Fluid Dynamics最新文献

The results of studies on the intensification of heat transfer in round and flat continuous diffusers for a number of diffusers’ opening angles are presented. Numerical modeling of heat transfer is carried out using a three-parameter differential RANS turbulence model supplemented with a transfer equation for a turbulent heat flow. It is shown that at the same opening angle in a round diffuser, the friction coefficient, Nusselt number, and turbulence intensity are significantly higher than in a flat diffuser, and these excesses increase with an increase in the opening angle. The possibility of using diffuser channels in plate and round pipe-in-pipe heat exchangers is considered.

The family of velocity profiles of a submerged jet, which are absolutely unstable in the plane-parallel approximation, is considered. The profiles are specified by two parameters: the first of them is responsible for the location of the only inflection point in the velocity profile, and the second is responsible for the shear layer thickness. An algorithm for determining the length of the region of local absolute instability of the jet with a given input velocity profile, that is, the distance at which absolute instability gives way to convective instability, has been implemented. The dependence of this length on the parameters defining the input profile is obtained. A connection between the characteristics of local absolute instability calculated in the plane-parallel approximation and global instability of the jet evolving in space is analytically obtained. The input velocity profile that corresponds to sufficiently large length of the zone of local absolute instability, at which global instability of spatially developing jet occurs, is demonstrated. Thus, the possibility of existence of global instability of plane submerged jets with special velocity distributions is demonstrated.

The results of a consistent computational and experimental study of acoustic self-oscillations in an annular cavity surrounding a circular pipe with a local narrowing are given. In the experiment, pressure fluctuations were measured on the outer wall of the annular cavity for various volume flow rates; air entered the pipe at the atmospheric pressure. It was found that the flow regime with excitation of acoustic self-oscillations in the cavity is implemented in a certain range of flow rates. The oscillation frequency corresponds to the first natural frequency, and the root-mean-square values of pressure fluctuations reach a level of 2300 Pa. Numerical simulation based on the RANS approach, carried out for the geometry and conditions of experiment, reproduces the observed effect of acoustic excitation of the cavity and gives similar values of the fluctuation amplitude. The oscillation modes developed at various volumetric flow rates are analyzed based on the obtained calculated data.

The processes of adsorption of N and O atoms on the surfaces of the thermal protection material SiO₂ are studied by the methods of quantum mechanics and molecular dynamics. The potential energy surface (PES) is calculated using the electron density functional theory. Based on the obtained PESs, the rate constants of adsorption of N and O atoms are determined by molecular dynamics methods in a wide range of surface temperatures of 500–2200 K and presented in the form of a generalized Arrhenius formula. The calculated rate constants are compared with the known phenomenological models and the calculation results based on the transition state theory.

Nonmodal development of stationary three-dimensional disturbances in a circular jet is numerically investigated at the Reynolds number Re = 2850. The operating conditions of a laboratory experiment performed earlier in the Institute of Mechanics of Moscow State University are reproduced. A method for calculating optimal disturbances under the conditions of downstream developing main flow is developed. The disturbances associated with different azimuthal numbers are calculated. The shape, character of development, and growth degree of optimal disturbances are determined.

We present the results of an experimental and numerical investigation of the floating-up of a solitary gas bubble in the presence and absence of dissolved surfactant at Reynolds numbers Re > 1. An original numerical technique is proposed, which makes it possible to investigate the dynamics of a solitary bubble with account for the effects occurring when a surfactant is introduced into the fluid. The effect of the surfactant concentration on the bubble dimensions, shape, and rise velocity are analyzed. Three stages of the bubble rise in the presence of a surfactant are established; they characterize its accumulation and distribution over the free surface of the bubble.

The splashes of a highly viscous fluid (glycerol) resulting from its pulsed displacement from a gap between two rapidly approaching disks are studied. It is found that, outside the disks, the splash has the form of a thin film bounded by an annular rim. A physical model of the splash is formulated, and analytical solutions describing its trajectory are given. The calculation results are compared with experimental data. The effects of fluid viscosity, surface tension, and film breakdown are analyzed. It is shown that the key influence on the splash development scenarios is exerted by surface tension of the film connecting the rim to the disks.

Using thermal nonequilibrium physicochemical kinetics models, which make it possible to numerically study the reacting gas flows with allowance for the processes of vibrational and electronic-translational relaxation and exchange, as well as the chemical and plasmachemical reactions, the profiles of the Gladstone–Dale constant and refractive index are calculated for two model problems: relaxation of the air behind the shock wave (SW) front and expansion of hydrogen and syngas combustion products in a supersonic nozzle. The required data on the electrical properties (polarizability and dipole moment) of the neutral and charged gas components in their ground and excited states are taken from the literature or estimated using recently developed analytical models. For each of the model gas-dynamic problems, the accuracy of various approximations when calculating the optical properties of a gas is analyzed.

Numerical simulation of convection of a colloidal binary mixture in a Hele-Shaw cell under vertical vibrations of finite amplitude has been carried out. The boundary of convective instability in a modulated gravity field is found. Bifurcation diagrams are constructed and the concentration distributions of nanoparticles corresponding to various solutions are analyzed. Thermal vibration convective flows are studied in the case of positive thermal diffusion of nanoparticles and their gravitational stratification. It is shown that vibrations, depending on the frequency, can both increase the intensity of the convective flow and weaken it.