one to avoid the loss of solution accuracy caused by the difference in the bronchus scales; moreover, the time needed to calculate the air flow in the lungs can be reduced by several times by using this technique.

The results of numerical simulation within the framework of a two-dimensional shallow water model of the interaction of a solitary wave with a stationary semi-submerged body with a rectangular cross section are presented. A comparison of these results with the results of calculations based on a model of irrotational three-dimensional flows showed that for small amplitudes of the incident wave, the modeling accuracy is satisfactory. It has been established that neglecting the diffraction of a solitary wave on the surface of a cylinder when using a one-dimensional model of shallow water leads to overestimates of the maximum values of the wave runup on the edge of the cylinder and the force loads on it.

A high-velocity flow in an axisymmetric nozzle containing a central body and pylons is studied. The influence of the geometry of the main and additional pylons on the gas-dynamic and thrust characteristics at the nozzle exit in the flow regime with n_pr= 2.25 ( n_pr is the ratio of the pressure in the settling chamber to the ambient pressure) is determined. Azimuthal nonuniformity of the flow at the nozzle exit is detected. The maximum azimuthal nonuniformity is observed in the wake behind the pylons. It is shown that a three-dimensional transonic flow is formed in the nozzle duct with the pylons mounted in the minimum free-area cross section; local supersonic regions closed by weak shock waves are formed in this flow. It is found that the formation of such a shock wave structure is responsible for nozzle thrust reduction by 12%.

Approximations of the profiles of the longitudinal and transverse velocity components in the boundary layer calculated for the flows around a swept wing and a body of revolution by means of solving the full Navier-Stokes equations and using the boundary layer profiles from the self-similar one-parameter family of the Falkner-Skan-Cooke profiles and two-parameter family of profiles proposed by Gaster are compared. A significant advantage of using the approximation of the numerical profiles near three-dimensional separation by profiles from the two-parameter family is demonstrated.

The process of occurrence of an internal crack of a mixed type (I and II fracture modes) in the wall of a pipe made of an ideal elastoplastic material under the action of combined tensile (compression) and bending loads has been studied. The process of destruction of such materials is described using a modified Leonov-Panasyuk-Dugdale model, which uses an additional parameter - the diameter of the plastic zone (prefracture zone width). The case of complex loading is considered, when the crack trajectory is necessarily curved, therefore the angle of the trajectory break is determined using a force integral criterion based on the asymptotics of the stress field in the vicinity of the crack tip. To obtain critical fracture parameters, a two-parameter (dual) strength criterion is proposed in the case of a complex stress state. An analysis of the parameters included in the resulting analytical model was carried out. The dimensionless geometric parameters of the structure are found numerically using the finite element method. Diagrams of quasi-brittle fracture have been constructed.

The deformation of an ideal elastoplastic adhesive layer of a sample in the form of an elastic double-cantilever beam is considered. Taking into account all diagonal components of the stress tensor in the layer, the values of the J -integral were found for a number of adhesives. It is shown that when using an elastoplastic model of layer deformation, the type of plane problem can have a significant impact on the value of the J -integral. It has been shown that during normal tearing in the zone of irreversible deformation of the adhesive in a plane stressed state, the presence of compressive stresses is possible.

Using the molecular dynamics method, the rigidity constants of five structural configurations of grapheme-carbon monolayer in which the atoms are arranged in a special way and have sp - and sp²-hybritization were calculated. It has been established that the arrangement of atoms in the graphene layer has a significant effect on the stiffness constants. It was found that the γ₂-carafe has the highest stiffness constant c₁₁ (1091 GPa), and the α-carafe has the smallest (258 GPa). It is shown that the β₃-carafe and γ₂-carafe are highly anisotropic structures.

A linear theory of static cylindrical bending of a thin plate is constructed without using Kirchhoff's hypotheses. The transverse shear, thickness compression and the resulting longitudinal force are taken into account. Taking into account the change in the areas of both surfaces during bending, the transverse distributed force is determined. It is assumed that the average pressure on the plate is several orders of magnitude greater than the pressure drop. Bending under conditions of plane deformed and stressed states is considered.

An experimental technique has been developed that allows one to visualize the occurrence and spread of damage in each layer of transparent armor. The dynamics of damage development in multilayer packages consisting of four and eight layers with an area of 480´480 mm and 300´300 mm, respectively, have been analyzed. The evolution of damage was obtained under the influence of a 12.7 mm caliber bullet with a two-component core with a total mass of 59.2 g and a 7.62 mm caliber bullet with a two-component core with a total mass of 10.9 g, both normal and at an acute angle in the range of impact speeds 777÷797 m/s. The experimental data are compared with the results of numerical modeling of the interaction of the striker with a multilayer transparent barrier.

A method has been developed for predicting creep and long-term strength based on data on the behavior of a previously tested sample (leader sample, prototype) under ductile fracture conditions. It is assumed that the material, when loaded, does not have instantaneous plastic deformation and the first stage of creep, and the incompressibility hypothesis is satisfied for it. It is shown that if for a leader sample the creep curve at constant stress and the time until its failure are known, then to obtain a diagram of the rheological deformation and long-term strength of the material at other stress values it is enough to know only the initial (at the initial moment of time) minimum creep strain rate of the samples for these voltage values. The adequacy of the developed method was checked with experimental data during tension of samples made of corrosion-resistant steel grade 12X18H10T at a temperature of 850 °С and samples from a titanium alloy at a temperature of 600 °С, as well as during tension and torsion of samples from the D16T alloy at a temperature of 250 °С. It is shown that the prediction results do not depend on the choice of the leader sample from a number of samples tested at different voltages. The possibility of using the developed method in conducting experimental studies of materials under conditions of creep up to their destruction is discussed.

An original numerical framework is developed in the present research work in order to estimate the free field sound radiation from baffled structural panels subjected to nonhomogeneous turbulent boundary layer flow-induced excitation. A sequence of semi-analytical methods is used to estimate the nonhomogeneous turbulent boundary layer wall pressure spectrum, which is decomposed using the Cholesky technique to obtain the random wall pressure in the frequency domain. Structural panels are modeled using the finite element technique, and a coupled finite element-boundary element modeling technique is developed to estimate the sound power level radiated into the free field. Results are obtained for laminated composite structural panels with various fiber orientations.