Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E32: Particle-Laden Flows: Particle-Resolved Simulations |
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Chair: Andrea Prosperetti, Johns Hopkins University Room: 2020 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E32.00001: Direct numerical simulation of gravity-driven avalanches immersed in a viscous fluid Thomas Bonometti, Edouard Izard, Laurent Lacaze This work deals with direct numerical simulations of sediment transport at the scale of O(10$^{3}$) grains. A soft-sphere discrete element method is coupled to an immersed boundary method in order to compute the flow around moving and colliding grains in an incompressible Newtonian fluid. A lubrication force is added for representing fluid-particles interaction near contact. The numerical method is shown to adequately reproduce the effective coefficient of restitution measured in experiments of the normal and oblique rebound of a grain on a wall. An analytical model is proposed and highlights the importance of the grain roughness and Stokes number on the rebound phenomenon. Three-dimensional configurations of gravity-driven dense granular flows in a fluid, namely the granular avalanche on an inclined plane and the collapse of a granular column, are performed. The granular flow regimes (viscous, inertial and dry) observed in experiments are identified as a function of the grain-to-fluid density ratio and the Stokes number. In particular, the simulations provide insights on the grain and fluid velocity profiles and force balance in each regime. In the second case, results agree well with experiments and the pore pressure feedback is observed for the first time in direct numerical simulations. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E32.00002: Turbulent channel flow laden with finite-size neutrally-buoyant particles Francesco Picano, Wim-Paul Breugem, Luca Brandt Dense suspensions are widely encountered in many applications and in environmental flows. While their rheological features in laminar flows have been longly studied, much less is known on their behavior in turbulent/inertial regime. The present works aims to fill this gap by investigating the turbulent channel flow of a Newtonian fluid laden with rigid neutrally-buoyant spheres at relatively high volume fractions. An Immersed Boundary Method has been used to account for the phase interaction performing Direct Numerical Simulation in the range of volume fractions $\Phi=0-0.2$ and a typical particle radius of 10 wall units. The results show that the mean velocity profiles are significantly altered by the presence of a solid phase with a decrease of the von Karman constant in the log-law. The overall drag is found to monotonically increase with the volume fraction. At the highest volume fraction here investigated, $\Phi=0.2$, the velocity fluctuation intensities and the Reynolds shear stress are found to decrease. The analysis of the mean momentum balance shows that the particle-induced stresses govern the dynamics in the dense cases and are responsible of the the overall drag increase since the turbulent shear stress is reduced with respect the unladen case. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E32.00003: Lattice Boltzmann simulation of particle inertial focusing in micro channels Yu Chen, Moran Wang We perform three dimensional lattice Boltzmann simulations to study particle inertial focusing in micro channels. Interpolation based curved boundary condition is employed to accurately treat the non-slip boundary condition of the particle surface. Force evaluation is via the corrected momentum exchange method recently proposed by our group, which ensures Galilean invariance and smooth force transition as the particle move across lattice nodes. Our results show good agreement with experiments, four equilibrium positions were found in square channel and two were found in rectangle channel. The two stage focusing is observed in our simulations which is also reported by others. For curving channels, additional force from dean flow further reduces equilibrium positions. A large portion of the curving channel needs to be simulated, as periodic boundary condition may not be valid here. By utilizing the parallel computing advantage of LBM, we perform large scale simulations of inertial focusing in curving channels. Detailed flow information and precisely monitored particle motion may provide valuable insight to understanding the mechanism of inertial focusing in micro channels and inspire developing of new designs. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E32.00004: Fully resolved simulations of particle sedimentation Adam Sierakowski, Yayun Wang, Andrea Prosperetti Progress in computational capabilities -- and specifically in the realm of massively parallel architectures -- render possible the simulation of fully resolved fluid-particle systems. This development will drastically improve physical understanding and modelling of these systems when the particle size is not negligible and their concentration appreciable. Using a newly developed GPU-centric implementation of the Physalis method for the solution of the incompressible Navier-Stokes equations in the presence of finite-sized spheres, we carry out fully resolved simulations of more than one thousand sedimenting spheres. We discuss the results of these simulations focusing on statistical aspects such as particle velocity fluctuations, particle pair distribution function, microstructure, and others. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E32.00005: Fully resolved simulation of the settling motion of a finite-sized spherical particle in a cellular flow field Jungwoo Kim For particle-laden flows related to particle transport and dispersion, a knowledge of particle settling velocity is one of the important subjects. In that respect, for last several decades, many numerical studies with point particle approaches have been done. However, existing analytical expressions and empirical correlations used in point particle approaches are made based on many assumptions including the fact that the particle size is much smaller than the typical length scale of a given flow field. So, the settling velocity of a finite-sized particle in turbulent flows remains an unresolved issue. Therefore, we perform fully resolved simulations of the settling motion of a finite-sized spherical particle in a cellular flow field. The cellular flow field considered has been regarded as one of the good model problems for the study of the particle settling. One of the important parameters is the ratio of the particle diameter (d) and the cell size in the cellular flow (L). In this study, the change of the particle settling velocity is examined in the range of 0.01$\le $d/L$\le $0.1. In addition, the instantaneous drag and lift force components are compared with existing expressions for the corresponding force on the particle. Those results would show the validity and limitation of the present point particle approach in understanding the settling motion of a spherical particle in turbulent flows. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E32.00006: Particle Resolved DNS of Turbulent Oscillatory Flow Over a Layer of Fixed Particles Chaitanya Ghodke, Javier Urzay, Sourabh Apte Particle resolved direct numerical simulations are performed using fictitious domain approach (Apte et al., JCP 2009) to investigate oscillatory turbulent flow over a layer of fixed particles representative of a sediment layer in coastal environments. Five particle Reynolds numbers in the range, $Re_D = 660-4240$ are studied and results are compared against available experimental data (Keiller \& Sleath, JFM 1976). Flow is characterized in terms of coherent vortex structures, Reynolds stress variation, turbulent cross-correlations and PDF distributions. The nature of the unsteady hydrodynamic forces on particles and their correlation to sweep and burst events is reported. The net lift coefficient remains positive over the cycle and is well correlated with phase averaged near-bed velocity. Maximum in the lift coefficient occurs when the strength of the horseshoe vortices is maximum. At this phase the lift fluctuations are correlated negatively with pressure and positively with velocity fluctuations in the region above the particle bed. Preliminary analysis shows non-Gaussian distribution for velocity fluctuation and follows 4th order Gram-Charlier. These detailed findings could eventually be useful in improving the existing criterion for predicting sediment incipient motion. [Preview Abstract] |
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