S. Elghobashi | University of California, Irvine (original) (raw)

Papers by S. Elghobashi

APS Meeting Abstracts, 2001

Title: On the decay rate of particle-laden isotropic turbulence. Authors: Ferrante, A.; Elghobash... more Title: On the decay rate of particle-laden isotropic turbulence. Authors: Ferrante, A.; Elghobashi, S. ... Publication: American Physical Society, 54th Annual Meeting of the Division of Fluid Dynamics November 18 - 20, 2001 San Diego, California Meeting ID: DFD01, abstract #JA.002. ...

Bulletin of the American Physical Society, 2010

Submitted for the DFD10 Meeting of The American Physical Society Numerical simulation of particle... more Submitted for the DFD10 Meeting of The American Physical Society Numerical simulation of particle dispersion in an acoustic field J. CLECKLER, F. LIU, S. ELGHOBASHI, University of California, Irvine-Particles with small relaxation time, τ p , subjected to sound waves for many acoustic periods execute both periodic motion and mean drift. Particle acceleration in an acoustic flow field is often modeled via a linearized Stokes drag law. This simple model can predict the oscillatory particle velocity amplitude for large particle-tofluid density ratios, ρ p /ρ f , and small velocity-amplitude acoustic waves. However, this model is not accurate for other conditions and does not predict particle drift velocities. We present the results of two-dimensional numerical simulations in which the particle trajectories are obtained via the complete Lagrangian particle motion equation which includes the forces due to non-linear Stokes drag, Basset's unsteady viscous drag, pressure gradient, virtual mass and gravity. Particle behavior is found to depend on three non-dimensional parameters: (ρ p /ρ f), (ωτ p), where ω is the acoustic frequency, and the Mach number, M , which is the ratio of the acoustic wave velocity amplitude to the speed of sound. Results for large ρ p /ρ f are in good agreement with the experimental results of Gonzalez et al. (2000) for the range of frequencies tested. Results for other conditions agree with a perturbation solution of the Lagrangian particle motion equation for moderate strength acoustic waves. Particle model simplifications are recommended for important ranges of the three parameters, (ρ p /ρ f), (ωτ p) and M .

26th Joint Propulsion Conference, 1990

ABSTRACT A three-dimensional numerical simulation is conducted for a liquid jet injected transver... more ABSTRACT A three-dimensional numerical simulation is conducted for a liquid jet injected transversely into a gaseous crossflow in a rectangular chamber. The flow under consideration is laminar and subsonic. A case of low jet-to-gas density and momentum flux ratios (which simulates a high pressure situation in reality) is examined by solving the Navier-Stokes equations and a conservation equation of the liquid volume fraction in an Eulerian mesh. The liquid volume fraction is used with a surface-tracking method to construct the liquid/gas interface and, thus, separate the two immiscible phases in the calculation. The surface-tension effects and liquid breakup phenomena are not included in this study. Results show the three-dimensional cross sections of the chamber. They indicate that the cross section of the jet perpendicular to the main flow direction seems to elongate in the transverse direction after an initial expansion in the spanwise direction of the chamber. The gas around the injector is displaced by the jet and multiple interacting and recirculating regions are present around the jet.

Mechanical Engineering Series, 2005

Approximating suspended small particles, drops or bubbles as points is a natural first step in th... more Approximating suspended small particles, drops or bubbles as points is a natural first step in the investigation of turbulent multiphase flows, and indeed this simplified model was adopted very early in the history of the subject (see e.g. Hinze Turbulence, McGraw-Hill, New York, 1959, p. 460). In the late 70's and early 80's the advent of computers with (at the time, still marginally) adequate power, and the development of methods for the direct numerical simulation of turbulence, opened new paths in this line of investigation. At about the same time, the increasingly widespread technique of laser Doppler velocimetry was beginning to generate data on particle-turbulence interaction, thus imparting additional impetus to the theoretical and computational studies. As a consequence of these developments, we are now looking back at more than three decades of intense research and several hundred papers on particle-laden turbulent flows based on the point-particle model. We felt that it would be worth while to invite several practitioners to write review articles with a summary of their results and their views on the model itself: strengths, weaknesses and-most importantly-future prospects. We are very grateful to the colleagues who graciously accepted our invitation. Their papers, collected in this special issue of the International Journal of Multiphase Flow, depict a vivid image of the current state of research on this crucial aspect of our discipline. We sincerely hope that their efforts will stimulate further progress and interest in the subject.

Aerosol Science and Technology, 2009

In this article, numerical simulation of the Navier-Stokes equations was performed for the large-... more In this article, numerical simulation of the Navier-Stokes equations was performed for the large-scale structures of a two-dimensional temporally developing cylinder flow and the associated dispersion patterns of particles were simulated. The time-dependent Navier-Stokes equations were integrated in time using a mixed explicit-implicit operator splitting rules. The spatial discretization was processed using spectral-element method. Nonreflecting conditions were employed at the outflow boundary. Particles with different Stokes numbers were traced by the Lagrangian approach based on one-way coupling between the continuous and the dispersed phases. The simulation results of the flow field agree well with experimental data. Due to the effects of the coherent structures, the particles demonstrate a more organized dispersion process in the space and a periodic dispersion characteristic in the time. Particle dispersion increases with the flow Reynolds number and so does for particle concentration, which is independent of particle size. However, for particles at different Stokes numbers, the dispersion patterns are different. The particles at smaller Stokes number congregate mainly in the vortex core regions and the particles at larger Stokes number disperse much less along the lateral direction with the even distribution. The higher density distribution at the outer boundary of large-scale vortex structure characterizes the dispersion pattern of particles at the Stokes numbers of order of unity. Furthermore, these particles disperse largely along the lateral direction and show the nonuniform distribution of concentration.

International Journal of Heat and Mass Transfer, 1998

34th Aerospace Sciences Meeting and Exhibit, 1996

The equations governing the motion of a spherical particle proposed by previous workers are exami... more The equations governing the motion of a spherical particle proposed by previous workers are examined and compared with the results of the numerical solution of the full Navier-Stokes equations for unsteady, axisymmetric flow around a freely moving sphere initially injected into an oscillating flow and for unsteady, three-dimensional flow around a freely moving sphere interacting with a large vortex tube. As a result, we propose a modified equation of the particle motion and demonstrate its superiority to the previously proposed equations for both rectilinear and two-dimensional motion over a wide range of Reynolds number and of density ratio.

International Journal of Multiphase Flow, 1997

An idealized representation of the interaction of spherical particles with turbulent eddies of co... more An idealized representation of the interaction of spherical particles with turbulent eddies of comparable length scale is considered by means of a three-dimensional, unsteady finite-difference Navier-Stokes solution of the interaction between a fixed rigid sphere and a pair of advecting vortex tubes. Initially the sphere is suddenly placed in the flow and held fixed in space. First, a doubly symmetric interaction with vortices of opposite rotation is considered. The resulting time-dependent drag differs from the drag in axisymmetric flows; however, the lift and torque on the sphere remain zero. Next, an interaction with two vortices of like rotation is studied. Here, non-zero lift and torque, as well as drag deviation from the axisymmetric case occur and would result in a deflection in the trajectory of a nonfixed sphere. The flow in this case behaves like that of a single vortex. Finally, a linear array of like-rotating vortices, interacting with a freely moving sphere, is considered. The two-dimensional deflection depends strongly upon the sphere/fluid density ratio and initial sphere Reynolds number. Lift and moment coefficients are shown to be linearly proportional to the maximum induced velocity due to the vortices. Moment coefficients are an order of magnitude less than lift coefficients.

DNS of bubble-laden isotropic decaying turbulence are performed using the two-fluid formulation (... more DNS of bubble-laden isotropic decaying turbulence are performed using the two-fluid formulation (TF) instead of the Eulerian-Lagrangian approach (EL). The latter requires considerable computational resources especially for the case of two-way coupling. The TF formulation is developed by spatially averaging the instantaneous equations of the carrier flow and bubble phase over a scale of the order of the Kolmogorov length scale which, in our case, is much larger than the bubble diameter. On that scale, the bubbles are treated as a continuum (without molecular diffusivity) characterized by the bubble phase velocity field and concentration (volume fraction). The bubble concentration, C, is assumed small enough (C <= 10-3) to neglect the bubble-bubble interactions. DNS of the bubble-laden decaying turbulence are performed for both cases of one-way and two-way coupling. Here, the bubble diameter and response time are much smaller than the Kolmogorov length and time scales respectively....

International Journal for Numerical Methods in Biomedical Engineering

High-fidelity, predictive fluid flow simulations of the interactions between the rising thermal p... more High-fidelity, predictive fluid flow simulations of the interactions between the rising thermal plumes from forced air warming blower and the ultra-clean ventilation air in an operating room (OR) are conducted to explore whether this complex flow can impact the dispersion of squames to the surgical site. A large-eddy simulation, accurately capturing the spatiotemporal evolution of the flow in 3 dimensions together with the trajectories of squames, is performed for a realistic OR consisting of an operating table (OT), side tables, surgical lamps, medical staff, and a patient. Two cases are studied with blower-off and blower-on together with Lagrangian trajectories of 3 million squames initially placed on the floor surrounding the OT. The large-eddy simulation results show that with the blower-off, squames are quickly transported by the ventilation air away from the table and towards the exit grilles. In contrast, with the hot air blower turned on, the ventilation airflow above and below the OT is disrupted significantly. The rising thermal plumes from the hot air blower drag the squames above the OT and the side tables and then they are advected downwards toward the surgical site by the ventilation air from the ceiling. Temporal history of the number of squames reaching 4 imaginary boxes surrounding the side tables, the OT, and the patient's knee shows that several particles reach these boxes for the blower-on case.

Progress is reported in research on turbulent two-phase flow which occurs during the combustion o... more Progress is reported in research on turbulent two-phase flow which occurs during the combustion of pulverized coal. Progress to date in the mathematical study includes derivation of the complete time-averaged conservation equations of the two-phase turbulent flow. These are the conservation equations of: continuity of the solid phase; momentum of the solid phase in the axial and radial directions; momentum of the gaseous phase in the axial and radial directions; turbulence kinetic energy; and dissipation rate of the turbulence kinetic energy. The experimental study included the design of the experimental apparatus, hardware fabrication, and the beginning of equipment assembly. (LCL)

Annual Review of Fluid Mechanics

This review focuses on direct numerical simulations (DNS) of turbulent flows laden with droplets ... more This review focuses on direct numerical simulations (DNS) of turbulent flows laden with droplets or bubbles. DNS of these flows are more challenging than those of flows laden with solid particles due to the surface deformation in the former. The numerical methods discussed are classified by whether the initial diameter of the bubble/droplet is smaller or larger than the Kolmogorov length scale and whether the instantaneous surface deformation is fully resolved or obtained via a phenomenological model. Also discussed are numerical methods that account for the breakup of a single droplet or bubble, as well as multiple droplets or bubbles in canonical turbulent flows.

International Journal of Multiphase Flow, 1989

A two-equation model for low Reynolds number turbulence has been developed for dispersed dilute t... more A two-equation model for low Reynolds number turbulence has been developed for dispersed dilute two-phase confined flows. The two equations describe the conservation of turbulence kinetic energy and dissipation rate of that energy for the carrier fluid. The model is based on the closure for high Reynolds number two-phase flows reported previously. In order to validiate the proposed model, a turbulent two-phase pipe flow (air laden with spherical uniform-size particles) is predicted. The predictions of the mean flow properties of the two phases and the turbulence characteristics of the carrier phase show good agreement with the available experimental data.

Turbulent Shear Flows 7, 1991