Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session E8: Particle-Laden Flows III: Particle-Turbulence Interaction |
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Chair: David Richter, University of Notre Dame Room: 330 |
Sunday, November 24, 2013 4:45PM - 4:58PM |
E8.00001: Feedback effect on the large-scale fluid motion in wall-bounded gas-solid disperse flow Yoichi Mito Influence of the forces, exerted by dispersed particles, in a channel, in which gas is flowing turbulently, is examined using a direct numerical simulation to calculate the gas velocities seen by the particles and a point force method to calculate the forces exerted by the particles on the gas. Influence of gravity and inter-particle collisions is ignored. Distributions of the mean streamwise body forces, exerted on the fluid by the turbulence and by the particles, are calculated to show the mean large-scale motions of the fluid phase and of the disperse phase. The fluid turbulence forces decrease with increasing volume fraction to accommodate the inter-phase body forces. Thus the large-scale fluid motions, which make a major contribution to the fluid turbulence, are damped. The turbophoretic velocities, which represent the mean drifts, show that mean contribution of each particle to the mean large-scale motion of the disperse phase decreases with increasing volume fraction. This is caused by the decreases in the fluid turbulence and the turbulent transport, with increasing volume fraction. [Preview Abstract] |
Sunday, November 24, 2013 4:58PM - 5:11PM |
E8.00002: Effects of small particles on coherent structures in particle-laden near-wall turbulence Junghoon Lee, Changhoon Lee In near-wall turbulence, particles interact effectively with coherent structures, such as the quasi-streamwise vortices near the wall. The quasi-streamwise vortices play a significant role in turbulence production and regeneration. In this study, we investigate the modification of the quasi-streamwise vortices due to the presence of particles using direct numerical simulation of turbulent channel flow. The particles considered are smaller than the Kolmogorov length scale and the particle Reynolds numbers are small. Therefore, a point-force approach was used in imposing the particle reaction force on the fluid. Since particles are assumed to be heavier than the fluid, the particle equation of motion was established considering only Stokes drag. In this study, the particle Stokes numbers based on wall units range from 0.5 to 25. It is shown that particles with the lowest Stokes number augment turbulence while particles with higher Stokes numbers attenuate it. The lowest-Stokes-number particles are found to enhance the low- and high-speed streaks around the quasi-streamwise vortices, affecting vortex regeneration cycle. Consequently, the frequency of the quasi-streamwise vortices is increased. However, particles with higher Stokes numbers directly damp the quasi-streamwise vortices. [Preview Abstract] |
Sunday, November 24, 2013 5:11PM - 5:24PM |
E8.00003: Laminar-turbulent transition of channel flows: the effect of neutrally buoyant finite-size particles Micheline Abbas, Vincent Loisel, Olivier Masbernat, Eric Climent Numerical simulations were performed on channel flows laden with resolved finite-size neutrally buoyant particles at moderate volumetric concentration. In the case of fluctuating flows close to laminar-turbulent transition, the particle volume fraction is homogeneously distributed in the channel except an accumulation layer in the near-wall region (particle migration is driven by inertia). Particles increase the level of perturbations close to the wall leading to significant enhancement of both the velocity fluctuations and the wall friction coefficient. Additionally, particles break down the large-scale flow structures into smaller, more numerous and sustained eddies. When the flow Reynolds number is decreased, flow relaminarization occurs at critical Reynolds number $Re_{cS}$ (based on the effective suspension viscosity) significantly below the critical Reynolds number $Re_c$ of single-phase flow transition. In the case of laminar flows, the suspension segregates into pure fluid and particle laden wall layers due to cross-stream migration. An instability is observed characterized by the formation of dune-like patterns at the separation between pure fluid and concentrated suspension. Increasing the Reynolds number yields transition to turbulence at a threshold above $Re_{cS}$. [Preview Abstract] |
Sunday, November 24, 2013 5:24PM - 5:37PM |
E8.00004: Anisotropy of inertial-particle clustering in homogeneous turbulent shear flow Parvez Sukheswalla, Lance Collins We study the clustering of inertial particles dispersed in homogeneous turbulent shear flow (HTSF), with a view towards characterizing the effects of flow-anisotropy on clustering as a function of Stokes numbers, separation distance, and time. Recent experiments [Nicolai et al., \emph{Phys. Fluids} (in review)] have shown preferential orientation of clusters along the plane of maximum mean-strain, for separations larger than the Kolmogorov scale ($\eta$). High-resolution ($2048\times1024\times1024$ grid) direct numerical simulations at similar flow conditions are performed using a hybrid Pseudospectral-WENO scheme, that allows well-resolved, long-time simulations of HTSF at high Reynolds numbers. Inertial particles at different Stokes numbers are tracked, and their angular distribution functions (ADFs) are analyzed. Consistent with Nicolai et al., we observe the particle concentrations are maximal along the extensional axis of the strain component of the imposed uniform mean shear. We quantify the anisotropy by the harmonic decomposition of the ADFs. The first harmonic is found to peak between $5$ and $10\eta$ for all particle classes. The results pave the way for future studies of the role anisotropy plays in aerosol processes such as collision and gravitational settling. [Preview Abstract] |
Sunday, November 24, 2013 5:37PM - 5:50PM |
E8.00005: Preferential Concentration Driven Instability of Sheared Gas-Solid Suspensions Mohamed Kasbaoui, Donald Koch, Ganesh Subramanian, Olivier Desjardins Through a linear stability analysis of a gas-solid suspension of particles with low Stokes number and moderate mass loading, we demonstrate that the modulation of the gravitational force exerted on the suspension due to preferential concentration of particles in regions of low vorticity can destabilize a homogeneous linear shear flow of a gas-solid suspension. Since the fastest growing modes are found to be those with wavelengths small compared with the characteristic length scale ($U/\Gamma$) where $U$ is the settling velocity and $\Gamma$ is the shear rate, we apply an asymptotic multiple scale analysis using the WKB method. This analysis reveals that the instability comes from the coupling of a particle number density mode driven by preferential concentration in regions where the velocity disturbance reduces the base state vorticity and a momentum mode driven by the particle number density variations. The growth of the amplitude of particle concentration and fluid velocity disturbances is characterized as a function of the wave number and Reynolds number using both the asymptotic theory and a numerical solution of the linearized equations. [Preview Abstract] |
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