Expiratory Events Research Articles

Influence of thermal stratification on the transport of polydispersed expiratory particles

The fluid dynamics of expiratory events are complex, and understanding how indoor air conditions affect this and the spread of exhaled material is crucial to the prevention of large-scale spread of diseases. It is known that thermal stratification can trap contaminants in the lower levels of a room; however, there is a lack of studies that investigate the influence of vertical temperature gradients on the transport of expiratory particles at room scale. To this effect, we used Eulerian–Lagrangian large-eddy simulations to investigate the effect of thermal stratification on the transport of polydispersed expiratory particles during speaking in two different sized rooms. Cases with increasing temperature gradient were compared to an isothermal base case, and the influence of stratification on the exhalation jet and the particles suspended within is analyzed. The particle volume fraction was computed to quantify the spatiotemporal evolution of different particle size categories. Our results show that thermal stratification leads to an increased concentration of aerosols in the breathing zone and extends their forward reach. Aerosols up to a size threshold between 12 and 20 μm are locked up at different heights by stratification—beyond this threshold, they fall out continuously. In all cases, aerosols <20 μm traveled up to 4 m from the source, showing that physical distancing guidelines alone may be inadequate for controlling cross-infection risk for long-term exposures. Particles >60 μm are unaffected by stratification and do not follow a ballistic trajectory, falling out within 0.5 m of the infectious individual in all cases.

Influence of two-dimensional expiratory airflow variations on respiratory particle propagation during pronunciation of the fricative [f

Propagation of respiratory particles, potentially containing viable viruses, plays a significant role in the transmission of respiratory diseases (e.g., COVID-19) from infected people. Particles are produced in the upper respiratory system and exit the mouth during expiratory events such as sneezing, coughing, talking, and singing. The importance of considering speaking and singing as vectors of particle transmission has been recognized by researchers. Recently, in a companion paper, dynamics of expiratory flow during fricative utterances were explored, and significant variations of airflow jet trajectories were reported. This study focuses on respiratory particle propagation during fricative productions and the effect of airflow variations on particle transport and dispersion as a function of particle size. The commercial ANSYS-Fluent computational fluid dynamics (CFD) software was employed to quantify the fluid flow and particle dispersion from a two-dimensional mouth model of sustained fricative [f] utterance as well as a horizontal jet flow model. The fluid velocity field and particle distributions estimated from the mouth model were compared with those of the horizontal jet flow model. The significant effects of the airflow jet trajectory variations on the pattern of particle transport and dispersion during fricative utterances were studied. Distinct differences between the estimations of the horizontal jet model for particle propagation with those of the mouth model were observed. The importance of considering the vocal tract geometry and the failure of a horizontal jet model to properly estimate the expiratory airflow and respiratory particle propagation during the production of fricative utterances were emphasized.

Numerical simulations of the flow and aerosol dispersion in a violent expiratory event: Outcomes of the “2022 International Computational Fluid Dynamics Challenge on violent expiratory events”

This paper presents and discusses the results of the “2022 International Computational Fluid Dynamics Challenge on violent expiratory events” aimed at assessing the ability of different computational codes and turbulence models to reproduce the flow generated by a rapid prototypical exhalation and the dispersion of the aerosol cloud it produces. Given a common flow configuration, a total of 7 research teams from different countries have performed a total of 11 numerical simulations of the flow dispersion by solving the Unsteady Reynolds Averaged Navier–Stokes (URANS) or using the Large-Eddy Simulations (LES) or hybrid (URANS-LES) techniques. The results of each team have been compared with each other and assessed against a Direct Numerical Simulation (DNS) of the exact same flow. The DNS results are used as reference solution to determine the deviation of each modeling approach. The dispersion of both evaporative and non-evaporative particle clouds has been considered in 12 simulations using URANS and LES. Most of the models predict reasonably well the shape and the horizontal and vertical ranges of the buoyant thermal cloud generated by the warm exhalation into an initially quiescent colder ambient. However, the vertical turbulent mixing is generally underpredicted, especially by the URANS-based simulations, independently of the specific turbulence model used (and only to a lesser extent by LES). In comparison to DNS, both approaches are found to overpredict the horizontal range covered by the small particle cloud that tends to remain afloat within the thermal cloud well after the flow injection has ceased.

Time-resolved particle-scale dynamics of a particle-laden jet

Particle-laden jet flow is important to both jet-related industry applications and transmission of the virus through violent expiratory events, such as coughing and sneezing. To help understand its dynamics from the particle level, we develop a time-resolved, three-dimensional (3D), particle tracking velocimetry method, coupled with particle image velocimetry measurement of gas flow, and perform experiments on a dilute particle-laden gas jet. The spatial distributions of velocity and fluctuating velocity of the gas and particles are obtained. It is found that the presence of particles significantly changes the gas turbulence and stretch the gas flow field to the downstream. The probability density function of axial particle velocity shows non-Gaussian distribution and deviates much from those of the spanwise velocities, indicating strong non-equilibrium and anisotropic states. A new drag model is derived based on the reconstructed particle trajectories and gas flow field near the ejector exit with particle Reynolds numbers between 30 and 300. It is found in better agreement with the experimental data than the standard single-particle drag model. A simple model relating the particle volume fraction with particle displacement is developed based on the self-similarity theory of jet, showing good agreement with the experimental measurement.

Sensitivity of puff dynamics and airborne droplet nuclei distribution to variations in violent expiration events

We use large eddy simulations to investigate the puff and droplet dynamics from violent expiratory events such as coughs and sneezes in the first few seconds following an ejection. For each of the eleven simulations considered, over 60,000 droplets are ejected and individually tracked using the point-particle Euler–Lagrange approach. We test the sensitivity of the puff and droplet dynamics to various parameters including the ejection volume, momentum, and orientation. We also explore the effect of the mouth shape on the aforementioned dynamics by considering elliptical and circular inlet cross-sections. The results from the simulations compare favorably with a recent theoretical framework put forth by Balachandar et al. (2020) in terms of the puff size and propagation velocity. More importantly however, the theory is able to accurately predict the number and size spectra of the potentially virus-laden droplet nuclei that remain airborne within the puff. We observe that the ejection angle and mouth shape do not significantly affect the puff and droplet dynamics. Additionally, we quantify the carrying capacity of the detached puff portions in terms of the number and size spectra of droplets/droplet nuclei suspended within.

Intelligent Dynamic Indoor Aerosol Sensing Using Terahertz Band Wireless Communication Systems

We investigate how an IRS (Intelligent Reflective Surface) deployment within a terahertz (THz) band communication system, can detect and classify indoor vapour clouds expelled in violent expiratory events, such as coughing. This detection is of interest for public health systems. Since indoor spaces are sites for deployment of wireless communication infrastructure in any case, combined sensing and communication is an attractive solution. Using 6G networks to accomplish this while not affecting communication performance, is a relevant and useful application of THz signal’s properties. We use machine learning to distinguish between violent expiratory events and other aerosol emissions.

Flow and aerosol dispersion from wind musical instruments

In the midst of the COVID-19 pandemic, many live musical activities had to be postponed and even canceled to protect musicians and the audience. Orchestral ensembles face a particular challenge of contamination, because they are personally heavy and instrumentally diverse. A chief concern is whether wind instruments are vectors of contamination through aerosol dispersion. This study, made possible by the participation of members of The Philadelphia Orchestra, brings insight into the modes of production and early life of aerosols of human origin emitted by wind instruments. We find that these instruments produce aerosol levels that are comparable to normal speech in quantity and size distribution. However, the exit jet flow speeds are much lower than violent expiratory events (coughing and sneezing). For most wind instruments, the flow decays to background indoor-air levels at approximately 2 m away from the instrument's opening. Long range aerosol dispersion is, thus, via ambient air currents.

Improved guidelines of indoor airborne transmission taking into account departure from the well-mixed assumption

We use large eddy simulations with the Euler-Lagrange point-particle approach to assess the appropriateness of the well-mixed theory in predicting indoor risk of contagion. A novel `statistical overloading' technique allows us to analyze infinitely many scenarios, with only one data-rich simulation. The figure shows the droplet nuclei (left half of the room; see color map for radius of nuclei `r') and an iso-surface of velocity magnitude (in yellow). The inset (bottom right) shows one of the infinitely many expiratory events that can be analyzed.

Influence of expiratory flow pulsatility on the effectiveness of a surgical mask.

Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport expiratory particles. The World Health Organization recommends wearing face masks to reduce the airborne transmission of diseases such as SARS-CoV-2 (COVID-19). However, masks are not perfect as flow leakage occurs around the mask, and their effectiveness under realistic (multi-pulse) coughing conditions is unknown. To assess the influence of expiratory flow pulsatility on the effectiveness of a surgical face mask by quantifying and classifying the flow leakage around the mask. A custom-built pulsatile expiratory flow simulator is used to generate single- and multi-pulsed coughing events. Flow visualization and particle image velocimetry are used to assess the penetration distance and volume of leakage flow at the top and sides of a surgical mask. Leakage flow velocity profiles at the top and sides of a surgical mask take the form of a wall jet and a free-shear jet, respectively. Multi-pulsed expiratory flow events are found to generate greater leakage flow around the mask than single-pulsed events. For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough. The novelties of this study are: First, flow field measurements are used to quantify and classify the leakage flow fields around the top and sides of a surgical mask, providing a benchmark for quantitative modeling of leakage flow velocity profiles. Second, the influence of pulsatility on the effectiveness of surgical face masks is studied by quantifying the leakage volume. For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough, as multi-pulsed expiratory flow events are found to generate greater flow leakage around the mask than single-pulsed events.

A model to predict the short-term turbulent indoor dispersion of small droplets and droplet nuclei released from coughs and sneezes

We propose a simple model to predict the short-term indoor turbulent dispersion of the aerosol cloud produced by violent expiratory events. Once the air injection ceases, the turbulent jet transitions to a thermal puff that progressively decays due to viscous effects. According to recent literature, the expelled liquid droplets of saliva and sputum smaller than 20–30 μm in diameter stay afloat within this decaying turbulent puff. In contrast, droplets larger than 100 μm tend to leave the puff following quasi-ballistic trajectories and landing on the floor at relatively short times after release. The model presented here is capable of providing good estimates for the shape and dimensions of the cloud composed of the lighter fraction of droplets as a function of the intensity and the duration of the flow injection and the density difference between the exhaled and the ambient air. Predictions agree with Direct Numerical Simulations and experiments reported in the literature. This model can be used as an operational tool to determine the short-term spatial range of expelled droplets and provides realistic initial conditions for simulations of long-term dispersion of pathogen-laden clouds in indoor environments with forced and natural ventilation.

Large eddy simulation of cough jet dynamics, droplet transport, and inhalability over a ten minute exposure.

High fidelity simulations of expiratory events such as coughing provide the opportunity to predict the fate of the droplets from the turbulent jet cloud produced from a cough. It is well established that droplets carrying infectious pathogens with diameters of remain suspended in the air for several hours and transported by the air currents over considerable distances (e.g., in meters). This study used a highly resolved mesh to capture the multiphase turbulent buoyant cloud with suspended droplets produced by a cough. The cough droplets' dispersion was subjected to thermal gradients and evaporation and allowed to disperse between two humans standing 2 m apart. A nasal cavity anatomy was included inside the second human to determine the inhaled droplets. Three diameter ranges characterized the droplet cloud, , which made up 93% of all droplets by number; 5 to 100 μm comprised 3%, and m comprising 4%. The results demonstrated the temporal evolution of the cough event, where a jet is first formed, followed by a thermally driven puff cloud with the latter primarily composed of droplets under 5 μm diameter, moving with a vortex string structure. After the initial cough, the data were interpolated onto a more coarse mesh to allow the simulation to cover ten minutes, equivalent to 150 breathing cycles. We observe that the critical diameter size susceptible to inhalation was , although most inhaled droplets after 10 min by the second human were approximately . These observations offer insight into the risk of airborne transmission and numerical metrics for modeling and risk assessment.

Filtering efficiency model that includes the statistical randomness of non-woven fiber layers in facemasks.

Facemasks have become important tools to fight virus spread during the recent COVID-19 pandemic, but their effectiveness is still under debate. We present a computational model to predict the filtering efficiency of an N95-facemask, consisting of three non-woven fiber layers with different particle capturing mechanisms. Parameters such as fiber layer thickness, diameter distribution, and packing density are used to construct two-dimensional cross-sectional geometries. An essential and novel element is that the polydisperse fibers are positioned randomly within a simulation domain, and that the simulation is repeated with different random configurations. This strategy is thought to give a more realistic view of practical facemasks compared to existing analytical models that mostly assume homogeneous fiber beds of monodisperse fibers. The incompressible Navier-Stokes and continuity equations are used to solve the velocity field for various droplet-laden air inflow velocities. Droplet diameters are ranging from 10nm to 1.0µm, which covers the size range from the SARS-CoV-2 virus to the large virus-laden airborne droplets. Air inflow velocities varying between 0.1m·s-1 to 10m·s-1 are considered, which are typically encountered during expiratory events like breathing, talking, and coughing. The presented model elucidates the different capturing efficiencies (i.e., mechanical and electrostatic filtering) of droplets as a function of their diameter and air inflow velocity. Simulation results are compared to analytical models and particularly compare well with experimental results from literature. Our numerical approach will be helpful in finding new directions for anti-viral facemask optimization.

Respiratory droplets interception in fibrous porous media.

We investigate, by means of pore-scale lattice Boltzmann simulations, the mechanisms of interception of respiratory droplets within fibrous porous media composing face masks. We simulate the dynamics, coalescence, and collection of droplets of the size comparable with the fiber and pore size in typical fluid-dynamic conditions that represent common expiratory events. We discern the fibrous microstructure into three categories of pores: small, large, and medium-sized pores, where we find that within the latter, the incoming droplets tend to be more likely intercepted. The size of the medium-sized pores relative to the fiber size is placed between the droplet-to-fiber size ratio and a porosity-dependent microstructural parameter , with ϵ being the porosity. In larger pores, droplets collection is instead inhibited by the small pore-throat-to-fiber size ratio that characterizes the pore perimeter, limiting their access. The efficiency of the fibrous media in intercepting droplets without compromising breathability, for a given droplet-to-fiber size ratio, can be estimated by knowing the parameter . We propose a simple model that predicts the average penetration of droplets into the fibrous media, showing a sublinear growth with . Permeability is shown also to scale well with but following a superlinear growth, which indicates the possibility of increasing the medium permeability at a little cost in terms of interception efficiency for high values of porosity. As a general design guideline, the results also suggest that a fibrous layer thickness relative to the fiber size should exceed the value in order to ensure effective droplets filtration.

Investigation of droplets released during digestive endoscopy using a high-speed camera (with video): a pilot study.

BackgroundA large release of droplets is often expected around the periphery of the digestive endoscope insertion site. Therefore, a sense of alarm over infection because of droplets that may be released during digestive endoscopy examination is increasing. This study aimed to investigate the droplets released during digestive endoscopy using a high-speed camera.MethodsWe utilized a high-speed camera (FASTCAM SA-3, Photron Limited) capable of recording small, transparent droplets with a black background and high-brightness lighting. The obtained video files were analyzed using post-processing software. We divided the 20 models into the control (a spray bottle model and a cough model) and experimental groups (digestive endoscopy models). The sedative, proficiency of digestive endoscopy and the amount of gas injected were modulated to change the level of released droplets.ResultsFor the control groups, droplets were clearly observed using a high-speed camera. However, no droplet larger than 10 µm in size was observed in the experimental groups. Furthermore, the changes in the sedative, proficiency of digestive endoscopy, and amount of gas injected did not affect droplet formation.ConclusionsBased on high-speed camera photography, the risk of droplet generation during digestive endoscopy was not higher than that during violent expiratory events, such as coughing and sneezing.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00464-021-08581-0.

Precipitation dynamics of surrogate respiratory sessile droplets leading to possible fomites

HypothesisThe droplets ejected from an infected host during expiratory events can get deposited as fomites on everyday use surfaces. Recognizing that these fomites can be a secondary route for disease transmission, exploring the deposition pattern of such sessile respiratory droplets on daily-use substrates thus becomes crucial. ExperimentsThe used surrogate respiratory fluid is composed of a water-based salt-protein solution, and its precipitation dynamics is studied on four different substrates (glass, ceramic, steel, and PET). For tracking the final deposition of viruses in these droplets, 100 nm virus emulating particles (VEP) are used and their distribution in dried-out patterns is identified using fluorescence and SEM imaging techniques. FindingsThe final precipitation pattern and VEP deposition strongly depend on the interfacial transport processes, edge evaporation, and crystallization dynamics. A constant contact radius mode of evaporation with a mixture of capillary and Marangoni flows results in spatio-temporally varying edge deposits. Dendritic and cruciform-shaped crystals are majorly seen in all substrates except on steel, where regular cubical crystals are formed. The VEP deposition is higher near the three-phase contact line and crystal surfaces. The results showed the role of interfacial processes in determining the initiation of fomite-type infection pathways in the context of COVID-19.

Role of pulsatility on particle dispersion in expiratory flows.

Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport respiratory particles. In this work, direct numerical simulation (DNS) of turbulent pulsatile jets, coupled with Lagrangian particle tracking of micron-sized droplets, is performed to investigate the role of secondary and tertiary expulsions on particle dispersion and penetration. Fully developed turbulence obtained from DNS of a turbulent pipe flow is provided at the jet orifice. The volumetric flow rate at the orifice is modulated in time according to a damped sine wave, thereby allowing for control of the number of pulses, duration, and peak amplitude. Thermodynamic effects, such as evaporation and buoyancy, are neglected in order to isolate the role of pulsatility on particle dispersion. The resulting vortex structures are analyzed for single-, two-, and three-pulse jets. The evolution of the particle cloud is then compared to existing single-pulse models. Particle dispersion and penetration of the entire cloud are found to be hindered by increased pulsatility. However, the penetration of particles emanating from a secondary or tertiary expulsion is enhanced due to acceleration downstream by vortex structures.

Multi-phase simulation of infected respiratory cloud transmission in air

In the face of the increasing death toll of the COVID-19 global pandemic, countries around the world have instituted restrictive measures to mitigate the serious effects of the pandemic. Human-to-human transmission of COVID-19 occurs primarily through large droplets that are expelled with sufficient momentum to come in direct contact with the recipients’ mouth. Therefore, the physics of flow is central to transmission of COVID-19. Respiratory infections increase the frequency of violent expiration, including coughing and sneezing that are particularly effective in dispersing virus-carrying droplets. Moreover, the high viral load in droplets of asymptomatic hosts that are expelled during respiratory activities is contributing to the rapid growth of the COVID-19 global pandemic. The present study uses 2D smoothed-particle-hydrodynamics multi-phase simulations of the fluid dynamics of violent expiratory events in order to obtain a deeper understanding of the multi-phase nature of respiratory clouds, which can help determine separation distances from an infected person needed to minimize respiratory transmission. Our results indicate that there are three phases of jet cloud flow: the first is dominated by no-buoyancy jet-like dynamics characterized by a high speed, the second is dominated by negative buoyancy, and the third is dominated by gravity that deflects the cloud downward. Moreover, two modes of jet behavior that differ in dilution have been identified to be a function of distance from the human mouth. This work is of direct relevance to studies on the spread of COVID-19 and similar outbreaks in the future.

Turbulence role in the fate of virus-containing droplets in violent expiratory events

Violent expiratory events, such as coughing and sneezing, are highly nontrivial examples of a two-phase mixture of liquid droplets dispersed into an unsteady turbulent airflow. Understanding the physical mechanisms determining the dispersion and evaporation process of respiratory droplets has recently become a priority given the global emergency caused by the SARS-CoV-2 infection. By means of high-resolution direct numerical simulations (DNS) of the expiratory airflow and a comprehensive Lagrangian model for the droplet dynamics, we identify the key role of turbulence on the fate of exhaled droplets. Due to the considerable spread in the initial droplet size, we show that the droplet evaporation time is controlled by the combined effect of turbulence and droplet inertia. This mechanism is clearly highlighted when comparing the DNS results with those obtained using coarse-grained descriptions that are employed in the majority of the current state-of-the-art investigations, resulting in errors up to $100\%$ when the turbulent fluctuations are filtered or completely averaged out.

Accurate Representations of the Microphysical Processes Occurring during the Transport of Exhaled Aerosols and Droplets.

Aerosols and droplets from expiratory events play an integral role in transmitting pathogens such as SARS-CoV-2 from an infected individual to a susceptible host. However, there remain significant uncertainties in our understanding of the aerosol droplet microphysics occurring during drying and sedimentation and the effect on the sedimentation outcomes. Here, we apply a new treatment for the microphysical behavior of respiratory fluid droplets to a droplet evaporation/sedimentation model and assess the impact on sedimentation distance, time scale, and particle phase. Above a 100 μm initial diameter, the sedimentation outcome for a respiratory droplet is insensitive to composition and ambient conditions. Below 100 μm, and particularly below 80 μm, the increased settling time allows the exact nature of the evaporation process to play a significant role in influencing the sedimentation outcome. For this size range, an incorrect treatment of the droplet composition, or imprecise use of RH or temperature, can lead to large discrepancies in sedimentation distance (with representative values >1 m, >2 m, and >2 m, respectively). Additionally, a respiratory droplet is likely to undergo a phase change prior to sedimenting if initially <100 μm in diameter, provided that the RH is below the measured phase change RH. Calculations of the potential exposure versus distance from the infected source show that the volume fraction of the initial respiratory droplet distribution, in this size range, which remains elevated above 1 m decreases from 1 at 1 m to 0.125 at 2 m.

Investigation of the protection efficacy of face shields against aerosol cough droplets

Simple plastic face shields have numerous practical advantages over regular surgical masks. In light of the spreading COVID-19 pandemic, the potential of face shields as a substitution for surgical masks was investigated. In order to determine the efficacy of the protective equipment we used a cough simulator. The protective equipment considered was placed on a manikin head that simulated human breathing. Concentration and size distribution of small particles that reached the manikin respiration pathways during the few tens of seconds following the cough event were monitored. Additionally, water sensitive papers were taped on the tested protective equipment and the manikin face. In the case of frontal exposure, for droplet diameter larger than 3 μm, the shield efficiency in blocking cough droplets was found to be comparable to that of regular surgical masks, with enhanced protection for portions of the face that the mask does not cover. Additionally, for finer particles, down to 0.3 µm diameter, a shield blocked about 10 times more fine particles than the surgical mask. When exposure from the side was considered, the performance of the shield was found to depend dramatically on its geometry. While a narrow shield allowed more droplets and aerosol to penetrate in comparison to a mask under the same configuration, a slightly wider shield significantly improved the performance. The source control potential of shields was also investigated. A shield, and alternatively, a surgical mask, were placed on the cough simulator, while the breathing simulator, situated 60 cm away in the jet direction, remained totally exposed. In both cases, no droplets or particles were found in the vicinity of the breathing simulator. Conducted experiments were limited to short time periods after expiratory events, and do not include longer time ranges associated with exposure to suspended aerosol. Thus, additional evidence regarding the risk posed by floating aerosol is needed to establish practical conclusions regarding actual transmittance reduction potential of face shields and surgical face masks.