Vol 60, No 3 (2019)

The energy capabilities of acceleration devices are theoretically analyzed, including various ramjet-in-tube options for gas-dynamic acceleration of massive (1 to 40 kg) projectiles under ground conditions up to velocities of 2–3 km/s. Simple quasi-one-dimensional models for a perfect gas are used in the computations. It is demonstrated that the use of a ramjet-in-tube accelerator with a closed exit allows the velocities of acceleration of massive projectiles to be increased to 3 km/s, which is twice greater than the values that can be obtained with available gunpowder-based methods.

The influence of a sudden change in the external magnetic field on the flow of an electrically conducting Bingham fluid in a two-dimensional channel is considered. It is demonstrated that a method of independent descriptions of the flows in plastic and viscous regions can be used for studying magnetohydrodynamic flows of the Bingham fluid. An exact equation is derived for the position of the plastic flow region boundary as a function of time and magnetic field induction. It is shown that the corresponding Cauchy problem has a unique asymptotically stable solution. Results of numerical integration for some values of parameters are presented; these result confirm the qualitative conclusions.

A mathematical model for reducing the pressure of natural gas in a direct acting pressure regulator is developed. It is shown that, if an outlet-inlet pressure ratio typical for practice is lower than a critical value, the pressure reduction process comes down to two stages: partial expansion of gas with throttling in the orifice plate of the pressure regulator and subsequent expansion in its body. The values of the gas temperature in the characteristic sections of the pressure regulator are determined. It is established that the temperature drops at the first stage of pressure reduction and rises as the flow in the pressure regulator body expands and decelerates. A method for minimizing the possibility of formation of gas hydrates in the pressure regulator is proposed.

The problem of waves generated in a fluid and an ice sheet by a pressure region moving on the free surface of the fluid along the edge of the semi-infinite ice sheet is solved using the Wiener–Hopf technique. The load applied in some region simulates an air cushion vehicle, and the ice sheet is modeled by a thin elastic plate of constant thickness on the surface of an ideal incompressible fluid of finite depth. In a moving coordinate system, the plate deflection and the fluid elevation are assumed to be steady. The wave forces, the elevation the fluid free surface, and the deflection and deformation of the plate at various speeds of the load are investigated. It is found that at near-critical load speeds, the ice sheet has a significant effect on the wave forces (wave resistance and side force) acting on the body moving on the free surface, and this effect is most pronounced at small distances from the edge. It is shown that for some values of the speed, ice thickness, and load pressure, breaking of the ice sheet near the edge is possible.

During the recently passed last few years, viscous flows due to continuously shrinking surfaces have become very much popular among the researchers working in this particular area. Based upon the literature published over these years, it has been established that, different from the continuous stretching surface case, the flow due to a continuous shrinking surface does not admit a meaningful solution in the absence of sufficient wall suction and does admit multiple solutions if a sufficient amount of wall suction velocity is introduced. Furthermore, it has also been believed that shrinking surface flows offer more nonlinear phenomena by exposing the “interesting” characteristics of the boundary-layer flow. Using a correct self-similar formulation for the two-dimensional shrinking sheet flow, the objective of this study is to prove that all the so-called “interesting” features of the shrinking sheet flow discovered in the previous studies are also exhibited by the stretching sheet flow. This fact consequently negates all such fascinations attributed to the shrinking sheet flow.

A numerical experiment has been carried out to study the influence of the change in the main pipeline cross-section due to hydrate formation on hydraulic resistance and the temperature and pressure dynamics taking into account quasi-stationary heat transfer with permafrost ground. The case where a wet gas is supplied to the pipeline is considered, and the dynamics of hydrate formation is determined along with other parameters. The calculations are carried out until the outlet pressure becomes lower than the standard one. The results of the experiment show that a model assuming a constant hydraulic resistance coefficient leads to a significant underestimation of the allowable pipeline operation time. Consequently, in mathematical modeling of hydrate formation in natural gas pipelines taking into account the relationship between heat transfer and viscous friction is critical.

This paper presents the theoretical description of steady motion of a moderately large spherical aerosol particle in the external temperature gradient field in the Stokes approximation with Reynolds and Peclet numbers much smaller than unity. It is assumed that the average temperature of the particle surface significantly differs from the temperature of its gaseous environment. Gas dynamics equations are solved with account for the power dependence of molecule transport coefficients (viscosity and thermal conductivity) and the density of the gaseous environment on temperature. Boundary conditions are written in the linear approximation based on the Knudsen number. It is shown that the thermophoretic force and velocity substantially depend on the Knudsen number and the average temperature of the particle surface.

Experimental dependences of the strength of frozen sandy soil on strain rate are analyzed using a structural–temporal approach. Results of dynamic uniaxial compression tests at a temperature of −18°C and strain rates of 400 to 2600 s⁻¹ of frozen sandy soil specimens of two types with an ice mass fraction of 10 and 18% measured at room temperature are presented. The strain rate dependence at different freezing temperatures of frozen sand with an ice mass fraction of 30% is studied using known experimental data.

This paper proposes a refined formulation of linearized problems of internal nonuniformly scaled flat buckling modes of a rigid lamina consisting of fibers and fiber bundles with allowance for their interaction with the surrounding matrix. Fibers are the structural elements of fibrous composites and in a prebuckling (unperturbed) state under the action of shear stresses and tensile (compression) stresses in the transverse direction. The problems are formulated using equations constructed by reducing the version of geometrically nonlinear equations of the elasticity theory to one-dimensional equations of the theory of rectilinear rods. These equations are based on the use of the refined Timoshenko shear model with allowance for tension-compression strains in the transverse direction for the rigid lamina and the transverse-soft layer model with immobile boundary planes in a perturbed state for the epoxy layers. It is shown that loading samples with a structure is accompanied by constant changes in the composite structure due to implementation and alternation of the internal buckling modes with a varying wave formation parameter. This particularly allows explaining the changing of the effective shear modulus of the fibrous composite with increasing shear strains.

A size-dependent cracked Timoshenko beam model is established based on the nonlocal strain gradient theory and flexibility crack model. Expressions of the higher-order bending moment and shear force are derived. Analytical expressions of the deflection and rotation angle of the cross section of a simply supported microbeam with an arbitrary number of cracks subjected to uniform loading are obtained. The effects of the nonlocal parameter, the material length scale parameter, the presence of the crack, and the slenderness ratio on the bending behaviors of the cracked microbeam are examined. It is found that the material length scale parameter plays an important role in the cracked microbeam bending behavior, while the nonlocal parameter is not decisive. Furthermore, the cracked microbeam also exhibits a stiffening or softening effect depending on the values of the two scale parameters; if the two parameters are equal, the bending deformation of the nonlocal cracked microbeam may not be reduced to that of the classical elastic cracked Timoshenko beam. Additionally, the influence of the size effect on beam stiffening and softening becomes more significant as the slenderness ratio decreases.

A prototype of a pulsed X-ray machine with an operating voltage of 600–800 kV based on a combined spiral generator has been manufactured and tested. Compared to the classical spiral generator, the total length of the spiral winding is increased by adding one-line winding coils in order to match the wave travel time along the spiral generator line to the oscillation time in the generator according to the Tesla model. As a result, a high efficiency was obtained in this modification of the transformer. A theoretical model describing the operation of the combined generator is proposed.