Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2019 number 1

The paper deals with the problem of ascertaining the value of air flow velocity necessary to prevent electric current through the motion of ionized air between two asymmetrical electrodes. By means of this experiment, the authors aim to prove the ion basis for the phenomenon of a mechanical force generated by a system of asymmetrical electrodes connected to high voltage. Particle image velocimetry (PIV) is used to precisely map the vector field of the air flow around the electrodes. The calculated and measured values of air flow velocity are compared, and good agreement between them is found.

The possibility of using a combined method of control of an incompressible turbulent boundary layer on the NACA 0012 airfoil is studied experimentally and numerically. The method is based on air blowing and suction through finely perforated walls, which are parts of the airfoil. The study is performed for Reynolds numbers Re_c=0.7 · 10⁶ and angles of attack α =-6 - 60. It is demonstrated that the mechanism responsible for changes in the aerodynamic characteristics of the airfoil is the same for all variants of the control action in the form of air blowing and suction: an increase in pressure due to air blowing on one side of the airfoil and a decrease in pressure due to air suction on the opposite side lead to an increase in the lift force and, finally, to a gain in the lift-to-drag ratio of the airfoil, which reaches three units at α ≈ 0^o. Unfortunately, the efficiency of this method of boundary layer control decreases with increasing α.

A problem of a far field generated by internal gravity waves from an oscillating point source of perturbations that moves in a vertically infinite stratified medium with variable buoyancy. For a simulated quadratic distribution of buoyancy frequencies, analytical solutions of the main boundary-value problem are obtained, expressed through parabolic cylinder functions. Asymptotic solutions constructed in this paper make it possible to describe amplitude-phase characteristics of far fields generated by internal gravity waves in a stratified medium with a variable Brunt- Väiää frequency.

A Couette flow of liquid, described by a modified Vinogradov-Pokrovsky model with a non-monotonic flow curve is simulated. It is shown that the analytical solution of the stationary problem has an infinite set of solutions. The time-dependent problem is numerically simulated in the assumption that the components of the structural tensor take values corresponding to a current change in the velocity field. It is determined that the time it takes for the plate velocity to reach a given value significantly affects the velocity profile and the dependence of tangential stresses on an operating shear rate. It is shown that, as this time decreases, the shear banding of the flow is observed not only for shear rates corresponding to the downward branch of the flow curve, but also in the entire domain of its ambiguity.

The filtration of a graphene suspension through a core sample has been studied experimentally. It has been found that at the oil-water interface in the porous medium of the core sample, a transient multilayer microheterophase structure consisting of planar graphene nanoparticles, hydrocarbon oil molecules and surfactants is formed. It has been found that with an increase in the concentration of graphene nanoparticles, the volume fraction of the displaced oil increases and the volume fraction of water in oil decreases.

This paper presents the results of the numerical calculation of gas filtration in a low-permeability gas formation with a fractured well. It is assumed that finite-conductivity filtering is described by Darcy law. The Peng-Robinson equation was used to take into account the dependence of the PVT properties of the gas on pressure. We studied the effect of fracture conductivity and formation permeability on the distribution of pressure and gas properties in a “fracture-formation” system and on the dynamics of pressure of the well and the gas production rate in the well during well-testing.

Longitudinal waves in a saturated porous medium simulated by a double-phase medium are under study. Exact expressions for a wave propagation velocity and attenuation coefficient are obtained. High-frequency and low-frequency wave asymptotics is investigated, and partial cases of soft and dry soils are considered.

The boiling in a superheated water jet discharged from a short cylindrical nozzle in the presence of a passive swirler has been experimentally investigated. The evolution of the disintegration of the boiling liquid jet with an increase in the degree of superheating has been studied. Characteristic structures and forms of the flow have been identified. The correlation between the jet shape and boiling regimes has been found. Significant fluctuations and flow instability at certain temperatures was noted. The possibility of influencing the flow pattern has been experimentally shown.

A mathematical model is constructed to describe a steady one-dimensional temperature distribution in a hollow circular cylinder made of a polymer dielectric with a constant difference in the electric field potentials on the cylinder surfaces. Based on systematized data on the dependence of the thermal conductivity and electrical conductivity of polymer materials used as dielectrics in engineering on temperature, a qualitative analysis of the model is performed for a prescribed density of the heat flux supplied to the inner surface of the cylinder and intensity of convective heat transfer on the outer surface. The results obtained in the study allow one to determine the area of applicability of polymer dielectrics used in various electrical engineering applications, including electrical insulation of DC high-voltage cables, and to formulate the conditions for the thermal breakdown of the cylindrical layer of the dielectric.

This paper presents the results of an experimental study of the occurrence and control of flaming combustion and thermal decomposition of typical forest combustible materials by creating a fire break in the form of a wetted layer with a fixed width, length, and depth and a given volume of the contained liquid. The ranges of parameter values that ensure stable suppression of the flaming combustion and thermal decomposition of forest combustible material were determined. Experiments were performed for all major types of forest combustible materials: leaves, needles, twigs, and mixtures of the listed materials. Prediction of the required (minimum) water volumes and sizes of the fire break for guaranteed control of the combustion of forest combustible materials of the types studied was made.

Results of a series of firing tests of two model nozzles made of a carbon--carbon composite material are reported. This material can be used to fabricate nozzles for a large-scale liquid-propellant rocket motor. The tests are performed in an experimental setup operating on oxygen and hydrogen. It is shown that the thermochemical loads on the nozzle material in model tests are greater than those under real conditions. The temperature of the outer surface of the nozzle is measured in each experiment by thermocouples and an infrared imager. The level of linear erosion of the wall material during the entire test period is determined. It is demonstrated that the results of firing tests can be used for estimating the operation performance of a large-size motor. A simple analytical dependence is proposed for recalculating the results of model tests on material erosion to full-scale conditions.

This paper deals with the problem of steady-state creep of a rectangular membrane in a rigid matrix with a ratio of the height to width of not less than 0.5 in the case of piecewise constant dependence lateral velocity over time. The creep of the membrane with a single change in the rate of transverse pressure over time is investigated. Two variants of the contact conditions of the membrane and matrix: perfect slip and no-slip are considered. Three stages of creep of the membrane are investigated. The analysis is carried out until the time at which the membrane is in almost complete contact with the walls of the matrix. Time dependences of the variable thickness of the membrane and the stress intensity in the membrane are obtained. It is shown that in this problem, the rule of summation of partial times is not performed.

A problem of compression of an isotropic medium with a periodic system of variable width slits compared with elastic displacements is under consideration. The edges of the slits are under the action of internal pressure. It is assumed that the ends of the slits have zones of plastic deformation. The presence of several zones of contact of the edges of each slit is studied in the assumption that one of the section may have slippage. The problem of equilibrium of the periodic system of slits with partially contacting edges under the action of a compressive load reduces to the problem of linear coupling of analytical functions. Contact stresses and the sizes of contact zones, adhesion zones, and end zones of plastic deformations are determined.

The problem of determining the transversely isotropic tensor closest in Euclidean norm to a given anisotropic elastic modulus tensor is considered. An orthonormal basis in the space of transversely isotropic tensors for any given axis of symmetry was obtained by decomposition of a transversely isotropic tensor in the general coordinate system into an isotropic part, two deviator parts, and a nonoric part. The closest transversely isotropic tensor was obtained by projecting the general anisotropy tensor onto this basis. Equations for five coefficients of the transversely isotropic tensor were derived and solved. Three equations that are stationary conditions were obtained for the direction cosines of the axis of rotation (symmetry). Solving these equations yields the absolute minimum distance from the transversely isotropic tensor to the given anisotropic elastic modulus tensor. The transversely isotropic elastic modulus tensor closest to the cubic symmetry tensor was found.

The Lord-Shulman theory of generalized thermoelasticity based on a memory-dependent derivative is employed to study the propagation of plane harmonic waves in a two-dimensional semi-infinite thermoelastic medium. The numerical solution is analyzed for various values of the time delay parameter.

This paper describes a problem of the theory of elasticity for a three-layer material in which two layers located symmetrically relative to the central layer have identical properties. The central layer has a normal detachment crack whose tops are located in the neighboring layers. The solution of this problem is reduced to solving a system of Fredholm integral equations of the second kind. A stress intensity factor for normal detachment cracks is determined, and its dependence on the problem parameters is investigated.

The purpose of this study is to prepare specimens of 10 and 20% nano Al₂O₃ particle reinforced LM25 metal matrix composites (MMCs) by stir casting. Another goal is to develop models using the response surface methodology (RSM) approach for predicting the surface roughness parameters and cutting force components during machining of the MMCs by a CBN7020 tool. With the help of the model developed, comparisons of 10 and 20 wt. % nano Al₂O₃ particle reinforced LM25 MMCs are performed. Combined effects of three cutting parameters (cutting speed, feed rate, and depth of cut) on the surface roughness parameters and cutting force components are explored by using the analysis of variance (ANOVA). The resultant values of the parameters are found to agree well with available experimental values.

This paper describes the simplest cases of a rectangular cantilever beam loaded by an concentrated force that is distributed by the load or dead weight. There are five geometric parameters whose optimal values can be calculated on the basis of five requirements: rigidity (or concentrated elastic energy), strength, equal strength, ratio of sectional sizes, and resistance to the shear force (strength condition in interlaminar shear). The parameter range is determined, outside of which the linear beam theory cannot be used to correctly calculate the bending of equal strength beams loaded by distributed forces or dead weight.

The main regularities of the degradation of the strength properties of structural materials (metals and alloys) resulting in a decrease in long-term strength were investigated. A mathematical model describing the viscoplastic deformation and damage accumulation in polycrystalline structural alloys in creep was developed using the equations of the mechanics of damaged media. The paper presents the results of experimental studies of the short-term creep of 12Kh18N9 steel and VZh-159 high-temperature alloy at constant temperatures and different stresses set in the samples and the results of experimental studies of creep recovery in some structural steels. The deformation and damage accumulation were studied numerically and the results were compared with the data of field experiments.

Four brittle materials are subjected to shock tests: gabbrodiabase, two modifications of cast iron, melted quartz, and beryllium. An interferometer is used to locally probe a free surface in order to calculate its velocity, thereby determining the criterion of the initial stage of brittle fracture of all materials tested - the formation of horizontal steps at the forefront of a compression pulse, which indicate the presence of local sources of fracture of the material. It is determined that the brittle material resists the shock compression as long as it has the reversible transfer of momentum and energy of the processes occurring at meso- and macrolevels.

Based on the elastic theory and the seepage flow theory, a new analytical solution with consideration of the seepage force is proposed to determine the external stress on the lining of a circular TBM tunnel. According to this solution, the relationships between the permeability coefficient of rock masses and the maximum allowable drainage flow are studied. The influence of the controlled drainage flow and the elastic modulus of surrounding rocks on the external stress is discussed. Moreover, in order to validate the results obtained from the elastic analytical solution developed in this paper, a comparison between the results obtained from the solution considering the seepage force and that without consideration of the seepage force is performed.