Experimental investigation of single particle settling in turbulence generated by oscillating grid (original) (raw)
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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...
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.
Experimental evidence of settling retardation in a turbulence column
Physical Review Fluids, 2020
Settling experiments were conducted in a turbulence column to investigate the effect of turbulence on the effective fall velocity of solid particles slightly denser than the fluid (ρ p ρ f). Five types of particles of different materials and shapes were tested, their size ranging between o(1)η and o(10)η, where η is the Kolmogorov viscous length scale. Thus, the particles were of finite size with an unknown analytical form for the fluidparticle forces. The density ratio ranged as (ρ p − ρ f)/ρ f = {0.13 : 1.6}, and the still-fluid particle Reynolds number as Re 0 p = {75 : 981}. The turbulence levels characterized with the integral-scale Reynolds number ranged as Re L = {34 : 510}. Two-dimensional (2D) particle image velocimetry was used to obtain flow statistics, the residual mean circulation, and the turbulence statistics, while 2D particle tracking was performed to measure particle settling velocities. For all types of particles tested, settling retardation is observed as the turbulence intensity is increased. It is found that if both the effective fall velocity W s and the turbulent fluid velocity W f ,rms are nondimensionalized by the still-fluid particle terminal velocity W 0 , the settling retardation can be described by a unique relation independent of the particle type, W s /W 0 = f (W f ,rms /W 0), for the given range of flow regimes. Using analytical descriptions of the loitering and nonlinear drag effects, this scaling is shown to have a solid physical basis.
Dispersion of fine settling particles from an elevated source in an oscillatory turbulent flow
European Journal of Mechanics - B/Fluids, 2008
The present paper examines the stream-wise dispersion of suspended fine particles with settling velocities in an oscillatory turbulent shear flow with or without a non-zero mean over a rough-bed surface when the particles are being released from an elevated continuous source. A finite-difference implicit method is employed to solve the unsteady turbulent convective-diffusion equation. A combined scheme of central and four-point upwind differences is used to solve the steady state equation and the Alternating Direction Implicit (ADI) method is adopted for unsteady equation. It is shown how the mixing of settling particles is influenced by the tidal oscillatory current and the corresponding eddy diffusivity when the initial distribution of concentration regarded as a line-source. The vertical concentration profiles of suspended fine particles with settling velocities are presented for different downstream stations for various values of settling velocity and the frequency of the oscillation in tidal flow. For two-dimensional unsteady dispersion equation, the behaviour of iso-concentration lines for different values of settling velocity, frequency of the oscillation, dispersion time and releasing height is studied in terms of the relative importance of convection and eddy diffusion.
Modification of grid-generated turbulence by solid particles
Journal of Fluid Mechanics, 1993
The effects of almost neutrally buoyant plastic particles and heavy glass particles on grid-generated turbulence were studied experimentally in a water flow facility. From measured velocities of both the solid and liquid phases, drag and slip velocities of the particles and energy spectra and dissipation rates of the liquid phase were estimated.
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.
International Journal of Heat and Fluid Flow, 2000
Experimental results dealing with the modulation of grid-generated turbulence by coarse glass particles in a vertical downward channel¯ow are presented. Glass beads with an average diameter of 700 lm and a mass loading up to 0.1 kg dust/kg air were brought into the¯ow by an individually designed particle feeder allowing to vary the initial velocity slip. In order to determine the gas¯ow characteristics, TiO 2 particles with an average size of about 2 lm were used as tracers. Two mono-plane grids with a mesh size of 4.8 and 10 mm generated an isotropic turbulence which decayed along the test section of 2 m length. The mesh Reynolds numbers for these grids, based on a mean air velocity of U 9X5 m/s, were Re M 3045 and Re M 6340, respectively. Distributions of the mean velocity, the turbulence intensity along the channel axis and in dierent cross-sections obtained by Laser Doppler Anemometer (LDA) are presented. A degradation of signals from the glass particles due to coating of their surfaces by TiO 2 particles was observed during the measurements. The decay curves of the turbulence intensity in the streamwise direction showed an attenuation of turbulence intensity of the¯ow induced by the particles for both grids, while the presence of particles decreased the energy spectra at high frequencies. Ó