Journal of Applied Mechanics and Technical Physics最新文献

The group classification problem is solved for equations describing the vibrations of a nonlinear elastic plate in gas flow. Exact solutions of these equations are given which can be used as test solutions in numerical calculations.

The concept of plasma-assisted combustion, which has a series of advantages (reduced ignition time, improved mixing, flame front stabilization), is considered. The work provides the results of a study of the ignition process using a longitudinal direct current discharge of a fuel–air mixture injected at supersonic speed into the core of a supersonic air flow. In order to exclude the influence of mixing, the fuel (ethylene) was premixed with the oxidizer (air). The constructed system for mixing ethylene with air and the system for injecting the resulting mixture into the test section channel are described. For this setup configuration, the results of gas flow simulation in the FlowVision package and the results of experiments on fuel ignition in a supersonic flow are provided.

The initiation of a mixed-type (modes I and II) inclined edge crack in a thin strip of sheet steel under tensile loads is considered. During rolling, the initially isotropic sheet metal typically acquires significant anisotropy characterized by a difference in plastic properties in the rolling and transverse directions. Plastic anisotropy is described using Hill’s quadratic plasticity criterion. The fracture of such materials is described by a modified Leonov–Panasyuk–Dugdale model. Under complex loading, the crack path is bent, and the path bending angle is determined using a force integral failure criterion. In asymptotic representations of the stress components in the vicinity of the crack tip, nonsingular terms ((T)-stresses) are taken into account. A two-parameter strength criterion is proposed to obtain the critical fracture parameters of a strip with an inclined edge crack. The parameters included in the resulting analytical model are analyzed. The dimensionless geometric parameters of the structure were determined numerically using the finite element method. A system of two nonlinear equations is obtained for the critical length of the prefracture zone and the critical load in the case of a complex stress state. The shape and dimensions of the plastic zone near the crack tip in a plastically anisotropic material are determined using Hill’s plasticity criterion.

A one-dimensional evolutionary system of equations is proposed that describes the motion of a thin bottom gravity current in a submerged environment of a lighter fluid in the Boussinesq approximation, taking into account the development of shear instability and the formation of an intermediate mixing layer. For hydrostatic flows, the propagation velocities of disturbances are determined and the concept of subcritical (supercritical) flow is formulated. An unsteady problem of the mixing layer is considered. Evidently, a monotonic or an oscillating mixing layer as a function of the Froude number of the incoming flow is formed. In the first case of a monotonic mixing layer, maximum entrainment is attained and a stationary solution is determined over a finite interval. For flows with a nonhydrostatic pressure distribution in the lower layer, steady solutions are constructed in the form of second-mode solitary waves adjacent to a given constant flow. Unsteady computations of the formation and propagation of large-amplitude bottom waves are performed.

The structure of an upward bubbly flow in an inclined circular tube is experimentally studied. Local void fraction profiles are obtained using a point conductivity sensor. Wall shear stress is determined using the electrochemical method. Evidently, the tube orientation greatly affects the local characteristics of a gas-liquid flow. Gas phase concentration significantly increases in the upper part of the inclined tube, leading to an increment of maximum values of local void fraction near the wall and to an increase in wall shear stress as compared to a single-phase flow. The most significant increase in friction occurs at inclination angles from 40° to 60°.

The interaction of a streamwise vortex generated by a jet vortex generator with a flat-plate turbulent boundary layer is studied experimentally. The measurements are mostly performed using the PIV method. The results are used to obtain Reynolds stresses and analyze their contribution to the Navier–Stokes equations. Data analysis yields integral dependences characterizing the influence of the streamwise vortex intensity on the turbulent boundary layer. It is found that the presence of the jet vortex generator can lead to a decrease in energy dissipation in the turbulent boundary layer.

Pressure fluctuations on a semi-infinite flat obstacle impinged by a supersonic underexpanded jet are studied. The nozzle exit Mach number is ({{text{M}}_a} = 1), the jet off-design parameter (n = 2.1), and the distance from the nozzle exit to the obstacle (h{text{/}}{D_a} = 2{-} 15) (({D_a}) is the nozzle exit diameter). It is shown that tangential injection of six microjets can lead to a significant reduction in pressure fluctuations on the obstacle. The mass flow rate through all microjets is 0.3% of the mass flow rate through the main jet. A self-oscillating mass-flow-rate regime of jet–obstacle interaction is found to occur at (h{text{/}}{D_a} < 3), a self-oscillating regime with acoustic feedback at (3 < h{text{/}}{D_a} < 8), and a turbulent aperiodic interaction regime at (h{text{/}}{D_a} > 8).

A mathematical model is developed and used as a basis for numerical experiments pertaining to the study of degradation of ice-containing permafrost rocks, accumulations of metastable (relict) gas hydrates, and free gas under the influence of thermal solutions and brine with account for the osmotic effect. Series of simulations with varying parameters are used to study the patterns and rates of permafrost degradation and methane release. Some of the data obtained using the model with corresponding experimental data shows good agreement.

This paper presents and discusses the results of finite element calculations of the continuity and stress fields (in a coupled formulation) near a creep crack front performed to determine the asymptotic distributions of stresses and continuity (damage) near the notch edge. The shape of the material damage region during creep growth ahead of the notch tip is determined for various values of material constants. An analysis of the radial stress distributions obtained in the finite element calculations shows that damage accumulation changes the asymptotic behavior of stresses near the crack tip in a power-law creeping material. It is shown that in the absence of damage accumulation, the numerical solution of the problem approaches the Hutchinson–Rice–Rosengren asymptotics, whereas accounting for damage accumulation affects the stress field near the notch or crack. The characteristic sizes of the regions of dominance of various asymptotics near the crack tip can be determined using finite-element radial distributions of stresses and continuity.

We report an experimental study of the wear and fracture of ZrN-coated VT6 alloy in its coarse-grained (as-prepared) and ultrafine-grained states during high-rate dynamic erosion by solid corundum particles with an average size of 109, 58, and 23 μm in an air flow at velocities in the range 50–250 m/s and exposure times of 30, 60, and 300 s. The experimental data were used to determine the fraction of viscous fracture, the erosion damage depth, the change in the microstructure of the coated and uncoated alloy near the erosion surface, and also the weight wear and surface roughness. The results demonstrate that, in the case of high-rate erosion, the alloy wear and damage processes depend significantly on the exposure time, erosion rate (velocity of abrasive particles), powder particle size, and substrate structure.