Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A37: Particle-Laden Flows I |
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Chair: Andrew Bragg, Duke University Room: Georgia World Congress Center B409 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A37.00001: The Maxey-Riley equation as a boundary condition to the 1-D heat equation Vishal Vasan, S. Ganga Prasath, Rama Govindarajan The Maxey-Riley equation describes the motion of a small spherical particle in an ambient flow field. It has a non-local contribution in the form of the Basset history integral due to the time-dependent motion of the particle. A major hurdle in studying the collective behaviour of particles has been the accurate and efficient computation of this integral. Previously, it has been either neglected or approximated by terms that are hard to rigorously justify. We show that the Maxey-Riley equation in its entirety can be mapped as a modified-Robin boundary condition to the 1-D heat equation. Exploiting this reformulation we obtain exact solutions for the particle velocity in any homogeneous time-dependent flow. We find that for short times, the particle relaxes faster than the exponential decay due to Stokes drag and for large times it relaxes as $t^{-3/2}$ in a still environment. We provide a numerical method with spectral accuracy for general flow fields at a fixed memory cost, without approximating the history integral, unlike traditional approaches. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A37.00002: Structural evolution of fractal particle agglomerates in a Taylor-Green vortex Xuan Ruan, Sheng Chen, Shuiqing Li Micron particles generally exist in the form of agglomerates, such as those in pulverized coal combustion, electrostatic agglomerators and flocculation during water treatment. Prediction of the structure of agglomerate in the flow field is a crucial but highly challenging problem. In this work, the structural evolution of agglomerates in a pseudo 2D Taylor-Green vortex is numerically investigated using an adhesive discrete-element method. We focus on the influence of the initial agglomerate structure, the interparticle adhesive force and flow field parameters on the structure of agglomerates. We find that loosely-packed agglomerates become more compact in the region with a higher shear rate. A stronger interparticle adhesion and a more compact structure both hinder the breakage and restructuring of the agglomerates. Based on an extensive simulation runs, we present a phase diagram in the form of the flow shear rate and the interparticle adhesion to summarize all possible deformation modes for an agglomerate in a vortex. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A37.00003: Predicting particle phase velocity statistics using fluctuating force-fluctuating torque (F3T) model Partha Sarathi Goswami, Swagnik Ghosh Particle laden flows have wide range of applications in industrial and natural processes. Even though, with fast computational facility it has become feasible to perform direct numerical simulations (DNS) of such flows, to address the dynamics of a system having size of practical interest is still not feasible. Therefore, modeling such flows becomes important. We have applied a fluctuating force and fluctuating torque (F3T) model to simulate the dynamics of particle phase in the limit of low volume fraction and high Stokes number. In this model, effect of fluid velocity and vorticity on the particle is modeled as anisotropic Gaussian white noise. Effect of inelasticity and roughness in particle-particle and particle-wall interactions on particle velocity statistics has been predicted. Simulations have also been preformed to investigate the system size dependence on particle phase statistics. All the results, predicted through model have been compared with DNS results. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A37.00004: PIV measurements of dilute suspension flows over porous medium models Eileen A. Haffner, Byron Erath, Parisa Mirbod Porous media encompasses a variety of materials, with unique engineering applications. This study specifically examines pressure-drive flow of dilute suspensions ranging from particle volume fractions of 0.01 to 0.03 over porous media. We consider the flow of neutrally buoyant, non-colloidal, non-Brownian suspensions of rigid, spherical particles in a Newtonian fluid when both particle and suspension Reynolds number are very low. The porous media was designed to have varying porosities ranging from 0.3 to 0.9 in order to show how the structure of the media affects the flow dynamics. To analyze the coupled flow and to conduct detailed velocity measurements inside and over the porous media models, we utilized particle image velocimetry (PIV). We also characterized the slip velocity at the interface of the porous media and the suspension flow. It was found that values of the slip velocity normalized by either the maximum velocity or by shear rate in the free flow region depend on the property and the thickness of porous media, the size and concentrations of the particles in the suspension. We also discuss in detail the comparison between the experimental data and our theoretical models. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A37.00005: A novel approach for separation of inertial particles Kimberly Liu, Mahdi Esmaily Moghadam, Ali Mani In this work, we study the segregation of inertial particles of different size in spatially and temporally varying flows. At first, we focus on flows with spatially uniform velocity gradient tensor but with time-harmonic amplitudes. The clustering or mixing behavior of particles in these flows is strongly governed by particle density and size. Specifically, it has been shown that under certain conditions, clustering occurs over a highly narrow band of Stokes number (Esmaily-Moghadam and Mani, 2017, arXiv:1704.00370). This study aims to analyze design concepts that utilize this phenomenon for precise separation applications. We present three performance measures that help assess these devices: a separation resolution, a minimum device size requirement, and a measure of robustness to perturbations (e.g., due to particle collisions). Furthermore, by extending the design space to systems involving periodic but non-harmonic signals, we develop key guidelines for the design of such separation devices. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A37.00006: Computational particles, parcels, or super-particles? What are these things? Jeremy Horwitz, Gianluca Iaccarino, Ali Mani The Eulerian-Lagrangian (EL) paradigm has become a leading strategy in the study of particle-laden flows. This method has the advantage of explicitly accounting for the non-continuum dispersed phase while simultaneously accounting for the continuum physics of the carrier fluid. However, practical flows may harbor an enormous number of particles. Therefore, the computational cost associated with simulating each particle can become prohibitively expensive. One strategy to reduce this cost in EL simulations is to simply reduce the number of particles tracked in the system. Each particle tracked in the simulation represents the properties of several particles related to the original system. This strategy, often dubbed the method of ‘computational particles’ is not a new idea. However, there seems to be few systematic studies examining the robustness of this strategy. We will study computational particles in homogeneous turbulence to understand how the ratio of computational to physical particles can be varied along with other parameters such as the Stokes number, volume fraction, and mass loading ratio to ensure that reduction in the number of particles simulated does not result in a significant change in reported statistics of the particle and fluid phases. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A37.00007: Inertial focusing of spherical particles suspended in laminar circular tube flows Saki Nakayama, Hiroshi Yamashita, Tomoaki Itano, Masako Sugihara-Seki Neutrally buoyant spherical particles suspended in laminar circular tube flows migrate laterally due to inertia toward an equilibrium radial position, called the Segre-Silberberg (S-S) annulus. Recent experimental studies reported that another annulus located closer to the tube center, called the inner annulus, appeared at relatively high Reynolds numbers (Re). However, other experimental studies showed the fading of this inner annulus at further downstream cross sections if Re is lower than a certain critical value. Thus, it has not been revealed yet whether the inner annulus is an equilibrium position or not. In this study, we investigated experimentally the cross-sectional distribution of spherical particles in circular tube flows with the particle-to-tube-diameter ratio (size ratio) from 0.083 to 0.15 at Re≲1,000. We found three types of particle focusing patterns in the downstream cross section, depending on Re, such as (A) S-S annulus only, (B) S-S and inner annuli and (C) inner annulus only. The pattern (A) was observed in the lowest Re range, the pattern (B) in higher Re range, and the pattern (C) in the highest Re range. Thus, the inner annulus could be an equilibrium radial position for Re larger than a critical value, which depends on the size ratio. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A37.00008: Direct numerical simulations of heat transfer in fluidized beds of spherical particles. Mehdi Niazi Ardekani, Luca Brandt Controlling heat and mass transfer in particulate suspensions has many important applications such as packed and fluidized bed reactors and industrial dryers. In this work, we study the effect of particle volume fraction and of the Grashof number (Gr) on the heat and mass transfer within a suspension of rigid spherical particles in a vertical box, using the immersed boundary method (IBM) to account for the solid-fluid interactions and a volume of fluid (VoF) method to resolve the temperature equation both inside and outside the particles. Different Grashof numbers 0, 10000 and 40000 are simulated for particle volume fractions 1, 5 and 10% where a cold flow and hot particles with density ratio of 1.02 are introduced at the bottom of the computational domain. The average particle velocity and temperature are monitored, aiming to maximise the mass transfer while the particles are efficiently cooled down during their rise to the top of the box. Our results indicate that increasing the Grashof number increases the average particle velocity for the large volume fractions while its effect on the average temperature is almost negligible. Detailed statistics of the fluid and particle phase will be presented at the conference. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A37.00009: A new correction scheme for two-way coupled point-particle methods for anisotropic grids Mahdi Esmaily, Jeremy A. K. Horwitz, Shankar Subramaniam, Mohammad Mehrabadi The accuracy of point-particle methods can degrade when the particle and fluid momentum equations are two-way coupled. In such cases, the fluid velocity at the location of the particle, which is often used as an estimation of the undisturbed fluid velocity in the discretized setting, can be altered by the particle, creating an error in the prediction of coupling forces. In this talk, we discuss a correction scheme to reduce this error by providing a more accurate estimation of the undisturbed fluid velocity. The proposed scheme can be applied to anisotropic rectilinear grids with arbitrary aspect ratio, arbitrary interpolation scheme, and particles that have a different size relative to the grid and density relative to the fluid. We evaluate the accuracy of the proposed scheme by comparing the computed settling velocity of individual and pair of particles under gravity on anisotropic rectilinear grids against analytical solutions, showing up to two orders of magnitude reduction in error in cases where the particle is up to 5 times larger than the grid that may have an aspect ratio of over 10. Additionally, a comparison against the particle-resolved simulation of decaying isotropic turbulence demonstrates the excellent accuracy of the proposed scheme. |
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