卷 60, 编号 2 (2019)

The exact solution of the Euler equations of relativistic hydrodynamics of an compressible fluid—a relativistic analog of the Ovsyannikov vortex (singular vortex) in the classical gas dynamics—was found and investigated. A theorem was proved which shows that the factor system can be represented as a union of a noninvariant subsystem for the function defining the deviation of the velocity vector from the meridian and an invariant subsystem for the function defining thermodynamic parameters, the Lorentz factor, and the radial component of the velocity vector. Compatibility conditions of the overdetermined noninvariant system were obtained. The stationary solution was studied in detail. It was proved that the invariant subsystem reduces to an implicit differential equation. The branching manifold of the solutions of this equations was studied, and many singular points were found. It is proved that there exist two flow regimes, i.e., the solutions describing the vortex source of a relativistic gas, was proved. One of these solutions is defined only at a finite distance from the source, and the other is an analog of supersonic gas flow from the surface of a sphere.

The development of perturbations of a tangential discontinuity surface separating two stationary flows of an ideal incompressible fluid slowly varying in space is studied taking into account surface tension. Perturbations are described using complex Hamiltonian equations. The dependences of the amplitude of perturbations on the coordinate and time are obtained.

Equations of ideal magnetohydrodynamics that describe stationary flows of an inviscid ideally electrically conducting fluid are considered. Classes of exact solutions of these equations are described. With the use of the natural curvilinear coordinate system, where the streamlines and magnetic force lines play the role of the coordinate curves, the model equations are partially integrated and converted to the form that is more convenient for the description of the magnetic lines and streamlines of particles. As the coordinate system used is related to the initial coordinate system by a nonlocal transformation, the group admitted by the system can change. An infinite-dimensional (containing three arbitrary functions of time) group of symmetries is calculated for the system in the natural coordinates. An optimal system of subgroups of dimensions 1 and 2 is constructed for this group. For one of the optimal system subgroups, an invariant exact solution is found, which describes the electrically conducting fluid flow of the vortex source type with swirling magnetic lines and streamlines.

A possibility of using the 4D Qflow protocol, which is commonly applied for medical diagnostics by magnetic resonance imaging, for determining the structure of the three-dimensional fluid flow in the human blood circulation system is considered. Specialized software is developed for processing DICOM images taken by a magnetic resonance scanner, and the retrieved unsteady three-dimensional velocity field is analyzed. It is demonstrated that magnetic resonance measurements allow one to detect the existence of the swirling flow in blood vessel models and also to study the degree of its swirling (helicity) both qualitatively and quantitatively.

A problem of changing of the orientation of a solid in a space by means of motion of the internal mass is under consideration. It is shown that it is possible for a solid to be arbitrarily reoriented due to special motions of the internal mass. Approaches to controlling the internal motions ensuring this reorientation are proposed.

The nonstationary motion of a spherical layer of an ideal fluid is investigated taking into account the adiabatic distribution of gas pressure in the internal cavity. The existence of nonlinear oscillations of the layer is established, and their period is determined. It is shown that there is only one equilibrium state of the layer. Amplitude equations taking into account the action of capillary forces on the surfaces of the layer in a linear approximation are obtained and used to study the stability of nonlinear oscillations of the layer. The limiting cases of a spherical bubble and soap film are considered.

This paper describes the inverse problems of heat transfer, arising in designing multilayered spherical cloaking shells and other functional devices for controlling thermal fields. It is assumed that shells are comprised of a finite number of layers, each filled by a homogeneous isotropic or anisotropic medium. An exact solution for a partial case of a single-layer homogeneous anisotropic shell is presented and analyzed. An optimization approach used to reduce the inverse problems under consideration to control problems. A numerical algorithm for solving them is proposed, which is based on a particle swarm optimization, and the results of numerical experiments are discussed.

A generalized phenomenological Leith model of wave turbulence in a medium with unsteady viscosity is under study. Group analysis methods are used to obtain the main models possessing nontrivial symmetries. All invariant submodels are determined for each model. Invariant solutions describing these submodels are either determined in explicit form or satisfy the integral equations obtained. The main models are used to study turbulent processes. At an initial instance and with a fixed value of the wave number modulus, either turbulence energy spectrum and its gradient or turbulence energy spectrum and the rate of its variation are specified for the above-mentioned models. It is determined that solutions of the problems describing these processes exist and are unique under certain conditions.

Horizontal shear motion of a homogeneous fluid in an open channel is considered in the approximation of the shallow water theory. The main attention is paid to studying the mixing process induced by the development of the Kelvin–Helmholtz instability and by the action of bottom friction. Based on a three-layer flow pattern, an averaged one-dimensional model of formation and evolution of the horizontal mixing layer is derived with allowance for friction. Steady solutions of the equations of motion are constructed, and the problem of the mixing layer structure is solved. The bottom friction produces a stabilizing effect and reduces the growth of the mixing layer. Verification of the proposed one-dimensional model is performed through comparisons with available experimental data and with the numerical solution of the two-dimensional equations of the shallow water theory.

A fundamentally new unsaturated technique for the numerical solution of the Dirichlet–Neumann problem for the Laplace equation was developed. This technique makes it possible, due to the smoothness of the sought solution of the problem, to take into account the axisymmetric specificity of the problem which is an insurmountable obstacle to any saturated numerical methods, i.e., methods with a leading error term.