Improved DRW Model for Prediction of Deposition and Dispersion of Nano- and Micro-Particles in Turbulent Flows

2020 ◽  
Author(s):  
Amir A. Mofakham ◽  
Goodarz Ahmadi
Keyword(s):  
Turbulent Flows ◽  
Time Scale ◽  
Correction Term ◽  
Particle Dispersion ◽  
Nano Particles ◽  
Concentration Profiles ◽  
Drift Correction ◽  
Velocity Fluctuations ◽  
Fluctuation Velocity ◽  
Point Particles

Abstract In this study, the accuracy of the discrete random walk (DRW) stochastic model in generating the instantaneous velocity fluctuations as seen by micro- and nano-particles in inhomogeneous turbulent flows were examined. Particular attention was given to the effects of the non-uniform normal RMS velocity fluctuations and turbulence time scale on the DRW model predictions. The trajectories of randomly injected point-particles with diameters ranging from 10 nm to 30 μm in a duct were evaluated using an in-house Matlab particle tracking code. The particle equation of motion included the drag and Brownian forces. The fully developed mean velocity and RMS fluctuation velocity profiles were exported from the RANS (v2f) simulations and were used for the particle dispersion and transport analysis. It was assumed that the particle-laden flow is sufficiently dilute so that the particle-particle collisions and the two-way coupling effects of particles on the flow could be ignored. To incorporate the instantaneous turbulence velocity fluctuations effects on particle dispersion, the Conventional-DRW model (in the absence of drift corrections), which was originally developed for homogenous turbulent flows, was first used. It was shown that the Conventional-DRW model leads to superfluous migration of fluid-point particles toward the wall and erroneous particle deposition rate. The Modified-DRW model with an appropriate velocity gradient drift correction term was also tested. It was found that the predicted concentration profiles of tracer particles still are not uniform. It was hypothesized that the reason for this erroneous prediction is due to the inhomogeneous turbulence time macroscale in the channel flow. A new drift correction term as a function of gradients of both RMS fluctuation velocity and the turbulence time macroscale was proposed. It was shown that the new Improved-DRW model with the velocity and time scale drift corrections leads to uniform distributions for fluid-point particles and reasonable concentration profiles for finite-size particles. It was shown that the predicted deposition velocities of different size particles by the proposed Improved-DRW model are in good agreement with the available experimental data as well as the predictions of the empirical models and earlier DNS results.

Improved Discrete Random Walk Stochastic Model for Simulating Particle Dispersion and Deposition in Inhomogeneous Turbulent Flows

2020 ◽  
Vol 142 (10) ◽  
Author(s):  
Amir A. Mofakham ◽  
Goodarz Ahmadi
Keyword(s):  
Random Walk ◽  
Turbulent Flows ◽  
Particle Deposition ◽  
Finite Size ◽  
Particle Dispersion ◽  
Concentration Profiles ◽  
Velocity Fluctuations ◽  
Point Particles ◽  
Deposition Velocities ◽  
Gradient Drift

Abstract The performance of different versions of the discrete random walk models in turbulent flows with nonuniform normal root-mean-square (RMS) velocity fluctuations and turbulence time scales were carefully investigated. The OpenFOAM v2−f low Reynolds number turbulence model was used for evaluating the fully developed streamwise velocity and the wall-normal RMS velocity fluctuations profiles in a turbulent channel flow. The results were then used in an in-house matlab particle tracking code, including the drag and Brownian forces, and the trajectories of randomly injected point-particles with diameters ranging from 10 nm to 30 μm were evaluated under the one-way coupling assumption. The distributions and deposition velocities of fluid-tracer and finite-size particles were evaluated using the conventional-discrete random walk (DRW) model, the modified-DRW model including the velocity gradient drift correction, and the new improved-DRW model including the velocity and time gradient drift terms. It was shown that the conventional-DRW model leads to superfluous migration of fluid-point particles toward the wall and erroneous particle deposition rate. The concentration profiles of tracer particles obtained by using the modified-DRW model still are not uniform. However, it was shown that the new improved-DRW model with the velocity and time scale drift corrections leads to uniform distributions for fluid-point particles and reasonable concentration profiles for finite-size heavy particles. In addition, good agreement was found between the estimated deposition velocities of different size particles by the new improved-DRW model with the available data.

On Non-Spherical Nanoparticle Dynamics in Turbulent Flows

2018 ◽  
Author(s):  
L. Tian ◽  
G. Ahmadi
Keyword(s):  
Turbulent Flows ◽  
Mean Free Path ◽  
Particle Dispersion ◽  
Transport Processes ◽  
Nano Particles ◽  
Brownian Diffusion ◽  
Coupled Equations ◽  
Micro Particles ◽  
Rotational Motions ◽  
Human Airways

Dispersion and deposition of non-spherical nano- and micro-particles suspended in turbulent flows was studied. Due to the non-spherical morphology, the coupled equations of translational and rotational motions of particles were solved and the corresponding particle trajectories were evaluated. For nano-particles with the scale comparable to the gas mean free path, the Brownian diffusion effects that becomes important was included in both translational and rotational motions. Particular attention was given to the interactions of nano-ellipsoidal particles and fluid motions at different scales. Examples of nano-ellipsoidal particle dispersion and deposition in ducts and respiratory tracks were presented and discussed. Such information is useful for understanding the transport processes of asbestos and nano-fibers in human airways for inhalation risk assessment studies.

Numerical study of vertical dispersion by stratified turbulence

2009 ◽  
Vol 631 ◽  
pp. 149-163 ◽  
Author(s):  
G. BRETHOUWER ◽  
E. LINDBORG
Keyword(s):  
Potential Energy ◽  
Numerical Simulations ◽  
Turbulent Flows ◽  
Time Scale ◽  
Numerical Study ◽  
Turnover Time ◽  
Particle Dispersion ◽  
Stratified Turbulence ◽  
The Mean ◽  
Simulation Results

Numerical simulations are carried out to investigate vertical fluid particle dispersion in uniformly stratified stationary turbulent flows. The results are compared with the analysis of Lindborg & Brethouwer (J. Fluid Mech., vol. 614, 2008, pp. 303–314), who derived long- and short-time relations for the mean square vertical displacement 〈δz〉 of fluid particles. Several direct numerical simulations (DNSs) with different degrees of stratification and different buoyancy Reynolds numbers are carried out to test the long-time relation 〈δz2〉 = 2ϵPt/N2. Here, ϵP is the mean dissipation of turbulent potential energy; N is the Brunt–Väisälä frequency; and t is time. The DNSs show good agreement with this relation, with a weak dependence on the buoyancy Reynolds number. Simulations with hyperviscosity are carried out to test the relation 〈δz2〉 = (1+πCPL)2ϵPt/N2, which should be valid for shorter time scales in the range N−1 ≪ t ≪ T, where T is the turbulent eddy turnover time. The results of the hyperviscosity simulations come closer to this prediction with CPL about 3 with increasing stratification. However, even in the simulation with the strongest stratification the growth of 〈δz2〉 is somewhat slower than linear in this regime. Based on the simulation results it is argued that the time scale determining the evolution of 〈δz2〉 is the eddy turnover time, T, rather than the buoyancy time scale N−1, as suggested in previous studies. The simulation results are also consistent with the prediction of Lindborg & Brethouwer (2008) that the nearly flat plateau of 〈δz2〉 observed at t ~ T should scale as 4EP/N2, where EP is the mean turbulent potential energy.

Accuracy of the CRW Models for Prediction of the Deposition and Dispersion of Particles in Inhomogeneous Turbulent Channel Flows

2019 ◽  
Author(s):  
Amir A. Mofakham ◽  
Goodarz Ahmadi
Keyword(s):  
Experimental Data ◽  
Stochastic Model ◽  
Turbulent Flows ◽  
Deposition Velocity ◽  
Finite Size ◽  
Channel Flows ◽  
Concentration Profiles ◽  
Velocity Fluctuations ◽  
Deposition Velocities ◽  
Turbulent Channel Flows

Abstract The accuracy of the continuous random walk (CRW) stochastic model for prediction of dispersion and deposition of suspended particles in inhomogeneous turbulent channel flows was explored. The Reynolds-averaged Navier-Stokes (RANS) equations in conjunction with the Reynolds Stress Transport model was used to evaluate the mean flow and RMS velocity fluctuation characteristics of a fully developed turbulent channel flow at shear Reynolds number of 219. Then, spherical particles with diameters ranging from 10 nm to 30 μm and dimensionless relaxation times of 10−4 to 50 (in wall units) were uniformly introduced into the channel and their trajectories were evaluated by using the equation of particle motion including the Stokes drag and Brownian excitation. The particle laden flow was assumed to be sufficiently dilute so that the particle-particle collisions and the effects of particles on the flow could be ignored. To incorporate the effects of turbulence velocity fluctuations on particle motions, first, the Conventional-CRW stochastic model, which was originally proposed for homogenous turbulent flows, was used. The particles were tracked for the duration of 10,000 wall units of time and the deposition of particles on the walls was evaluated. By conducting ensemble averaging, the steady-state concentration profiles and deposition velocity of the particles were calculated. Comparison of the predicted results with the direct numerical simulation (DNS) and experimental data suggests that the deposition velocity was overestimated. In addition, unrealistic accumulation of fluid-point particles in the near-wall regions, and overestimation of the turbophoresis effects on finite-size particles were also observed. The poor agreement of the concentration profiles and deposition velocities resulting from the conventional (homogenous flow) CRW model with the experimental and the DNS data pointed to the lack of accuracy of the Conventional-CRW model in generating instantaneous fluid velocity fluctuations seen by ultrafine and finite-size particles in inhomogeneous turbulent flows. Then, the normalized Langevin equation with a drift correction term that was suggested by Bocksell and Loth [1] was used as an improved CRW model for applications to inhomogeneous flows. The simulations for the same range of particle sizes were repeated and the corresponding concentration profiles and the deposition velocity were evaluated. It was shown that the improved CRW model led to a reasonable uniform concentration profile for the ultrafine particles and the predicted concentration profiles of finite-size particles quantitatively matched with the DNS data. In addition, the evaluated deposition velocities from the improved CRW model were also in a good agreement with the experimental data and empirical model predictions.

Optical Response of High-Density Solid Micro- & Nano-Particles in Turbulent Flows

2015 ◽  
Vol 1086 ◽  
pp. 120-127
Author(s):  
Sriram P. Kalathoor
Keyword(s):  
Turbulent Flows ◽  
Laser Diagnostics ◽  
Flow Regimes ◽  
Optical Response ◽  
Particle Dispersion ◽  
Phase Changes ◽  
Nano Particles ◽  
Laser Scattering ◽  
Seed Particles ◽  
Optical Visualization

Optical Visualization Techniques are gaining steady ground as a much favored method for observing and recording the physics of complex flows. These methods use a solid particle dispersed into the concerned flow to analyze the fluid motion. The particles are assumed to be of spherical shape (an acceptable relaxation), wherein the diameter is assumed to be constant this means that the particles have a normal distribution of size, with a sharp peak at the given diameter. Scattering of high-frequency laser light by these particles is the basis of image formation which depicts the flow physics. The fluid phase, which is the carrier phase, is required to impart sufficient momentum to the particulate phase for the particles to trace the flow of the fluid. For this reason, the particles are required to be of sizes not greater than 100 microns, in keeping with the increasing density with size. Alumina and Titania (Al2O3and TiO2) are the most commonly employed seed particles for this purpose. Laser scattering is also dependent on the luminescence prop-erties of the seed particles interactions between particle luminescence and external illumination plays a major role in determining the quality of results. With time, porous particles and artificial materials have found their acceptable places for use in this field. Silica, Glass and Ceramics provide a good balance of density (or weight) and surface area, which is difficult to obtain in metallic particles. The present paper proposes to evaluate the optical response characteristics of dispersed particles vis-a`-vis the operating/flow conditions and the laser systems used. The flow regimes com-monly considered for such studies include turbulence, mixing, reactions, phase changes and so on. Keywords Optical Response; Flow Seeding; Visualization; Multi-phase Flow: Flow Regimes; Particle Dispersion; Laser Diagnostics

Effect of Particle Residence Time on Particle Dispersion in a Plane Mixing Layer

1993 ◽  
Vol 115 (4) ◽  
pp. 751-759 ◽  
Author(s):  
Tsuneaki Ishima ◽  
Koichi Hishida ◽  
Masanobu Maeda
Keyword(s):  
Gas Phase ◽  
Residence Time ◽  
Time Scale ◽  
Characteristic Time ◽  
Mixing Layer ◽  
Particle Dispersion ◽  
Laser Doppler Velocimeter ◽  
Characteristic Time Scale ◽  
Two Phase ◽  
Particle Residence Time

A particle dispersion has been experimentally investigated in a two-dimensional mixing layer with a large relative velocity between particle and gas-phase in order to clarify the effect of particle residence time on particle dispersion. Spherical glass particles 42, 72, and 135 μm in diameter were loaded directly into the origin of the shear layer. Particle number density and the velocities of both particle and gas phase were measured by a laser Doppler velocimeter with modified signal processing for two-phase flow. The results confirmed that the characteristic time scale of the coherent eddy apparently became equivalent to a shorter characteristic time scale due to a less residence time. The particle dispersion coefficients were well correlated to the extended Stokes number defined as the ratio of the particle relaxation time to the substantial eddy characteristic time scale which was evaluated by taking account of the particle residence time.

Observations on the turbulent fluctuations of a tidal current

1949 ◽  
Vol 199 (1058) ◽  
pp. 311-327 ◽  
Keyword(s):  
Time Scale ◽  
Tidal Current ◽  
Velocity Fluctuations ◽  
Turbulent Fluctuations ◽  
Long Period ◽  
Auto Correlation ◽  
Short Period ◽  
Wide Range ◽  
First Time ◽  
A Current

Records have been obtained of fluctuations in the speed of the tidal current in the Mersey estuary, using a current meter in a stand on the bottom, and compared with other records taken with the meter suspended freely at various depths. The fluctuations covered a wide range of periods but could be separated into two main types: ‘short period’, having periods of the order of a few seconds, and ‘long period’, with periods from 30 sec. to several minutes. The amplitudes, periods and auto-correlation of the short-period fluctuations have been examined in some detail, and it is concluded that the fluctuations observed near the bottom are evidence of the turbulence associated with bottom friction. It is believed to be the first time that the presence of turbulent velocity fluctuations of this time-scale in the sea has been established experimentally. The long-period fluctuations resemble those found in previous investigations and show features consistent with their being turbulent in origin also, although turbulence of the time-scale involved in their case would probably be mainly horizontal.

Analysis of Alfvénic flows with Solar Orbiter: particle and magnetic observations down to kinetic scales.

2021 ◽  
Author(s):  
Philippe Louarn ◽  
Andrei fedorov ◽  
alexis Rouillard ◽  
Benoit Lavraud ◽  
Vincent Génot ◽  
...  
Keyword(s):  
Solar Wind ◽  
Fine Structure ◽  
Time Resolution ◽  
Time Scale ◽  
Residual Energy ◽  
Distribution Functions ◽  
Strongly Correlated ◽  
Temperature Anisotropy ◽  
Velocity Fluctuations ◽  
Turbulence Parameters

<p>The magnetic and velocity fluctuations of the solar wind may be strongly correlated. This characterizes the  ‘Alfvenic’ flows. Using the observations of the Proton Alfa sensor (PAS/SWA) and the magnetometer (MAG) onboard Solar Orbiter, we analyze a period of 100 hours of such alfvenic flows, at different scales. Several parameters of the turbulence are computed (V-B correlation, various spectral indexes, cross-helicity, residual energy). We explore how these parameters may vary with time and characterize different turbulent states of the flow. More specifically, using the unprecedented time resolution of PAS during burst mode, especially its capability to measure 3D distribution functions at time scale below the proton gyroperiod, we study the connection of the turbulence to the dissipation domain and analyze the fine structure of the distribution functions and their evolutions at sub-second scales. The goal is to investigate whether some characteristics of the distributions, as their more or less pronounced temperature anisotropy, may be related to the turbulence parameters and the degree of V-B correlation.</p>

A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows

2014 ◽  
Vol 6 (06) ◽  
pp. 764-782 ◽  
Author(s):  
Jian-Hung Lin ◽  
Keh-Chin Chang
Keyword(s):  
Turbulent Flows ◽  
Particle Dispersion ◽  
Hard Sphere Model ◽  
Wall Roughness ◽  
Mass Loading ◽  
Particle Collisions ◽  
Physical Mechanisms ◽  
Parametric Studies ◽  
Low Mass ◽  
Bounded Particle

AbstractThree physical mechanisms which may affect dispersion of particle’s motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-numberk–εturbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be considerable in the bounded particle–laden flow. Since the investigated particles are associated with large Stokes numbers, i.e., larger thanO(1), in the test problem, the effects of turbulence on particle dispersion are much less considerable, as expected, in comparison with another two physical mechanisms investigated in the study.

scholarly journals Particle dispersion processes in two-dimensional turbulence: a comparison with 2-D kinematic simulation.

2007 ◽  
Vol 14 (2) ◽  
pp. 139-151 ◽  
Author(s):  
R. Castilla ◽  
J. M. Redondo ◽  
P. J. Gámez-Montero ◽  
A. Babiano
Keyword(s):  
Turbulent Flows ◽  
Particle Dispersion ◽  
Space Time ◽  
Relative Dispersion ◽  
Kinematic Simulation ◽  
Space Time Structure ◽  
Coherent Vortices ◽  
The One ◽  
Turbulent Particle Dispersion ◽  
Kinematic Simulations

Abstract. We study numerically the comparison between Lagrangian experiments on turbulent particle dispersion in 2-D turbulent flows performed, on the one hand, on the basis of direct numerical simulations (DNS) and, on the other hand, using kinematic simulations (KS). Eulerian space-time structure of both DNS and KS dynamics are not comparable, mostly due to the absence of strong coherent vortices and advection processes in the KS fields. The comparison allows to refine past studies about the contribution of non-homogeneous space-time 2-D Eulerian structure on the turbulent absolute and relative particle dispersion processes. We particularly focus our discussion on the Richardson's regime for relative dispersion.

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