Modeling the evaporation and dispersion of airborne sputum droplets expelled from a human cough

This study contributes a new model to simulate the evaporation and dispersion of sputum droplets from human coughs or sneezes. It is the first time different chemical components have been included in order to estimate the transport of sputum or similar biological droplets. This modified model demonstrates the ability to describe real-world phenomena that the widely used single droplet model lacks. Evaporation and dispersion of airborne sputum droplets originating from a human cough are simulated using this model combined with an initially buoyant turbulent jet. Constituents of sputum droplets such as NaCl, amino acids, carbohydrates, and lipids are included. Effects of these chemical components on evaporation rate, velocity, and temperature of droplets are investigated in detail. The results obtained for sputum droplets will provide a perspective of what conditions the viruses within a droplet might face upon being ejected from the mouth during a cough. Finally, computational fluid dynamics (CFD) and probability density function (PDF) techniques were used to complement the new model with a simulation of a cough jet and the dynamics of droplet nuclei in confined spaces. Numerical results indicate that a 10 microns sputum droplet will evaporate to become a droplet nucleus (3.5 microns) in 0.55 s at 0.8 or 80% RH, in 0.3 s at 0.5 or 50% RH, and in 0.25 s at 0.2 or 20% RH. The droplet temperature decreases rapidly from human body temperature to room temperature, which may affect the viability of any carried virus.     

Experimental investigation of far-field human cough airflows from healthy and influenza-infected subjects

Seasonal influenza epidemics have been responsible for causing increased economic expenditures and many deaths worldwide. Evidence exists to support the claim that the virus can be spread through the air, but the relative significance of airborne transmission has not been well defined. Particle image velocimetry (PIV) and hot-wire anemometry (HWA) measurements were conducted at 1 m away from the mouth of human subjects to develop a model for cough flow behavior at greater distances from the mouth than were studied previously. Biological aerosol sampling was conducted to assess the risk of exposure to airborne viruses. Throughout the investigation, 77 experiments were conducted from 58 different subjects. From these subjects, 21 presented with influenza-like illness. Of these, 12 subjects had laboratory-confirmed respiratory infections. A model was developed for the cough centerline velocity magnitude time history. The experimental results were also used to validate computational fluid dynamics (CFD) models. The peak velocity observed at the cough jet center, averaged across all trials, was 1.2 m/s, and an average jet spread angle of θ = 24° was measured, similar to that of a steady free jet. No differences were observed in the velocity or turbulence characteristics between coughs from sick, convalescent, or healthy participants.     

Fire development in a 1/3 train carriage mock-up

To study what parameters that control the initial fire spread and the development to local flashover in a metro carriage, a total of six fire tests were conducted in a mock-up of a metro carriage that is about 1/3 of a full wagon length. They were carried out under a large scale calorimeter in a laboratory environment. The focus was on the initial fire development in a corner scenario using different types of ignition source that may lead to a fully developed fire. The ignition sources used were either a wood crib placed on a corner seat or one litre of petrol poured on the corner seat and the neighbouring floor together with a backpack. The amount of luggage and wood cribs in the neighbourhood of the ignition source was continuously increased in order to identify the limits for flashover in the test-setup. The tests showed that the combustible boards on parts of the walls had a significant effect on the fire spread. In the cases where the initial fire did not exceed a range of 400–600 kW no flashover was observed. If the initial fire grew up to 700–900 kW a flashover was observed. The maximum heat release rate during a short flashover period for this test set-up was about 3.5 MW. The time to reach flashover was highly dependent on the ignition type: wood cribs or backpack and petrol. A full developed carriage fire was achieved as a result of intense radiation from the flames and ceiling smoke layer. This was mostly dependent on the amount of fire load nearby the ignition source and how strong the vertical flame spread on the high pressure laminate boards mounted to walls and ceiling above the ignition source was, leading to a ceiling flame. In such cases, the seats alone did not contain sufficient fuel for the fire to spread within the train, and additional fuel (luggage) is required near the seats. For fully developed carriage fires, the fire starting on the seat in the corner spread to the opposite seat on the same side of the aisle, then horizontally spread to seats on the other side of the aisle, and finally a longitudinal flame spread along the carriage was observed. When and where the fire stopped or whether it reached a fully developed stage was mostly dependent on the amount of fire load nearby the ignition source and how strong the vertical flame spread on the high pressure laminate boards mounted to walls and ceiling above the ignition source was.     

Evaporation and dispersion of respiratory droplets from coughing

Understanding how respiratory droplets become droplet nuclei and their dispersion is essential for understanding the mechanisms and control of disease transmission via droplet‐borne and airborne routes. A theoretical model was developed to estimate the size of droplet nuclei and their dispersion as a function of the ambient humidity and droplet composition. The model‐predicted dried droplet nuclei size was 32% of the original diameter, which agrees with the maximum residue size in the classic study by Duguid, 1946, Edinburg Med. J., 52 , 335 and the validation experiment in this study, but is smaller than the 50% size predicted by Nicas et al., 2005, J. Occup. Environ. Hyg., 2 , 143. The droplet nuclei size at a relative humidity of 90% (25°C) could be 30% larger than the size of the same droplet at a relative humidity of less than 67.3% (25°C). The trajectories of respiratory droplets in a cough jet are significantly affected by turbulence, which promotes the wide dispersion of droplets. We found that medium‐sized droplets (e.g., 60 μm) are more influenced by humidity than are smaller and larger droplets, while large droplets (≥100 μm), whose travel is less influenced by humidity, quickly settle out of the jet.     

Effectiveness of a personalized ventilation system in reducing personal exposure against directly released simulated cough droplets

The inhalation intake fraction was used as an indicator to compare effects of desktop personalized ventilation and mixing ventilation on personal exposure to directly released simulated cough droplets. A cough machine was used to simulate cough release from the front, back, and side of a thermal manikin at distances between 1 and 4 m. Cough droplet concentration was measured with an aerosol spectrometer in the breathing zone of a thermal manikin. Particle image velocimetry was used to characterize the velocity field in the breathing zone. Desktop personalized ventilation substantially reduced the inhalation intake fraction compared to mixing ventilation for all investigated distances and orientations of the cough release. The results point out that the orientation between the cough source and the breathing zone of the exposed occupant is an important factor that substantially influences exposure. Exposure to cough droplets was reduced with increasing distance between cough source and exposed occupant.     

The impact of indoor thermal stratification on the dispersion of human speech droplets

Exhaled jets from an infected person are found to be locked at a certain height when thermal stratification exists in rooms, causing a potential high risk of disease transmission. This work is focused on the theoretical analysis of the dynamic characteristics of human speech droplets and the residual droplet nuclei in both thermally uniform and stratified environments. Results show that most droplets generated from human speaking can totally evaporate or deposit to the ground within 1.5-2 m. For small droplets of < 80μm, thermal stratification shows a more significant impact on their residues. The lock-up height of the droplet nuclei is a function of droplet size and the temperature gradient, and within this lock-up layer, these droplet nuclei can travel a long distance, much more than 2m. For medium droplets of 80-180 μm, thermal stratification can weaken the evaporation and accelerate the deposition processes, equivalent to a higher relative humidity (RH). Accordingly, more droplets can deposit to the ground, reducing the exposure to large droplets in close proximity to the source. Large droplets of > 180μm show no dependence on stratification and RH. These findings can have implications for developing effective engineering methods to limit the spread of infectious disease.     

Flow dynamics and characterization of a cough

Abstract Airborne disease transmission has always been a topic of wide interests in various fields for decades. Cough is found to be one of the prime sources of airborne diseases as it has high velocity and large quantity of droplets. To understand and characterize the flow dynamics of a cough can help to control the airborne disease transmission. This study has measured flow dynamics of coughs with human subjects. The flow rate variation of a cough with time can be represented as a combination of gamma‐probability‐distribution functions. The variables needed to define the gamma‐probability‐distribution functions can be represented by some medical parameters. A robust multiple linear regression analysis indicated that these medical parameters can be obtained from the physiological details of a person. However, the jet direction and mouth opening area during a cough seemed not related to the physiological parameters of the human subjects. Combining the flow characteristics reported in this study with appropriate virus and droplet distribution information, the infectious source strength by coughing can be evaluated.

Practical Implications

There is a clear need for the scientific community to accurately predict and control the transmission of airborne diseases. Transportation of airborne viruses is often predicted using Computational Fluid Dynamics (CFD) simulations. CFD simulations are inexpensive but need accurate source boundary conditions for the precise prediction of disease transmission. Cough is found to be the prime source for generating infectious viruses. The present study was designed to develop an accurate source model to define thermo‐fluid boundary conditions for a cough. The model can aid in accurately predicting the disease transmission in various indoor environments, such as aircraft cabins, office spaces and hospitals.     

Study on transport characteristics of saliva droplets produced by coughing in a calm indoor environment

In this paper, the transport characteristics of saliva droplets produced by coughing are examined in a calm indoor environment. Three subjects are studied, with results indicating that more than 6.7 mg of saliva is expelled at speeds of up to 22 m/s during each individual cough, and that saliva droplets can travel further than 2 m. In addition, the dispersion processes of saliva droplets of different diameters expelled during coughing are analyzed using the Lagrangian equation. The results indicate that the transport characteristics of saliva droplets due to coughing change with size. The effects of gravity or inertia on droplets of 30 μm or less are negligible due to their small sizes, and therefore their transport is mostly influenced by the indoor flow field. Droplets of 50–200 μm, which are significantly affected by gravity, fall as the flow-field weakens. Droplets of 300 μm or more, which are affected more by inertia than gravity, fall difficultly. Moreover, the analytical results also indicate that the droplets’ transport is greatly influenced by the spatial relationship between the air-conditioner and the subjects. Finally, based on the experimental and analytical results, droplet infection by saliva droplets due to coughing is examined.     

CFD analysis of a realistic reduced-scale modeling equipped with axial jet fan

Typically, in the experimental scale road tunnel model, the air flow induced by ventilation system is provided by an external fan. In this paper, the authors have numerically simulated full and reduced-scale tunnel in order to evaluate the possibility to realize a reduced scale of a road tunnel model with a realistic ventilation system consisting of impulsive jet fans.In particular, two different types of longitudinal ventilation systems were considered, traditional and alternative. The last one was equipped with jet fans that have the inlet/outlet sections inclined at a fixed pitch angle (α=6°) toward the tunnel floor. The jet fan was simulated as a simple momentum source that provides a pressure rise (pressure drop) across them as a function of the outflow air velocity.The analyzed tunnel consists in a 800 m one directional bore with circular cross section 5.05 m radius; the jet fans were installed at 5.67 m from the floor. Furthermore a burning Heavy Good Vehicle (HGV), placed at 450 m far away the tunnel entrance, was considered. To simulate numerically the burning vehicle, the species transport equation combustion model with Eddy-Dissipation-Concept (EDC) model was adopted.In order to create a reduced-scale model from a full scale, Froude method was applied to preserve geometrical, kinematical and dynamical similitude. Temperature and axial velocity profiles, in different tunnel sections for both considered models (full and scaled) and ventilation systems, were provided. The numerical results showed a good agreement for the both ventilation systems.     

CFD study of exhaled droplet transmission between occupants under different ventilation strategies in a typical office room

This paper investigated the transmission of respiratory droplets between two seated occupants equipped with one type of personalized ventilation (PV) device using round movable panel (RMP) in an office room. The office was ventilated by three different total volume (TV) ventilation strategies, i.e. mixing ventilation (MV), displacement ventilation (DV), and under-floor air distribution (UFAD) system respectively as background ventilation methods. Concentrations of particles with aerodynamic diameters of 0.8 μm, 5 μm, and 16 μm as well as tracer gas were numerically studied in the Eulerian frame. Two indexes, i.e. intake fraction (IF) and concentration uniformity index R_C were introduced to evaluate the performance of ventilation systems. It was found that without PV, DV performed best concern protecting the exposed manikin from the pollutants exhaled by the polluting manikin. In MV when the exposed manikin opened RMP the inhaled air quality could always be improved. In DV and UFAD application of RMP might sometimes, depending on the personalized airflow rate, increase the exposure of the others to the exhaled droplets of tracer gas, 0.8 μm particles, and 5 μm particles from the infected occupants. Application of PV could reduce R_C for all the three TV systems of 0.8 μm and 5 μm particles. PV enhanced mixing degree of particles under DV and UFAD based conditions much stronger than under MV based ones. PV could increase the average concentration in the occupied zone of the exposed manikin as well as provide clean personalized airflow. Whether inhaled air quality could be improved depended on the balance of pros and cons of PV.     

Frequent recovery of influenza A but not influenza B virus RNA in aerosols in pediatric patient rooms

Influenza transmission occurs through the air, but the relative importance of small droplets, or aerosols, in influenza transmission especially within healthcare facilities remains uncertain. Detections of influenza virus in aerosols in cough and exhaled breath from infected patients and from the air in outpatient or inpatient healthcare facilities have been studied, but most studies were done in adults with very few data involving children. We aimed to assess the potential of influenza transmission via aerosols in pediatric patient rooms. Two-stage cyclone (NIOSH) air samplers were used to collect the air in 5-bed pediatric patient rooms with patients with influenza-like illness. Influenza A virus RNA was recovered in 15/19 (79%) air sampling occasions with ≥1 patient with laboratory-confirmed influenza A virus infections, in all air size fractions (>4 µm, 1-4 µm and <1 µm). Influenza B virus RNA was significantly less detected (2/10 occasions, 20%). We estimated a ventilation rate of 1.46 ACH in a similar but unoccupied 5-bed patient room. High quantities of influenza A virus RNA detected in the air in pediatric patient rooms suggests other individuals in pediatric patient rooms including other patients, visitors, caretakers and healthcare workers could be exposed to influenza A virus in aerosols while caring for infected children.     

Experimental studies of thermal environment and contaminant transport in a commercial aircraft cabin with gaspers on

Gaspers installed in commercial airliner cabins are used to improve passengers' thermal comfort. To understand the impact of gasper airflow on the air quality in a cabin, this investigation measured the distributions of air velocity, air temperature, and gaseous contaminant concentration in five rows of the economy‐class section of an MD‐82 commercial aircraft. The gaseous contaminant was simulated using SF₆ as a tracer gas with the source located at the mouth of a seated manikin close to the aisle. Two‐fifths of the gaspers next to the aisle were turned on in the cabin, and each of them supplied air at a flow rate of 0.66 l/s. The airflow rate in the economy‐class cabin was controlled at 10 l/s per passenger. Data obtained in a previous study of the cabin with all gaspers turned off were used for comparison. The results show that the jets from the gaspers had a substantial impact on the air velocity and contaminant transport in the cabin. The air velocity in the cabin was higher, and the air temperature slightly more uniform, when the gaspers were on than when they were off, but turning on the gaspers may not have improved the air quality.     

Transport behavior of functionalized multi-wall carbon nanotubes in water-saturated quartz sand as a function of tube length

A series of one-dimensional column experiments was conducted to examine the effects of tube length on the transport and deposition of 4-ethoxybenzoic acid functionalized multi-wall carbon nanotubes (MWCNTs) in water-saturated porous media. Aqueous MWCNTs suspensions were prepared to yield three distributions of tube lengths; 0.02-1.3 μm (short), 0.2-7.5 μm (medium), and 0.2-21.4 μm (long). Results of the column studies showed that MWCNT retention increased with increasing tube length. Nevertheless, more than 76% of the MWCNT mass delivered to the columns was detected in effluent samples under all experimental conditions, indicating that the functionalized MWCNTs were readily transported through 40-50 mesh Ottawa sand. Examination of MWCNT length distributions in the effluent samples revealed that nanotubes with lengths greater than 8 μm were preferentially deposited. In addition, measured retention profiles exhibited the greatest MWCNT deposition near the column inlet, which was most pronounced for the long MWCNTs, and decreased sharply with travel distance. Scanning electron microscope (SEM) images showed that MWCNTs were deposited on sand surfaces over the entire column length, while larger MWCNT bundles were retained at grain intersections and near the column inlet. A mathematical model based on clean bed filtration theory (CBFT) was unable to accurately simulate the measured retention profile data, even after varying the weighting function and incorporating a nonuniform attachment rate coefficient expression. Modification of the mathematical model to account for physical straining greatly improved predictions of MWCNT retention, yielding straining rate coefficients that were four orders-of-magnitude greater than corresponding attachment rate coefficients. Taken in concert, these experimental and modeling results demonstrate the potential importance of, and need to consider, particle straining and tube length distribution when describing MWCNT transport in water-saturated porous media.     

Entrainment and its implications on microclimate ventilation systems: Scaling the velocity and temperature field of a round free jet

Research on microclimate ventilation systems, which mostly involve free jets, points to delivery of better ventilation in breathing zones. While the literature is comprehensive, the influence of contaminant entrainment in jet flows and its implications on the delivery of supplied air is not fully addressed. This paper presents and discusses entrainment characteristics of a jet issued from a round nozzle (0.05 m diameter), in relation to ventilation, by exploring the velocity and temperature fields of the jet flow. The results show a trend suggesting that increasing the Reynold number (Re) reduces ambient entrainment. As shown herein, about 30% concentration of ambient air entrained into the bulk jet flow at Re 2541 while Re 9233 had about 13% and 19% for Re = 6537/12 026 at downstream distance of 8 diameters (40 cm). The study discusses that “moderate to high” Re may be ideal to reduce contaminant entrainment, but this is limited by delivery distance and possibly the risk of occupant discomfort. Incorporating the entrainment mixing factor (the ratio of room contaminants entrained into a jet flow) in performance measurements is proposed, and further studies are recommended to verify results herein and test whether this is general to other nozzle configurations.     

Estimates of particulate matter inhalation doses during three-dimensional printing

Harmful emissions including particulates, volatile organic compounds, and aldehydes are generated during three-dimensional (3D) printing. Ultrafine particles are particularly important due to their ability to penetrate deep into the lung. We modeled inhalation exposure by particle size during 3D printing. A total of six thermoplastic filaments were used for printing under manufacturer's recommended conditions, and particle emissions in the size range between 10 nm and 10 μm were measured. The inhalation exposure dose including inhaled and deposited doses was estimated using a mathematical model. For all materials, the number of particles between 10 nm and 1 μm accounted for a large proportion among the released particles, with nano-sized particles being the dominant size. More than 1.3 × 10⁹ nano-sized particles/kgbw/g (95.3 ± 104.0 ng/kgbw/g) could be inhaled, and a considerable amount was deposited in respiratory regions. The total deposited dose in terms of particle number was 3.1 × 10⁸ particles/kgbw/g (63.6% of the total inhaled dose), and most (41.3%) were deposited in the alveolar region. The total mass of particles deposited was 19.8 ± 16.6 ng/kgbw/g, with 10.1% of the total mass deposited in the alveolar region. Given our findings, the inhalation exposure level is mainly determined by printing conditions, particularly the filament type and manufacturer-recommended extruder temperature.     

Simulating longitudinal ventilation flows in long tunnels: Comparison of full CFD and multi-scale modelling approaches in FDS6

The accurate computational modelling of airflows in transport tunnels is needed for regulations compliance, pollution and fire safety studies but remains a challenge for long domains because the computational time increases dramatically. We simulate air flows using the open-source code FDS 6.1.1 developed by NIST, USA. This work contains two parts. First we validate FDS6’s capability for predicting the flow conditions in the tunnel by comparing the predictions against on-site measurements in the Dartford Tunnel, London, UK, which is 1200 m long and 8.5 m in diameter. The comparison includes the average velocity and the profile downstream of an active jet fan up to 120 m. Secondly, we study the performance of the multi-scale modelling approach by splitting the tunnel into CFD domain and a one-dimensional domain using the FDS HVAC (Heating, Ventilation and Air Conditioning) feature. The work shows the average velocity predicted by FDS6 using both the full CFD and multi-scale approaches is within the experimental uncertainty of the measurements. Although the results showed the prediction of the downstream velocity profile near the jet fan falls outside the on-site measurements, the predictions at 80 m and beyond are accurate. Our results also show multi-scale modelling in FDS6 is as accurate as full CFD but up to 2.2 times faster and that computational savings increase with the length of the tunnel. This work sets the foundation for the next step in complexity with fire dynamics introduced to the tunnel.     

Predicting airflow distribution and contaminant transport in aircraft cabins with a simplified gasper model

This study investigated the air distribution and contaminant transport in aircraft cabins with gaspers by using computational fluid dynamics (CFD). If the detailed gasper geometry were used in the CFD simulations, the grid number would be unacceptably high. To reduce the grid number, this investigation proposed a method for simplifying the gasper geometry. The method was then validated by two sets of experimental data obtained from a cabin mockup and a real aircraft cabin. It was found that for the cabin mockup, the CFD simulation with the simplified gasper model reduced the grid number from 1.58 to 0.3 million and the computing cost from 2 days to 1 hour without compromising the accuracy. In the five-row economy-class cabin of the MD-82 airplane, the CFD simulation with the simplified gasper model was acceptable in predicting the distribution of air velocity, air temperature, and contaminant concentration.     

Simulation of vertical concentration gradient of influenza viruses in dust resuspended by walking

Particles are resuspended from the floor by walking and are subject to turbulent transport in the human aerodynamic wake. These processes may generate a vertical concentration gradient of particles. To estimate the magnitude of turbulence generated by walking, we measured the velocity field in the wake from floor to ceiling at 10‐cm intervals with a sonic anemometer. The resulting eddy diffusion coefficients varied between 0.06 and 0.20 m²/s and were maximal at ~0.75–1 m above the floor, approximately the height of the swinging hand. We applied the eddy diffusion coefficients in an atmospheric transport model to predict concentrations of resuspended influenza virus as a function of the carrier particle size, height in the room, and relative humidity, which affects the resuspension rate coefficient and virus viability. Results indicated that the concentration of resuspended viruses at 1 m above the floor was up to 40% higher than at 2 m, depending on particle size. For exposure to total resuspended viruses, the difference at 1 vs. 2 m was 11–14%. It is possible that shorter people are exposed to higher concentrations of resuspended dust, including pathogens, although experimental evidence is needed to verify this proposition.     

连续大跨钢结构支座滑移监测研究及应用

论述超声传感监测系统的特点、功能和适用环境,针对某连续大跨钢结构工程支座滑移监测项目,介绍传感系统的布置,并对支座滑移监测数据进行深入分析。结果表明:支座滑移量随温度增加基本呈线性变化;受平面、立面呈曲线布置的结构形式的影响,同一位置处各支座的滑移量存在差异;结构的温度场不均匀及日照会对各支座滑移量产生影响;监测数据与结构理论计算结果吻合良好。采用超声传感系统进行大跨连续结构支座滑移监测的数据稳定性及可靠性好、精度高,完全适用于大型结构在自然环境下的位移监测。     

Numerical study of the lock-up phenomenon of human exhaled droplets under a displacement ventilated room

This paper adopts an Eulerian-Lagrangian approach to investigate the lock-up phenomenon (or trap phenomenon) of human exhaled droplets in a typical office room under displacement ventilation (DV). A particle-source-in-cell (PSI-C) scheme is used to correlate the concentration with the Lagrangian particle trajectories in computational cells. Respiratory droplets with sizes of 0.8 μm, 5 μm and 16 μm are released from a numerical thermal manikin (NTM). The influence factors including indoor temperature gradient, heat source configuration and exhalation modes are studied. It is found that large temperature gradient would result in trap phenomenon of small exhaled droplets (smaller than 5 μm). The intensive heat source near the NTM could help to transport the small droplets to the upper zone and decrease the concentration level in the trapped zone. Both nose-exhaled and mouth-exhaled small droplets would be trapped at the breathing height when temperature gradient is sufficiently high. However, the trap height of the droplets from mouth is a little bit higher. Because of large gravitational force, it is difficult for the thermal plume to carry 16 μm respiratory droplets to the upper zone.