Particle Transport Mechanisms Research Articles

Transport of ballistic projectiles during the 2015 Aso Strombolian eruptions

Large pyroclasts—often called ballistic projectiles—cause many casualties and serious damage on people and infrastructures. One useful measure of avoiding such disasters is to numerically simulate the ballistic trajectories and forecast where large pyroclasts deposit. Numerical models are based on the transport dynamics of these particles. Therefore, in order to accurately forecast the spatial distribution of these particles, large pyroclasts from the 2015 Aso Strombolian eruptions were observed with a video camera. In order to extrapolate the mechanism of particle transport, we have analyzed the frame-by-frame images and obtained particle trajectories. Using the trajectory data, we investigated the features of Strombolian activity such as ejection velocity, explosion energy, and particle release depth. As gas flow around airborne particles can be one of the strongest controlling factors of particle transport, the gas flow velocities were estimated by comparing the simulated and observed trajectories. The range of the ejection velocity of the observed eruptions was 5.1–35.5 m/s, while the gas flow velocity, which is larger than the ejection velocity, reached a maximum of 90 m/s, with mean values of 25–52 m/s for each bursting event. The particle release depth, where pyroclasts start to move separately from the chunk of magmatic fragments, was estimated to be 11–13 m using linear extrapolation of the trajectories. Although these parabolic trajectories provide us with an illusion of particles unaffected by the gas flow, the parameter values show that the particles are transported by the gas flow, which is possibly released from inside the conduit.

Diffusiophoresis in Cells: A General Nonequilibrium, Nonmotor Mechanism for the Metabolism-Dependent Transport of Particles in Cells.

The more we learn about the cytoplasm of cells, the more we realize that the cytoplasm is not uniform but instead is highly inhomogeneous. In any inhomogeneous solution, there are concentration gradients, and particles move either up or down these gradients due to a mechanism called diffusiophoresis. I estimate that inside metabolically active cells, the dynamics of particles can be strongly accelerated by diffusiophoresis, provided that they are at least tens of nanometers across. The dynamics of smaller objects, such as single proteins, are largely unaffected.

Particle transport within the pulsar wind nebula HESS J1825–137

Context. We present a detailed view of the pulsar wind nebula (PWN) HESS J1825–137. We aim to constrain the mechanisms dominating the particle transport within the nebula, accounting for its anomalously large size and spectral characteristics. Aims. The nebula was studied using a deep exposure from over 12 years of H.E.S.S. I operation, together with data from H.E.S.S. II that improve the low-energy sensitivity. Enhanced energy-dependent morphological and spatially resolved spectral analyses probe the very high energy (VHE, E > 0.1 TeV) γ-ray properties of the nebula. Methods. The nebula emission is revealed to extend out to 1.5° from the pulsar, ~1.5 times farther than previously seen, making HESS J1825–137, with an intrinsic diameter of ~100 pc, potentially the largest γ-ray PWN currently known. Characterising the strongly energy-dependent morphology of the nebula enables us to constrain the particle transport mechanisms. A dependence of the nebula extent with energy of R ∝ Eα with α = −0.29 ± 0.04stat ± 0.05sys disfavours a pure diffusion scenario for particle transport within the nebula. The total γ-ray flux of the nebula above 1 TeV is found to be (1.12 ± 0.03stat ± 0.25sys) × 10−11 cm−2 s−1, corresponding to ~64% of the flux of the Crab nebula. Results. HESS J1825–137 is a PWN with clearly energy-dependent morphology at VHE γ-ray energies. This source is used as a laboratory to investigate particle transport within intermediate-age PWNe. Based on deep observations of this highly spatially extended PWN, we produce a spectral map of the region that provides insights into the spectral variation within the nebula.

Assessment of forces acting on fine particles on a traveling-wave electric field conveyor: Application to powder manipulation

The purpose of this work is to investigate the transport mechanism of fine dielectric particles using a traveling-wave electric field conveyor. In the first part of this study, an experimental bench is set up to evaluate the efficiency of particle displacement according to their size and to visualize their motion. The displacement efficiency is evaluated based on the ratio of the mass of transported particles to the total mass of particles deposited. The experimental setup allows us to measure the charge of particles as well. In the second part of this study, we discuss the experimental results based on the balance of forces exerted on particle in such a device. The particles studied are spherical PMMA particles having sizes ranging between 50 and 500 μm. The results show that the displacement efficiency and charge per mass ratio of the particles decrease as the particle diameter increases. This is explained by the dominance of the gravitational force for large particles. The balance of forces also shows that the adhesion force is dominant for small particles. Coulomb's force is predominant in a certain particle size range. It is the only repulsive force, which make it the responsible of the particles movement.

Analytical study on quantifying the magnitude and rate of subgrade fines migration into subbase under flexible pavement

Abstract Particle transport and subsequent deposition in porous media play a significant role in the performance of civil infrastructure. For example, in pavement systems, the migration (also known as pumping) of fine-grained soil from saturated subgrades into subbase layers can compromise the pavement serviceability and performance. In this paper, an analytical framework is formulated to understand the mechanisms of particle transport in pavements and to provide a reasonable prediction of the amount of fines migration from a saturated pavement subgrade to subbase. The model is verified using results from a set of experimental investigations previously conducted to quantify the magnitude and rate of subgrade fines migration. The tests were performed on two classes of flexible pavement roadways, using one-third scale model mobile load simulator (MMSL3), an accelerated pavement testing (APT) device. Non-plastic saturated silt and partially saturated aggregate were used as subgrade and subbase, respectively. The study suggests that the migration of the subgrade soil into subbase is governed by hydraulic condition, grain size of subgrade, grain size and pore size of subbase, permeability of subbase, and viscosity of pore fluid in the subbase. Moreover, magnitude and duration of applied loads play key roles in the amount of pumping in pavement.

Probing the Puzzle of Behind-the-limb γ-Ray Flares: Data-driven Simulations of Magnetic Connectivity and CME-driven Shock Evolution

Abstract Recent detections of high-energy γ-rays from behind-the-limb (BTL) solar flares by the Fermi Gamma-ray Space Telescope pose a puzzle and challenge on the particle acceleration and transport mechanisms. In such events, the γ-ray emission region is located away from the BTL flare site by up to tens of degrees in heliographic longitude. It is thus hypothesized that particles are accelerated at the shock driven by the coronal mass ejection (CME) and then travel from the shock downstream back to the front side of the Sun to produce the observed γ-rays. To test this scenario, we performed data-driven, global magnetohydrodynamics simulations of the CME associated with a well-observed BTL flare on 2014 September 1. We found that part of the CME-driven shock develops magnetic connectivity with the γ-ray emission region, facilitating transport of particles back to the Sun. Moreover, the observed increase in γ-ray flux is temporally correlated with (1) the increase of the shock compression ratio and (2) the presence of a quasi-perpendicular shock over the area that is magnetically connected to the γ-ray emitting region, both conditions favoring the diffusive shock acceleration (DSA) of particles. These results support the above hypothesis and can help resolve another puzzle, i.e., long-duration (up to 20 hr) γ-rays flares. We suggest that, in addition to DSA, stochastic acceleration by plasma turbulence may also play a role, especially in the shock downstream region and during the early stage when the shock Alfvén Mach number is small.

An improved particle swarm optimization method for locating time-varying indoor particle sources

The indoor transmission of airborne particles can spread disease and have health-related and even life-threatening effects on occupants, thus necessitating effective ways to locate indoor particle sources. The identification of particle sources from concentration distributions is a difficult task because particles are often released at a time-varying rate, and particle transport mechanisms are more complex than those of gas. This study proposes an improved multi-robot olfactory search method for locating two types of time-varying indoor particle sources: 1) periodic sources such as occupants’ respiratory activities and 2) decaying sources such as laboratory leaky containers with hazardous chemicals. The method considers both particle concentrations and indoor air velocities by including an upwind term in the standard particle swarm optimization (PSO) algorithm, preventing robots from becoming trapped into a local optimum, which occurs when using other algorithms. We also considered two ventilation types (mixing ventilation and displacement ventilation) when particles are emitted from different source types, comprising four scenarios. For each scenario, particle concentration and air velocity were simulated using computational fluid dynamics (CFD) and then fed to the PSO algorithm for source localization. In addition, we validated the CFD approach for one scenario by comparing experimental data (e.g., velocities and particle concentrations) under laboratory settings. The results showed that the proposed method can locate the two types of particle sources within approximately 55 s, and the success rates of source localization exceeding 96%, which is a much higher level than levels achieved from the standard PSO and wind utilization II algorithms.

Influence of septic system wastewater treatment on titanium dioxide nanoparticle subsurface transport mechanisms.

Engineered nanomaterials (ENMs) are commonly incorporated into food and consumer applications to enhance a specific product aspect (i.e., optical properties). Life cycle analyses revealed ENMs can be released from products during usage and reach wastewater treatment plants (WWTPs), with titanium dioxide (TiO2) accounting for a large fraction. As such, food grade (FG) TiO2, a more common form of TiO2 in wastewater, was used in this study. Nanomaterials in WWTPs have been well characterized, although the problematic septic system has been neglected. Elution and bioaccumulation of TiO2 ENMs from WTTPs in downriver sediments and microorganisms has been observed; however, little is known about mechanisms governing the elution of FG TiO2 from the septic drainage system. This study characterized the transport behavior and mechanisms of FG TiO2 particles in porous media conditions after septic waste treatment. FG and industrial grade (IG) TiO2 (more commonly studied) were introduced to septic tank effluent and low-ionic strength electrolyte solutions prior to column transport experiments. Results indicate that FG TiO2 aggregate size (200-400nm) remained consistent across solutions. Additionally, elution of FG and IG TiO2 was greatest in septic effluent at the higher nanoparticle concentration (100ppm). FG TiO2 was well retained at the low (2ppm) concentration in septic effluent, suggesting that particles that escape the septic system may still be retained in drainage field before reaching the groundwater system, although eluted particles are highly stabilized. Findings provide valuable insight into the significance of the solution environment at mediating differences observed between uniquely engineered nanomaterials. Graphical abstract.

Interfacial Targeting of Sessile Droplets Using Electrospray.

We report on the use of electrospray atomization to deliver nanoparticles and surfactant directly to the surface of sessile droplets. The particles delivered to the target droplet remained adsorbed at its interface since they arrived solvent-free. Upon complete evaporation, the interface of the target drop was mapped to the underlying substrate, forming a nanoparticle deposit. The use of electrospray permitted the exploration of the interfacial particle transport and the role of surfactants in governing particle motion and deposit structure. When no surfactant was present in the sprayed solution, there was no observable convection of the interfacial particles. When Tween 80, a high-molecular-weight surfactant, was added to the sprayed solution, the surface flow was similarly suppressed. Only when small surfactants (e.g., sodium dodecyl sulfate) were present in the sprayed solution was Marangoni flow, directed toward the droplet apex, induced at the interface. This flow drove the interfacial particles to the apex of the target droplet, creating a particle-dense region at the center of the final deposit. We found that small surfactants were capable of desorbing from the interface at a sufficiently high rate relative to the evaporation time scale of the target droplet. Once inside the drop, the desorbed surfactant was convected to the contact line where it accumulated, inducing a surface tension gradient and a solutal Marangoni flow. Numerical modeling using the lattice Boltzmann-Brownian dynamics method confirmed this mechanism of particle transport and its relationship to deposit structure. The use of sacrificial targets combined with electrospray may provide a unique capability for building colloidal monolayers with organized structure in a scalable way.

Flow pattern maps for particle-fluid mixture transport in a horizontal pipe - application of a three-layer model

ABSTRACTA modified three-layer model was applied to model particle-fluid mixture flow in a horizontal pipe, the viscoelastic properties of carrier polymer solution were taken into consideration, and the Deborah number was used to calculate solid-fluid friction factor. An energy equation was applied to determine temperature distribution of carrier fluid along a horizontal pipe to accurately represent the rheological properties of carrier fluid. During the transport process, particles quickly settle out of carrier fluid and accumulate on pipe bottom forming a particle bed, so a particle bed load flow is observed. The transport mechanisms of particles in moving particle bed are govern by fluidization, which causes the height of this layer to be small and equal to 2∼5 times of particle diameter. In addition, the pressure drop is composed of solid-fluid and solid-solid friction loss, which dominate the hydrodynamic performance at different stages.

Experimental investigation on particle transport of coal fines in unsteady terrain slug flow

The coal fine output from coalbed methane (CBM) formation during the production of CBM wells seems to be a common occurrence. At the lower section of wellbore, the aggregation of coal fine particles may occur, that lead to the increase of local coal fine concentration and the negative effect on the work of downhole equipment. To investigate the migration of coal fine particles in gas-liquid flow, the gas-liquid-solid three-phase flow experiments were carried out. The experiments were conducted in a downbent V-shaped pipe with different incline angles (5°, 15° and 25°), and recorded the real-time liquid holdup at certain positions of the lower elbow with the conductivity method. The terrain-induced slugs can be identified on the time series curves of liquid holdup. And the characteristic parameters of each slug, such as slug length and translational velocity, were calculated. By injecting marked coal fine particles into the multiphase flow loop and observing whether they can be discharged from the V-shaped pipe, the lower limits of gas and liquid velocities to void the particle retention at the lower elbow were collected. According to the experimental observation of the coal fine particle movement in one slug unit, we defined dimensionless particle velocity function FD to analyze the particle transport mechanism in the terrain slug flow.The FD value distribution of terrain slugs, calculated by the use of slug parameters, changed with the slug dissipation and coalescence in the experimental incline pipeline. Detail analyses of FD statistical parameters show that the mean values of FD increase with the rise of mixture flow velocity, and the decrease of inclination angle. The mean values of FD at the critical particle discharge conditions are positively correlated to particle concentration. And a linear relationship between the critical means of FD and the particle concentrations is presented.

On the transport, segregation, and dispersion of heavy and light particles interacting with rising thermal plumes

A systematic numerical analysis is carried out on the multiplicity of patterns produced by inertial particles dispersed in a fluid and localized gravitational convection developing in the form of a rising thermal plume. In particular, specific numerical examples are presented to provide inputs for an increased understanding of the underlying flow-particle interaction mechanisms and cause-and-effect relationships. A rich spectrum of convective dynamics is obtained at the relatively high value of the considered Rayleigh number (Ra = 108), which naturally allows the investigation of several intriguing effects (including, but not limited to, particle interaction with plume jet, associated vortices, shear instabilities, and symmetry breaking phenomena). An important degree of freedom is introduced in the problem by changing the particle viscous drag through proper tuning of the related Stokes number (St). Similarly, inertia and weight of solid matter are varied parametrically by performing numerical simulations for both light and heavy particles at different values of the Froude number. This framework lets us identify the average behavior of particles by revealing the mean evolution. We connect such statistics to the behavior of the temporally evolving thermal plume, giving deeper insights into the particle transport mechanisms and associated dissipative dynamics.

Performance of a depth fibrous filter at particulate loading conditions. Description of temporary and local phenomena with structure development

Optimization of a depth filter structure requires information about the filter's performance during its loading with particles. In this paper a macroscopic model of filtration process was proposed. It was based on the description of the loading process of three different gradient fibrous filters produced in the melt-blown technology process. Due to the contamination deposition on fibers, a new method of determination of the filter porosity transition, was suggested. It was based on growth of the deposit mass accumulated in individual filter layers, which was defined upon the scanning electron microscope images, the clearance of layer samples, and the overall increase of the filter mass. Local phenomena, i.e. the agglomerate structure, its spatial distribution and resuspension, were exposed through a two-stage character of the process. The empirical correlations obtained for deposition kinetics and pressure drop development are helpful in filter design. By knowing the desired contamination removal, fiber diameter and porosity can be verified mathematically, so that the filter will work the most efficiently. The proposed description of the structural loading of the multilayer filter is simple and reliable. Further research on biotic and abiotic particle transport mechanisms should be performed.

Physics of Proppant Transport Through Hydraulic Fracture Network

Horizontal drilling with successful multistage hydraulic fracture treatments is the most widely applied and effective method to enable economic development of hydrocarbon-bearing shale reservoirs. Once fracture networks are established, they must be propped open to maintain pathways for fluid migration through the production phase. As such, the design and application of effective and efficient proppant treatment is considered a key step to successfully develop the targeted resource. Unfortunately, the available literature and simulation tools to describe proppant transport in complex fracture networks are inadequate, and some of the fundamental mechanisms of proppant transport are poorly understood. The present study provides a critical review of relevant published literature to identify important mechanisms of particle transport and related governing equations. Based on that review, a mathematical model was developed to quantitatively predict the transport behavior of proppant particles in model fracture networks. Aspects of this mathematical model are compared against computational fluid dynamic (CFD) simulation, and implications of this work are discussed.

Stratification-induced reorientation of disk settling through ambient density transition

Settling due to gravity force is a basic transport mechanism of solid particles in fluids in the Earth. A large portion of particles occurring in nature and used in technical applications are non-spherical. Settling of particles is usually studied in homogeneous ambient conditions, however, stratification is inherent of natural fluids. It has been acknowledged that stratification modifies the velocity of settling spheres and amorphous aggregates. However, the effect of particle shape on the dynamics of settling through density-stratified ambient fluid has not been recognized well enough. Here I show experimental evidence that continuous density transition markedly modifies the settling dynamics of a disk in terms of settling velocity and orientation of a particle. Settling dynamics of a disk are more complex than dynamics of spheres and aggregates studied previously. I found that in a two-layer ambient with density transition, a disk settling in a low Reynolds number regime undergoes five phases of settling with the orientation varying from horizontal to vertical, and it may achieve two local minimum settling velocities in the density transition layer. Moreover, I found that the settling dynamics depends on a density difference between upper and lower homogeneous layers, stratification strength and thickness of density transition.

Detailed deposition analysis of inertial and diffusive particles in a rat nasal passage

Rats have been widely used as surrogates for evaluating the health effects of inhaled airborne particulate matter. To provide a thorough understanding of particle transport and deposition mechanisms in the rat nasal airway, this article presents a computational fluid dynamics (CFD) study of particle exposure in a realistic rat nasal passage under a resting flow condition. Particles covering a diameter range from 1 nm to 4 µm were passively released in front of the rat’s breathing zone, and the Lagrangian particle tracking approach was used to calculate individual particle trajectories. Detailed particle deposition analysis shows the deposition of inertial particles >2 µm is high in the rat nasal vestibule and more than 70% of all inhaled inertial particles were trapped in this region. While for diffusive nanoparticles, the vestibule filtration effect is reduced, only less than 60% of inhaled nanoparticles were blocked by the anterior nasal structures. The particle exposure in the olfactory region only shows notable deposition for diffusive nanoparticles, which peaks at 9.4% for 5 nm particles. Despite the olfactory deposition remains at a low level, the ratio between the olfactory and the main passage is kept around 30–40% for 10–800 nm particles, which indicates a particle-size-independent distribution pattern in the main nasal passage and olfactory. This study provides a deep understanding of particles deposition features in a rat nasal passage, and the research findings can aid toxicologist in inter-species exposure-response extrapolation study.

Flushing Enhancement with Vibration and Pulsed Current in Electrochemical Machining

This research aims to understand flushing of by-products in electrochemical machining (ECM) by modeling and experimentally verifying mechanism of particle transport in inter-electrode gap under low frequency vibration. A series of hole were drilled on steel plates to evaluate the effect of vibration on material removal rate and hole quality. Infinite focus optical technique was used to capture and analyze the three-dimensional images of ECM'ed features. Experimental results showed that maximum machining depth and minimum taper angle can be achieved when vibrating the workpiece at 40 Hz and 10 µm amplitude. Simulation results showed that the highest average flushing speed of 0.4 m/s was obtained at this vibration frequency and amplitude. Machining depth and material removal rate has a positive correlation with the average flushing speed. Sharper ECM’ed profile is obtained since the taper angle is favorably reduced at high average flushing speed.

Visualization of local deposition of nebulized aerosols in a human upper respiratory tract model

Knowing aerosol deposition distribution (ADD) in the respiratory tract is crucial in assessing dose–response relationship and optimizing inhalation therapy. However, characterization of ADD inside the airway is challenging, due to its inaccessibility to standard measurement instruments. Radioactive imaging techniques such as gamma scintigraphy, SPECT, and PET are capable of characterizing ADD and have proven valuable in drug-device designs, but these techniques are costly, complex-to-use, and pose radiation risks to patients. The objective of this study is to develop and validate a simple and practical technique to visualize nebulized aerosol deposition in a human airway model. An anatomically accurate mouth–lung airway model was used in both experiments and numerical simulations for validation purpose. A sectional hollow cast replica was developed and fabricated using 3D printing. Sar-Gel was utilized to visualize the aerosol deposition distribution on the inner walls of the upper respiratory airway. Computational simulations were conducted to understand the underlying mechanisms of particle transport and deposition. The deposition distribution obtained from the Sar-Gel experiments and numerical simulations resembled each other to a high degree. Specifically, the deposition hot spots in the upper trachea were nearly identical between experiment and simulation, suggesting that the computational model is adequate in capturing the mechanisms of particle transport and deposition. Considering the inhalation effect, more drug particles were delivered to the lungs at 10 L/min than at 30 L/min. The Sar-Gel based method in combination with sectional upper airway casts appears to be a practical approach to visualize regional depositions with nebulizers. Computational modeling and Sar-Gel tests can be used as complementary in optimizing inhalation therapies.

Modeling of the transport and deposition of polydispersed particles: Effects of hydrodynamics and spatiotemporal evolution of the deposition rate.

A time-distance-dependent deposition model is built to investigate the effects of hydrodynamic forces on the transport and deposition of polydispersed particles and the evolution of deposition rates with time and distance. Straining and the heterogeneity of the particle population are considered to play important roles in the decreasing distribution of deposition rates. Numerical simulations were applied in a series of sand column experiments at different fluid velocities for three different porous media. The effects of hydrodynamics forces are elaborated with the systematic variations of deposition dynamic parameters of the proposed model. With retention distributions with particle size as well as temporal and spatial evolutions of deposition rates, the transport and deposition mechanisms of polydispersed particles will be elucidated through the interplay of the variation of the particle size distribution of mobile particle populations and the geometrical change of the porous medium due to retention (straining and blocking).

Intrinsic suppression of turbulence in linear plasma devices

Plasma turbulence is the dominant transport mechanism for heat and particles in magnetised plasmas in linear devices and tokamaks, so the study of turbulence is important in limiting and controlling this transport. Linear devices provide an axial magnetic field that serves to confine a plasma in cylindrical geometry as it travels along the magnetic field from the source to the strike point. Due to perpendicular transport, the plasma density and temperature have a roughly Gaussian radial profile with gradients that drive instabilities, such as resistive drift-waves and Kelvin–Helmholtz. If unstable, these instabilities cause perturbations to grow resulting in saturated turbulence, increasing the cross-field transport of heat and particles. When the plasma emerges from the source, there is a time, , that describes the lifetime of the plasma based on parallel velocity and length of the device. As the plasma moves down the device, it also moves azimuthally according to E × B and diamagnetic velocities. There is a balance point in these parallel and perpendicular times that sets the stabilisation threshold. We simulate plasmas with a variety of parallel lengths and magnetic fields to vary the parallel and perpendicular lifetimes, respectively, and find that there is a clear correlation between the saturated RMS density perturbation level and the balance between these lifetimes. The threshold of marginal stability is seen to exist where . This is also associated with the product , where is the drift-wave linear growth rate, indicating that the instability must exist for roughly 100 times the growth time for the instability to enter the nonlinear growth phase. We explore the root of this correlation and the implications for linear device design.