Fluid Dynamics最新文献

The nonlocal boundary conditions are formulated for mathematical modeling of wave dynamics of stratified media; these conditions take into account two substantial physical circumstances: the linear theory is valid at large distances from perturbation sources and there are no other sources of wave perturbations outside the mixing zone of the stratified medium. Using these boundary conditions allows correctly describing outgoing linear internal gravity waves excited by a region of partially mixed stratified medium. It is shown that with the results it is possible to determine the further dynamics of internal gravity waves far away from these perturbation sources by a given distribution of parameters of the stratified medium, assuming the validity of using the linear model of wave dynamics at large distances.

In this work, internal gravity waves are considered that propagate in a stratified medium of finite depth with variable background shear flows. In the linear formulation, using the Fourier method, the author constructed the Green function of the corresponding equation of internal gravity waves. The asymptotic of the Green function in the far zone expressed through the square of the Airy function is studied. It is shown that, for real distributions of buoyancy frequency and shear flow vector, the wave zone is bounded by two closed curves: head and rear fronts. In the case when the components of shear velocity vector less vary over the vertical, the head wave front is a circle expanding with a maximum group velocity of the separate wave mode without flows and shifted by the flow with a velocity equal to the average value of the shear velocity vector taken with some weight.

In this work, the analytical representations of far fields of internal gravity waves from local and nonlocal perturbation sources in a layer of arbitrarily stratified medium of finite depth are considered. The internal criteria of applicability of various asymptotic representations are formulated that provide a predetermined accuracy of asymptotic calculations of far wave fields. It is shown that for elongated perturbation sources, in the case when the characteristic dimensions of the source are comparable to the width of the first pulse of the wave, the corresponding convolution provides the maximum values of the elevation field of internal gravity waves in the neighborhood of the wave front and, oscillating, rapidly decays with distance from it, because the neighboring half-waves mitigate each other far from the wave front.

The merging of a falling drop with a water surface gives rise to a series of hydrodynamic phenomena that differ in time and space scales. Among them, fast unsteady flows are distinguished, in the process of which deformed underwater caverns and gas cavities separated from them are formed. When the surface of the separating gas cavity closes, the formed bubble undergoes volumetric oscillations, which, in turn, generate short acoustic packets propagating under water and in the air. Experimental data are presented in which the sequence of the formation of caverns is traced, and the processes of the formation and detachment of bubbles and accompanying acoustic signals are identified and detailed. In the composition of the sound packets recorded in the water and air environments, there are differences associated with the characteristics of propagation (transient damping at the water-air boundary) and the influence of the transmission functions of the hydrophone and microphone. In the case of detachment, reattachment, and reseparation of an air bubble from successive caverns, the frequency of the emitted signal increases. Despite the highly irregular shape of the emitting bubbles, the emission frequency remains constant, which indicates the volume of the bubble as the governing parameter of acoustic emission.

A diffusion-drift two-liquid two-temperature model of a gas-discharge plasma is used to describe the electrodynamic structure of the Penning discharge in molecular hydrogen at a pressure of the order of 1 mTorr. The effect of the discharge chamber’s geometrsic parameters, variations in the EMF of the power supply of an external electrical circuit, and the axial magnetic field induction on the discharge structure is studied by numerical simulation. The determining role of the flow of high-energy electrons formed during the azimuthal motion of plasma flows around the axis of symmetry of the discharge chamber on the ionization of neutral gas is shown. The modes of the quasi-monotonic and oscillatory averaged motion of charged particle fluxes in the axial regions are found.

In this work the authors construct the asymptotics of solutions describing internal gravity waves excited by a motionless localized perturbation source in a layer of an arbitrarily stratified medium. The analytical expressions for an individual mode in the stratified rotating medium are obtained both far from and near the wave fronts. The estimates for the effect of rotation of the stratified medium as a whole on the main characteristics of far wave fields are presented. The qualitative pattern of propagation of far wave fields is investigated. The peculiarities of wave fields at large time and limited distances are studied. It is shown that the field of internal gravity waves at large times appears to be more localized at a depth of source immersion and relatively distributed over the vertical. The impact of various parameters of the perturbation source nonlocality on the spatial patterns of far fields of internal gravity waves is investigated.

In this work the peculiarities of modeling the wave dynamics of stratified media are considered taking into account compressibility and viscosity. It is shown that the boundary layers may have a thickness of up to several meters under real ocean conditions, which is several percent of the half-wavelength of internal gravity waves for lower modes, and, because the several lower modes are excited in the ocean, viscosity has no significant impact on the wave dynamics of natural stratified media, ocean and atmosphere. The main features of dispersion relationships describing the wave dynamics of acoustic-gravity wave pulses in stratified media are investigated taking into account their compressibility.

The features of argon radiation mixed with molecular and atomic additives in condensing supersonic jets are analyzed. Mixtures of argon (95%) with methane (5%) and argon (95%) with monosilane (5%) are used. The mixture’s particles are activated by a well-focused electron beam. The dependence of the radiation intensity of individual argon lines on the gas-dynamic parameters in the jet is studied. In a certain pressure range, different various compositions of mixtures, not only anomalously intense emission was recorded on individual spectral lines of argon (Ar_I) in mixtures with methane and monosilane, but also the role of clusters of a certain size and composition was revealed. At the same time, a similar effect is not detected in the spectrum of argon (Ar_II) ions. It is established that the cause of the anomaly is a highly efficient molecular cluster mechanism of the selective excitation of the individual levels of argon atoms, which is absent in noncondensing jets and weakens at the stage of the formation of large clusters. The main channels of energy transmission are reviewed and discussed. Based on the data obtained, an empirical model of the excitation-emission process is proposed.

We present a brief overview of the most popular turbulence models and the validation results available for them. A high-speed flow around a blunted spherical cone is simulated using various commercial software programs. The problem setups in different software systems are as close to each other as possible. The numerical values of the heat flux density on the cone surface are compared with the experimental data. We demonstrate the influence of the turbulence present in the calculation on the value of the surface heat flux density.

This paper presents the algorithms and results of calculations of the dynamics of a surface layer of a fluid under the action of currents that have emerged from a depth. Several approaches to model the velocity field in a horizontal flow round a fixed underwater obstacle are investigated. Formulas for calculating the velocity field on the free surface of an ideal homogeneous fluid are proposed. A computer program is developed that makes it possible to simulate the interaction of a stratified fluid flow with an underwater obstacle. The possibility of using asymptotic formulas for the far-field approximation to calculate the velocity field in a uniformly stratified fluid is studied.