Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2020 number 3

The transformation of a solitary two-dimensional wave into three-dimensional waves on a falling liquid film was studied using the high-speed method of laser-induced fluorescence allowing high spatial and temporal resolution, which allows measuring the thickness of liquid films with high spatial and temporal resolution, the process of decay of a solitary two-dimensional wave into three-dimensional waves in case of film flow along a vertical plate. It was shown that the package of three-dimensional waves resulting from the decay of a solitary two-dimensional wave had a strong perturbing effect on the falling film and leads to the formation of rivulets on its surface.

A series of experimental investigations is performed for determining the thermal and environmental characteristics of burning heavy fuel oil of the M-100 type sprayed by a jet of superheated steam in a laboratory prototype of a new burner. A scheme of liquid fuel burning preventing choking of narrow fuel channels of injectors is proposed; this scheme ensures effective burning of high-viscosity fuels and wastes. The process of stable burning of heavy fuel oil in a low-power (7 kW) burner with low concentrations of toxic emissions and high combustion efficiency is realized.

The wave structure of a liquid film blown by a high-velocity gas flow with entrainment of droplets from the film surface has been investigated. The paper presents an analysis of existing methods of measuring the thickness of the local fluid film and a review of the classical concepts of wave hydrodynamics of annular dispersed flow obtained in early studies using crude methods. A systematic description of the structure, evolution, and interaction of waves of various types is made based on analysis of high-resolution spatio-temporal records of film thickness obtained by laser-induced fluorescence. The results are compared with the results of modern experiments, and the necessity of revising some classical concepts is shown. Directions for further research are suggested.

Possible existence of vortex pairs of coaxially rotating helical filaments with circulation in the opposite directions is theoretically studied. Such pairs are generated, e.g., by a rotating blade of the rotor with identical intensities of the tip and shifted axial vortices. Conditions that ensure uniform rotation of the vortex pair of helical filaments are found, which is needed for modeling vortex wakes behind rotor blades.

This paper presents the results of experimental studies of the spatial flow structure and coherent structures in the initial region of swirling jets at a Reynolds number of 5000 and various swirl parameters. The contribution of these structures to the intensity of turbulent transfer of momentum and mass (impurity) was first quantitatively evaluated. In addition, the effect of coherent structures on pulsations of the intensity of local heat release due to the deformation of the flame front during combustion of mixtures of methane at an air excess coefficient of 1.43 was investigated for the case of strong swirling with vortex core breakdown.

A swirling flow in a tangential-type hydrodynamic chamber with stationary vortex structures is experimentally investigated. The main attention is paid to determining the geometric and kinematic parameters necessary for a correct description of helical vortices and their spatial structure, in particular, to testing the possibility of a theoretical description of the limiting case where the vortex becomes stationary, i.e., its rotation frequency (or precession frequency) becomes zero. It is shown that the theory of helical vortices ensures a sufficiently accurate prediction of conditions for the appearance of their stationary (immobile) states.

This paper presents the results of an experimental study of mixing fluid flows with different temperatures in a T-junction. The experiments were performed with liquids having substantially different physical properties. The distributions of the average and fluctuation temperatures on the surface of the wall of the T-junction and the structure of the temperature fields in the region of mixing of flows of cold and hot liquids were obtained.

In a thin-layer approximation, a problem of a free liquid film vertically bounded by solid walls and being under the influence of gravity and thermocapillary forces is considered. A solution in which the film thickness is constant and temperature is a linear function of the longitudinal coordinate is investigated analytically for stability using the matching asymptotic expansions method and numerically using the orthogonalization method for various values of gravitational acceleration. The results obtained analytically and numerically are in good agreement. It is shown that the solution is unstable, but the increment of perturbations is small even under terrestrial gravity.

The flow in a Ranque tube with a square cross-section channel was studied within the framework of the hydraulic concept of strongly swirling flow with a circulation zone. Using experimental data, it has been found that the flow in the vortex tube satisfies the hydraulic condition of flow crisis: the longitudinal speed becomes equal to the speed of propagation of centrifugal waves at the boundary between the vortex and the circulation zones. It is shown that the temperature change along the flow occurs mainly in the region of the working channel in which the ratio of the longitudinal velocity at the vortex boundary to the propagation velocity of centrifugal waves fluctuates about unity assumption made The existence of a previously unknown mechanism of energy separation due to the presence of hydraulic jumps during the development of internal waves in a Ranque pipe was assumed.

This paper presents the results of numerical simulation of the injection of liquid sulfur dioxide into a porous reservoir accompanied by the formation of sulfur dioxide gas hydrate. The case is considered where the reservoir is initially saturated with methane and water and has a finite length and an impermeable outer boundary. It is shown that at the initial stage, the displacement of methane from the reservoir can occur with or without the formation of methane gas hydrate, depending on the permeability of the porous medium and pressure. It is found that over time, the thermodynamic conditions in the region of methane filtration become close to the formation conditions of methane gas hydrate.

The problem of scattering of a normally incident surface wave train by an obstacle in the form of a thick rectangular vertical bottom-standing barrier submerged in finite-depth water with a free surface is extended here with an ice cover modelled as a thin uniform elastic plate. The problem is reduced to a set of singular integral equations of the first kind. The Galerkin approximation in terms of simple polynomials multiplied by appropriate weight functions whose form is dictated by the behavior of the fluid velocity near the barrier edges is used for solving the integral equations. The reflection and transmission coefficients are obtained numerically, and they are depicted graphically against the wavenumber for various ice cover parameters. It is observed that the barrier thickness plays a significant role in modelling of efficient breakwaters.

A spallation target is one of the key components of the accelerator driven system. A liquid metal windowless target is an advanced type of a spallation target and has great application prospects. In this study, the free surface flow behavior and development in a scaled-down liquid metal windowless target are investigated in experiments and simulations. The experiments are carried out based on a liquid gallium experimental loop. The CFD simulations are performed based on a three-dimensional target model by using ANSYS CFX. The main working parameters of the target are measured through experiments and then set as the boundary conditions for simulations. The simulated values of the free surface height are in good agreement with the experimental data. The free surface height gradually decreases with an increase in the inlet flow rate and with a decrease in the outlet pressure. The simulation results reveal that a flow stagnation zone presenting large eddies and a reverse flow are formed below the outlet of the inner tube.

Steady axisymmetric regimes of the fluid flow in a thermogravitational boundary layer near the free surface with a nonuniform temperature distribution on this boundary are calculated for equations of fluid motion in the Oberbeck-Boussinesq approximation, where the viscosity and thermal diffusivity are small. With due allowance for the thermocapillary effect, it is demonstrated that nonhomogeneous fluid flow regimes with rotation can arise in the boundary layer owing to a bifurcation in the case of local cooling of the free surface, whereas rotation outside this layer is absent.

This paper describes a theoretical study of the steady motion of a large solid nonvolatile aerosol spherical particle, which contains thermal sources within itself, in a concentration gradient of binary gas mixture components. It is assumed that an average particle surface temperature significantly differs from the temperature of the binary gas mixture surrounding it. Equations of gas dynamics are solved taking into account the power-law dependence of the molecular transfer coefficients (viscosity, thermal conductivity, and diffusion) and the density of the gaseous medium on temperature. Under boundary conditions, diffusion and thermal slip are taken into account. Numerical estimates show that the diffusion and photophoretic forces and velocity substantially depend on the average particle surface temperature.

Nonlinear interaction and energy loss by harmonics of perturbations propagating upstream are investigated. Formation of new harmonics due to nonlinear interaction of harmonics is shown.

The current study is devoted to investigating the duality of the solution for an unsteady stretching cylinder flow subjected to wall normal suction. It is demonstrated by using numerical methods that the dual solutions exist for various values of the curvature parameter regardless of the presence or absence of wall suction.

On the basis of a minimax criterion and stress intensity factor minimization, the optimal preload is determined for the case of elastic fibers fit into a doubly periodic system of holes in an isotropic elastic binder. A selection criterion is proposed for an interference fit that prevents the fracture of a composite reinforced with unidirectional fibers and ensures optimal stress distribution.

The use of steel fiber-reinforced concrete panels to strengthen masonry walls is examined. Within the scope of this study, 18 of a total of 24 specimens are manufactured by using concrete panels containing steel fibers at specific ratios to strengthen hollow brick and clay brick walls, while the remainder serve as reference specimens. The shear strength, deformation and energy consumption capacities are then evaluated for each of the different strengthening methods.

Volumetric samples of pure ultra-high molecular polyethylene and volumetric samples reinforced with metal interlayers (steel grids and perforated titanium plates) are obtained by cyclic shock pressing. Experiments are carried out in which the properties of compacts made of ultra-high molecular polyethylene with and without metal interlayers are studied during perforation by a lead ball (shot) with a diameter of 8 mm flying at a velocity of approximately 370 m/s. The depth of penetration of a fraction into a compact is experimentally determined. To estimate the penetration depth of a spherical body into a polymer, a model is proposed that describes the motion of a ball in this medium.

Acoustic emission activity and longitudinal and volumetric deformations in rock salt samples subjected to uniaxial mechanical loading with a constant strain rate and thermal stress are measured. The features of acoustic emission during deformation under various thermobaric experimental conditions are analyzed. It is shown that, in contrast to the deformation parameters, the change in the activity of acoustic emission at the boundaries of the indicated stages is nonmonotonic in nature, as well as features that make it possible to accurately determine each stage and estimate the elastic and strength properties of the rock salt.

Natural vibrations of heterogeneous orthotropic cylindrical shells reinforced by annular ribs and filled with fluid are studied. Dependences of a frequency response on various geometric and physical parameters of the problem are described.

The numerical solution of the direct scattering problem for a system of the Zakharov-Shabat equations is considered. Based on the Marchuk identity, a fourth order method of approximation accuracy is proposed. The numerical simulation of the scattering problem is carried out using an example of two characteristic boundary value problems with known solutions. The calculations have confirmed high accuracy of the algorithm proposed, which is necessary in a number of practical applications for optical and acoustic sensing of media in optics and geophysics applied.

We consider an unstudied optimization problem of summing the elements of the two numerical sequences: Y of length N and U of length q ≤ N . The objective of the optimization problem is to minimize the sum of differences of weighted convolutions of sequences of variable lengths (which are not less than q ). In each of the differences, the first convolution is the unweighted autoconvolution of the sequence U nonlinearly expanded in time (by repetitions of its elements), and the second one is the weighted convolution of an expanded sequence with a subsequence of Y . The number of differences is given. We show that the problem is equivalent to that of approximation of the sequence Y by an element of some exponentially sized set of sequences. Such a set consists of all the sequences of length N which include, as subsequences, a given number M of admissible quasiperiodic (fluctuating) repetitions of the sequence U . Each quasiperiodic repetition is generated by the following admissible transformations of the sequence U : (1) shifting U in time, so that the differences between consecutive shifts do not exceed T_max ≤ N , variable expansion of U in time consisting in repeating each element of U , with variable multiplicities of the repetitions. The optimization objective is minimizing the sum of the squares of element-wise differences. We demonstrate that the optimization problem in combination with the corresponding approximation problem are solvable in polynomial time. Specifically, we show that there exists an algorithm which solves the problems in the time O(T³_max M N ). If T_max is a fixed parameter of the problem, then the algorithm running time is O( M N ). In the examples of numerical modeling, we show the applicability of the algorithm to solving applied problems of noise-robust analyzing electrocardiogram-like and photoplethysmogram-like signals.

In this paper a new numerical method with a low-dissipation of a numerical solution, based on a combination of the Godunov method with the Roe scheme, and a piecewise parabolic method on a local stencil is described. The construction of the numerical method is described in considerable detail, the method is validated on the one-dimensional Riemann problem. The results of the numerical simulation of the collision of two relativistic gas spheres are presented.

The propagation of a wave from the point source in the case when the velocity in the medium v is expressed as v = 1/√ y is considered. Exact solutions of the corresponding eikonal equation are obtained and numerically verified. The ambiguity of the solution to this question in the case when the point source is situated at the origin is shown.

This paper considers numerical methods for approaching and simulate the Stokes-Darcy problem, with a new boundary condition. We study herein a robust stabilized fully mixed discretization technique, this method ensures the stability of the finite element scheme and does not use any Lagrange multipliers to introduce a stabilization term in the temporal Stokes-Darcy problem discretization. The well-posedness of the finite element scheme and its convergence analysis are also derived. Finally, the effciency and accuracy of the numerical methods are illustrated by different numerical tests.

A source identification algorithm for the systems of nonlinear ordinary differential equations of the production-destruction type is applied to the inverse problem for the discretized Smoluchowski equation. An unknown source function is estimated by time series of measurements of the specific size particles concentration. Based on an ensemble of adjoint equations solutions, the sensitivity operator is constructed that links the perturbations of the sought for model parameters with perturbations of the measured values. This reduces the inverse problem to a family of quasilinear operator equations. To solve the equations, an algorithm of the Newton-Kantorovich type is used with r -pseudoinverse matrices. The eficiency and properties of the algorithm are numerically studied.

A characteristic analysis of the equations of a single-velocity heat-conducting vapor-gas-drop mixture is carried out, in which the interfraction heat exchange is taken into account and their hyperbolicity is shown. The computational formulas of the Godunov method with a linearized Riemann solver are presented with whose use a number of the mixture flows are calculated.

This paper concerns the solution of the inverse boundary value problem for the equation of thermal conductivity and the estimation of the approximate solution error. The Fourier transform with respect to time, which allows one to obtain an error estimate, is not applicable to the problem to be solved. Therefore, in the equation of thermal conductivity, the variable was replaced, which led to the synthesis of problems and allowed obtaining an estimate.

An unstable laminar flow over a wavy region of a plate placed parallel to the oncoming air flow is studied. The results have been obtained by the hot-wire method in a low-turbulent wind tunnel at low subsonic velocities. Flow near a wall with transverse undulation, whose amplitude is sufficiently large and can contribute to the formation of local regions of boundary layer separation, is considered. The external harmonic (acoustic) forcing of the periodic flow leads to the suppression of perturbations that dominate in the spectrum of velocity disturbances near the model surface. The results of this work may be useful in developing methods for controlling spatially inhomogeneous flows.

The study presents the experimental results for inter-assembly interaction of coolant flows in the core of the VVER consisting of fuel assemblies TVSA-T and TVSA-T.mod.2. The coolant flowing in fuel assemblies (FA) was modeled on an aerodynamic stand. The studies were carried out on the model of the VVER core fragment and consisted in measuring the velocity vector modulus in the characteristic zones of both the TVSA and the inter-assembly space of the VVER core. Measurements were carried out by a five-channel pneumometric probe. The analysis of the spatial distribution of the projections of the absolute flow velocity allowed detailing the pattern of streamlining of the spacer, mixing and combined spacer grids of the TVSA by the coolant flow. Results of investigation of inter-assembly interaction of the coolant between adjacent fuel assemblies TVSA-T and TVSA-T.mod.2 were adopted for practical use in JSC "OKBM Afrikantov" to assess the thermal reliability of VVER cores and included in the database to verify the programs of computational fluid dynamics (CFD-codes) and to provide a detailed cell calculation of the VVER core.

The paper considers nonlinear waves generated on a surface of a horizontal liquid layer put into an assigned stress field at the gas-liquid interface. The nature of branching for wavy modes from the undisturbed flow was studied. To accomplish this, the solution of a model nonlinear equation written for the deviation of the layer thickness from the undisturbed layer is found. Analytical solutions were constructed for nonlinear steady state-travelling solutions of this equation with the wavenumbers belonging to the vicinity of neutral wavenumbers. Steady state-travelling periodic solutions for the first family were simulated for the case of wavenumbers beyond this vicinity.

The paper deals with the theoretical analysis of viscous liquid films flowing down along a single element of a structured packing with large ribs and microtexture in the form of a wavy two-dimensional surface or three-dimensional surface of the “undulating” type. The effect of inertia forces, surface tension, coarse corrugation, and fine texture geometry on the averaged characteristics of the film flow is considered. New equations for calculating a three-dimensional film flow along a plate with double corrugation structure are obtained. In the calculations, the main attention is paid to the effect of various types of microtextures on film spreading. The type, amplitude, angle of fine texture inclination, and Reynolds number of liquid are varied. It is found that the microtexture of three-dimensional “undulating” type has a small effect on liquid film spreading over a single element of the structured packing; the averaged liquid flow rates along and across large ribs depend weakly on the amplitude and inclination angle of fine texture of this type. It is revealed that only the average film thickness is sensitive to a change in the amplitude of microsurface of the “undulating” type. This is very different from the effect that a wavy two-dimensional microtexture has on film spreading along and across the large ribs. In this case, the effect of waviness on spreading hydrodynamics depends substantially on fine texture amplitude and inclination angle.

Specific features of the phenomenon of vapor bubble collapse in hot tetradecane (with a temperature of 663 K) are considered for various values of pressures in the liquid in the range from 13 to 100 bar. At the beginning of the collapse, the vapor in the bubble is in the state of saturation with a pressure of 10.3 bar, and with the initial radius of the bubble to equal 500 mm. It is shown that, at a liquid pressure lower than 13 bar, a nearly-uniform vapor compression is realized in the bubble, whereas at higher pressure values, compression is realized by means of radially converging isentropic waves (at 14-18 bar) and shock waves (starting from 19 bar). The degrees of vapor compression, estimated from the vapor pressure, density and temperature at the boundary of a small central region of the bubble of 0.25-mm radius, are compared with the degrees of vapor compression realized when a similar vapor bubble collapses in cold acetone at a temperature of 273 K (as in known experiments on acoustic cavitation of deuterated acetone). It is found that the degrees of compression comparable with those achieved in the case of acetone at a pressure of 15 bar, equal to the amplitude of the acoustic action exercised in the mentioned experiments, are achieved in the case of tetradecane at a pressure of 70 bar. In the latter case, the maximum rate of bubble collapse in tetradecane is 10 times lower than that in acetone (110 m/s versus 1100 m/s).

Acoustic streaming in a vibrating air-filled cylindrical cavity is numerically investigated. Adiabatic and isothermal boundary conditions are considered, as well as the case in which adiabatic and isothermal boundary conditions are set respectively at the ends of the cavity and on its lateral surface. The value of the cavity radius at which the axial component of acoustic streaming velocity attains its maximum is found. The influence of the radius of the cavity and boundary conditions at its ends on the free oscillations, at the initial stage of the process, on the period average temperature, and on the acoustic streaming pattern is described.

The developed mathematical model was applied for study of fluid dynamics in a rotational bioreactor for bone tissue engineering by in vitro technology. The research goal is finding an optimal mode for rotation ensuring proper cyclic loading from fluid upon the cell-seeded biomaterial. The basis for developing a mathematical model of a bioreactor was a design of rotational type biological reactor used in medical research; the liquid flow is generated through viscosity mechanism due to surface rotation. Mathematical description of flow in a reactor cavity was performed with Navier-Stokes equations. It was assumed that flow regime in the boundary layer is laminar. Numerical algorithm was accomplished using a fluid flow solver “Fluent” in the code package ANSYS-12. Four variants of generating the rotational motion in the reactor cavity were considered. A series of parametric computations was performed for the rotation frequency f in the range 0.05 £ f £ 0.25 Hz. The paper offers visualization of velocity fields in the vertical plane. The distributions for shear stress and pressure in the working zone of reactor were calculated and analyzed. Simulations demonstrated that a method of fluid rotation by driving the outer cylinder with an offset axis is the best for arranging a cyclic pressure and cyclic shear stress on the biological material.

Expressions describing the peculiarities of the temperature field formation in the photoacoustic cell, taking into account the temperature dependence of not only the thermophysical parameters of all layers and the absorptivity, but also of the optical absorption coefficient of the sample have been obtained. A system of nonlinear algebraic equations has been derived to determine the dependence of temperatures of the illuminated layer of the sample and its reverse side on the intensity of the incident beam. As a result of numerical solution of this system, nonlinear dependences of temperatures of both sides of the sample on the beam intensity have been obtained. The sign of the thermal coefficient of the temperature dependence of the optical absorption coefficient has been found to significantly affect the nature of the dependences of the increment of the irradiated surface temperature and the rear side of the sample on the intensity. Significant effect of the substrate on the formation of the temperature field in the photoacoustic cell has been shown as well.

A heat storage using the latent heat of the solid-liquid phase change was experimentally investigated. To intensify heat exchange in its design, hyper-heat-conducting plates were used. During the experiments, the temperature values and the volume changes of the working substance (hexadecane) were measured. The analysis of the thermal balance and phase changes in the volume of the heat storage was carried out. The efficiency of the considered design of the heat storage and high heat-distribution capacity of hyper-heat-conducting plates have been confirmed.

The paper presents the results of numerical studies of the influence of thermochemical activation of pulverized coal flows on the processes of heat and mass transfer occurring in the areas of real geometry (furnaces) with burning power fuel. The aerodynamic flow pattern, velocity, temperature, and concentration fields were obtained, the analysis of which allows the conclusion that plasma activation of fuel increases the efficiency of its combustion and reduces emissions of harmful substances into the atmosphere.

The paper describes the design of an experimental setup for plasma gasification of organic waste with its discrete supply to the loading unit. The experimental results of determining the percentage composition of synthesis gas and its calorific value depending on time for gasification of wood sawdust and polyethylene granules are presented. The amount of methane in synthesis gas composition has been calculated depending on temperature and oxygen flow rate. It is shown that when the waste is fed in separate portions, the synthesis gas composition, its yield and calorific value change over time significantly.