A frontal air intake may improve the natural ventilation in urban buses

Abstract

In this report we analyze the air flow across the open windows (natural ventilation) of an urban bus model and the consequent dispersion of aerosols emitted in the passengers area. The methods include computational fluid dynamics simulations and three ways to characterize the dispersion of passive tracers: a continuous concentration-based model, a discrete random model and a parametric scalar based on the so-called mean age of air. We also conducted experiments using a 1:10 scale bus model and $$\text{CO}_{2}$$ CO 2 as a passive tracer to assess the ventilation characteristics. We found that dispersion and expulsion of aerosols is driven by a negative pressure in the standard bus design equipped with lateral windows. Also, the average age of air is 6 minutes while the air flow promotes aerosol accumulation to the front (driver’s area). To speed up the expulsion of aerosols and reduce their in-cabin accumulation, we propose a bus bodywork prototype having a frontal air intake. All the numerical models and experiments conducted in this work agreed that the expulsion of aerosols in this novel configuration is significantly increased while the average age of air is reduced to 50 seconds. The average air flow also changes with the presence of frontal air intakes and, as a consequence, the expulsion of aerosols is now driven by a frontal velocity field.

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] .…”

“…The size of ventilation openings is also a factor that affects natural ventilation. Several studies found a notable improvement in the ventilation rate when opening windows increased to two pairs from one pair [48 , 83 , 84] . Two pairs of opening windows could supply ventilation rates close to even exceeding WHO recommended per-person requirements for high-risk clinical areas [84] .…”