A. A. Chesnokov

Mixing layer under a free surface

In the long-wave approximation, the flow of a homogeneous fluid with a free surface in the gravity field is considered. Mathematical models of the surface turbulent layer in shear flows are derived. Steady solutions of the problem of evolution of the mixing layer under the free surface and formation of a surface turbulent jet are constructed. In particular, the problem of the structure of a turbulent bore in a supercritical flow is solved, and the conditions for the formation of a local subcritical zone ahead of the obstacle are studied.

Propagation of nonlinear perturbations in a quasineutral collisionless plasma

For the nonlinear kinetic equation describing the one-dimensional motion of a quasineutral collisionless plasma, perturbation velocities are determined and conditions of generalized hyperbolicity are formulated. Exact (in particular, periodical) solutions of the model are constructed and interpreted physically for the class of traveling waves. Differential conservation laws approximating the basic integrodifferential equation are proposed. These laws are used to perform numerical calculations of wave propagation, which show the possibility of turnover of the kinetic distribution function.

Stability criterion of shear fluid flow and the hyperbolicity of the long-wave equations

It is shown that the conditions of hyperbolicity of the integrodifferential equations of long waves correspond to the stability criteria of shear flows of an ideal fluid.

Special class of solutions of the kinetic equation of a bubbly fluid

A new class of solutions is constructed for the kinetic model of bubble motion in a perfect fluid proposed by Russo and Smereka. These solutions are characterized by a linear relationship between the Riemann integral invariants. Using the expressions following from this relationship, the construction of solutions in the special class is reduced to the integration of a hyperbolic system of two differential equations with two independent variables. Exact solutions in the class of simple waves are obtained, and their physical interpretation is given.

Characteristic properties and exact solutions of the kinetic equation of bubbly liquid

The paper considers a kinetic model for the motion of incompressible bubbles in an ideal liquid that takes into account their collective interaction in the case of one spatial variable. Generalized characteristics and a characteristic form of the equations are found. Necessary and sufficient hyperbolicity conditions of the integrodifferential model of rarefied bubbly flow are formulated. Exact solutions of the kinetic equation for the class of traveling waves are derived. A solution of the linearized equation is obtained.

Axisymmetric Vortex Fluid Flow in a Long Elastic Tube

A mathematical model for axisymmetric eddy motion of a perfect incompressible fluid in a long tube with thin elastic walls is proposed. Necessary and sufficient conditions for hyperbolicity of the system of equations of motion for flows with monotonic radial velocity profiles are formulated. The propagation velocities of the characteristics of the system under study and the characteristic shape of this system are calculated. The existence of simple waves continuously attached to a given steady shear flow is proved. The group of transformations admitted by the system is found, and submodels that determine invariant solutions are given. By integrating factor‐systems, new classes of exact solutions of equations of motion are found.

Roll Wave Structure in Long Tubes with Compliant Walls

We consider a flow of a fluid in a long vertical tube with elastic walls and show that, for certain parameters of the flow, small perturbations of the flow at the inlet section of the tube give rise to roll waves. Depending on the properties of the closing relation, either regular or anomalous roll waves are formed. In the latter case, a roll wave is characterized by two strong discontinuities that connect regions of continuous flow. We present the results of numerical simulations of the development of a pulsatile flow mode for convex and nonconvex closing relations that demonstrate the formation of regular and anomalous roll waves. We also construct a two-parameter class of exact periodic solutions and obtain existence diagrams for roll waves.

On the Propagation of Long‐Wave Perturbations in a Two‐Layer Free‐Boundary Rotational Fluid

A mathematical model for the propagation of long‐wave perturbations in a free‐boundary shear flow of an ideal stratified two‐layer fluid is considered. The characteristic equation defining the velocity of perturbation propagation in the fluid is obtained and studied. The necessary hyperbolicity conditions for the equations of motion are formulated for flows with a monotonic velocity profile over depth, and the characteristic form of the system is calculated. It is shown that the problem of deriving the sufficient hyperbolicity conditions is equivalent to solving a system of singular integral equations. The limiting cases of weak and strong stratification are studied. For these models, the necessary and sufficient hyperbolicity conditions are formulated, and the equations of motion are reduced to the Riemann integral invariants conserved along the characteristics.