Aerosol transmission in passenger car cabins: Effects of ventilation configuration and driving speed

Abstract

Identifying the potential routes of airborne transmission during transportation is of critical importance to limit the spread of the SARS-CoV-2 virus. Here, we numerically solve the Reynolds-averaged Navier–Stokes equations along with the transport equation for a passive scalar in order to study aerosol transmission inside the passenger cabin of an automobile. Extending the previous work on this topic, we explore several driving scenarios including the effects of having the windows fully open, half-open, and one-quarter open, the effect of opening a moon roof, and the scaling of the aerosol transport as a function of vehicle speed. The flow in the passenger cabin is largely driven by the external surface pressure distribution on the vehicle, and the relative concentration of aerosols in the cabin scales inversely with vehicle speed. For the simplified geometry studied here, we find that the half-open windows configuration has almost the same ventilation effectively as the one with the windows fully open. The utility of the moonroof as an effective exit vent for removing the aerosols generated within the cabin space is discussed. Using our results, we propose a “speed–time” map, which gives guidance regarding the relative risk of transmission between driver and passenger as a function of trip duration and vehicle speed. A few strategies for the removal of airborne contaminants during low-speed driving, or in a situation where the vehicle is stuck in traffic, are suggested.

COVID-19 transmission and control in land public transport: A literature review

“…Natural ventilation is a low-cost, economical, and energy-saving alternative in land public transportation. It can effectively reduce the infection risk, which has been verified in real cases [67 , 68 , 70] , experiments [35 , 39 , 66] , and modeling [38 , 48 , 50 , [53] , [54] , [55] , 72 , 81 , 82] . When the vehicle is driving, the external airflow generates a pressure distribution over the vehicle - the areas near the front are lower-than-atmospheric pressures, while the areas toward the middle and rear of the vehicle are neutral or higher-than-atmospheric pressures ( Fig.…”

“…The same applied to the minibus and taxis, only opening the front windows provided the most ventilation compared to only opening the middle windows or the rear windows [84] . Moreover, inlet ports at the frontal and sunroof could also effectively carry fresh air into the vehicle [48 , 54 , 81 , 85] . Two studies revealed that when the infector was near ventilation openings, especially the windows in the row in front of the infector, could effectively remove his exhaled droplets and thereby reduce the infection risk of other passengers [50 , 55] Meanwhile, droplets generated by the front-seated person were easier to be discharged by opening windows than that generated by rear-seated passengers [38 , 53] .…”

“…Under the same window opening conditions, the amount of ventilation brought by various vehicle speeds will be different. Generally, increased bus speed can enhance natural ventilation [38 , 54 , 66 , 72] . The air exchange rates in the train with all 12 windows opened to 10 cm increased from 10/h to 42/h as the train speed increased from 20 km/h to 57 km/h [66] .…”

“…Hence, we should pay more attention to the ventilation of vehicles driving at low speeds or stuck in traffic. The study by Mathai et al [54] recommended opening the left front window near the driver and running the air conditioner to the maximum to eliminate the aerosol contaminants released by the driver/rear passenger at low speeds. Nikam and Borse [86] suggested opening all windows on the bus at low operating speed to ensure adequate ventilation.…”

“…In addition, the recirculated air in the HVAC system may lead to secondary infections [65 , 68] . Nature ventilation is an energy-saving ventilation mode but is influenced by the outdoor environment and other elements (e.g., vehicle speed, location of window, window opening size) that cannot be accurately controlled [54 , 66 , 84] . Therefore, how to coordinate nature and mechanical ventilation according to the specific transportation environment need to be further investigated.…”