Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2020 number 5

Stability of a subsonic boundary layer with heat supply into its narrow band is studied. The maximum length of the stable boundary layer in the case of heat supply is found to be approximately three times smaller as compared to the case of a thermally insulated wall without heat supply. The frequency range of disturbances and their growth rates are approximately doubled. In contrast to the case without heat supply, three-dimensional disturbances are the most rapidly growing disturbances. In the case of a heated wall, the growth rate of disturbances decreases after heat supply into the boundary layer, which may lead to an increase in the length of the laminar region of the boundary layer.

Results of experimental and numerical modeling of a supersonic flow around a cylinder with a frontal gas-permeable high-porosity insert aligned at different angles of attack are presented. The experiments are performed in a supersonic wind tunnel at the Mach number M¥ =7 and unit Reynolds number Re1=1.5 х 106 m-1 in the range of the angles of attack 0-25oC. The numerical simulations are performed by means of solving three-dimensional Reynolds-averaged Navier-Stokes equations with the use of a three-dimensional annular skeleton model of the porous material. The drag and lift coefficients for a cylinder with a 95% porosity and pore diameter of 2 mm are obtained for different values of the insert length and angle of attack.

Flight experiments aimed at studying the transition from the laminar to turbulent flow in the boundary layer on the glider wing at different levels of atmospheric turbulence performed in the 1980s are reviewed. The results are obtained by means of hot-wire measurements and visualization of the flow on the wing surface by the method of sublimating coatings. The transition is found to proceed in several stages: emergence, evolution, and failure of a discrete packet of instability waves in the region of the adverse pressure gradient. The data obtained in the study are compared with the results of similar investigations carried out in a large wind tunnel on the same real glider wing at realistic Reynolds numbers.

This paper describes a study of a laminar separated flow in a compression angle for the Mach number of the incident flow M = 6; 8. The shock-wave structure of the flow in an attachment region is considered. The formation mechanism of a high-pressure layer, which is a narrow gas region located above the boundary layer downstream from the attachment line, is investigated.

The interaction of a laminar boundary layer with a shock wave at a Mach number M = 1.43 is studied by numerical simulation. The results obtained by direct numerical simulation are compared with the results of calculations using the Reynolds-averaged Navier-Stokes (RANS) equations supplemented with different turbulence models describing laminar-turbulent transition. The possibility of determining the position of the flow turbulence zone based on linear stability theory and the eN-method is estimated. Comparison of the numerical simulation with experimental data shows that the engineering RANS methods can be used to study supersonic flows in which transition to turbulence occurs in regions of interaction of the shock wave with the boundary layer.

This paper presents a brief overview of the most significant studies on magneto-plasma aerodynamics performed at the Khristianovich Institute of Theoretical and Applied Mechanics Siberian Branch, Russian Academy of Sciences in the last 20 years.

This paper describes a study of curvature of gas trajectory at the initial section of a supersonic nonisobaric jet on the features of unsteady perturbations from the Kelvin-Helmholtz instability class. It is shown that, in the presence of a barrel-shaped structure, steady Taylor-Gortler perturbations in the form of longitudinal structures (banded formations) arise. Studies for a mixing layer with a Mach number M=1.5 are carried out. The possibility of amplifying and suppressing the growth of Kelvin-Helmholtz perturbations by steady Taylor-Gortler waves. A nonlinear problem is solved within the framework of three-wave resonance interactions in a local-parallel approximation. A pumping wave is a steady Taylor-Gortler wave. It is shown that, at the initial section, small-amplitude traveling waves can be both amplified and suppressed.

A PIV method is used to measure the flow velocity in a zone of interaction between a shock wave and a boundary layer on a plate in the case where the Mach number is M=1.43. Two states of the incident boundary layer are considered (laminar and turbulent), and a boundary layer thickness can be two orders of magnitude smaller than the characteristic longitudinal scale of the flow. The results of measuring the velocity in the boundary layer, obtained using the PIV method at different settings of the equipment, and the results of measurements performed using various algorithms for determining the tracer particle displacement and reconstructing velocity fields from them. It is shown that the main constraint for increasing the spatial resolution is the tracer particle inertia.

This paper presents an experimental study of diffusion jet combustion of gas at different flow rates of CH4-N2 and a constant flow rate of coal. Entrainment curves for a diffusion flame for different nozzle diameters are obtained. It is shown that an increase in the nozzle diameter causes a decrease in the minimum residual flow rate of the fuel gas at which stable combustion is possible. As coal is supplied, the residual flow rate of the fuel gas also becomes slightly lower. The fluorescent glow of OH, which makes it possible to analyze the position and dynamics of the flame front is recorded.

Structural changes in a supersonic flow in a variable-section channels with axial injection of kerosene induced by the action of a jet generating a throttling effect are numerically simulated. Reynolds-averaged Navier-Stokes equations closed by the k-e turbulence model are solved. Kerosene combustion if modeled by equations of simplified chemical kinetics. The evolution of the processes in the channel is analyzed numerically and analytically for different types of the fuel (hydrogen, ethylene, and kerosene), and comparisons with experimental data are performed. It is found that high-intensity gas-dynamic pulses have to be used to form a transonic region in the case where kerosene is used as a fuel.

A hybrid model (combining the molecular dynamics and Monte Carlo methods) for computing the gas flow in a cylindrical channel with allowance for heat exchange between the gas molecules with the surface and adsorption of molecules on the channel surface is developed and improved. Two variants of the channel flow are considered: the flow in which the total number of molecules in the system remains unchanged in time and the flow with simulation of an infinite volume of vessels or a vessel with a constant pressure. Three variants of molecule interaction with the wall are analyzed: 1) the molecule experiences elastic scattering with a probability WE; 2) inelastic scattering (scattering with energy exchange) occurs with a probability WT; 3) deposition (adsorption) of the molecule onto the solid surface occurs with a probability WA. The sum of the probabilities if equal to unity. It is found that the allowance for adsorption exerts a significant effect on the velocity of the center of mass of molecules and on the density of the flux of molecules in computational volumes.

A technology of growing the bone tissue on a thin scaffold in a rotational bioreactor is developed. An optimal regime of cell bulk cultivation is determined for testing the method, and an optical method for diagnostics of the bone tissue evolution in the course of its growing is developed. The fluid flow in the bioreactor is numerically simulated, which allows significant simplification of the medical experiment and appropriate choice of the optimal values of the rotation frequencies and shear stresses acting on the cell material located on the scaffold. The optical diagnostics of the scaffold samples in the course of dynamic cultivation in the bioreactor is performed by the method of laser-induced fluorescence. An algorithm based on the principal component method is applied to analyze the spectral data; as a result, the spectra of excitation and fluorescent emission of the basic fluorescent substances in the sample (tyrosine and tryptophase aminoacids, structural protein (collagene), and fluorescent structures of polycaprolactone) are calculated. It is found that the contribution of the component corresponding to collagene increases in the course of dynamic cultivation in the samples, which testifies to effective formation of the extracellular matrix of the bone.

This paper presents the results of a numerical study of the effect of a positive longitudinal pressure gradient in a sudden pipe expansion on the turbulent two-phase flow structure and local heat transfer. It is shown that the longitudinal pressure gradient has a significant effect on flow characteristics and heat transfer in a separated gas-droplet flow. Increasing the opening angle of the diffuser leads to a significant increase in the degree flow turbulence (almost twofold increase compared to gas-droplet flow in pipe with a sudden expansion at φ = 0 ^oC). It is found that in the flow under study, the length of the recirculation zone increases in comparison with separation gas-droplet flow at φ = 0 ^oC and the point of maximum heat transfer rate shifts downstream. In this case, the coordinate of the point of maximum heat transfer rate does not coincide with the coordinate of the reattachment point of the detached two-phase flow.

This paper presents a brief overview of experimental studies performed at the Institute of Thermophysics SB RAS to model jets of low-thrust liquid rocket engines used as control and orientation thrusters of spacecrafts, including the International Space Station. Modeling criteria are formulated for the parameters of both the gas flow and the near-wall film of the liquid used for nozzle cooling. The experimental setup, diagnostic methods, and experimental results on the interaction of the near-wall liquid film with co-current gas flow inside the supersonic nozzle and during jet expansion into vacuum are described. The problem of minimizing backflows leading to contamination of the spacecraft external surfaces is discussed.

This paper describes the results of experimental studies of a flow using the prototype of a propeller-type microhydraulic turbine. The tests are carried out on a testbed in which atmospheric air is used as a working medium. The measurements carried out with the help of a two-component laser-Doppler anemometer are used to obtain velocity distributions behind an impeller in the case where the operating modes of the device change in a wide range. It is shown that the created model microhydraulic turbine has optimal parameters for the conditions set during the design, and a change in the operating mode of the device from nominal parameters to underload or overload increases the residual swirl of the flow and the generation of strong hydrodynamic instability in the form of a precessing vortex bundle. In this case, axial velocity over the cross section is distributed unevenly and the flow pulsation level is increased.

The aerodynamics of a laboratory model of an improved furnace with a four-vortex coal combustion scheme is studied using laser Doppler anemometry. For the isothermal case, the distributions of the averaged velocity and velocity pulsations are obtained for various flow regimes. The main features of the flow are established. Comparison of the experimental results with previous PIV measurements shows that they are in good agreement.

A method of gas-phase deposition of diamond structures with the use of a high-velocity jets for transporting gases activated in a microwave plasma to the substrate is developed. The diamond structures are synthesized from a hydrogen-methane mixture with the methane concentration of 1%. The influence of the gas mixture flow rate on the synthesis of the diamond structures with an unchanged composition of the mixture, pressure in the deposition chamber, and substrate temperature is studied. It is found that an increase in the flow rate leads to an increase in the polycrystalline film density and in the size of the diamond crystals forming the film. The composition of the mixture at the discharge chamber exit is numerically analyzed for different flow rates of the gas mixture. A correlation of the mixture composition with the growth rate and quality of the diamond structures is demonstrated.

Problems arising in the analysis of the physical and mechanical properties of composite materials on polymer matrices are under study. The structural features of such materials and the resulting difficulties in the development and implementation of corresponding models are discussed. A technique for determining the effective strain-strength, thermophysical, and electrophysical properties of the compositions is proposed, and a number of examples are considered. Particular attention is paid to solving computer design problems in order to determine the composition and structure of composites under specified constraints on their effective properties. Initial data for solving these problems are obtained both in field and computational experiments. Directions for further research are identified.

A review of works devoted to the study of the mechanical properties, stability, and buckling of graphene and carbon nanotubes is presented. Most of these results are obtained by means of molecular dynamics and molecular mechanics, which make it possible to effectively investigate the mechanical properties and stability of nanostructures. The data on the strength of graphene are presented, and bending modes for armchair and zigzag graphene sheets are analyzed. The stability and bending modes of natural vibrations of zigzag' and armchair nanotubes are analyzed.

A series of computations based on experimental data on the depth of projectile penetration into targets is performed for verification of parameters of ballistic stability of plates made of Al2O3, B4C, and SiC ceramics. Interaction of a heart-shaped steel projectile and a target made of an aluminum alloy with a variable-thickness ceramic plate positioned ahead of the target is simulated. The computed results are compared with experimental data for various ceramic targets 1-5 mm thick and projectile velocities of 810-850m/s.

A series of numerical simulations of the impact parameters of projectiles of different shapes with massive targets is performed. It is experimentally demonstrated that the dependences of the dimensionless depths of the craters formed by projectiles of different shapes on the dimensionless kinetic energy is described by a single curve of the modeling process.

This paper considers an energy approach to assessing the state of a cerebral aneurysm as a hydroelastic system consisting of an elastic vessel wall and oncoming blood flow. Assuming that the elastic energy of a vessel with an aneurysm in combination with the bending energy and kinetic energy is spent only on viscous flow dissipation in the structure, we performed a series of numerical calculations for model configurations of a fusiform aneurysm in the absence and in the presence of a diverticulum of different sizes relative to the size of the aneurysm body. It is shown that pressure-velocity diagrams are in good agreement with clinical data. Using numerical simulation, it is shown that a diverticulum of small size has a significant effect on hemodynamics inside the body of the aneurysm, and at a large diverticulum size, the vortex induced inside the diverticulum is almost completely localized in it.

Results of an experimental study of the microstructure and microhardness of a functionally graded material based on 316L stainless steel and ceramic particles of tungsten carbide (WC) created by the method of additive laser fusion are reported. Defect-free high-quality metal-ceramic coatings with different fractions of ceramics are obtained. It is demonstrate by the methods of optical and scanning electron microscopy and energy dispersion analysis that the laser action leads to dissolution of WC particles. The composition of the resultant coating, which has a dendrite character, includes iron and chromium carbides. The range of the coating microhardness is found to be HV_0.3 = 280-430.

The formation of single-layer and multilayer grapheme by chemical vapor deposition has been studied experimentally. The structure of the coatings formed at different temperatures and compositions of the gas mixture have been analyzed. Modes for transferring graphene structures to various substrates have been developed. Transparent flexible conductive polyethylene terephthalate-ethylene vinyl acetate-graphene and polymethylmethacrylate-graphene compositeshave been obtained.

Effect of nanosized silicon dioxide on mechanical properties, texture, and structure of a polymer paint-and-lacquer coating made of gray KhV-16 enamel based on perchlorovinyl and glyphthalic resins is studied. It is established that the abrasion strength, hardness, and elastic modulus are larger than in the case of the original coating. Various methods are used to analyze the structure of the resulting samples of paint-and-lacquer coatings. It is established that the structure changes and the mechanical properties of the coatings improve because of the formation of new structure-forming centers in the composite coating due to the addition of nanosized silicon dioxide.