Particle velocity distributions and velocity fluctuations of non-Brownian settling particles by particle-resolved direct numerical simulation (original) (raw)
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Flow Scales of Influence on the Settling Velocities of Particles with Varying Characteristics
PLOS ONE, 2016
The settling velocities of natural, synthetic, and industrial particles were measured in a grid turbulence facility using optical measurement techniques. Particle image velocimetry and 2D particle tracking were used to measure the instantaneous velocities of the flow and the particles' trajectories simultaneously. We find that for particles examined in this study (Re p = 0.4-123), settling velocity is either enhanced or unchanged relative to stagnant flow for the range of investigated turbulence conditions. The smallest particles' normalized settling velocities exhibited the most consistent trends when plotted versus the Kolmogorov-based Stokes numbers suggesting that the dissipative scales influence their dynamics. In contrast, the mid-sized particles were better characterized with a Stokes number based on the integral time scale. The largest particles were largely unaffected by the flow conditions. Using proper orthogonal decomposition (POD), the flow pattern scales are compared to particle trajectory curvature to complement results obtained through dimensional analysis using Stokes numbers. The smallest particles are found to have trajectories with curvatures of similar scale as the small flow scales (higher POD modes) whilst mid-sized particle trajectories had curvatures that were similar to the larger flow patterns (lower POD modes). The curvature trajectories of the largest particles did not correspond to any particular flow pattern scale suggesting that their trajectories were more random. These results provide experimental evidence of the "fast tracking" theory of settling velocity enhancement in turbulence and demonstrate that particles align themselves with flow scales in proportion to their size.
Size segregation and particle velocity fluctuations in settling concentrated suspensions
Rheologica Acta, 2009
We investigate the sedimentation of concentrated suspensions at low Reynolds numbers to study collective particle effects on local particle velocity fluctuations and size segregation effects. Experiments are carried out with polymethylmetacrylate (PMMA) spheres of two different mean diameters (190 and 25 μm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labelled PMMA spheres and a particle image velocimetry method. We also report measurements of the interstitial fluid pressure during settling. Experiments show that size segregation effects can occur during the sedimentation of concentrated suspensions of either quasi-monodisperse or bidisperse spheres. Size segregation is correlated to the organisation of the sedimentation velocity field into vortex-like structures of finite size. A loss of size segregation together with a significant decrease of the fluid pressure gradient in the bulk suspension is observed when the size of vortex-like structures gets on the order of the container size. However, the emergence of channels through the settling zone prevents a complete loss of size segregation in very concentrated suspensions.
Physical Review E, 2005
We report a systematic experimental study of concentration and velocity patterns formed in a horizontal rotating cylinder filled completely with a monodisperse suspension of non-Brownian settling particles. The system shows a series of concentration and velocity patterns, or phases, with varying rotation rate and solvent viscosity. Individual phases are studied using both side and cross-sectional imaging to examine the detailed flow structures. The overall phase diagram of the system is mapped out as a function of the rotation rate and solvent viscosity. Attempts are made to analyze the functional form of the phase boundaries in order to understand the transition mechanism between different phases.
Measurement and modeling of the settling velocity of isometric particles
Powder Technology, 2008
The sedimentation of solid particles of simple form and complex form has been studied. The applied data in the case of isometric particles have been compared to those of spherical particles of same volume. This comparison allows highlighting an equivalent sedimentation diameter concept.
Journal of Fluid Mechanics, 1993
The average settling velocity in homogeneous turbulence of a small rigid spherical particle, subject to a Stokes drag force, has been shown to differ from that in still fluid owing to a bias from the particle inertia (Maxey 1987). Previous numerical results for particles in a random flow field, where the flow dynamics were not considered, showed an increase in the average settling velocity. Direct numerical simulations of the motion of heavy particles in isotropic homogeneous turbulence have been performed where the flow dynamics are included. These show that a significant increase in the average settling velocity can occur for particles with inertial response time and still-fluid terminal velocity comparable to the Kolmogorov scales of the turbulence. This increase may be as much as 50% of the terminal velocity, which is much larger than was previously found. The concentration field of the heavy particles, obtained from direct numerical simulations, shows the importance of the inertial bias with particles tending to collect in elongated sheets on the peripheries of local vortical structures. This is coupled then to a preferential sweeping of the particles in downward moving fluid. Again the importance of Kolmogorov scaling to these processes is demonstrated. Finally, some consideration is given to larger particles that are subject to a nonlinear drag force where it is found that the nonlinearity reduces the net increase in settling velocity .
arXiv: Fluid Dynamics, 2015
The settling velocities of natural, synthetic, and industrial particles were measured in a grid turbulence facility using optical measurement techniques. Particle Image Velocimetry and 2D Particle Tracking were used to measure the instantaneous velocities of the flow and the particles' trajectories simultaneously. We find that for particles examined in this study (Rep = 0.4 - 123), settling velocity is either enhanced or unchanged relative to stagnant flow for the range of investigated turbulence conditions. The smallest particles scaled best with a Kolmogorov-based Stokes number indicating the dissipative scales influence their dynamics. In contrast, the mid-sized particles scaled better with a Stokes number based on the integral time scale. The largest particles were largely unaffected by the flow conditions. Using Proper Orthogonal Decomposition (POD), the flow pattern scales are compared to particle trajectory curvature to complement results obtained through dimensional anal...
Settling velocities of particulate systems
International Journal of Mineral Processing, 2000
In one space dimension, the phenomenological theory of sedimentation-consolidation processes predicts the settling behaviour of a flocculated suspension in dependence of two constitutive material-specific functions, the Kynch batch flux density function and the effective solid stress. These functions depend only on the local solids concentration. In this paper, we determine these functions from published data of several experimental studies. The mathematical model is then solved numerically making use of these functions. The numerical results are compared to the respective measurements. General good agreement between numerical and experimental data confirms the validity of the phenomenological theory.
The settling velocity of heavy particles in an aqueous near-isotropic turbulence
Physics of Fluids, 2003
The ensemble-average settling velocity, V s , of heavy tungsten and glass particles with different mean diameters in an aqueous near-isotropic turbulence that was generated by a pair of vertically oscillated grids in a water tank was measured using both particle tracking and particle image velocimetries. Emphasis is placed on the effect of the Stokes number, St, a time ratio of particle response to the Kolmogorov scale of turbulence, to the particle settling rate defined as (V s ϪV t)/ V t where V t is the particle terminal velocity in still fluid. It is found that even when the particle Reynolds number Re p is as large as 25 at which V t /v k Ϸ10 where v k is the Kolmogorov velocity scale of turbulence, the mean settling rate is positive and reaches its maximum of about 7% when St is approaching to unity, indicating a good trend of DNS results by Wang and Maxey ͑1993͒ and Yang and Lei ͑1998͒. This phenomenon becomes more and more pronounced as values of V t /v k decrease, for which DNS results reveal that the settling rate at V t /v k ϭ1 and Re p Ͻ1 can be as large as 50% when StϷ1. However, the present result differs drastically with Monte Carlo simulations for heavy particles subjected to nonlinear drag (Re p Ͼ1) in turbulence in which the settling rate was negative and decreases with increasing St. Using the wavelet analysis, the fluid integral time (I), the Taylor microscale (), and two heavy particles' characteristic times (c1 , c2) are identified for the first time. For StϽ1, c1 Ͻ I and c2 Ͻ , whereas c1 Ϸ I and c2 Ϸ for StϷ1. This may explain why the settling rate is a maximum near StϷ1, because the particle motion is in phase with the fluid turbulent motion only when StϷ1 where the relative slip velocities are smallest. These results may be relevant to sediment grains in rivers and aerosol particles in the atmosphere.