Fluid Dynamics最新文献

A laboratory technique has been developed to study the effect of dissolved substances on the condensational growth of spherical droplets of water in a self-arranged droplet cluster levitating above a locally heated water surface, as well as on the equilibrium droplet size obtained by infrared heating of the cluster. Inorganic salts such as potassium and sodium chlorides were shown to significantly influence the condensation/evaporation process of water droplets even at low solute concentrations. In contrast, the influence of typical substances used in plant treatments is negligible. The new experimental results can be used to model various technological processes involving aqueous aerosols. These results might also be useful in studies of moisture transfer and precipitation formation in the atmosphere.

An experimental study was conducted to investigate the thermal fields in the boundary layer along the wall of a gas-dynamic channel near a rectangular insert. The study focused on conditions following the passage of a shock wave and during the initiation of a pulsed surface discharge in the flow. The heating and cooling dynamics of the region affected by the pulsed sliding discharge along the dielectric surface in the flow separation zone were examined. Registration of the radiation of the channel walls in the range of 1.5–5.1 µm was carried out through the side windows of the test (discharge) chamber of the shock tube, transparent both for the thermal radiation of the walls and for the visible radiation of the discharge. It is shown that the cooling of the insert region, heated by a localized nanosecond discharge in the leeward zone, occurs in less than a millisecond; on the shock-heated surface of the channel in the windward zone of the insert, cooling occurs in several milliseconds. The study measured radiative, conductive and convective components of heat fluxes in various supersonic flow configurations. The experiments were conducted in the range of shock wave Mach numbers ({{{text{M}}}_{0}} = 2{-} 4) and high-speed flows behind them, respectively, with Mach numbers ({text{M}} = 1.1{-} 1.4).

One-sided density-driven convection in a porous medium is simulated numerically with reference to hydrodynamic processes occurring during injection of carbon dioxide into underground porous formations. When carbon dioxide dissolves in water or oil, the density of solution increases. This leads to the growth of instability. A hydrodynamic model that includes the continuity equation, the equation of motion (in the form of Darcy equation), and the convection-diffusion equation has been used. The equation of state that relates the density of the fluid phase to the concentration of carbon dioxide is nonlinear. The density of solution reaches a maximum at a certain concentration, which varies. A new computational code based on the finite-difference method has been developed to solve the problem. The effect of the concentration that gives the maximum density on the parameters of convective motion and mass transfer is investigated. In particular, it is found that if the maximum density occurs at a higher concentration, the amount of carbon dioxide that is transported downward by the convective flow increases. This means that, in this case, convective dissolution is more effective in trapping of carbon dioxide at depth.

Heat transfer in double-pipe heat exchangers with diffuser channels with small opening angles with gas and liquid coolants is numerically simulated. In the calculations, a three-parameter differential RANS turbulence model supplemented with a transfer equation for a turbulent heat flow is used. It is shown that due to the intensification of heat transfer in heat exchangers with diffuser channels, the amount of heat transferred from the hot coolant to the cold one increases compared to heat exchangers with channels with a constant cross section.

In this paper, the first experimental results of measuring the time profiles of the atomic oxygen concentration by atomic resonance absorption spectroscopy (ARAS, 130.5 nm) obtained using the developed experimental complex combining shock-wave heating and pulsed laser photolysis (LP, 193 nm) of gas mixtures are presented. Using the example of photolysis of oxygen molecules and the reaction of O atoms with methane, the capabilities of the developed setup for studying the kinetics of elementary reactions are demonstrated. The temperature dependence of the absorption cross section of oxygen and methane molecules for a wavelength of 130.5 nm is obtained. The efficiency of oxygen atom formation during the laser photolysis of oxygen molecules is determined in the temperature range of 700–1900 K at laser pulse energies of 300–400 mJ. The rate constant of the reaction of oxygen atoms with methane at temperatures of 770–1600 K and pressures of 3–4 bar is obtained. Additionally, numerical modeling of experimental profiles is carried out using current kinetic schemes of hydrocarbon combustion.

For the compressible turbulent boundary layer, the results of the numerical study using the three-parameter RANS turbulence model are compared with the results of direct numerical simulation (DNS). It is shown that the calculation results using the RANS model are in satisfactory agreement with the DNS results at the Mach numbers from 6 to 14. This makes it possible to recommend the use of the RANS model in engineering calculations of the hypersonic boundary layer when there is no need for powerful computing systems.

Extended (up to several tens of centimeters) high-current (hundreds of amperes) electric arcs in various gases at the atmospheric pressure are studied experimentally and theoretically. Such discharges have been studied on the electric discharge stand of the P-2000 facility of the Institute of Mechanics of Moscow State University. The data on the influence of an external magnetic field on the stability of such discharges and the formation of branched current channels are clarified. One of the areas of the research is the study of the effect of the orientation of the magnetic field imposed on the arc on the processes of development of the discharge in various gas media, such as air, CO₂, Ar, and N₂. The data for argon and nitrogen are presented most fully. The experiments were carried out in a chamber with transparent walls. The calculation and the theoretical study are carried out on the basis of an electrical engineering model using the empirical data on the volt-ampere characteristics of arcs between graphite electrodes. It is found that the stability of high-current arcs is significantly affected by electrode jet-flame dynamics. At the same time, the traditional models of arcs in the external magnetic field without taking these factors into account show that the direction of the external axial field does not affect the stability of the arcs, affecting only the direction of their twisting during the development of helical instability.

The impact of droplets on surfaces is widely applied in inkjet printing and additive manufacturing. Industrial fluids often exhibit non-Newtonian properties (e.g., shear-thinning or viscoelasticity) due to additives. While existing studies focus on Newtonian fluids, this work investigates non-Newtonian droplet dynamics using a numerical model combining the volume of fluid (VOF) and level set methods to track phase interfaces. The effects of polymer concentration on the droplet impact behavior are analyzed. The results show that increase in the polymer concentration enhances viscous dissipation during impact, leading to significant morphological changes. Specifically, the higher concentrations reduce the maximum dimensionless spreading diameter, increase the maximum dimensionless height, delay the splashing onset, elevate secondary droplet positions, and amplify lateral deviation from the centerline. Upon impacting the high-temperature surfaces, the surface heat flux of polymer droplets initially increases and then decreases due to field synergy effects. These findings establish predictive correlations for controlling droplet deposition in oil–water separation applications, emphasizing the critical role of rheological tailoring in optimizing impact outcomes.

Free internal waves in a uniformly stratified fluid are considered in the Boussinesq approximation with regard for the Earth’s rotation. It is shown that the dispersion relation, derived with taking into account the horizontal component of the angular velocity of the Earth’s rotation at constant wave frequency, is reduced to the canonical equation for second-order curves in the plane of horizontal wave numbers. If the wave frequency is higher than the inertial frequency and less than the Brunt-Väisälä frequency, the frequency isolines are ellipses. If the wave frequency is higher than the buoyancy frequency, then the frequency isolines are hyperbolas; and if the wave frequency is equal to the Brunt-Väisälä frequency, then the isolines are two straight lines parallel the direction to the east. The vertical wave momentum fluxes are obtained as functions of the direction of wave propagation. It is shown that the fluxes are maximum in absolute value when the wave propagates to the north or to the south. A comparison of the vertical momentum fluxes of internal and sub-inertial waves at the same length and the maximum wave amplitude is carried out. It is shown that the vertical momentum flux of sub-inertial waves is higher than that of internal waves and weakens with weakening of stratification.

The stability of spatially periodic flows of homogeneous and stratified fluid is investigated with regard for bottom friction. The Galerkin method with three basis Fourier harmonics is used to solve the stability problem. A system of ordinary differential equations for the amplitudes of the Fourier harmonics is formulated. A solution to the linearized version of the system is obtained and an expression for the increment of disturbance growth is found. It is established that at the nonlinear stage of development the exponential growth of linear disturbances is replaced by the regime of establishing steady-state periodic disturbances in form of closed cells. These disturbances reduce the averaged horizontal velocity of the flow. Analytical expressions for the spatial period and amplitude of steady-state disturbances are obtained.