Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2019 number 4

The Maxwell equations for a composite two-component layered material with a periodic structure in the field of a time-harmonic source acting along the layer are considered. Two-scale homogenization of the equations is performed with allowance for complex conductivity of interphase layers and their thickness. The boundary-value problem for systems of differential equations with boundary conditions is reduced to a problem in a weakly variational formulation. Unique solvability of the problem is established. The case of low frequencies of interphase surface currents of different intensities with allowance for the frequency-dependent wave length and skin layer length is analyzed. Macro-equations are derived, and effective material constants are determined, such as the magnetic and dielectric permeabilities and electrical conductivity. Conditions at which the effective parameters depend on interphase currents are described. It is found that the effective dielectric permeability can be negative at specially chosen parameters of interphase layers, if the effective dielectric permeability is determined on the basis of the effective wave number.

The rotational motion of finned shaped-charge liners is studied. It is shown that the main factors affecting the rotation of a shaped-charge jet are the position of the jet-forming layer and the distribution of the circumferential velocity in the liner cross-section. The proposed method can be used to estimate the average angular velocity of the jet with consideration of the size of the finned shaped-charge liner and the properties of its material. It is shown that the results are consistent with available experimental data.

A thermodynamically equilibrium model is applied for simulating thermodynamic parameters of shock loading of both pure materials and mixtures of homogeneous and porous materials. The model includes a modified equation of state, which has only one fitting parameter determined on the basis of experimental data. The thermodynamic parameters of shock loading of copper and copper-based mixtures with porosities of 1-10 at pressures above 5 GPa are calculated. The results of these calculations are compared to available experimental data (Hugoniot adiabats, double compression by shock waves, and temperature estimates). The possibility of modeling the compression of the mixture as a whole and each component separately is demonstrated.

Results of studying stability of the Blasius boundary layer on a two-layer compliant coating in the linear formulation are reported. The computations are based on experimental parameters of viscoelasticity of a real coating, which reveal the dependences of its elasticity modulus and loss coefficient on frequency. Parametric investigations of the influence of the coating layer thicknesses and free-stream velocity on flow stability, in particular, on the critical Reynolds number, are performed. Regions of a nonmonotonic behavior of the critical Reynolds number are found, which allow one to determine the optimal thicknesses of the upper and lower layers for intense interaction with the flow. An explanation of this effect is proposed.

Starting of an air-breathing engine with fuel injection distributed along the combustion chamber is numerically simulated. Issues of principal importance are the presence of a compressed are jet generating a throttling effect and preliminary deceleration of the flow to transonic velocities. Averaged Navier-Stokes equations closed by the SST, SST κ-ω, or κ-ε turbulence models are solved. Hydrogen and ethylene combustion is simulated by one reaction. The computations are performed for various values of the turbulent kinetic energy in the flow. A pulsed transonic regime of hydrogen and ethylene combustion is discovered.

This paper considers the possibility of using measuring sections with inductive sensors to detect hypersonic particles in time in the simulation of collisions of objects with space debris. A method for determining the average velocity of hypersonic particles simulating the space debris and a method of starting the recording equipment for optical contactless detection of these particles are proposed. Results of experimental studies of the interaction of particles with a simulator of the mesh shield of the spacecraft.

The influence of the shape of the jet head on its impact on a wall covered with a thin layer of liquid has been studied. The conditions characteristic of the impact of the jet arising on the surface of a cavitation bubble upon its collapse near the wall have been considered. It has been established that changing the shape of the jet head can lead to a significant change in the size of the area of maximum loading of the wetted wall and in the magnitude and pattern of loading. In particular, with an increase in the degree of sharpening of the jet head, the pressure on the wall decreases and its spatial distribution becomes more uniform. Dependences of the maximum pressure and integrated wall load on the jet shape were obtained.

The influence of the dimensionless angular and linear parameters of interaction of water droplets shaped as a sphere, an ellipsoid, and a conventionally liquid disk on the characteristics (regimes) of their collisions in air (rebound, coagulation, spreading, or fragmentation) is studied by using a system of high-speed video recording. Conditions of stable realization of this interaction are determined. Charts of the corresponding regimes are constructed and compared with available data. The characteristic sizes, the number of liquid fragments formed in collisions, and total areas of the evaporation surface are calculated. It is demonstrated that the liquid surface area in the case of collisions of conventionally liquid disks is significantly (by several times) greater than that in the case of spherical droplets.

Three-dimensional deformation of two bubbles and bubbles in a cluster in an ideal incompressible fluid exposed to an acoustic field is investigated using the boundary element method for potential flows. The dependence of the dynamics of two interacting bubbles on the frequency and amplitude of the acoustic field and the distance between them is studied. The values of the parameters of the acoustic field and the cluster for which jets are formed and the bubbles are deformed or remain spherical are determined. The behavior of two central bubbles in a structured cluster exposed to an acoustic field with different frequency and amplitude is investigated as a function of the distance between the bubbles of the cluster. A comparative analysis of the deformation of the investigated bubbles in the presence and absence of adjacent bubbles is performed.

Theoretical model describing the movement of particles of saline water in oil flowing over a larger droplet of fresh water has been developed to solve the problem of cleaning oil from salts. The results of calculations of the effect of the radius and the initial velocity of a drop of fresh water on the coagulation of fresh and saline water droplets are presented.

The nonstationary process of extracting material from a porous body simulated by a system of semi-infinite capillaries into a moving liquid is studied; the transfer rate of the material in the flow is a linear function of the cross flow coordinate. The case where the liquid velocity at the interface becomes zero is considered. It is assumed that the diffusion in the flow is quasi-stationary. Analytical dependences at the boundary of the porous material and the flow region are found for mass transfer characteristics of practical interest (concentration, diffusion flow, total diffusion flux and the total yield of the target component through the cross section of the porous body).

Stress relaxation in a nano-sized rod containing structural defects in the course of constant-rate uniaxial straining is studied, and the reasons for the onset of this phenomenon are determined. Under the assumption that structural defects can serve as carriers of irreversible deformation of a higher level than dislocations, the problem is solved by the molecular dynamics method. It is found that stress relaxation is accompanied by the transition of the entire system to a steady state with a deeper potential minimum as compared to the system energy before the stress relaxation process, resulting in a temperature increase and reduction of the strain tensor components.

The stress state in the neighborhood of the common edge of a freely supported composite wedge-shaped plate under transverse load has been studied. The materials of the constituent plates are cylindrical orthotropic. The problem is solved using the classical theory of anisotropic plates. Equations are derived for the hypersurface which, in the space of physical and geometric parameters, is the boundary of the region of parameters for which the neighborhood of the common edge of the composite plate is in a low-stress state (the stresses at the points of the common edge of the composite plate are limited).

A variational principle is used to study the oscillations of a reinforced thin cylindrical shell with a filler under axial compression with allowance for friction on their contact surface. The dependences of frequencies of natural oscillations on the number of waves in a circumferential direction are constructed.

Nonlinear elasticity model in current state variables is used to study the plane strain dynamics of an incompressible body, and a system of nonlinear equations of displacement and pressure is obtained. In the absence of bulk forces and in the assumption of quadratic elastic potential, the resulting equations that describe plane waves, self-similar motions, and stress fields in these motions are solved and studied. It is shown that several solutions of a certain type are possible both in wave and self-similar cases.

The dynamic buckling of an elastic cylindrical shell with a general boundary condition (composed of stiffness and damping) under an axial impact by a rigid body is considered. A dynamic equation is derived to obtain the axial stress and radial displacement of the shell. Then, by substituting the results into the energy equation, the critical condition for the dynamic buckling of the shell is obtained. The influence of the general boundary condition on the critical velocity of the impactor is analyzed. The results reveal that the boundary condition exerts no effect on the dynamic buckling of the shell before the stress wave becomes reflected from the fixed end face of the shell. After reflection, the critical velocity decreases with increasing impactor mass and stiffness, but increases with increasing damping. At times smaller than the instant when the stress wave reaches the fixed end face of the shell, the dynamic buckling occurs earlier at greater values of damping and stiffness. After stress wave reflection, the earlier dynamic buckling is observed at smaller values of damping and stiffness.

Ballistic experiments, numerical simulations, and theoretical model investigations of the penetration performance of homogeneous and jacketed rods into a semi-infinite target are presented. The striking velocities vary between 0.9 and 3.3 km/s. The effects of the jacket material, striking velocity, and initial kinetic energy on the penetration performance and damage mechanisms are analyzed. The results show that jacketed rods provide better penetration performance than homogeneous rods with the same initial kinetic energy. For a fixed ratio of the jacket radius to the core radius, it is preferable to use a jacket material with a lower density and strength that can provide the lowest required flexural stiffness.

Synthesis of a composite of powder mixtures is experimentally studied in a thermal explosion. A mathematical model for initiating a reaction that allows for the main physical and chemical phenomena changing the phase composition of a compact is formulated. It is shown that simulation results qualitatively agree with experimental data.

A simplified two-dimensional model of a lapped adhesion bond is proposed. The problem of a stress state of an adhesion bond, along the surfaces of which nonglued regions are located, is solved analytically in the assumption that the cross-sectional displacements of carrier layers equal zero. The resulting solution is a functional series, with eigenfunctions being nonorthogonal. It is shown that the presence of nonglued regions may significantly increase stresses near the edge of the adhesion layer.

The theory of large deformations is used to solve a problem of an elastic-viscoplastic material placed in a gap between two coaxial cylindrical surfaces, with one of them rotating with an alternating velocity and the other one at rest. It is shown that an increase in the stresses in a cylindrical layer due to a mechanical influence on it initially causes irreversible creep strains because of viscosity of the material and then the accumulation of plastic strains because of the arrival of stress states at the loading surface. The unloading is accompanied by a plastic strain and then a viscous one. The stress-strain parameters of the medium with a varying rotation velocity of the cylinder are calculated. Stress relaxation after its complete stop is described.

The influence of the laser beam parameters (power, motion velocity, and focus position) on the characteristics of the track being formed (size, elemental composition, and microhardness) is studied. If the difference in the laser radiation absorption coefficients in the heat conduction and “knife-like” regimes is taken into account, then the track sizes can be determined by a unified dependence on the energy parameter. The effect of the laser beam on the chemical composition and microhardness of cermet (WC-NiCrBSi) tracks is studied. Regardless of the track formation regime, these parameters are determined by the dimensionless parameter, which describes the degree of dilution of chemical substances. It is found that a track with the maximum mass fraction of tungsten and the greatest value of microhardness is formed at small values of the dimensionless parameter, which corresponds to the heat conduction regime. The microhardness of the deposited cermet structure is observed to be 4-5 times higher than the microhardness of the substrate material.

Acceleration of projectiles in ballistic devices by means of high-rate extrusion of polyethylene through a conical nozzle is experimentally and theoretically studied. Quasi-one-dimensional gas-dynamic model of a polydisperse mixture of gas and powder particles and a viscoplastic model of a deformed piston are used. The results of numerical simulation of the process under study and the results of a series of shots from a powder ballistic device are given. The influence of the geometry of a conical transition and input velocity of the deformed body on the output velocity of a projectile is experimentally investigated.

Metal shields of various thickness made of various steels and used to protect the shells of explosion chambers from damage by debris and reduce the loads on the structural elements have been studied. Dependences of the equivalent stresses in the protective shields and shells explosion chambers and the final size of the gap between the shields and the shell on wall thickness, steel grade, and initial gap size were obtained by numerical simulation. Examples of the explosion chamber designs with protective shields are given.