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Vol 60, No 1 (2019)
- Year: 2019
- Articles: 22
- URL: https://journals.rcsi.science/0021-8944/issue/view/9803
Article
Influence of an External Air Flow on a System of High-Voltage Asymmetrical Electrodes
Abstract
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.
1-6
Development of the Air Blowing and Suction Technology for Control of a Turbulent Flow on an Airfoil
Abstract
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 Rec = 0.7 · 106 and angles of attack α from − 6 to 6°. 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°. Unfortunately, the efficiency of this method of boundary layer control decreases with increasing α.
7-15
Internal Gravity Waves Excited by a Moving Oscillating Source in a Stratified Medium with Variable Buoyancy
Abstract
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–Väisälä frequency.
16-21
Effect of Different Flows on the Shear Branding of a Liquid with a Non-Monotonic Flow Curve
Abstract
A Couette flow of liquid, described by a modified Vinogradov–Pokrovskii 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 apparent 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.
22-30
Oil Filtration in a Porous Medium in the Presence of Graphene Nanoparticles
Abstract
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 displaced oil increases and the water volume fraction in oil decreases.
31-34
Gas Filtration in a Low-Permeability Formation with a Hydraulically Fractured Well
Abstract
This paper presents the results of numerical calculation of gas filtration in a low-permeability gas formation with a fractured well. It is assumed that in the low-permeability formation and in the finite-conductivity fracture is described by the 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 the “fracture–formation” system and on the dynamics of well pressure and gas production rate during gas well testing.
35-45
Longitudinal Waves in a Saturated Porous Medium
Abstract
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.
46-52
Effect of a Passive Swirler on Superheated Liquid Outflow
Abstract
Experimental studies have been performed to investigate flashing in a superheated water jet discharged from a short cylindrical nozzle in the presence of a passive swirler (twisted tape). The evolution of the jet atomization of flashing liquid with increasing superheat has been analyzed. Characteristic flow structures and shapes have been identified. The correlation between the jet shape and flashing modes has been found. Significant fluctuations and instability of the jet at certain temperatures have been observed. Experiments have shown the possibility of influencing the jet shape.
53-58
Temperature State of a Hollow Cylinder Made of a Polymer Dielectric with Temperature-Dependent Characteristics
Abstract
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.
59-67
Experimental Determination of the Fire-Break Size and Specific Water Consumption for Effective Containment and Complete Suppression of the Front Propagation of a Typical Local Wildfire
Abstract
This paper presents the results of an experimental study of the initiation and containment of the flaming combustion and thermal decomposition of typical forest fuels by creating a fire break in the form of a wet layer of fixed width, length, and depth with a known volume of contained liquid. The ranges of parameter values that ensure stable suppression of the flaming combustion and thermal decomposition of forest fuel material were determined Experiments were performed for all major types of forest fuels: leaves, needles, branches, and mixtures thereof. The minimum water volumes and fire-break sizes required for guaranteed containment of combustion of the investigated forest fuels are predicted.
68-79
Long-Run Testing of Model Nozzle Extensions Made of a Carbon-Carbon Composite Material in a Liquid-Propellant Rocket Engine Operating on Hydrogen and Oxygen
Abstract
Results of a series of firing tests of two model nozzle extensions made of a carbon-carbon composite material are reported. This material can be used to fabricate nozzle extensions for a large-scale liquid-propellant rocket engine. 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 extension is measured in each experiment by thermocouples and an infrared imager. The level of wall material ablation 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-scale engine. A simple analytical dependence is derived for recalculating the results of model tests on material ablation to full-scale conditions.
80-86
Steady-State Creep of a Long Membrane in a Rigid Matrix with a Piecewise Constant Dependence of the Rate of Change in Transverse Pressure on Time
Abstract
The problem of steady-state creep of a rectangular membrane in a rigid matrix with a height-to-width ratio of not less than 0.5 is studied in the case of a piecewise constant dependence of the rate of change in transverse pressure on time. The creep of the membrane with a single change in transverse pressure over time is investigated. Two types of contact conditions between the membrane and matrix are considered: perfect slip and no-slip. Three stages of creep of the membrane are investigated. The analysis is carried out to 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 valid.
87-96
Contact Problem of a Periodic System of Variable Width Slits with Partially Interacting Edges in the Presence of End Zones of Plastic Deformation
Abstract
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.
97-105
Transversely Isotropic Tensor Closest in Euclidean Norm to a Given Anisotropic Elastic Modulus Tensor
Abstract
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 one isotropic part, two deviator parts, and one nonor 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 describing 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 a given anisotropic elastic modulus tensor. The transversely isotropic elastic modulus tensor closest to the cubic symmetry tensor was found.
106-122
Plane Harmonic Waves in Thermoelastic Materials with a Memory-Dependent Derivative
Abstract
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.
123-131
Fracture of a Three-Layer Material with an Opening Mode Central Crack
Abstract
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 an opening mode 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 opening mode cracks is determined, and its dependence on the problem parameters is investigated.
132-135
Production and Machining Performance Study of Nano Al2O3 Particle Reinforced LM25 Aluminum Alloy Composites
Abstract
The purpose of this study is to prepare specimens of 10 and 20%(wt.) 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.
136-143
Analyzing the Correctness of Equal Strength Composite Profiled Beam Bending Problems
Abstract
This paper describes the simplest cases of a rectangular cantilever beam loaded by an concentrated force, distributed load, or dead weight. There are five geometric parameters whose optimal values can be calculated on the basis of five requirements: rigidity (or accumulated 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 deflection of equal strength beams loaded by distributed forces or dead weight.
144-155
Constitutive Relations of the Mechanics of a Damaged Medium for Evaluating the Creep-Rupture Strength of Structural Alloys
Abstract
The main regularities of the degradation of structural materials (metals and alloys) resulting in a decrease in creep-rupture 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 temperature and different stresses set in the samples and the results of experimental studies of creep recovery in some structural steels. Deformation and damage accumulation were studied numerically and the results were compared with the data of full-scale experiments.
156-166
Localized Structural Instability and Dynamic Strength of Brittle Materials
Abstract
Four brittle materials are subjected to shock tests: gabbrodiabase, two modifications of cast iron, fused 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.
167-175
Analytical Solution for the External Stress Acting on the Lining in a Deep-Buried Circular TBM Tunnel Considering the Seepage Field
Abstract
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.
176-185
Erratum
Erratum to: The Model of Electric Connection of a Low-Conductivity Liquid in High-Frequency Electric Field
Abstract
In the original publication, there are several misprints.
1. In page header, the author’s name was misspelled. It should read “Ponomareva” instead of “Ponomarev.”
2. In title, it should read “The Model of Electric Convection of a Low-Conductivity Liquid in High-Frequency Electric Field” instead of “The Model of Electric Connection of a Low-Conductivity Liquid in High-Frequency Electric Field.”
186-186