CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Engineering

Volume 11 No. 2 June 2005

Special issue on interaction phenomena in multiphase flows EUROMECH Colloquium No. 447

Preface; 89–90

Jüri Engelbrecht and Ülo Rudi

Comparison between Lagrangian and Eulerian particle simulations coupled with DNS of homogeneous isotropic decaying turbulence; 91–105

André Kaufmann, Jerome Helie, Olivier Simonin and Thierry Poinsot

Abstract. A direct numerical simulation (DNS) approach to Eulerian–Eulerian dispersed two-phase flow is discussed. The need for a stress term in the momentum transport equation of the dispersed phase is identified and a simple model for this stress term is proposed. Measurements of the dispersed phase quantities such as number density, mesoscopic velocity and stress tensor components from a reference computation, using Lagrangian particle tracking, allows to validate the predictions of the Eulerian–Eulerian computation.

Key words: direct numerical simulation, solid particle, turbulent dispersion, continuum approach, preferential concentration.

Physics of inertial microparticles transfer in turbulent boundary layers; 106–125

Alfredo Soldati

Abstract. Turbulent dispersed flows in boundary layers are crucial in a number of industrial and environmental applications. In most applications, the key information is particle spatial distribution and it is known that inertial particles in turbulent flows distribute preferentially avoiding strong vortical regions and segregating into straining regions. It is also known that, in the specific case of boundary layers, this preferential accumulation occurs along the wall in a macroscopic way. Numerical and experimental works show that fluid motions in turbulent boundary layers are intermittent and have a strongly organized and coherent nature, represented by the large scale motions. These motions, even though not exactly repeatable and only quasi-deterministic, control the transport of the dispersed species. In this work, direct numerical simulations of turbulence in upward boundary layers in pipes and channels and Lagrangian tracking of inertial particles are employed to examine particle transfer in connection with the dynamics of the flow structures populating the boundary layer. It is argued that the local syncronicity between the events in the turbulence regeneration cycle and the mechanisms, controlling particle fluxes toward and away from the wall, determine the macroscopic net particle fluxes.

Key words: particles, turbulence, boundary layer, coherent structures.

Particles scattering in particle–wall collisions and its effect on the particle-phase flow; 126–139

Yury Tsirkunov and Sergei Panfilov

Abstract. The scattering of particles, reflected from a rigid surface, is numerically studied. The effects of surface roughness and particle non-sphericity are analysed separately and in combination. A high-speed subsonic gas–particle flow in a two-dimensional channel with a wedge step is simulated. The particle-phase flow patterns and concentration profiles are obtained for rough wedge surface and for particles, distributed in size. These results are compared with those for smooth wedge surface and monosized particles.

Key words: gas–particle flow, particle bouncing, surface roughness, non-spherical particles.

Particle dynamics and mixing in an oscillating viscous vortex pair; 140–153

Felix Kaplanski, Sergei Sazhin and Ylo Rudi

Abstract. A model of a viscous vortex pair, based on a solution of the Stokes equation, is applied for studying particle dynamics and mixing in vortex-pair-like structures. The perturbed flow field and dynamics of small spherical particles, contained in this flow, are studied on the basis of this solution. The particle-path equations and well-established techniques, such as computing of Poincaré maps, is used. It is shown that the flow inside the vortex pair can behave chaotically when a relatively thick pair (core size of the pair comparable with its radius) is under the influence of a periodic perturbation. This is expected to lead to better mixing of the fluid. However, an increase of the perturbation frequency causes the appearance of regions where a bounded quasi-periodic motion occurs. These regions behave like barriers in the phase space, reducing mixing and transport processes in the fluid. Introduction of the perturbation causes changes in the trajectories of the spherical aerosol-type particles. For a certain range of Stokes numbers (St < 10), long-term accumulation inside the vortex pair is observed for these particles, while the same particles in the unperturbed flow are forced out of the pair into the ambient flow.

Key words: viscous flow, Stokes equation, vortices, vortex pair.

Models of fuel spray penetration; 154–160

Sergei Sazhin, Cyril Crua, Jin-Sik Hwang, Soo-Young No and

Morgan Heikal

Abstract. A brief review of models of diesel fuel spray penetration, developed at the University of Brighton, are presented. These refer to the initial stage of spray penetration and the two-phase flow stage, when the relative velocity between droplets and gas can be ignored. The predictions of the two-phase models of spray penetration are compared with the results of experimental studies. A rapid compression diesel spray rig, based at Brighton University, and a high-pressure dimethyl ether spray chamber, based at Chungbuk National University, have been used. In both cases the experimental results are shown to be in agreement with the prediction of theoretical models.

Key words: diesel fuel spray, dimethyl ether spray, two-phase flow, spray penetration.

An experimental study of the effect of particles on the shear stress in particulate turbulent pipe flow; 161–168

Alexander Kartushinsky, Anatoly Mulgi, Sergei Tisler and Efstathios E. Michaelides

Abstract. Experimental data on the shear stress, exerted by a flowing mixture of solid particles in air, are presented. The data were obtained in a facility with a steel or vinyl test section. The Reynolds numbers were close to 10⁵, thus the flow was turbulent. Several types of particles with different material properties were used. Low as well as intermediate-to-high values of the loading ratio were investigated. It was found that the reduction of the data with respect to the Gastershtadt coefficient highlights certain trends of the flow and makes it possible to derive some general conclusions on the behaviour of the mixture.

Key words: gas–solid pipe flow, turbulence, loading ratio, shear stress, Gastershtadt coefficient.

Experimental study of the effect of velocity slip and mass loading on the modification of grid-generated turbulence in gas–solid particles flows; 169–180

Medhat Hussainov, Alexander Kartushinsky, Ylo Rudi,

Igor Shcheglov and Sergei Tisler

Abstract. Experimental data on the effects of the velocity slip and mass loading on a grid-generated turbulence in gas–solid particles flow are presented. Glass beads (700 mm) were used as the dispersed phase. Velocities of both phases were measured with a Laser Doppler Anemometer. Turbulence decay curves, obtained for different grids, show that particles enhance turbulence for small grids and attenuate it for the large ones. Turbulence enhancement and attenuation are intensified with the increase of the flow mass loading. The particles effect on turbulence changes from turbulence attenuation for a small velocity slip to its enhancement for a large velocity slip. A criterion for the evaluation of turbulence modification in gas–solid particles flow is proposed.

Key words: gas–solid particles flow, grid-generated turbulence, turbulence modification, mass loading ratio, velocity slip.

Instructions to authors; 181–183