Post by Admin on Dec 8, 2020 6:26:00 GMT
Airflows inside passenger cars and implications for airborne disease transmission
Science Advances 04 Dec 2020:
eabe0166
DOI: 10.1126/sciadv.abe0166
Abstract
Transmission of highly infectious respiratory diseases, including SARS-CoV-2, is facilitated by the transport of exhaled droplets and aerosols that can remain suspended in air for extended periods of time. A passenger car cabin represents one such situation with an elevated risk of pathogen transmission. Here we present results from numerical simulations to assess how the in-cabin microclimate of a car can potentially spread pathogenic species between occupants, for a variety of open and closed window configurations. We estimate relative concentrations and residence times of a non-interacting, passive scalar–a proxy for infectious particles–being advected and diffused by turbulent air flows inside the cabin. An air flow pattern that travels across the cabin, farthest from the occupants can potentially reduce the transmission risk. Our findings reveal the complex fluid dynamics during everyday commutes, and non-intuitive ways in which open windows can either increase or suppress airborne transmission.
Introduction
Outbreaks of respiratory diseases, such as influenza, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and now the novel coronavirus (SARS-CoV-2), have taken a significant toll on human populations worldwide. They are redefining a myriad of social and physical interactions as we seek to control the predominantly airborne transmission of the causative, severe acute respiratory syndrome coronavirus disease-2 (1–3). One common and critical social interaction that must be reconsidered is how people travel in passenger automobiles, as driving in an enclosed car cabin with a co-passenger can present a significant risk of airborne disease transmission. Most megacities (e.g., New York City) support over a million of such rides every day with median figures of 10 daily interactions per rider (4). For maximum social isolation, driving alone is clearly ideal but this is not widely practical or environmentally sustainable, and there are many situations in which two or more people need to drive together. Wearing face masks and using of barrier shields to separate occupants do offer an effective first step toward reducing infection rates (5–10). However, aerosols can pass through all but the most high-performance filters (8, 11) and virus emissions via micron-sized aerosols associated with breathing and talking, let alone coughing and sneezing, are practically unavoidable (12–21). Even with basic protective measures such as mask-wearing, the in-cabin micro-climate during these rides falls short on a variety of epidemiological guidelines (22) with regard to occupant-occupant separation and interaction duration for a confined space. Preliminary models indicate a build- up of the viral load inside a car cabin for drives as short as 15 min (23, 24), with evidence of virus viability within aerosols of up to 3 hours (25, 26).
To assess these risks, it is critical to understand the complex airflow patterns that exist inside the passenger cabin of an automobile, and furthermore, to quantify the air that might be exchanged between a driver and a passenger. Although the danger of transmission while traveling in a car has been recognized (27), published investigations of the detailed air flow inside the passenger cabin of an automobile are surprisingly sparse. Several works have addressed the flow patterns inside automobile cabins, but only in the all-windows-closed configuration (28–30) – most commonly employed so as to reduce noise in the cabin. However, intuitively a means to minimize infectious particles is to drive with some or all of the windows open, presumably enhancing the fresh air circulating through the cabin.
Motivated by the influence of pollutants on passengers, a few studies have evaluated the concentration of contaminants entering from outside the cabin (31) and the persistence of cigarette smoke inside the cabin subject to different ventilation scenarios (32, 33). However, none of these studies have addressed the micro-climate of the cabin, and the transport of a contaminant from one specific person (e.g., the driver) to another specific person (e.g., a passenger). In addition to this being an important problem applicable to airborne pathogens in general, the need for a rigorous assessment of such air-flow patterns inside the passenger cabin of an automobile seems urgent in the current COVID-19 worldwide public health crisis.
The current work presents a quantitative approach to this problem. Although the range of car geometries and driving conditions is vast, we restrict our attention to that of two people driving in a car (five seater), which is close to the average occupancy and seating configuration in passenger cars in the United States (34). We then ask the question: What is the transport of air and potentially infectious aerosol droplets between the driver and the passenger, and how does that air exchange change for various combinations of fully open and closed windows?
To address this question, we conducted a series of representative Computational Fluid Dynamics (CFD) simulations for a range of ventilation options in a model four-door passenger car. The exterior geometry was based on a Toyota Prius, and we simulated the flow patterns associated with the moving car while having a hollow passenger cabin and six combinations of fully open and closed windows, named as front-left (FL), rear-left (RL), front- right (FR) and rear-right (RR) (Fig. 1). We consider the case of two persons traveling in the car – the driver in the front left-hand seat (assuming a left-hand-drive vehicle) and the passenger sitting in the rear right-hand seat, thereby maximizing the physical distance (≈1.5 m) between the occupants. For the purposes of simulation, the occupants were modeled simply as cylinders positioned in the car interior.
Fig. 1 Schematic of the model car geometry, with identifiers the front-left (FL), rear-left (FL), front-right (FL), and rear-right (FL) windows.
The two regions colored in black represent the faces of the driver and the passenger. Table on the right summarizes the six configurations simulated, with various combinations of fully open- and closed windows.
As a reference configuration (Fig. 1, Config. 1), we consider driving with all four windows closed and a typical air-conditioning flow – with air intake at the dashboard and outlets located at the rear of the car – that is common to many modern automobiles (35). The intake air was modeled to be fresh (i.e., no re-circulation) with a relatively high inflow rate of 0.08 m3/s (36).
The numerical simulations were performed using ANSYS-Fluent package, solving the three-dimensional, steady, Reynolds-averaged Navier-Stokes (RANS) equations using a standard k -E turbulence model (for details see Methods section). The RANS approach for turbulence, despite its known limitations (37), represents a widely-used model for scientific, industrial and automotive applications (38). A more accurate assessment of the flow patterns and the droplet dispersion is possible using Large Eddy simulations (LES) or using fully resolved Direct Numerical Simulations (DNS), which have a significantly higher computational cost. This is beyond the scope of the present work.
We simulated a single driving speed of ν = 22 m/s (50 mph) and an air density, ρa = 1.2 kg/m3. This translates to a Reynolds number of 2 million (based on the car height), which is high enough that the results presented here should be insensitive to the vehicle speed. The flow patterns calculated for each configuration were used to estimate the air (and potential pathogen) transmission from the driver to the passenger, and conversely from the passenger to the driver. These estimates were achieved by computing the concentration field of a passive tracer “released” from each of the occupants and evaluating the amount of that tracer reaching the other occupant (see Methods).
In this paper, we first describe the pressure distributions established by the car motion and the flow induced inside the passenger compartment. Following that we describe the passenger-to-driver and driver-to-passenger transmission results for each of the ventilation options, and finally conclude with insights based on the observed concentration fields, and general conclusions and implications of the results.
Science Advances 04 Dec 2020:
eabe0166
DOI: 10.1126/sciadv.abe0166
Abstract
Transmission of highly infectious respiratory diseases, including SARS-CoV-2, is facilitated by the transport of exhaled droplets and aerosols that can remain suspended in air for extended periods of time. A passenger car cabin represents one such situation with an elevated risk of pathogen transmission. Here we present results from numerical simulations to assess how the in-cabin microclimate of a car can potentially spread pathogenic species between occupants, for a variety of open and closed window configurations. We estimate relative concentrations and residence times of a non-interacting, passive scalar–a proxy for infectious particles–being advected and diffused by turbulent air flows inside the cabin. An air flow pattern that travels across the cabin, farthest from the occupants can potentially reduce the transmission risk. Our findings reveal the complex fluid dynamics during everyday commutes, and non-intuitive ways in which open windows can either increase or suppress airborne transmission.
Introduction
Outbreaks of respiratory diseases, such as influenza, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and now the novel coronavirus (SARS-CoV-2), have taken a significant toll on human populations worldwide. They are redefining a myriad of social and physical interactions as we seek to control the predominantly airborne transmission of the causative, severe acute respiratory syndrome coronavirus disease-2 (1–3). One common and critical social interaction that must be reconsidered is how people travel in passenger automobiles, as driving in an enclosed car cabin with a co-passenger can present a significant risk of airborne disease transmission. Most megacities (e.g., New York City) support over a million of such rides every day with median figures of 10 daily interactions per rider (4). For maximum social isolation, driving alone is clearly ideal but this is not widely practical or environmentally sustainable, and there are many situations in which two or more people need to drive together. Wearing face masks and using of barrier shields to separate occupants do offer an effective first step toward reducing infection rates (5–10). However, aerosols can pass through all but the most high-performance filters (8, 11) and virus emissions via micron-sized aerosols associated with breathing and talking, let alone coughing and sneezing, are practically unavoidable (12–21). Even with basic protective measures such as mask-wearing, the in-cabin micro-climate during these rides falls short on a variety of epidemiological guidelines (22) with regard to occupant-occupant separation and interaction duration for a confined space. Preliminary models indicate a build- up of the viral load inside a car cabin for drives as short as 15 min (23, 24), with evidence of virus viability within aerosols of up to 3 hours (25, 26).
To assess these risks, it is critical to understand the complex airflow patterns that exist inside the passenger cabin of an automobile, and furthermore, to quantify the air that might be exchanged between a driver and a passenger. Although the danger of transmission while traveling in a car has been recognized (27), published investigations of the detailed air flow inside the passenger cabin of an automobile are surprisingly sparse. Several works have addressed the flow patterns inside automobile cabins, but only in the all-windows-closed configuration (28–30) – most commonly employed so as to reduce noise in the cabin. However, intuitively a means to minimize infectious particles is to drive with some or all of the windows open, presumably enhancing the fresh air circulating through the cabin.
Motivated by the influence of pollutants on passengers, a few studies have evaluated the concentration of contaminants entering from outside the cabin (31) and the persistence of cigarette smoke inside the cabin subject to different ventilation scenarios (32, 33). However, none of these studies have addressed the micro-climate of the cabin, and the transport of a contaminant from one specific person (e.g., the driver) to another specific person (e.g., a passenger). In addition to this being an important problem applicable to airborne pathogens in general, the need for a rigorous assessment of such air-flow patterns inside the passenger cabin of an automobile seems urgent in the current COVID-19 worldwide public health crisis.
The current work presents a quantitative approach to this problem. Although the range of car geometries and driving conditions is vast, we restrict our attention to that of two people driving in a car (five seater), which is close to the average occupancy and seating configuration in passenger cars in the United States (34). We then ask the question: What is the transport of air and potentially infectious aerosol droplets between the driver and the passenger, and how does that air exchange change for various combinations of fully open and closed windows?
To address this question, we conducted a series of representative Computational Fluid Dynamics (CFD) simulations for a range of ventilation options in a model four-door passenger car. The exterior geometry was based on a Toyota Prius, and we simulated the flow patterns associated with the moving car while having a hollow passenger cabin and six combinations of fully open and closed windows, named as front-left (FL), rear-left (RL), front- right (FR) and rear-right (RR) (Fig. 1). We consider the case of two persons traveling in the car – the driver in the front left-hand seat (assuming a left-hand-drive vehicle) and the passenger sitting in the rear right-hand seat, thereby maximizing the physical distance (≈1.5 m) between the occupants. For the purposes of simulation, the occupants were modeled simply as cylinders positioned in the car interior.
Fig. 1 Schematic of the model car geometry, with identifiers the front-left (FL), rear-left (FL), front-right (FL), and rear-right (FL) windows.
The two regions colored in black represent the faces of the driver and the passenger. Table on the right summarizes the six configurations simulated, with various combinations of fully open- and closed windows.
As a reference configuration (Fig. 1, Config. 1), we consider driving with all four windows closed and a typical air-conditioning flow – with air intake at the dashboard and outlets located at the rear of the car – that is common to many modern automobiles (35). The intake air was modeled to be fresh (i.e., no re-circulation) with a relatively high inflow rate of 0.08 m3/s (36).
The numerical simulations were performed using ANSYS-Fluent package, solving the three-dimensional, steady, Reynolds-averaged Navier-Stokes (RANS) equations using a standard k -E turbulence model (for details see Methods section). The RANS approach for turbulence, despite its known limitations (37), represents a widely-used model for scientific, industrial and automotive applications (38). A more accurate assessment of the flow patterns and the droplet dispersion is possible using Large Eddy simulations (LES) or using fully resolved Direct Numerical Simulations (DNS), which have a significantly higher computational cost. This is beyond the scope of the present work.
We simulated a single driving speed of ν = 22 m/s (50 mph) and an air density, ρa = 1.2 kg/m3. This translates to a Reynolds number of 2 million (based on the car height), which is high enough that the results presented here should be insensitive to the vehicle speed. The flow patterns calculated for each configuration were used to estimate the air (and potential pathogen) transmission from the driver to the passenger, and conversely from the passenger to the driver. These estimates were achieved by computing the concentration field of a passive tracer “released” from each of the occupants and evaluating the amount of that tracer reaching the other occupant (see Methods).
In this paper, we first describe the pressure distributions established by the car motion and the flow induced inside the passenger compartment. Following that we describe the passenger-to-driver and driver-to-passenger transmission results for each of the ventilation options, and finally conclude with insights based on the observed concentration fields, and general conclusions and implications of the results.