Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D3: ParticleLaden Flows: Density Effects 
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Chair: Luca Brandt, Linne FLOW Center, KTH Mechanics Room: 102 
Sunday, November 22, 2015 2:10PM  2:23PM 
D3.00001: Sedimentation of finitesize particles in quiescent and turbulent environments Luca Brandt, Walter Fornari, Francesco Picano Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows. We present Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using an Immersed Boundary Method to study the behavior of finitesize particles in homogeneous isotropic turbulence. The particle radius is approximately 6 Komlogorov lengthscales, the volume fraction 0.5{\%} and 1{\%} and the density ratio 1.02. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reduction with respect to a single particle in quiescent fluid is about 12{\%} in dilute conditions. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails are associated to the intermittent fast sedimentation of particle pairs in draftingkissingtumbling motions. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulas and show that non stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment. [Preview Abstract] 
Sunday, November 22, 2015 2:23PM  2:36PM 
D3.00002: DNS Study of ParticleBedTurbulence Interactions in an Oscillatory WallBounded Flow Chaitanya Ghodke, Sourabh Apte Particleresolved direct numerical simulations are performed to investigate the effects of an oscillatory flow field over a roughbed, corresponding to the experimental setup of Keiller \& Sleath (1976) for transitional and turbulent flows over a range of Reynolds numbers (95400) based on the Stokeslayer thickness. It is shown that the roughness modulates the nearbed turbulence, distorts and breaks the streamwise horseshoe structures, and reduces the largescale anisotropy. A doubleaveraging of the flow field reveals spatial inhomogeneities at the roughness scale and alternate paths of energy transport in TKE budget. The unsteady nature of hydrodynamic forces on particles and their crosscorrelations with measurable flow variables are also studied. Temporal correlations showed drag and lift to be positively correlated with a phase difference, which is approximately equal to the Taylor microscale related to drag/lift correlations. Spatiotemporal correlations between the flow field and particle force showed that the lift force is well correlated with the streamwise velocity fluctuations up to distances of the same order as the particle diameter, whereas pressure fluctuations are correlated and anticorrelated with the lift force in the front and aft regions of the particle. [Preview Abstract] 
Sunday, November 22, 2015 2:36PM  2:49PM 
D3.00003: Similarity between particles and bubbles as microadditives in turbulent channel flow Yoichi Mito The acceleration of turbulent fluid flow in a vertical channel by the use of a uniform distribution of microparticles and of microbubbles has been examined by using a direct numerical simulation to calculate the fluid velocities seen by the additives. The flows considered are the downward gas flow to which solid particles of density ratio of $10^3$ are added and the upward liquid flow to which bubbles of density ratio of $10^{3}$ are added. Both additives, ranging in volume fraction up to $2 \times 10^{3}$, are represented as solid spheres. The Froude numbers are chosen so as to have similar effects in both flows by the use of the same volume fraction of the additives. The fluidphase momentum balance, integrated over the domain, is used to examine the changes in drag, wall friction and averaged feedback force of the nonstationary flow models. The feedback force per volume fraction is unchanged in the bubble flow. It decreases with increasing volume fraction and inertia of particles in the particle flow. Similarities between the two disperse flows are seen at small times for small volume fractions. Drag is reduced by both additives. The amount of reduced drag decreases with time at large times in the bubble flow, due to the increases in the accumulation of bubbles above walls. [Preview Abstract] 
Sunday, November 22, 2015 2:49PM  3:02PM 
D3.00004: DNS with Discrete Element Modeling of Suspended Sediment Particles in an Open Channel Flow Pedram Paksereht, Sourabh Apte, Justin Finn Interactions of glass particles in water in a turbulent open channel flow over a smooth bed with gravity perpendicular to the mean flow is examined using direct numerical simulation (DNS) together with Lagrangian DiscreteElementModel (DEM) for particles. The turbulent Reynolds number ($Re_{\tau}$) is $710$ corresponding to the experimental observations of Righetti \& Romano (JFM, 2004). Particles of size $200$ microns with volume loading on the order of $10^{3}$ are simulated using fourway coupling with standard models for drag, added mass, lift, pressure, and interparticle collision forces. The presence of particles affect the outer as well as inner region of the wall layer where particle inertia and concentration are higher. The DNSDEM is able to capture the fluidparticle interactions in the outer layer accurately. However, in the inner layer, an increase in mean as well as rms fluid velocity, as observed in the experiments, is not predicted by the DNSDEM model. It is conjectured that particles slide and roll on the bottom wall, creating sliplike condition. Predictions using different models for drag and lift forces, as well as strong torque coupling are explored and compared with experimental data. [Preview Abstract] 
Sunday, November 22, 2015 3:02PM  3:15PM 
D3.00005: Entrainment in sediment laden flows Jorge Salinas, Mrugesh Shringarpure, Mariano Cantero, S. Balachandar The process of entrainment in classical problems of plumes, jets and wakes has been studied extensively. In this work we are interested in understanding the entrainment process in particle laden gravity currents. This process is influenced by the stable stratification produced by suspended particles. Moreover, particle settling can produce detrainment. We have analyzed two different kinds of flows. First, we have studied the problem of entrainment of a sediment layer sequestered at the bottom of a pressuredriven turbulent channel flow. These kinds of conditions are characteristic of river flows moving over erodible beds. This scenario leads to entrainment of sediment mass only, and we present the selfsimilar transient process of particle entrainment and the approach to a Roussian profile at long times. The effect of settling velocity on this process has been studied. In the second scenario, we study the entrainment process in the body of submarine turbidity currents. In this case the flow is driven by the density difference produced by sediment. Because the ambient is quiescent, we are able to study the process of entrainment of both mass (sediment) and momentum (turbulence). The effect of settling velocity of sediment particles and Richardson number on entrainment is studied. [Preview Abstract] 
Sunday, November 22, 2015 3:15PM  3:28PM 
D3.00006: Resuspension of a granular bed by thermal convection Cyprien Morize, Eric Herbert, Yves D'Angelo, Alban Sauret The transport, dispersion and resuspension of particles occur in industrial fluid dynamical processes as well as environmental and geophysical situations. Whereas the resuspension of an immersed granular bed by fluid flows such as vortices or shear flows has been the focus of many studies, the ability to fluidize particles with a vertical gradient of temperature remains poorly understood. Using laboratory experiments with a localized heat source, we observe that a massive entrainment of particles into the fluid volume occurs beyond a threshold temperature. The buoyancy driven fluidized bed then leads to the transport of solid particles through the generation of particleladen plumes. We show that the destabilization process is driven by the thermal conductivity inside the granular bed and demonstrate that the threshold temperature depends on the thickness of the granular bed and the buoyancy number, i.e. the ratio of the stabilizing density contrast to the destabilizing thermal density contrast. [Preview Abstract] 
Sunday, November 22, 2015 3:28PM  3:41PM 
D3.00007: Study of interactions between sediment particles in sheet flow using CFDDEM Rui Sun, Heng Xiao CFDDEM simulation is a promising approach in the study of sediment transport. However, the study of sediment transport in sheet flow is still lacking. In this work, a parallelized CFDDEM solver sediFoam, which has been extensively validated, is applied to the study of sediment transport. Numerical simulations of sediment bed of different characteristics under various waves are studied and compared with the results obtained in the literature, including sediment transport flux, solid volume fraction, and movable bed height. The microstructures (e.g., contact network) and macroscopic solid phase stresses of the sediment bed are analyzed to gain insight into the interparticle interactions during the transport process. Additionally, the microstructural properties and solid phase stresses in waveinduced sheet flow are compared to those obtained in unidirectional currents, and significant difference is observed. [Preview Abstract] 
(Author Not Attending)

D3.00008: Simultaneous Measurement of Fluid and Particle Motion in Shear Induced Erosion Paul S. Krueger, Zhongfeng An Fluid particle interaction is fundamental to shear induced particle erosion, but experimental measurements of this interaction are challenging due to differing optical characteristics of the fluid and particles and because of the high particle volume fraction in the particle bed. To address these challenges, monodisperse glass beads were used with a refractiveindex matched aqueous solution of NaI flowing horizontally over the particle bed. Two cameras separately imaged the fluid and particle phase motion using optical filters to isolate the emission bands of the fluorescent fluid tracer particles and dye added to the fluid for the fluid and particle phase cameras, respectively. Then digital particle image velocimetry and particle tracking were used to obtain the fullfield, timevarying evolution of the fluid and particle motion simultaneously. The results showed rapid, fluctuating particle transport near flow initiation for sufficiently high fluid flow rates. Increased slip in mean particle velocities was observed above from the particle bed surface and an approximately linear relationship was observed between particle and fluid velocity fluctuations. [Preview Abstract] 
Sunday, November 22, 2015 3:54PM  4:07PM 
D3.00009: Effect of particle size distribution on the hydrodynamics of dense CFB risers Akhilesh Bakshi, Samir Khanna, Raj Venuturumilli, Christos Altantzis, Ahmed Ghoniem Circulating Fluidized Beds (CFB) are favorable in the energy and chemical industries, due to their high efficiency. While accurate hydrodynamic modeling is essential for optimizing performance, most CFB riser simulations are performed assuming equallysized solid particles, owing to limited computational resources. Even though this approach yields reasonable predictions, it neglects commonly observed experimental findings suggesting the strong effect of particle size distribution (psd) on the hydrodynamics and chemical conversion. Thus, this study is focused on the inclusion of discrete particle sizes to represent the psd and its effect on fluidization via 2D numerical simulations. The particle sizes and corresponding mass fluxes are obtained using experimental data in dense CFB riser while the modeling framework is described in Bakshi et al 2015. Simulations are conducted at two scales: (a) fine grid to resolve heterogeneous structures and (b) coarse grid using EMMS subgrid modifications. Using suitable metrics which capture bed dynamics, this study provides insights into segregation and mixing of particles as well as highlights need for improved subgrid models. [Preview Abstract] 
Sunday, November 22, 2015 4:07PM  4:20PM 
D3.00010: Particledriven gravity currents in nonrectangular crosssection channels Tamar Zemach Particledriven gravity currents are suspensions of dense particles that spread into an ambient fluid due to the difference between the density of the suspension and that of the ambient fluid. During the evolution of the current, particles continually sediment and are deposited from the flow. Particledriven gravity currents are important in many environmental situations (volcanic ash flows, turbidity currents). In the present work we consider the propagation of a highReynoldsnumber gravity current generated by suspension of heavier particles in fluid of density $\rho_{i}$ propagating along a channel into an ambient fluid of the density $\rho_{a}$ . The bottom and top of the channel are at $z=0,H$, and the crosssection is given by the quite general $f_1(z)\le y \le f_2(z)$ for $0 \le z \le H$. The flow is modeled by the onelayer shallowwater equation. We solve the problem by the finitedifference numerical code to present typical height $h(x,t)$, velocity $u(x,t)$ and volume fraction of particles $\phi(x,t)$ profiles. The methodology is illustrated for flow in typical geometries: powerlaw, trapezoidal and circle. The presence of the particles reduces the speed of propagation, however the details are depend on the geometry of the crosssection. [Preview Abstract] 
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