Showing papers in "Journal of Applied Mechanics and Technical Physics in 2015"

Engineering modeling of the laminar–turbulent transition: Achievements and problems (Review)

Abstract: Currently available methods of computing the laminar–turbulent transition (LTT), including methods used in gas-dynamic software packages, are analyzed from the viewpoint of LTT simulation accuracy.

Mixed convection in a lid-driven cavity with an inside hot obstacle filled by an Al 2 O 3 -water nanofluid

Abstract: The present paper focuses on the problem of a mixed convection fluid flow and heat transfer of an Al2O3–water nanofluid with the thermal conductivity and effective viscosity dependent on temperature and nanoparticle concentration inside a lid-driven cavity having a hot rectangular obstacle. The governing equations are discretized by using the finite volume method, and the SIMPLE algorithm is employed to couple the velocity and pressure fields. By using the developed code, the effects of the Richardson number and the diameter and volume fraction of Al2O3 nanoparticles on the flow, thermal fields, and heat transfer inside the cavity are studied. The obtained results show that the average Nusselt number for the entire range of the solid volume fraction decreases with an increase in the Richardson number and the nanoparticle diameter. The results also clearly indicate that addition of Al2O3 nanoparticles produces a remarkable enhancement on heat transfer with respect to that of the pure fluid.

Boundary layer flow due to a stretching sheet with a variable thickness and slip velocity

Abstract: This article presents a numerical solution for the flow of a Newtonian fluid over an impermeable stretching sheet with a power-law surface velocity, slip velocity, and variable thickness The flow is caused by nonlinear stretching of the sheet The governing partial differential equations are transformed into a nonlinear ordinary differential equation with appropriate boundary conditions for various physical parameters The remaining ordinary differential equation is solved numerically by using the Chebyshev spectral method The effects of the slip parameter and the wall thickness parameter on the flow profile and local skin friction are presented A comparison of obtained numerical results is made with previously published results in some special cases, and excellent agreement is noted

Modeling of a supersonic flow around a cylinder with a gas-permeable porous insert

Abstract: Results of an experimental and numerical study of a supersonic (M∞ = 4.85) flow around a streamwise-aligned cylinder with a gas-permeable porous insert on the frontal face in the range of Reynolds numbers Re D = (0.1–2.0) · 105 are presented. The numerical study is performed by using the Ansys Fluent software system and a porous medium model based on a quadratic law of filtration. The parameters of the quadratic dependence are calculated on the basis of experimental data for an air flow in a porous material. Flow fields are obtained, and the wave drag of the model is calculated as a function of the porous insert length and the Reynolds number. Results of numerical simulations are compared with wind tunnel measurements.

Determination of parameters of the Johnson-Cook model for the description of deformation and fracture of titanium alloys

Abstract: Stress-strain curves of dynamic loading of VT6, OT4, and OT4-0 titanium-based alloys are constructed on the basis of experimental data, and the Johnson-Cook model parameters are determined. Results of LS-DYNA simulations of the processes of deformation and fracture of the fan casing after its high-velocity impact with a fan blade simulator are presented.

Vibrations in an elastic beam with nonlinear supports at both ends

Abstract: Vibrations in an elastic beam supported by nonlinear supports at both ends under the influence of harmonic forces are analyzed in this study. It is hypothesized that the elastic Bernoulli-Euler beam is supported by cubic springs to simulate nonlinear boundary conditions. The dynamic behavior of the beam is described by using the Fourier expansion and the Bessel functions. The Hankel transform is then applied to obtain particular (nonhomogeneous) solutions. This study succeeds in describing the “jump” phenomenon (instantaneous transition of the system from one position to another) of the vibrating system at certain frequencies. Models based on linear boundary conditions are unable to capture this phenomenon. A larger modulus of elasticity in nonlinear supports increases the frequency of unstable vibrations in the first mode and also widens the frequency region of system instability. This influence is less prominent in the second mode, in which the largest amplitude is smaller than those observed in the first mode.

Parametric identification of crystals having a cubic lattice with negative Poisson’s ratios

Abstract: A two-dimensional model of an anisotropic crystalline material with cubic symmetry is considered. This model consists of a square lattice of round rigid particles, each possessing two translational and one rotational degree of freedom. Differential equations that describe propagation of elastic and rotational waves in such a medium are derived. A relationship between three groups of parameters is found: second-order elastic constants, acoustic wave velocities, and microstructure parameters. Values of the microstructure parameters of the considered anisotropic material at which its Poisson’s ratios become negative are found.

Oscillations of a cylindrical body submerged in a fluid with ice cover

Abstract: The linear plane problem of oscillations of an elliptic cylinder in an ideal incompressible fluid of finite depth in the presence of an ice cover of finite length is solved. The ice cover is modeled by an elastic plate of constant thickness. The hydrodynamic loads acting on the body are determined as functions of the oscillation frequency and the positions of the cylinder and plate.

Effect of the type of boundary conditions on the three-phase contact line on the flow characteristics during filling of the channel

Abstract: The effect of various models of the dynamics of the three-phase contact line on the flow characteristics during filling of a plane channel is investigated. The system of constitutive equations is solved numerically using a technique based on the combined use of the SIMPLE algorithm and the method of invariants. Different methods for calculating the motion of the contact point for slip and no-slip conditions for a dynamic contact angle θ = π, neglecting surface tension on the free boundary are considered. It is shown that in the whole computational domain, except in a small vicinity of the contact point, the kinematic characteristics of the flow and the distributions of the dissipation function and shear stress are slightly dependent on the selected calculation method of the motion of the contact point.

Transient natural convective flow of a nanofluid past a vertical plate in the presence of heat generation

Abstract: In this work, an exact analysis on the effects of heat generation and nanoparticle volume concentration on an unsteady free convective flow of a nanofluid past an impulsively started infinite vertical plate is presented. Nanofluids containing nanoparticles of aluminum oxide, copper, titanium oxide, and silver with a nanoparticle volume concentration range smaller than or equal to 0.04 are considered. The governing dimensionless partial differential equations are solved by using the Laplace transform technique. The effects of heat generation and nanoparticle volume concentration on the velocity and temperature profiles are represented graphically. The expressions for the skin friction coefficient and Nusselt number are derived. The effect of heat transfer is found to be more pronounced in a silver–water nanofluid than in the other nanofluids. Comparisons with other published results are found to be in excellent agreement.

Mechanical characteristics of high-quality laser cutting of steel by fiber and CO 2 lasers

Abstract: The quality of cutting of low-carbon and stainless steel by beams of fiber and CO2 lasers with oxygen or nitrogen being used as a process gas is compared. The cut surface roughness for sheets from 3 to 10 mm thick is determined. Domains of optimal (in terms of the minimum roughness criterion) application of lasers of various types in the space of dimensionless parameters (Peclet number and dimensionless power) are found. It is demonstrated that the CO2 laser is more effective for laser-oxygen cutting, while the fiber laser is more beneficial for cutting with the use of a neutral gas.

Wave propagation in two-dimensional block media with viscoelastic layers (Theory and experiment)

Abstract: A model of a medium consisting of parallel layers of elastic rectangular blocks separated by deformable viscoelastic interlayers is considered. The model is proposed for describing the lowfrequency part of the spectrum in waves propagating in media with such a structure. For a twodimensional assembly consisting of 36 blocks, the results of numerical calculations are compared with experimental data.

Fracture of brittle geomaterial with a circular hole under biaxial loading

Abstract: The influence of boundary conditions on the fracture of brittle geomaterial in the stress concentration zone under biaxial loading with account for the size effect is theoretically and experimentally studied. The calculation results are compared with the experimental data.

Reflection and refraction of acoustic waves at the interface between a gas and a disperse systems

Abstract: The reflection and refraction of acoustic waves at different angles of incidence on the interface between a vapor-gas-droplet system and air are studied. From an analysis of analytical solutions, it has been found that in the case of incidence on the interface from the side of the vapor-gas-droplet medium, there is a critical angle of incidence at which the wave is completely reflected from the boundary, i.e., total internal reflection takes place. It is shown that for a certain angle of incidence on the interface both from the air side and from the mixture side and for a certain volume fraction of water in the disperse system, complete transmission of the acoustic wave through the medium is observed.

Two-Dimensional Nonstationary Problem of Elastic Diffusion for an Isotropic One-Component Layer

Abstract: A 2 D nonstationary problem of an orthotropic one-component elastic layer, accounting for diffusion, is considered. A locally balanced model of elastic diffusion is used, which includes a coupled set of equations of elastic body motion and a mass-transfer equation. On the layer boundaries normal displacement, tangential stress and diffusive flow are assumed. At an initial time, the layer is in an undisturbed state.

Effect of the volume concentration of a set of water droplets moving through high-temperature gases on the temperature in the wake

Abstract: The processes of heat and mass transfer and phase transformations during motion of a set of water droplets through high-temperature gases have been studied numerically. The regimes and conditions of formation of zones of significant joint influence of the droplets on the integral characteristics of heat and mass transfer are determined. The values of the dimensionless parameters describing the dependence of the temperature of the gases in the wake of a small set droplets on their volume concentration and arrangement are calculated. The variations in these dimensionless parameters during motion of droplets through high-temperature gases are described.

Experimental and theoretical studies of the influence of a tensile load on the relaxation of residual stresses in a hardened cylindrical specimen under creep conditions

Abstract: This paper presents an experimental and theoretical study of the influence of a tensile load on the relaxation of residual stresses in a hardened cylindrical specimen of ZhS6KP alloy under creep conditions at 800°C An experimental study was conducted to investigate the distribution of the axial residual stress tensor component across the thickness of the hardened layer after hardening by air shot blasting using microbeads and after creep loading for 50 and 200 h under a tensile load of 150 and 250 MPa A detailed theoretical analysis of the problem was performed In all loading regimes, the calculated and experimental values of the residual stresses were found to be in good agreement It was shown that at low tensile load, the relaxation rate decreased in comparison with the case of thermal exposure in the absence of a tensile load and, with increasing load intensity, it increased

Numerical simulation of the interaction of a transverse jet with a supersonic flow using different turbulence models

Abstract: This paper presents a numerical simulation of the flow resulting from transverse jet injection into a supersonic flow through a slot nozzle at different pressures in the injected jet and the crossflow. Calculations on grids with different resolutions use the Spalart–Allmaras turbulence model, the k–e model, the k–ω model, and the SST model. Based on a comparison of the calculated and experimental data on the wall pressure distribution, the length of the recirculation area, and the depth of jet penetration into the supersonic flow, conclusions are made on the accuracy of the calculation results for the different turbulence models and the applicability of these models to similar problems.

Development and deceleration of viscoplastic flow in a layer heated by friction on a rough plane

Abstract: This paper presents a solution of a sequence of coupled problems of thermoelastoplasticity which study the occurrence and development of flow in a material layer under pure shear conditions, and its subsequent deceleration by slowly removing the load. The homogeneity of the stress state of the layer is excluded due to the coupling of thermal and deformation processes in the presence of a temperature dependence of the yield point. An additional source of heat is taken to be its production by friction of the material layer on a rough plane. The conditions for the occurrence of viscoplastic flow in the deformable material layer and the laws of motion of the boundaries between the elastic and plastic regions in this layer are determined, and the flow velocities and large irreversible and reversible deformations are calculated. It is shown that reversible deformations cause stresses in the flow region and the moving elastically deformed core.

Peristaltic flow of a reactive viscous fluid through a porous saturated channel and convective cooling conditions

Abstract: This article addresses the heat transfer in a peristaltic flow of a reactive combustible viscous fluid through a porous saturated medium. The flow here is induced because of travelling waves along the channel walls. It is assumed that exothermic chemical reactions take place within the channel under the Arrhenius kinetics and the convective heat exchange with the ambient medium at the surfaces of the channel walls follows Newton’s law of cooling. The analysis is carried out in the presence of viscous dissipation and without consumption of the material. The governing equations are formulated by employing the long-wavelength approximation. Closed-form solutions for the stream function, axial velocity, and axial pressure gradient are obtained. It is found that the temperature decreases at high Biot numbers, and the Nusselt number increases with increasing reaction parameter. The Biot number and reaction parameter produce the opposite effects on the Nusselt number.

On some features of plane waves of thermonuclear burn

Abstract: The behavior of a slow burn wave propagating over a precompressed thermonuclear fuel heated by several shock waves generated by a laser pulse is studied. It is shown that such a burn wave can rapidly increase the fuel density ahead of the wave front and transform to a pair of detonation waves moving in the opposite directions. Hydrodynamic equations with a linear velocity profile are solved. It is found that the proton beam intensity necessary for ignition increases with the initial fuel density in accordance with the known formula generalizing results of two-dimensional simulations. A possibility of using results of one-dimensional simulations for determining the energy of ignition of a cylindrical target is discussed.

On the tolerance modelling of heat conduction in functionally graded laminated media

Abstract: The object of analysis is a heat conduction problem within the framework of tolerance modelling in laminated media with a functional gradation of effective properties. In contrast to the known asymptotic models (based on the homogenization technique), the characteristic feature of the tolerance model equations is that they make it possible to analyze the effect of the microstructure size on the overall behavior of the laminate. The proposed model equations describe heat conduction in laminates by mean of partial differential equations with smooth and slowly varying functions. This paper describes a certain extension of the unified tolerance modelling procedure, which makes it possible to analyze specific problems of heat conduction in laminates with a transversal gradation of effective properties.

Unsteady three-dimensional stagnation-point flow and heat transfer of a nanofluid with thermophoresis and Brownian motion effects

Abstract: An unsteady three-dimensional stagnation-point flow of a nanofluid past a circular cylinder with sinusoidal radius variation is investigated numerically. By introducing new similarity transformations for the velocity, temperature, and nanoparticle volume fraction, the basic equations governing the flow and heat and mass transfer are reduced to highly nonlinear ordinary differential equations. The resulting nonlinear system is solved numerically by the fourth-order Runge–Kutta method with the shooting technique. The thermophoresis and Brownian motion effects occur in the transport equations. The velocity, temperature, and nanoparticle concentration profiles are analyzed with respect to the involved parameters of interest, namely, unsteadiness parameter, Brownian motion parameter, thermophoresis parameter, Prandtl number, and Lewis number. Numerical values of the friction coefficient, diffusion mass flux, and heat flux are computed. It is found that the friction coefficient and heat transfer rate increase with increasing unsteadiness parameter (the highest heat transfer rate at the surface occurs if the thermophoresis and Brownian motion effects are absent) and decrease with increasing both thermophoresis and Brownian motion parameters. The present results are found to be in good agreement with previously published results.

Rabotnov damageparameter and description of delayed fracture: Results, current status, application to fracture mechanics, and prospects

Abstract: This paper presents a review of studies of delayed fracture and fracture mechanics problems in which the hypotheses and ideas of Yu. N. Rabotnov and L. M. Kachanov on the mechanisms of delayed fracture under creep conditions are extended to describe fracture processes using scalar and tensor measures of damage. The results of current research in the theory of elasticity, the mathematical theory of plasticity and creep, the mechanics of composites, and linear and nonlinear fracture mechanics, with material damage taken into account.

MHD flow and heat transfer of a viscous fluid over a radially stretching power-law sheet with suction/injection in a porous medium

Abstract: A steady boundary layer flow and heat transfer over a radially stretching isothermal porous sheet is analyzed. Stretching is assumed to follow a radial power law, and the fluid is electrically conducting in the presence of a transverse magnetic field with a very small magnetic Reynolds number. The governing nonlinear partial differential equations are reduced to a system of nonlinear ordinary differential equations by using appropriate similarity transformations, which are solved analytically by the homotopy analysis method (HAM) and numerically by employing the shooting method with the adaptive Runge-Kutta method and Broyden’s method in the domain [0,∞). Analytical expressions for the velocity and temperature fields are derived. The influence of pertinent parameters on the velocity and temperature profiles is discussed in detail. The skin friction coefficient and the local Nusselt number are calculated as functions of several influential parameters. The results predicted by both methods are demonstrated to be in excellent agreement. Moreover, HAM results for a particular problem are also compared with exact solutions.

Flow structure formed due to interaction of a supersonic jet with a porous obstacle

Abstract: Results of studying impingement of a supersonic underexpanded air jet onto a finite-thickness porous metal obstacle whose frontal plane is normal to the jet axis and whose side surface is impermeable for the gas flow are reported. The case of a non-porous obstacle of the same diameter is considered for determining the effect of porosity on gas-dynamic characteristics of jet–obstacle interaction.

Stress-strain state of ice cover during aircraft takeoff and landing

Abstract: We consider the linear unsteady motion of an IL-76TD aircraft on ice. Water is treated as an ideal incompressible liquid, and the liquid motion is considered potential. Ice cover is modeled by an initially unstressed uniform isotropic elastic plate, and the load exerted by the aircraft on the ice cover with consideration of the wing lift is modeled by regions of distributed pressure of variable intensity, arranged under the aircraft landing gear. The effect of the thickness and elastic modulus of the ice plate, takeoff and landing regimes on stress-strain state of the ice cover used as a runway.

Electrically exploded opening switches in high-current explosive magnetic generators

Abstract: This paper presents a review of publications on the use of electrically exploded foil opening switches to form current pulses up to 100 MA (up to 45 MA in experiments) with a rise time of 01–100 µs Physical schemes and models are considered, and the efficiency of foil opening switches for existing and advanced facilities is analyzed

Laser ceramics with disordered crystalline structure

Abstract: New ceramic materials based on yttrium oxide Y2O3 with isovalent (Yb2O3, Nd2 O3, and Lu2O3) and heterovalent (ZrO2 and HfO2) components are synthesized, and their spectroscopic properties are investigated. Possible channels of losses in the gain of stimulated radiation in the radiative transitions of Nd3+ and Yb3+ ions in ceramics with heterovalent additives are studied. The results of measurements of Y2O3 ceramics doped with zirconium and hafnium ions, the emission bandwidth and the lifetimes of the 4F3/2 and 2F5/2 levels of Nd3+ and Yb3+ ions, respectively, are presented. It is shown that the nonradiative population of the 4F3/2 levels of neodymium ions is due to their dipole-dipole interaction with Zr3+ and Hf3+ ions. Laser generation in [(Yb0.01Lu0.24Y0.75)2O3]0.88(ZrO2)0.12 ceramics with disordered crystalline structure was achieved at a wavelength of 1034 nm with a differential efficiency of 29%.

Computation of wave interference and relaxation of particles after passing of a shock wave

Abstract: Interaction of a shock wave with a system of motionless or relaxing particles is numerically simulated. Regimes of the gas flow around these particles are described, and the influence of the initial parameters of the examined phenomenon on the flow pattern is analyzed. The drag coefficient of particles is calculated as a function of the Mach number behind the shock wave at a fixed Reynolds number. The dynamics of heat exchange for particles of different sizes (10 μm–1 mm) is determined, and the laws of thermal relaxation after passing of a shock wave over the system of particles are found. The times of thermal and velocity relaxation of particles are estimated as functions of the Reynolds number, and the predicted relaxation time is compared with the corresponding empirical dependences.