Fluid Dynamics最新文献

This article studies an actual problem of a decrease in heat transfer because of the formation of a deposit on the walls of the heat transfer equipment. A new heat transfer similarity number of deposit formation that can describe its thermal and electric nature is presented here. New methods of experimental studies under free air convection with deposit on the heat transfer surface are presented and discussed. The deposit is a dense wet mass of NaCl salt. A new criterion equation for the natural air convection conditions is obtained and shown in the article. New methods for calculating the heat transfer during deposit formation based on the created heat transfer similarity number of deposit formation are developed.

The paper studies the dynamics of pressure waves in a pipe with water containing a water-air bubble cluster in the form of a hollow cylinder. A system of macroscopic equations for the laws of conservation of masses, the number of bubbles, momenta, and pressure in bubbles was used in the approximation of cylindrical symmetry. This system of equations was solved using an explicit scheme. The dependence of the maximum value of the pressure amplitude recorded in the pipe on the geometrical parameters of the cluster and pipe, as well as on the amplitude of the initial impact, was analyzed. It is shown that for the case of a bubble region in the form of a solid cylinder, the amplitude of the wave signal behind the bubble zone is several times higher than the amplitude for the case of a hollow cylindrical cluster.

A thermal nonequilibrium kinetic model for a syngas/air mixture taking into account the state-to-state (STS) vibrational kinetics of CO, N₂, O₂, H₂, and OH molecules is developed. The physical and chemical processes occurring in the mixture when CO molecules are highly vibrationally excited by the absorption of the resonant CO laser radiation are considered. It is shown that the nonequilibrium vibrational excitation of a CO molecule leads to the acceleration of chemical reactions and makes it possible to initiate effectively the combustion of a syngas/air mixture. Due to the radiation absorption and vibrational-chemistry coupling, strongly non-Boltzmann vibrational distributions of the molecules are formed, which significantly influence the syngas combustion kinetics. The results of the STS model are compared with the predictions of simplified models that do not take into account vibrational nonequilibrium or the disruption of local Boltzmann vibrational distributions.

In the asymptotic calculations of the first order of smallness by the dimensionless amplitude of capillary waves on the surface of charged jets of a polar liquid, the effect of the relaxation effect of surface tension on the regularities of their implementation is studied. Calculations are carried out on the model of an ideal incompressible electrically conductive fluid. It is shown that taking into account the effect of dynamic surface tension leads to an increase in the order of the dispersion equation, which has another damping root, describing the oscillations of the jet surface related to the destruction of the near-surface double electric layer (destruction of the ordering of polar molecules in the near-surface layer). At sufficiently large charges (prebreakdown in the sense of the ignition of a corona discharge in a gaseous medium), this solution becomes unstable, as a result of which the entire surface undergoes electrostatic instability. In the used mathematical model of an ideal fluid, the motion of the jet surface that occurs when the surface tension relaxation effect is turned on and the attenuation decrements of the capillary wave motions are purely of a relaxation nature.

The plasma-assisted ignition of a mixture of synthesis gas in air is considered, when either the air or synthesis gas is exposed to the discharge after which they are mixed and ignited, or otherwise their mixture is exposed to the discharge. The specific strengths of the electric field of the optimal discharge in terms of shortening the ignition time are determined. It is shown that the processes occurring during the transportation of discharge products to the mixing area can significantly alter the mechanism of the discharge effect on the ignition.

Within the framework of a three-dimensional boundary layer, the region of developed turbulent flow of an incompressible fluid under the action of a longitudinal unfavorable (positive) pressure gradient (hereinafter, referred to as UPG) in the diffuser is simulated. For a turbulent flow regime, the Reynolds-averaged Navier–Stokes equations in the boundary layer approximation are closed using a differential turbulence model based on the introduction of turbulent viscosity and the Kolmogorov–Prandtl hypotheses. Using the nonlinear least squares method (NLSM), correlation dependencies are found for the initial experimental data, which are further used in the numerical simulation. The calculated and experimental velocity profiles are compared in the region close to separation. Based on the numerical solutions of the turbulent boundary layer equations, the mechanisms of the interaction of the flows in the presence of transverse pressure gradients in an incompressible fluid are studied.

The ice cover is modeled by a thin elastic isotropic plate floating on the surface of a liquid of finite depth. The source of disturbances moves along the surface of the plate. The values of critical velocities at which the character of the wave disturbance changes are obtained. The angular zones in which the waves propagate are determined. The influence of the velocity of the source of disturbances, the thickness of the ice plate, and compression and stretching forces on the amplitudes of the formed waves is studied.

In this work, the far fields of internal gravity waves excited by a perturbation source moving in a vertically infinite stratified medium are studied. Propagation of waves in inviscid, incompressible medium with an exponential distribution of unperturbed density is considered. In the linear approximation and in the Boussinesq approximation, uniform asymptotics of excited fields of internal gravity waves far from the moving perturbation source are built, including those in the neighborhood of the traverse plane and the motion horizon. The obtained asymptotic solutions allow effectively computing the main amplitude-phase characteristics of excited far fields of internal gravity waves at certain modes of generation and, in addition, qualitatively analyzing the obtained solutions, which is important to correct formulation of more sophisticate mathematical models of wave dynamics of real natural stratified media.

The results of the calculation of the interaction of a supersonic jet flowing at a zero angle of attack from the frontal surface of the body of rotation to the opposite supersonic flow at different supersonic Mach numbers of the flow and jet are given. The influence of the total pressure in the gas flow and in the jet on the structure of the emerging gas flow is considered.

This publication presents an experimentally obtained video file for visualizing changes in the brightness of a radiation source based on a combination of continuous and repetitively pulsed optical discharges on a microsecond scale. The brightness was recorded with an electron-optical camera in the visible region of the spectrum in a nine-frame scan mode. A detailed description of the conditions and results of the experiments performed, the characteristics of the radiation source, as well as a discussion of the results obtained are published in [1, 2].