Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2018 number 3

A numerical investigation of the properties of two different ion-acoustic solitons has been performed by using analytical solutions of the Schamel equation. A new type of the analytical solution of the Schamel equation that describes soliton propagation with a negative phase velocity has been found for the first time. This new type of the solution has been applied to investigate the physical properties of two different plasmas and to understand the effect of nonextensivity and the effect of trapped electrons on ion-acoustic waves in a superthermal plasma. It is shown that the soliton amplitude and width depend on the nonextensivity parameter, superthermality of the electron distribution, and characteristic trapping parameter.

A circuit of a multicascade magnetocumulative generator based on a dynamic variation of the coupling coefficient of inductively coupled circuit is proposed. Each cascade contains two circuits including two pairs of inductively coupled coils. One pair of coils is subjected to simultaneous straining, and one of the coils in the other pair is arranged with the back-to-back connection. It is shown that the energy in the load can be gradually increased (from one cascade to another) by using additional cascades. By an example of a two-cascade system, the proposed circuit is compared to the known circuits of cascade system design based on the magnetic cumulation principle (generator with a step-up transformer and dynamic transformer). Within the framework of the model that ignores the ohmic resistance of conductors, it is demonstrated that the proposed circuit allows one to obtain a greater energy in the high-inductance load than the circuits with the step-up or dynamic transformer owing to a change in the sign of the magnetic flux in the secondary circuit. It is found that the increase in energy in the new circuit is independent of the coupling coefficient (at high values of this coefficient) and become greater as the number of cascades is increased.

Rotation bifurcation in a steady axisymmetric thermocapillary flow of an incompressible fluid filling a semi-infinite space bounded by the free surface with a nonuniform distribution of temperature is studied. The fluid flow is calculated on the basis of Navier-Stokes equations under the assumption of small diffusion coefficients. It is shown that the bifurcation triggers a rotational regime in a thin Marangoni boundary layer in the case of local cooling of the free boundary near the axis of symmetry and in the presence of an external flow; there is no rotation outside this layer. In the case of local heating of the free boundary, rotation is not observed

This paper touches upon an initial-boundary-value problem that describes the unidirectional thermogravitational motion of fluid in a plane channel in the case of solid immobile upper and lower walls with temperature distribution thereon and in the case of a heat-insulated upper wall. The motion is caused by a joint effect of the longitudinal temperature gradient and given nonstationary flow rate. The initial-boundary-value problem is inverse relative to the pressure gradient along the channel. An exact stationary solution is obtained. A solution of the nonstationary problems in Laplace images is determined, and the results of numerical calculations are presented.

The problem of injection of a hydrate-forming gas (methane) into a snow block, whose pores in the initial state are saturated by the same gas. Self-similar solutions describing the temperature and pressure fields and the snow, hydrate, and gas distributions in the block are constructed. It is shown that three characteristic zones can be distinguished, depending on the initial thermobaric state of the snow-methane system and the rate of gas injection into the filtration region: a near zone, in which snow completely is converted into hydrate and, consequently, the hydrate layer is saturated with gas; an intermediate zone, in which gas, snow, and hydrate are in phase equilibrium; a far zone filled with gas and snow. It is shown that the length of the heated zone decreases with increasing initial snow content in the block and with decreasing injected gas pressure. It is also shown that the length of the region of hydrate formation increases with increasing permeability. It is also noted that the heating of the intermediate zone occurs more rapidly.

A quasi-hydrodynamic system of equations describing flows of a heat-conducting viscous compressible multiphase multicomponent fluid is constructed taking into account surface effects. The system was obtained by generalizing the methods of obtaining a single-phase quasi-hydrodynamic system and a multicomponent flow model taking into account the surface effects based on the concept of microforces and microstresses. The equations are derived using the Coleman-Noll procedure. The results of the calculations show that the constructed model is applicable for modeling multiphase multicomponent flows with allowance for surface effects on the interfaces.

The effects of the camber ratio on the hydrodynamic and structural behaviors of a NACA-based ducted marine propeller in the wake flow behind an underwater axisymmetric body are numerically studied by computational fluid dynamics methods, in particular, the finite element method. The results are presented in terms of the efficiency, deflection, pressure coefficient, and natural frequencies. It is shown that the wake flow strongly affects the performance of the selected propulsion system. It is shown that the distributions of the camber ratio over the blades of the propeller nonlinearly changes its resistance against cavitation occurrence and deflection, and also changes its hydrodynamic performance and vibrational behavior.

This paper considers methods for controlling secondary flows arising near an oscillating circular cylinder by changing two process control parameters: the dimensionless amplitude and the vibrational Reynolds number. A direct numerical modeling study is performed. It is shown that by varying the indicated parameters in a relatively small range, it is possible not only to intensify mass transfer processes, but also change the direction of the main secondary flows.

A mathematical model of blood circulation in human lower limbs is proposed. The model is based on the laws of motion (filtration) of a viscous fluid in a heterogeneous medium consisting of two or more interpenetrating continua. It is assumed that the blood system consists of a distribution network of comparatively large vessels (arterioles) connected to small capillaries and a similarly structured collection network of small capillaries united into larger veins. A system of parabolic differential equations is derived, for which a problem with no initial data is posed. A periodic (in time) solution of the system corresponding to harmonic oscillations defined by the cardiac rhythm is found. Analytical solutions for particular cases of problems following from the general model of blood circulation are obtained. Numerical calculations are performed, and a numerical solution is found for a one-dimensional problem with parameters similar to those corresponding to real conditions of blood circulation with allowance for the cross-sectional area of the muscular tissue of the lower limb.

This paper presents the results of numerical modeling of gas hydrate formation upon injection of carbon dioxide into a finite-length reservoir saturated with methane and water. It is shown that at different stages, hydrate formation can occur on both the frontal surface and in a reservoir region of finite length. The effects of pressure on the boundaries of the reservoir and the permeability and initial water saturation of the reservoir on the hydrate formation process were studied. The dependences of the time of the complete conversion of water into gas hydrate state in the entire reservoir on the injection pressure and reservoir permeability were obtained.

Analytical frequency dependences of the absorption coefficient, wavenumber, and phase velocity were constructed for filtration-wave fields in a highly permeable interlayer bounded from above and from below by layers having high permeability in the vertical direction. It is shown that as the frequency decreases, the phase velocity of the wave decreases to values below this velocity in the porous medium, and low-frequency deceleration occurs.

A reverse experiment technique is used along with the technology of measuring rods to study the impact and penetration of a steel conical shocker in frozen sandy soil. This paper presents the dependences of maximum values of the force of resistance of cones with base diameters of 10.0, 12.0, and 19.8 mm to penetration into soil on the impact velocity in the range of values 100-400 m/s. The numerical solution of the problem in an axisymmetric formulation with the use of the “Dinamika-2” software package is used to show the effect of waves reflected from the walls of the container on the contact force. A comparative analysis of the forces of resistance to penetration of the shocker into compacted dry, water-saturated, and frozen sandy soils is carried out.

Within the framework of the anisotropic theory of elasticity, a three-dimensional contact problem of interaction of two massive transversely isotropic bodies, whose dimensions substantially exceed the size of the contact region. In this case, the isotropy planes of contacting elastic bodies are mutually perpendicular. Exact and numerical solutions of the problem are determined. Calculations for various transversely isotropic materials are carried out.

A method of thermal treatment of DI37-VI steel exhibiting precipitation hardening is developed with due allowance for the steel microstructure and determining the chemical composition of carbides and the matrix after each operation. In the case of low-temperature tempering, the mechanical properties of steel are found to be little different from its properties obtained by means of multiple high-temperature tempering (aging); in the latter case, however, the wear resistance of steel in the case of cutting of steel bars is 1.5 times higher. It is demonstrated that the wear resistance of steel is significantly affected by the chemical composition of carbides and the matrix.

A percolation model of nanocomposite reinforcement is under study. It is shown that the degree of reinforcement of polyurethane - carbon nanotube nanocomposites depends on the structure of nanofillers, which are annular formations. This structure is most accurately characterized by its fractal dimension. It is established that the creation of a structure with negative percolation indices allows for a significant increase in the degree of reinforcement of considered nanocomposites at low nanofiller concentrations.

This paper presents the results of experimental studies of damage accumulation in a metal-composite high-pressure tank in pneumatic strength tests. The deformation and fracture of the composite structure, accompanied by matrix cracking and fiber rupture, are analyzed. It is shown that the cracks and fractures generate acoustic-emission signals of various types. The results of acoustic-emission control were used to develop a criterion for ranking tanks based on the strength characteristics of the pressure composite shell.

A technique for identification of hereditary properties under conditions of short-time creep of Plexiglas is developed. The technique is based on experimental determination of shifting of the center of decaying flexural oscillations of vertically fixed test samples after their preliminary curing in a static bent state. Mathematically, the technique is based on using the finite element method and integral equations of the theory of hereditary viscoelasticity with the Koltunov-Rzhanitsyn hereditary kernel. An object function is constructed for identification of rheological parameters of this kernel. The minimum of this function is found by the direct search method, which does not require the function gradient to be known. The hereditary kernel of Plexiglas averaged on the basis of data for several test samples is obtained as a function of time.

A model of the explosive fragmentation of a thin ring was developed which take into account the statistical dispersion of the destructive relative deformation along the length of the ring. A formula is proposed for calculating the speed of motion of the boundary of the region near the plastic rupture in which the plastic flow of the ring material is stopped. Methods of numerical and analytical calculation of the average number of fragments of the ring were developed. The calculation results were compared with known experimental data.

The problem of steady radial vibrations of a thin electroelastic hollow disk in a preliminary inhomogeneous plane stress-strain state is solved. Vibrations are caused by a potential difference at the electrodes placed on the end surfaces of the disk. Equations of vibrations and boundary conditions are formulated. The preliminary stress state corresponding to the solution of the Lame problem was investigated. The direct problem of determining the displacement function is solved numerically by the shooting method. The inverse problem of determining the prestress parameter from the change in the natural frequency of the disk is formulated and solved. The accuracy of determining the prestressed state for initial put data specified with an error is analyzed.

A mathematical model describing the nonstationary creep of metals under complex loading is proposed. The results of numerical simulation of creep of St.304 steel in complex regimes of block multiaxial cyclic deformation are given. The numerical calculation results obtained are compared with the data of full-scale experiments. Creep is simulated in complex deformation processes accompanied by the rotation of main regions of stress, strain, and creep strain tensors.

Laser-assisted welding of the 1420 alloy of the Al-Mg-Li system is experimentally studied for the purpose of its optimization. Various regimes of thermal treatment of non-detachable joints obtained by means of laser welding are considered. Thermal treatment modes that allow significant enhancement of the welded joint density as compared to the as-received alloy are chosen. The ultimate normalized elongation of samples after aging is found to decrease by a factor of 3.

It is shown that a weld seam formed during explosion welding at a microlevel (at a wavelength of the order of several hundred micrometers) has a wavy geometry. Stress concentration caused by local geometry of the seam is calculated for various shapes and amplitudes of the weld seam wave and combinations of joined materials.