Home – Home – Jornals – Journal of Applied Mechanics and Technical Physics 2022 number 3

This paper presents the results of an experimental study of the turbulent structure of the flow of two parallel jets from long round tubes in the near field of the flow. Measurements were performed for distances between the axes of the tubes s/d = 1.2, 1.8, and 2.4 and for Reynolds numbers Re =5.5x103 and 1.1x104. It is shown that in the near field of the closely spaced twin jets, the longitudinal velocity component on their axis decreases more slowly than in single jets. In this case, the distance between the jets s/d has practically no effect on the transverse scale of the jet and turbulent fluctuations of the longitudinal velocity component. Turbulence anisotropy is practically absent throughout the flow field, except in the outer regions of the jets.

A partially invariant solution of a three-dimensional problem with a free boundary for the Navier-Stokes equations is studied. The flow domain under consideration is a horizontal layer bounded by a solid plane from below and by a flat free surface from above. The vertical velocity and pressure are independent of the x and y coordinates. Three flow modes can be formed for different initial velocities of the flow: stabilization to the quiescent state with time, solution blow up within a finite time, and self-similar regime in which the layer thickness unboundedly increases with time.

Model buoyancy frequency distribution and the Wentzel-Kramers-Brillouin method are used to obtain an asymptotic solution to a problem of constructing solutions that describe internal gravity waves in a stratified medium with a background shear flow slowly varying in depth. Dispersion relation asymptotics are expressed in terms of the Airy functions. Asymptotics for various model distributions of background shear flows are used to obtain analytical representations of dispersion relations and eigenfunctions. Exact and asymptotic results are compared for various distributions of background shear flows and generation regimes typical of a real ocean.

The turbulent mixing on interfaces of three-layer gas systems after shock-wave passage has been studied. The influence of the initial roughness of interfaces on the development of instability and mixing of gases with different density has been investigated. The development of Richtmyer-Meshkov instability has been simulated using the implicit large eddy method and the MIMOZA code. The results of the study have been compared with available experimental data. It has been shown that the width of the mixing region and the enstrophy and mass of mixed gases depend significantly on the degree of interface roughness.

The physical grounds of droplet breakdown in a flow behind a transient shock wave in accordance with one of possible sheet stripping mechanisms (shedding of a liquid boundary layer) are considered. The analysis is performed on the basis of data of high-speed shadowgraphy of the droplet behavior behind the shock wave in the range of the Weber numbers We = 200-2200, measurements of the breakdown induction period, and registration of the ablation character. The condition of instability of the liquid surface by the sheet stripping mechanism and the criterion of interface breakdown are formulated with the use of parameters of an adjoint boundary layer in the fluid. On this basis and with due allowance for previous investigations, a dependence of the delay of droplet breakdown by this mechanism on the fluid characteristics and flow parameters is derived.

Results of optical and gas-dynamic investigations of the microwave discharge plasma under conditions of gas-jet synthesis of diamond are reported. The data are analyzed for determining the temperature and composition of the mixture in the discharge chamber for the purpose of further optimization of the method. The influence of the flow rate of the injected gas mixture on the macroparameters of the gas mixture flow at the exit of the discharge chamber is studied. The temperature and pressure measured in the discharge chamber are used for the numerical analysis of the mixture composition depending on the injected gas flow rate. It is demonstrated that the mixture composition at the exit of the discharge chamber correlates with the rate of diamond synthesis.

Numerical analyses of turbulent boundary layer parameters and skin-friction drag reduction on a flat plate under the effect of air micro-blowing with the use of the SST k-w turbulence model are performed. The macroscale characteristics of a huge number of microjets are simulated by using a microporous wall model (MPWM) incorporated into ANSYS FLUENT by user-defined functions. Numerical results obtained within the Mach number range M=0.2-0.5 (Reynolds number Re=2.88x106-7.20x106) confirm the experimental data of other researchers. Furthermore, a slight increase in the boundary layer thickness, displacement thickness, and momentum thickness, as well as a decrease in the velocity gradient and shear friction are well captured. In comparison to a simple flat plate, applying air micro-blowing reduces the skin-friction coefficient by 51% at the Mach number M=0.4 and blowing fraction of 0.008. Additionally, the skin-friction coefficient decreases as the blowing fraction and Mach number increase.

This paper describes a problem of measuring a gas flow temperature using thermocouples for which the time it takes to reach an equilibrium temperature is shorter than the duration of the measurement process. Results for the numerical simulation of a gas flow in a sensor used to measure a stagnation temperature in short-term wind tunnels are presented. The conjugate problem of a sensor in a supersonic flow is solved and the flow field inside the stagnation chamber is calculated. The temperature of the thermocouple place at the end of the stagnation chamber is determined. The results of simulating the thermocouple readings depend on time and the oncoming flow parameters. The obtained readings of the stagnation temperature sensor are taken as virtual experiment data, which are processed using experimental aerodynamics methods. The “step process” and “two thermocouples” methods are used to restore the stagnation temperature. A difference in thermocouple readings is a normalized thermocouple hardware function. True temperature readings are restored by deconvolution. The restored readings are compared with the initial values of the stagnation temperature in the incoming flow to the sensor. The sources of measurement errors are determined, and the applicability of experimental methods for determining the stagnation temperature in short-term aerodynamic installations, including those with parameters decreasing with time, is substantiated.

The present study elaborates the response of a heat source along with thermomechanical loading in a modified Green-Lindsay generalized thermoelastic half-space with nonlocal and two-temperature parameters. The problem is formulated for the model under consideration by reducing the governing equations into a dimensionless form. The problem is solved by using the Laplace and Fourier transforms. The physical field quantities, such as the stresses, displacement vector components, thermodynamic temperature, and conductive temperature, are found in the domain obtained after the Laplace and Fourier transforms. Numerical inversion techniques are used to recover the equations in the physical domain. Results obtained by using various thermoelasticity theories are compared.

In the present study, entropy generation in a non-axisymmetric steady-state incompressible viscous flow over a single rotating porous disk is investigated analytically. The dimensionless form of the local and total entropy generation caused by frictional and thermal effects are derived. The effects of the injection rate and Brinkman number on the entropy generation are revealed. It is found that there exists an optimal injection rate for each Brinkman number, which minimizes the total entropy generation.

A mathematical model of unsteady oil flow to a vertical well in a circular reservoir taking into account degassing in the bottomhole zone is proposed. The effects of saturation pressure and gas factor on pressure recovery curves and their derivatives are investigated. It is shown that the saturation pressure value characterizes the radius of the degassing zone. The change in permeability in this zone is affected by both the saturation pressure and the gas factor. Based on the proposed model and using the theory of inverse problems, a method was developed for interpreting the results of hydrodynamic studies of vertical wells operating with bottomhole pressure below the saturation pressure. This method can be used to evaluate the saturation pressure and permeability of the reservoir from pressure recovery curves.

Fluid flow in a hydraulic fracture and the reservoir surrounding the fracture and the dynamics of pressure and flow rate in transient well operation were studied theoretically. Solutions describing the relationship between fluid flow rate and pressure evolution in the fracture were obtained. The main dependences of reservoir parameters and fracture characteristics on time were determined.

In order to improve the accuracy and efficiency of solving the simulation model of high-frequency pulse electrochemical machining of a cross groove, a multi-physics coupling model based on the turbulent bubble flow model coupling the electric field and temperature field models is established. The direct current average voltage is proposed to replace the high-frequency pulse voltage. Experimental results are also provided.

This paper describes the study of a linear formulation of a problem of the stability of boundary layers with a pressure gradient, which are formed in a flow around surfaces with a single-layer viscoelastic coating. Calculations account for a change in the external flow velocity in the longitudinal direction, and experimental dependences describing the coating characteristics and frequency are used. The e N method is used to estimate the influence of a compliant coating on the position of a laminar-turbulent transition region is estimated. It is shown that, even in the case of a sufficiently rigid coating, a laminar-turbulent transition in a boundary layer with a favorable pressure gradient can be delayed along the longitudinal coordinate by approximately 41%.

Effect of preliminary heat treatment of CuA_l10Fe₃Mn₂ bronze particles in the working chamber of a furnace with varying atmosphere on the process of applying coatings by cold gas-dynamic spraying and their properties is experimentally studied. It is shown that an temperature rise in the working chamber of the furnace reduces the microhardness of the material from HV_0.025 = 240 to HV_0.025 = 115. It is noted that the spraying process is accompanied by an increase in the specific mass of the coatings from 1.62 to 3.50 kg/m2. It is shown by the study of the surface structure and physical characteristics that, for the coatings obtained from the original bronze powder, porosity is 2%, the arithmetic mean of the absolute values of profile deviations within the base length R_a = 27 µm, and microhardness is HV₁ = 250. For the coatings obtained from heat-treated bronze powder, porosity is 5%, R_a = 21 µm, and microhardness is HV_0.1 = 300.

The problem of optimizing the bearing capacity of a stringer panel with a hole and a straight crack propagating from its contour is considered. The goal is to determine a hole contour at which the stress intensity factor in the vicinity of the crack tip is equal to zero (i.e., the crack is immobile) and at which the maximum circumferential stress is the smallest. A minimization problem is solved using the method of successive approximations, the minimax criterion, and the simplex algorithm.

This paper presents a volumetric feeder of non-free-flowing powders which is comparable in dosing precision to weighing systems. The efficiency of the main units was studied numerically (using the discrete element method) and experimentally. A continuous high-precision volumetric feeder design is proposed.

The spatial vibrations of a pipeline on two supports vibrating in the vertical direction with equal amplitudes and phases are considered. The pipeline movement consists of rotation (rolling) around a straight line passing through the supports and bending deformation. It is assumed that the bending occurs in the rolling plane without torsional deformation. Nonlinear equations of bending and angular motion of the pipeline are used. The pipeline movement was studied in the case where the bending is much more influenced by the internal pressure of the medium in the pipe than by its weight. The ranges of input parameters in which the upper and lower bent states of the pipeline are stable are determined by analogy with the Kapitza pendulum. Numerical calculations were carried out for a reinforced magnesium tube filled with methane.

The initiation of an edge annular longitudinal shear crack (III fracture mode) during torsion of a circular cross-section bar made of an elastoplastic material with ultimate strain is considered. The fracture process of such materials is described using a modified Leonov-Panasyuk-Dugdale model, which uses an additional parameter - the diameter of the plastic zone (the width of the prefracture zone). To obtain the critical parameters of quasibrittle fracture, a double (sufficient) strength criterion for mode III cracks in an elastoplastic material is proposed. Diagrams of quasibrittle fracture of a circular shaft with an edge crack are plotted. For quasiductile and ductile types of fracture, the ultimate loads are found numerically by the finite element method. The difference between the numerical and analytical models is that the characteristics of the investigated materials correspond to the deformation modes in full-scale yielding. It is found that the results of numerical experiments are in good agreement with the results of calculations using the analytical model of fracture of specimens during torsion in the mode of small-scale plastic flow.