Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2020 number 2

A slow laminar flame which is a thin (less than 1 mm) gas layer separating media of different densities is proposed as the inerface between gaseous media for laboratory studies of Richtmyer-Meshkov and Rayleigh-Taylor instabilities. The potential of the proposed approach to produce an interface is shown by the example of shock-wave interaction with a laminar flame in a lean (6 vol. %) hydrogen-air mixture. The development of Richtmyer-Meshkov instability at the interface between a heavy (cold) and a light (hot) mixtures was recorded by shadowgraphy.

This paper describes results of an experimental and numerical study of a magnetic-hydrodynamic (MHD) method for controlling a hypersonic ( M = 6) airflow in which a launched device of typical configuration is located. The experiments are carried out using an MHD testbed based on a shock tube. The flow in front of the solid is ionized using an electric discharge in an external magnetic field with an induction B =0.80-1.58 T. Conditions corresponding to the experimental conditions are numerically simulated using the Reynolds-average steady Navier-Stokes equations. The MHD interaction region is simulated by isolating a zone in front of the blunt part of the model with given force and heat sources. It is shown that, as a result of strong MHD interaction, the head jump moves away from the model surface and the heat flux to the body decreases with a value of the Stuart number S =0.1-0.3.

Stretching and breaking of shaped-charge jets and the behavior of the strained metal are studied with the use of dislocation mechanisms of plastic straining and with due allowance for shock wave processes at the stage of liner implosion and jet stretching. Regular features of shaped-charge jet stretching and breaking are found. Equations are derived for analyzing the influence of the strength and plastic properties of the material, its microstructure, geometric parameters, and kinematic characteristics on jet stretching and breaking.

To improve the penetration performance and distribution homogeneity of multi-explosively formed penetrators (MEFPs), the factors that affect their configuration, velocity, and distribution characteristics are studied using a three-dimensional LS-DYNA coupled hydro-code. Three different kinds of integral MEFPs are tested in experiments. The results show that the arc plane liner of uneven thickness is easier to form MEFPs with a large length to diameter ratio. Tantalum and copper MEFPs have good penetration performance due to a higher length to diameter value and kinetic energy. The arc bracket and the equidistant array pattern of the liners are beneficial for obtaining better distribution homogeneity of MEFPs.

A group analysis of the second-order equation including one-dimensional gas dynamics equations in Lagrangian coordinates as a particular case is performed. The use of Lagrangian coordinates makes it possible to consider one-dimensional gas dynamics equations as a variational Euler-Lagrange equation with an appropriate Lagrangian. Conservation laws are derived with the use of the variational presentation and Noether theory. A complete group classification of the Euler-Lagrange equation is obtained; as a result, 18 different classes can be classified.

In this study, an artificial neural network (ANN) is used to develop predictive models for estimating molecular diffusion coefficients of various gases at multiple pressures over a large field of temperatures. Two feed-forward neural networks NN1 and NN2 are trained using six physicochemical properties: molecular weight, critical volume, critical temperature, dipole moment, temperature, and pressure for NN1 and molecular weight, critical pressure, critical temperature, dipole moment, temperature, and pressure for NN2. The diffusion coefficients are regarded as the output. A set of 1252 gases (941 non-polar and 311 polar gases) is used for training and testing the ANN performance, and good correlations are found (R =0.986 for NN1 and R =0.988 for NN2). The result of the sensitivity analysis shows the importance of the six input parameters selected for modeling the diffusion coefficient. Moreover, the present ANN model provides more accurate predictions than other models.

This paper describes a numerical simulation of an unsteady axisymmetric flow in a shock tube, which arises as a strong shock wave incident on the end face of the tube is reflected under conditions in which a chain of shock waves is formed after the reflected shock wave, similar to a pseudo-jump in steady flows. The conditions under which this flow is formed are understudied. The flow is investigated in the case of the shock tube in which an incident shock wave is formed and moving at a velocity corresponding to the Mach number M =2.6. The numerical calculation is carried out using the Reynolds-averaged Navier-Stokes equations and the SST-(k-ω) model of turbulence. The resulting data are compared with the known experimental data and the results of numerical calculations.

Results of experimental and numerical modeling of supersonic under expanded jets exhausting into a slotted submerged space formed by two parallel disks are presented. It is demonstrated that the structure of the exhausting off-design jet depends on the distance between the disks. The friction force acting from the disks on the gas is responsible for significant changes in the supersonic jet structure. At a certain distance between the disks, there arises a curvilinear shock wave, followed by a subsonic flow region. In this case, the jet boundary acquires a fan-shaped structure rather than a typical barrel-shaped structure.

An axisymmetric laminar swirling jet of a viscous incompressible fluid flowing from a rotating semi-infinite tube in space filled with the same fluid is explored. The inner surface of the tube rotates with a constant angular velocity, the outer surface is stationary or rotates with the same angular velocity. It is shown that in the first case, the flow field far from the tube orifice is described by the Loitsyansky asymptotic solution, and in the second case (with a weak coflow flow), it is described by the Long-Goldshtik-Zoubtsov self-similar solution. The Goldshtik hidden invariant is generalized to arbitrary axisymmetric swirling jets, and its influence on the jet asymptotics is studied. Strong swirling jets are calculated, and the dependence of the parameters of the recirculation zone (vortex breakdown in a swirling jet) on the swirl number and the Reynolds number is examined.

A new experimental method for panoramic contactless determination of the laminar-turbulent transition position in a three-dimensional boundary layer is described. It is demonstrated that this method allows accurate determination of the transition region boundaries at both reduced and elevated degrees of free-stream turbulence.

A model of the motion of a granular medium is proposed which is based on the elastic interaction of particles, each of which is an absolutely solid body (a disk in the flat case and a ball in the spatial case) of mass m and radius r , surrounded by a sufficiently thin elastic shell. Algorithm for solving the formulated problem is proposed. At each step, the forces and moments of forces arising from the interaction with neighboring particles are calculated. The Cauchy problem for the resulting system of differential equations is numerically solved. Based on the mathematical model, a set of programs has been created that similates the behavior of granular media in moving containers.

A percolation model for the reinforcement of polymer nanocomposites is proposed which allows a quantitative evaluation of the degree of aggregation of nanofillers of arbitrary dimension. When using this model, the efficiency of a nanofiller is determined not by its initial characteristics, but by its ability to generate interfacial regions.

Based on the nonclassical theory of shells, an energy-consistent resolving system of dynamics equations for cylindrical shells made of metal-plastic is obtained as a result of minimization of the total-energy functional of a shell as a three-dimensional body. The numerical method for solving the formulated initial-boundary-value problem is based on an explicit variational-difference scheme. The validity of the method is confirmed by the results of comparison of numerical solutions and experimental data. The ultimate deformability and strength of homogeneous fiberglass cylindrical shells and double-layer metal-plastic shells are anlyzed for various reinforcing structures.

A local strain-stress state in the vicinity of a joint, which arises due to the wavelike structure of this joint, is studied. This thermoelastic problem is solved using a two-scale method.

This paper describes a stress state of membranes of variable thickness at large deformations, namely, the deformation of round continuous anisotropic and isotropic membranes with an initial variable thickness, which are under the action of a uniformly distributed load. It is assumed that the membrane materials are elastic, and generalized Hooke's law is used to describe their behavior. This problem is solved using the equation of equilibrium of the membrane element is used. True principal strains are expressed through dimensionless radial, annular, and normal stresses. An equation is obtained that describes the shape of the membrane after deformation, and the corresponding boundary conditions. Dimensionless stresses and the membrane shape after deformation are determined. Numerical calculations are carried out for various parameters of the problem.

Propagation of a semi-elliptical crack on the suction surface of a rotating blade of a gas turbine compressor is modeled and analysed. The analytical solution for the stress intensity factor (SIF) at the deepest point of the crack shows that the SIF increases with an increase in the crack depth and rotation speed, whereas the SIF decreases with an increase in the semi-elliptical aspect ratio and crack-rotation axis distance. The SIF increase rate with increasing crack depth is found to depend on the aspect ratio. Comparisons of theoretical and numerical calculation results reveal a relative error within 4%. The SIF distribution along the semi-elliptical surface crack front at different depths, positions, and aspect ratios are obtained by a numerical method.

A generator design with a pulse energy of more than 1 MJ connected to an external constant voltage source with limited current strength and power is proposed. The power supply of an active-inductive load is calculated numerically and analytically. Generator parameters for pulse-frequency operation are determined. Results of an experimental study of a small-sized transformer generator are given that confirm the efficiency of this generator and the validity of the method for calculating its characteristics.

This paper describes results of an experimental study of strength characteristics and a microstructure of a permanent joint of dissimilar materials (VT20 titanium alloy and B-1461 aluminum alloy), obtained by laser butt welding. Optical and electron microscopy, the results of X-ray spectral and phase analysis, and the transmission diffraction method with the help of synchrotron radiation are all used to investigate the properties of a welded joint. It is shown that the displacement of laser radiation relative to the contact surface of dissimilar materials significantly changes the microstructure and phase composition of the intermetallic interlayer formed between the materials welded, thereby affecting the strength of dissimilar welded joints.