Vol 58, No 5 (2017)
- Year: 2017
- Articles: 22
- URL: https://journals.rcsi.science/0021-8944/issue/view/9739
Article
Monitoring of hemodynamics of brain vessels
Abstract
This paper describes the results of intraoperational monitoring of hemodynamic parameters (velocity and pressure) in brain vessels, carried out within the framework of 50 neurosurgical operations using a Volcano ComboMap instrumental measuring system. It is established that the introduced parameter of specific load used during the neurosurgical operations is a significant parameter for the success of the operation.
Variational approach to the study of processes of geophysical hydro-thermodynamics with assimilation of observation data
Abstract
This paper presents a variational approach to solving direct and inverse problems based on the joint use of mathematical models and data monitoring of processes of geophysical hydro-thermodynamics. This approach is used to solve problems related to environmental protection. A variational principle with weak constraints is formulated to account for uncertainties and errors in models and data. The inclusion of uncertainties makes it possible to develop direct non-iterative algorithms for sequential assimilation of data obtained by various observation systems. Some criteria and functions for controlling the quality of the natural environment are introduced into the modeling system to solve inverse problems of environmental risk assessment. A problem with data assimilation is considered for the Novosibirsk agglomeration.
Rayleigh–Taylor instability of high-velocity condensed-matter liners
Abstract
A review of publications on the Rayleigh–Taylor instability arising during high-velocity implosion of liners is presented. Papers that describe experimental testing and numerical simulation of the development and suppression of this instability are also considered.
Characteristic properties of the system of equations for an incompressible viscoelastic Maxwell medium
Abstract
Characteristics of a system of equations that describe three-dimensional motion of an incompressible viscoelastic Maxwell medium with the upper and lower convective derivatives and the rotational Jaumann derivative being used in the rheological relation are calculated. An initial-boundary-value problem is formulated for the system linearized in the vicinity of the state at rest, and its unique solvability is established.
Simulation of supersonic flows on the basis of splitting algorithms
Abstract
For the numerical simulation of aerodynamics problems, the Euler and Navier — Stokes equations written in integral form are used to construct an implicit finite-volume predictor-corrector scheme. At the predictor stage, the splitting of equations into physical processes and spatial directions is introduced, which makes it possible to reduce the solution of the original system to the solution of individual equations on fractional steps by the scalar sweep method and ensure the stability of the algorithm as a whole. The paper also describes the supersonic gas flows in a narrowing channel with regular and non-regular reflection of the compression shock from the symmetry plane and the numerical substantiation of the existence of pulsating flow with a supersonic flow past a cylinder with a needle.
Internal wave bore in the shelf zone of the sea
Abstract
This paper presents the observation results for the internal wave bore in the coastal region of the Sea of Japan with the use of vertical thermistor chains. The data obtained is interpreted by the mathematical models of shallow water in which the effect of nonlinearity and dispersion on the propagation of internal wave trains is taken into account. Within the framework of the theory of multilayer shallow water, the problem of transformation of a solitary wave into an internal bore is solved, and the possibility of recovery of a space-time picture of the flow during the passage of an internal bore in the section between adjacent experimental bottom stations is demonstrated.
Spontaneous rotation in the exact solution of magnetohydrodynamic equations for flow between two stationary impermeable disks
Abstract
Magnetohydrodynamic (MHD) flow of a viscous electrically conducting incompressible fluid between two stationary impermeable disks is considered. A homogeneous electric current density vector normal to the surface is specified on the upper disk, and the lower disk is nonconducting. The exact von Karman solution of the complete system of MHD equations is studied in which the axial velocity and the magnetic field depend only on the axial coordinate. The problem contains two dimensionless parameters: the electric current density on the upper plate Y and the Batchelor number (magnetic Prandtl number). It is assumed that there is no external source that produces an axial magnetic field. The problem is solved for a Batchelor number of 0–2. Fluid flow is caused by the electric current. It is shown that for small values of Y, the fluid velocity vector has only axial and radial components. The velocity of motion increases with increasing Y, and at a critical value of Y, there is a bifurcation of the new steady flow regime with fluid rotation, while the flow without rotation becomes unstable. A feature of the obtained new exact solution is the absence of an axial magnetic field necessary for the occurrence of an azimuthal component of the ponderomotive force, as is the case in the MHD dynamo. A new mechanism for the bifurcation of rotation in MHD flow is found.
Direct numerical simulation of the transition to turbulence in a supersonic boundary layer on smooth and rough surfaces
Abstract
Direct numerical simulations of instability development and transition to turbulence in a supersonic boundary layer on a flat plate are performed. The computations are carried out for moderate supersonic (free-stream Mach number M = 2) and hypersonic (M = 6) velocities. The boundary layer development is simulated, which includes the stages of linear growth of disturbances, their nonlinear interaction, stochastization, and turbulent flow formation. A laminar–turbulent transition initiated by distributed roughness of the plate surface at the Mach number M = 2 is also considered.
Dynamics of discontinuity formation in a cavitating liquid layer under shock wave loading
Abstract
The problem of experimental modeling of discontinuity formation in a cavitating liquid layer under shock wave loading is considered. It is shown that the discontinuity takes the shape of a spherical segment and retains it up to the closure instant. The discontinuity surface becomes covered with a dynamically growing thin boundary layer consisting of bubbles, which transforms to a ring-shaped vortex bubble cluster at the instant of the discontinuity closure, generating a secondary shock wave. Specific features of the structure of the cavitating flow discontinuity arising at loading intensities lower than 0.1 and 5 kJ are discussed.
Turbulization of the wake behind a single roughness element on a blunted body at a hypersonic Mach number
Abstract
The influence of a cylinder-shaped single roughness element on the laminar–turbulent transition in the presence of an entropy layer is experimentally studied. The experiments are performed on a blunted cone model at the Mach number M = 5. The roughness element is located on the blunted tip of the model. Information about the mean and fluctuating parameters of the boundary layer in the wake behind the roughness element is obtained by using hot-wire anemometry. It is shown that flow turbulization behind the roughness elements occurs at the local Reynolds number calculated on the basis of the roughness element height and equal to 400–500. It is found that the presence of the roughness element exerts a significant effect on the unsteady characteristics of the boundary layer if the roughness element height is smaller than the effective value.
A new algorithm of mass flow rate determination in gas production and transportation systems via pressure measurement
Abstract
The current algorithm for calculating mass flow rate in gas production and transportation systems from outlet pressure measurements is generalized to the case where the inner cross section of the pipe changes with time and is also to be determined in the course of solving the general problem. The generalized algorithm is recommended for identification of gas hydrate formation in the above-mentioned systems. The identification of hydrates in a main gas pipeline in permafrost is considered as an example.
On the theory of seepage waves of pressure in a fracture in a porous permeable medium
Abstract
Seepage pressure waves in fractures in a porous permeable medium are studied. The effects of the reservoir and fracture porosity and permeability, the fracture width, and the rheological properties of the saturating fluid on the perturbation dynamics in the fracture are analyzed. It is shown that in porous permeable reservoirs, fractures are wave channels through which low-frequency fluctuations of borehole pressure propagate. Accurate solutions are obtained which describe the evolution of pressure fields in a fracture with an instantaneous change in the borehole pressure by a constant value. Based on these solutions, dependences of the fluid flow rate on time and interface pressure are determined.
Thermal limitations in a rapid-fire multirail launcher powered by a pulsed magnetodhydrodynamic generator
Abstract
The operation of rapid burst firing multirail electromagnetic launchers of solids is numerically simulated using unsteady two-dimensional and three-dimensional models. In the calculations, the launchers are powered by a Sakhalin pulsed magnetohydrodynamic generator. Launchers with three and five pairs of parallel rails connected in a series electrical circuit are considered. Firing sequences of different numbers of solid projectiles of different masses is modeled. It is established that the heating of the rails is one of the main factors limiting the performance of launchers under such conditions. It is shown that the rate of heating of the rails is determined by the nonuniformity of the current density distribution over the rail cross-section due to the unsteady diffusion of the magnetic field into the rails. Calculations taking into account the unsteady current density distribution in the rails of a multirail launcher show that with an appropriate of the mass of the projectiles (up to 800 g), their number in the sequence, and the material of the rails, it is possible to attain launching velocities of 1.8–2.5 km/s with moderate heating of the rails.
Synthesis of diamond structures from the jet of the H2 + CH4 mixture in a cocurrent axisymmetric hydrogen flow
Abstract
The flow of a hydrogen–methane mixture through heated coaxial cylindrical tungsten channels with a built-in tungsten wire is studied by the Direct Simulation Monte Carlo method. The purpose of the study is further development of the gas-phase method of deposition of diamond structures. The axial distributions of the concentrations of the components of the hydrogen–carbon mixture are calculated by means of solving a system of chemical kinetics equations. A series of experiments on deposition of diamond structures from various flows of the hydrogen–methane mixture is performed. The calculated results are compared with the experimental data. Based on these comparisons, it is concluded that numerical optimization of operation modes of gas-dynamic reactors can be used for deposition of diamond structures.
Experimental estimation of evaporation rates of water droplets in high-temperature gases
Abstract
Evaporation rates of water droplets in high-temperature gases were experimentally determined using high-speed video recording cameras and low-inertia thermocouples (for heated air flow as an example). The experiments were carried out for droplets of initial size (radius) 1–3 mm at an air temperature of 500–1000 K. Dependences of the evaporation rate of water droplets on time and gas temperature were obtained for various initial droplet sizes.
Numerical simulation of titanium dissolution in the aluminum melt and synthesis of an intermetallic compound
Abstract
Titanium dissolution in the aluminum melt and synthesis of an intermetallic compound at constant temperature and pressure are numerically simulated by the molecular dynamics method. Owing to titanium dissolution, the TiAl3 intermetallic compound is formed near the interface between the titanium crystal and aluminum melt. Based on the theory of weak solutions, a mathematical model of titanium dissolution in the aluminum melt is constructed. Dependences of the diffusion coefficient, equilibrium concentration of titanium, and dissolution rate on temperature are obtained.
Modified equations of finite-size layered plates made of orthotropic material. Comparison of the results of numerical calculations with analytical solutions
Abstract
This paper describes the modified bending equations of layered orthotropic plates in the first approximation. The approximation of the solution of the equation of the three-dimensional theory of elasticity by the Legendre polynomial segments is used to obtain differential equations of the elastic layer. For the approximation of equilibrium equations and boundary conditions of three-dimensional theory of elasticity, several approximations of each desired function (stresses and displacements) are used. The stresses at the internal points of the plate are determined from the defining equations for the orthotropic material, averaged with respect to the plate thickness. The construction of the bending equations of layered plates for each layer is carried out with the help of the elastic layer equations and the conjugation conditions on the boundaries between layers, which are conditions for the continuity of normal stresses and displacements. The numerical solution of the problem of bending of the rectangular layered plate obtained with the help of modified equations is compared with an analytical solution. It is determined that the maximum error in determining the stresses does not exceed 3 %.
Dynamics of deformation of an elastic medium with initial stresses
Abstract
The constitutive equations of motion of an elastic medium with given initial stresses are formulated in the form of a hyperbolic system of first order differential equations. Equations describing the propagation of small perturbations in a prestressed isotropic medium with an arbitrary dependence of the elastic strain energy on the strain tensor are derived, and equations for the quadratic dependence of elastic strain energy on the strain tensor are given.
Analytical modeling of the flexible rim of space antenna reflectors
Abstract
This paper presents a geometrically nonlinear analytical model of the flexible cylindrical rim of a deployable precision large space antenna reflectors made of shape-memory polymer composites. A nonlinear boundary-value problem for the rim in the deformed (folded) configuration is formulated and exact analytical solutions in elliptic functions and integrals describing the deformation modes of the rim are obtained. Exact analytical solutions based on the geometrically nonlinear model are obtained and can be used to determine preliminary geometric dimensions and optimal shape of the flexible rim along with the estimation of the accumulated energy.
Development of a system of plane radial cracks during explosion of linear blasthole and borehole charges
Abstract
Numerical algorithms have been developed to determine the shape and dimensions of radial cracks formed during confined explosions of linear charges in a monolithic rock massif at a great depth and near its surface. The influence of the charge length on the dimensions of radial cracks in a confined explosion has been studied. The burial depth of a borehole or blast charge for which the area of radial cracks is maximal is determined.
Effect of Mg and Cu on mechanical properties of high-strength welded joints of aluminum alloys obtained by laser welding
Abstract
Results of experimental investigations of welded joints of high-strength aluminum–lithium alloys of the Al–Cu–Li and Al–Mg–Li systems are reported. The welded joints are obtained by means of laser welding and are subjected to various types of processing for obtaining high-strength welded joints. A microstructural analysis is performed. The phase composition and mechanical properties of the welded joints before and after heat treatment are studied. It is found that combined heat treatment of the welded joint (annealing, quenching, and artificial ageing) increases the joint strength, but appreciably decreases the alloy strength outside the region thermally affected by the welding process.
Creation of heterogeneous materials on the basis of B4C and Ni powders by the method of cold spraying with subsequent layer-by-layer laser treatment
Abstract
A method is proposed for creating principally new functionally graded heterogeneous materials on the basis of B4C ceramic powders with different mass fractions in the original mixture and plastic metallic additive of Ni by a combined method of cold spraying with subsequent layer-by-layer laser treatment. Mechanical properties of the resultant tracks are examined. It is shown that the track microhardness increases with increasing B4C concentration in the original mixture. The track structure is found to depend on the size of ceramic particles in the interval from 3 to 75 μm. Reduction of the B4C particle size (approximately by a factor of 2–3) inside the track owing to fragmentation under the action of the laser beam is observed for the first time.