Aerosol Motion Research Articles

Revisiting the CFD modeling of a virtual impactor with inverted split ratio: Highlighting some determinants of representativeness

Dichotomous sampling of a mixture of gas and aerosol is looked for in many applications such as airborne SVOC (semivolatile organic compound) sampling. This article revisits the computational fluid dynamics simulation of the original SVOC aerosol dichotomous sampler (Kim & Raynor, 2009) to investigate which among some factors influence most the representativeness of such simulations. Considered modeling factors of influence are the 2D-axisymmetric reduction of space and the choice of turbulence model (realizable k-ε vs. k-ω BSL or k-ω SST). Considered physical factors of influence are the particles aerodynamic diameter (range 50 nm to 20 μm), the type of inlet condition for gas and particles (uniform or free-sampling condition), the particles turbulent dispersion and the particles initial radial position at inlet. Results are supported by an extensive numerical verification procedure and by available validation data. In this specific confined transitional two-phase flow, it is found that the axisymmetric simplification holds, that the turbulence model affects significantly the predicted flow pattern but less pressure drops and marginally the fate of aerosol particles. Turbulent dispersion of aerosols is found negligible, but the effect of inlet boundary condition appears decisive on aerosol motion despite a marginal influence on the flow pattern. In particular, for aerodynamic diameters above 2 μm, free-sampling conditions produce a strong focusing effect of particles which drastically helps in limiting wall deposition. These findings highlight particularly the importance of using a physically sound inlet condition for aerosol particles when simulating aerosol samplers, rejecting the usual simplistic uniform condition. Results also particularly emphasize the necessity of a thorough verification procedure for the computation of both phases when using CFD for similar samplers. Regarding the application to dichotomous sampling, it appears that any process that moves particles away from the wall upon entry improves the separation efficiency of the device while decreasing deposition.

Determination of the effective porosity of a single filter fiber

In this work, a numerical simulation of the aerosol motion when flowing around a single porous filter fiber with a diameter of 5 mm is carried out. The fiber is formed by a set of microfibers in a random arrangement. The size of the microfibers varies from 0.1 mm to 0.5 mm. For each fixed size of microfibers, a fiber model with different porosity of the medium was created. The porosity ranged from 0.7 to 0.9. The calculations were carried out in the ANSYS software package (v. 19.0). Studies have shown that a porous filter fiber model provides the maximum deposition efficiency for highly inert particles is provided by a porous filter fiber model with a microfiber size of 0.1 mm and a medium porosity of 0.9.

Numerical Simulation of Aerosol Scattering by Sneezing and Exhalation

Numerical simulations of aerosol motion induced by sneezing and exhalation were carried out by CFD (Computational Fluid Dynamics) with modeled particles. For sneezing analysis, with and without covering by an elbow and wearing a face mask were modeled to clarify the effect of how to prevent scattering. Aerosol scattering from a talking passenger in a ventilated train was also analyzed to show a realistic situation that may cause infections of the virus. Those results have been published on the web through several SNSs to promote understanding the effect of infection prevention manner.

Numerical Simulation of the Aerosol Particle Motion in Granular Filters with Solid and Porous Granules

In this work, a study was carried out to compare the filtering and hydrodynamic properties of granular filters with solid spherical granules and spherical granules with modifications in the form of micropores. We used the discrete element method (DEM) to construct the geometry of the filters. Models of granular filters with spherical granules with diameters of 3, 4, and 5 mm, and with porosity values of 0.439, 0.466, and 0.477, respectively, were created. The results of the numerical simulation are in good agreement with the experimental data of other authors. We created models of granular filters containing micropores with different porosity values (0.158–0.366) in order to study the micropores’ effect on the aerosol motion. The study showed that micropores contribute to a decrease in hydrodynamic resistance and an increase in particle deposition efficiency. There is also a maximum limiting value of the granule microporosity for a given aerosol particle diameter when a further increase in microporosity leads to a decrease in the deposition efficiency.

Mine aerology of dust aerosols under conditions of hydro-vortex coagulation

Dust suppression is the most popular method for depositing dust from the air to blast isolation. Improving the isolation system for probable coal dust explosions is an integral part of the problem of dust explosion protection of mining enterprises, the solution of which is one of the most important tasks in the field of occupational safety and health. Purpose: study of physics of the process and the construction of a mathematical model of the movement of aerosols in mine workings in a wide range of changes in the inertial kinematic parameters of the air flow. Research methods. Based on the Boussinesq equation, the Fourier rule and the similarity theorem of complex systems, the analogy of vorticity dispersion and kinematic viscosity, a mathematical model of the movement of dust aerosols under hydro-vortex coagulation conditions is constructed. Some similarity criteria are obtained that ensure the identity of experimental studies and real aerological processes in mines, as well as criterion equations for calculating the coefficient of aerodynamic drag and the time of unsteady inertial motion. Results. Equations are obtained that make it possible to identify aerodynamic processes of aerosol motion under hydro-cyclonic dust suppression through the criteria of Reynolds, Euler and Archimedes, corresponding to the overStokes and Stokes flow regimes. The possibility of reducing the aerodynamic resistance to 20% during hydro-vortex coagulation has been confirmed, which makes it possible to increase the energy efficiency of dust suppression by up to 15% and to increase the turbulization coefficient by at least 20%, thereby reducing the likelihood of a hazardous concentration of dust aerosols. Application. The use of the proposed mathematical model will make it possible to better calculate local aerological processes in mine workings and, as a consequence, to increase the efficiency of turbulization and dust suppression process control at mining enterprises.

An Overview of Experiments and Numerical Simulations on Airflow and Aerosols Deposition in Human Airways and the Role of Bioaerosol Motion in COVID-19 Transmission

Determining the hotspots and deposition efficiencies (DEs) for aerosols in human airways is important for both research and medical purposes. The complexity of the human airways and the breathing process limit the application of in vitro measurements to only two consecutive branches of the human airway. Herein, in-depth information on in vitro experiments and state-of-the-art review on various computational fluid dynamics (CFD) applications and finite element methods on airflow and aerosol motion in both healthy and obstructed human airways are provided. A brief introduction of the application of one-dimensional and two-dimensional mathematical models to investigate airflow and particle motion in the lungs are further discussed. As evident in this review, aerosol deposition in the upper and central human airway regions has been extensively studied under different inhalation statuses and conditions such as humidity as well as different aerosol sizes, shapes, and properties. However, there is little literature on the lower sections of the human airways. Herein, a detailed review of the fundamentals for both in vitro experiments and numerical simulation at different sections of human airways is done. Exceptional features and essential developments in numerical methods for aerosol motion in healthy and diseased human airways are also discussed. Challenges and limitations associated with the applications of in vitro experiments and CFD methods on both human-specific and idealized models are highlighted. The possibility of airborne transmission pathways for COVID-19 has been discussed. Overall, this review provides the most useful approach for carrying out two-phase flow investigations at different sections of the human lungs and under different inhalation statuses. Additionally, new research gaps that have developed recently on the role of bioaerosols motion in COVID-19 transmission, as well as the deposition of aerosols in impaired human airways due to coronavirus (COVID-19) are underlined.

Improving the efficiency of particle deposition on the filter fiber through its modification

In this work, we performed a numerical study of the parameters of a fiber filter that make the greatest contribution to the efficiency of particle deposition, using the example of a single fiber. The mechanism of inertial particle deposition is considered. The aerosol motion was numerically simulated in the ANSYS Fluent software package. The particle deposition efficiency was calculated for a single solid and porous fiber without a protrusion, with one protrusion and five protrusions. Based on the data obtained, the dependences of the particle deposition efficiency for a single fiber are graphically presented. A new method is proposed for increasing filter efficiency by creating modified fibers.

Introduction of a variable electrostatic field for targeted drug application following intraperitoneal aerosol chemotherapy (PIPAC).

e14211 Background: In the treatment of Peritoneal Metastasis, the delivery of pressurized chemo-aerosols into the capnoperitoneum has been reported to show superior qualities to the application of conventional liquid chemotherapy. However, there is little basic knowledge on how aerosol particle motion within the abdominal cavity may be influenced and directed by means of an electrostatic field. This study aims to investigate aerosol motion within the abdominal cavity using an ex-vivo model. Material and Methods: All experiments were performed in an ex-vivo box as well as on postmortem swine abdomen, both established ex-vivo PIPAC models previously introduced by our research team. Extra-abdominally created, unipolar charged aerosols are introduced to the abdominal cavity. A modulated electrostatic field within the abdominal cavity is established to examine aerosol particle collision via fluorescence microscopy and photography. Results: Aerosol particle collision was observed on the peritoneum in the varying electric field. Preliminary data suggests that tissue penetration rates change with a varying electric field (p < 0.05). Drug distribution pattern showed a significant improvement with a varying electric field vs. without (p < 0.05). Conclusions: Our data suggest that extra-abdominally created, highly drug concentrated aerosol particles have the capacity to achieve high drug penetration rates in the peritoneum. The addition of a varying electric field enhances particle collision as well as significantly improves drug distribution patterns. Further studies are required to investigate the in-vivo safety and efficacy of a varying electric field and the significance of extra-abdominally created chemo-aerosols with regards to possible clinical applications.

Design Principles for Sensing Particle Number Concentration and Mean Particle Size With Unipolar Diffusion Charging

Diffusion charging followed by particle capture and measurement is a promising method for determining particle number concentration of automotive exhaust. In addition, the method allows for the determination of a mean particle size, which is of high relevance with respect to health impacts. We investigate three different measurement configurations based on unipolar diffusion charging and electrostatic particle capture, using an analytical non-dimensional model, as well as a transient, multi-physical 3D model. The comprehensive modeling techniques enable prediction of the transient electrical current signals induced by the motion of charged aerosols. Comparison of the multi-physical modeling approach to analytical calculations and experimental data demonstrate model validation. The understanding gained from the modeling techniques enables a study of sensor design and calibration. Particle number concentration sensing is demonstrated for both a modulated precipitation configuration and a two-stage measurement configuration. With the latter, additional information on the mean particle size is extracted.

Targeting inhaled aerosol delivery to upper airways in children: Insight from computational fluid dynamics (CFD).

Despite the prevalence of inhalation therapy in the treatment of pediatric respiratory disorders, most prominently asthma, the fraction of inhaled drugs reaching the lungs for maximal efficacy remains adversely low. By and large drug delivery devices and their inhalation guidelines are typically derived from adult studies with child dosages adapted according to body weight. While it has long been recognized that physiological (e.g. airway sizes, breathing maneuvers) and physical transport (e.g. aerosol dynamics) characteristics are critical in governing deposition outcomes, such knowledge has yet to be extensively adapted to younger populations. Motivated by such shortcomings, the present work leverages in a first step in silico computational fluid dynamics (CFD) to explore opportunities for augmenting aerosol deposition in children based on respiratory physiological and physical transport determinants. Using an idealized, anatomically-faithful upper airway geometry, airflow and aerosol motion are simulated as a function of age, spanning a five year old to an adult. Breathing conditions mimic realistic age-specific inhalation maneuvers representative of Dry Powder Inhalers (DPI) and nebulizer inhalation. Our findings point to the existence of a single dimensionless curve governing deposition in the conductive airways via the dimensionless Stokes number (Stk). Most significantly, we uncover the existence of a distinct deposition peak irrespective of age. For the DPI simulations, this peak (∼ 80%) occurs at Stk ≈ 0.06 whereas for nebulizer simulations, the corresponding peak (∼ 45%) occurs in the range of Stk between 0.03-0.04. Such dimensionless findings hence translate to an optimal window of micron-sized aerosols that evolves with age and varies with inhalation device. The existence of such deposition optima advocates revisiting design guidelines for optimizing deposition outcomes in pediatric inhalation therapy.

Aerosol deposition predictions in computed tomography-derived skeletons from severe asthmatics: A feasibility study

BackgroundThe authors numerically investigated the correlation between airway skeletons of severe asthmatic human subjects and predicted aerosol deposition to shed light on the effect of environmental factors on asthma risk. We hypothesized that there are asthmatic subjects whose airway skeletal structure can expose the subject to a risk of higher local aerosol deposition compared to subjects with a more common/normal branching pattern. MethodsFrom a population of severe asthmatics studied at total lung capacity via computed tomography we randomly selected 8 subjects whose Forced Expiratory Volume in 1s, percent predicted fell below 45% predicted. To simulate aerosol motion in the human lungs, we employed in-house three-dimensional eddy-resolving computational fluid dynamics and particle tracking models utilizing 3 of the 8 severe asthmatic subjects. One of the 3 subjects was found to have a distinct, localized airway narrowing chosen for further investigation. In the simulation, we controlled flow rate and luminal area, i.e., Reynolds and Stokes numbers, in each branch of the computed tomography-derived airway skeletons. FindingsWe found a distinct enhancement of aerosol deposition associated with the narrowed branches of one subject even when the luminal area was numerically adjusted from its narrowed state to that of a non-asthmatic subject. The branching angle, freed of luminal narrowing persisted in demonstrating a marginally significant increase in local particle deposition compared with the subjects without the initial constriction. InterpretationThese results demonstrate the possibility that inherent airway structure may influence localized constriction found in severe asthmatics.

The role of anisotropic expansion for pulmonary acinar aerosol deposition

Lung deformations at the local pulmonary acinar scale are intrinsically anisotropic. Despite progress in imaging modalities, the true heterogeneous nature of acinar expansion during breathing remains controversial, where our understanding of inhaled aerosol deposition still widely emanates from studies under self-similar, isotropic wall motions. Building on recent 3D models of multi-generation acinar networks, we explore in numerical simulations how different hypothesized scenarios of anisotropic expansion influence deposition outcomes of inhaled aerosols in the acinar depths. While the broader range of particles acknowledged to reach the acinar region (dp=0.005–5.0μm) are largely unaffected by the details of anisotropic expansion under tidal breathing, our results suggest nevertheless that anisotropy modulates the deposition sites and fractions for a narrow band of sub-micron particles (dp~0.5–0.75μm), where the fate of aerosols is greatly intertwined with local convective flows. Our findings underscore how intrinsic aerosol motion (i.e. diffusion, sedimentation) undermines the role of anisotropic wall expansion that is often attributed in determining aerosol mixing and acinar deposition.

Capture and long-term suspension of aerosols in an open cavity flow

The motion of non-Brownian aerosols in an open laminar cavity flow is investigated by means of numerical and asymptotic methods. The cavity is a rectangular domain where air enters through an inlet located on the top, and exits in the horizontal direction through symmetric apertures near the bottom wall. While such flows are generally used to provide a clean (particle-free) environment, the introduction of any object inside the cavity can affect the flow topology and, in return, can lead to large aerosol residence times. In addition, it is known that the unsteadiness of the flow, though weak, significantly delays the exit of aerosols from the cavity. The goal of this paper is to analyze the conditions under which these phenomena occur, for heavy inertial particles with a small response time and a finite free-fall velocity, in a laminar cavity flow. A two-dimensional situation is considered and investigated by means of numerical simulations and perturbation methods, at moderate cavity Reynolds numbers. It is observed that any obstacle placed inside the cavity (along the symmetry axis) creates large quasi-steady triangular recirculation cells stretched by the outward flow, provided the Reynolds number is not too large. In particular, streamlines near the floor are reversed, so that deposited particles are swept inward and remain in the cavity. The probability of trapping has been calculated asymptotically, for inertia-free sedimenting particles, and compared to numerical results. Trapping in the wake of obstacles is observed to persist when the flow is slightly unsteady. In addition, it is shown that the small but finite unsteadiness of the flow can lead to the temporary trapping of a significant portion of particles by large recirculation cells near the upper corners of the cavity. This phenomenon takes place in spite of gravity and centrifuge effects due to the curvature of the cell, and leads to a long-term suspension which significantly increases the residence time of aerosols. By making use of separatrix map methods, the critical aerosol diameter below which this phenomenon occurs has been obtained and compared to numerical simulation results.

Revisiting pulmonary acinar particle transport: convection, sedimentation, diffusion, and their interplay.

It is largely acknowledged that inhaled particles ranging from 0.001 to 10 m are able to reach and deposit in the alveolated regions of the lungs. To date, however, the bulk of numerical studies have focused mainly on micrometer sized particles whose transport kinematics are governed by convection and sedimentation, thereby capturing only a small fraction of the wider range of aerosols leading to acinar deposition. Too little is still known about the local acinar transport dynamics of inhaled (ultra)fine particles affected by diffusion and convection. Our study aims to fill this gap by numerically simulating the transport characteristics of particle sizes spanning three orders of magnitude (0.01-5 m) covering diffusive, convective, and gravitational aerosol motion across a multigenerational acinar network. By characterizing the deposition patterns as a function of particle size, we find that submicrometer particles [formulae see text (0.1 m)] reach deep into the acinar structure and are prone to deposit near alveolar openings; meanwhile, other particle sizes are restricted to accessing alveolar cavities in proximal generations. Our findings underline that a precise understanding of acinar aerosol transport, and ultrafine particles in particular, is contingent upon resolving the complex convective-diffusive interplay in determining their irreversible kinematics and local deposition sites.

Analytical Model of the Transport of Aerosol Particles in a Circular Hole Inside a Porous Medium

The approximate analytical and numerical solutions of the problem of aerosol motion in a circular hole inside a porous medium are presented using the approximations of Poiseuille flow for the carrier phase and the convective-diffusive equation of transport of inertialess particles. Analytical expressions for the fluid velocity profile, the space distribution of particle concentration, and the penetration that include the Darcy number and a slip coefficient at the interface between the porous medium and an adjacent free fluid are obtained as parameters. The numerical solution of the problem of the air flow in the combined porous and free fluid regions was used in order to find the relation between the slip coefficient and the Darcy number.

Observation of macroscopic aerosol motion due to thermal creep on chamber walls at low Knudsen number in microgravity

In a rarefied gas the temperature field and the gas motion are closely related, and the temperature field can cause, in a confined flow geometry, a steady flow without the help of external forces. This is due to the creep of the fluid along the walls induced by a wall temperature gradient, as a consequence of the molecular transfer of momentum to the wall. Experiments performed in microgravity conditions in two small cells, with mono- and bi-atomic carrier gases (Ar, N2), and a thermal gradient between upper and bottom horizontal plates, allowed the measurement of the width of the thermal creep and the induced velocity of the gas near the vertical cell wall, due to thermal gradient. In addition, experiments demonstrated the existence of the thermal creep flow in the cells, even with Knudsen number as low as 10−5. The work evidences experimentally, and for the first time, the thermal creep flow in small cells, due to wall temperature gradient, with Knudsen number as low as 10−5. In view of these results, the no-slip boundary conditions of the Navier–Stokes law in the “continuum” regime can be inadequate in non-isothermal flow geometry and in microgravity conditions.

Critical trajectories for aerosol particles: Their determination for impaction in fibrous filters and in oscillating bubbles

Critical trajectories for aerosol particles in a gas flow are the ones which divide an aerosol flux into different parts, for example aerosol which is, and is not deposited. They can exist in all gas flows in which aerosol motion is governed by gas velocity rather than by diffusion and we describe two mathematical methods for their calculation. For deposition by impaction on a filter fibre it is necessary to solve the differential equations for particle motion and an efficient iterative procedure is used to obtain the critical trajectories.Jonas and Schütz (1988) have shown that aerosol impaction is an important mechanism for the removal of aerosol from an oscillating sodium vapour bubble formed during a hypothetical core disruptive accident in a fast reactor. For these one-dimensional oscillations, when the gas velocity within a bubble is a linear function of position, we extend their work by calculating critical trajectories directly from the integral equation describing a depositing particle for two models with different initial conditions. With initially entrained uniform aerosol, the percentage impacted is independent of the inclusion of gravity in the calculations as long as regions empty of aerosol do not appear in the bubbles. Numerical results are obtained for a wide range of amplitudes of bubble oscillations and aerosol in the size range 1-30μm. In agreement with Jonas and Schütz, we find that a considerable fraction of the aerosol at larger sizes is removed by impaction. The theory is also shown to apply to other types of bubble oscillation including those of a spherical bubble.

Two-Component Horizontal Aerosol Motion Vectors in the Atmospheric Surface Layer from a Cross-Correlation Algorithm Applied to Scanning Elastic Backscatter Lidar Data*

Abstract Two-component horizontal motion vectors of aerosol features were calculated by applying a cross-correlation algorithm to square image blocks extracted from consecutive pairs of elastic backscatter lidar scans. The resulting vector components were compared with corresponding horizontal wind components from tower-mounted sonic anemometers located at the center of the image blocks. In the analysis 180 245 pairs of vectors derived from 75 days of field data collected between 19 March and 11 June 2007 were used. Examples of time series comparisons from 4-h periods during light, strong, and changing wind conditions are presented. Mean signal-to-noise ratios (SNRs) of the block backscatter data, maxima of the cross-correlation functions (CCFs), observed wind speed, and turbulent kinetic energy (TKE) were also calculated for each velocity component comparison. The correlation between the lidar-derived motion components and sonic anemometer wind components tends to be highest during light wind conditions with low TKE. An empirical relationship is presented that enables the elimination of vectors that are likely to be significantly different than the anemometer measurement. When applied to the entire set of scans available, this quality control (QC) method increases the correlation between the two forms of measurements. Finally, the cross-correlation algorithm and QC method are applied to a mesh of locations over pairs of scans. Two examples of two-dimensional and two-component vector flow fields are shown. In one case, the flow field reveals a rotational circulation associated with a vortex and, in the other case, convergence and transport near the leading edge of a density current front.

Photophoretic motion of soot aerosols in field of shortwave solar radiation

The results of a theoretical analysis of the photophoretic motion of soot particles in the field of solar radiation under the conditions of stationary atmosphere are presented. In the refined radiation block of the model, integral fluxes of solar radiation were calculated by the Monte Carlo method in the context of the approximation of a plane-parallel horizontally homogeneous atmosphere. An analysis of the obtained results again verifies the possibility of significant photophoretic exhibitions for soot aerosol particles in the stratosphere; solar photophoresis can be considered an efficient mechanism of the vertical transport of absorbing submicron particles up to heights of the middle stratosphere. Forces of solar photophoresis can compete with forces of gravity up to heights of 30–35 km for particles of submicron and micron sizes. At the same time, the estimations again demonstrated the low efficiency of radiometric photophoresis for weakly absorbing, dense, and strongly heat-conducting particles of the atmospheric aerosol.

On the efficiency and correction of vertically oriented blunt bioaerosol samplers in moving air

The aspiration efficiency of vertical and wind-oriented Air-O-Cell samplers was investigated in a field study using the pollen of hazel, sweet chestnut and birch. Collected pollen numbers were compared to measurements of a Hirst-type Burkard spore trap. The discrepancy between pollen counts is substantial in the case of vertical orientation. The results indicate a strong influence of wind velocity and inlet orientation relative to the freestream on the aspiration efficiency. Various studies reported on inertial effects on aerosol motion as function of wind velocity. The measurements were compared to a physically based model for the limited case of vertical blunt samplers. Additionally, a simple linear model based on pollen counts and wind velocity was developed. Both correction models notably reduce the error of vertically oriented samplers, whereas only the physically based model can be used on independent datasets. The study also addressed the precision error of the instruments used, which was substantial for both sampler types.