Improving air quality in urban environments and transportation systems is crucial. Concerns are related to health and environmental issues associated with huge costs. Car cabin is a microenvironment where pollutants can accumulate with possible risks for occupants. In automotive engineering, it has then become mandatory to study the path and dispersion of such pollutants emitted from the tailpipe of a car. In the present paper, the relation between the flow topology and the dispersion of ultrafine particles (UFP) in the wake of a vehicle is discussed. Experiments were undertaken at a reduced scale using simplified car models. Experimental conditions were defined to be representative of a vehicle in an urban environment. Based on experimental data, a simplified analytical model is developed, which aims at describing the concentration fields of UFP in the wake of a single vehicle for different rear slant angles. The strengths and limits of the present model are discussed and ways of improvements are suggested. Additional experiments are presented to assess the influence of the inter-vehicle distance on this recirculation region. Critical inter-vehicle distances were determined based on defined criteria for different rear slant angles of the leading vehicle and compared to safety clearances.Atmosphere 2020, 11, 39 2 of 23 systems such as cars, buses and trains, pollutants can be either particles including fine (FP, PM 2.5 ), ultrafine (UFP, PM 0.1 ) and nanoparticles (PM 0.05 ) or gases such as carbon dioxid (CO 2 ), ozone (O 3 ), nitrogen oxide (NO) and nitrogen dioxid (NO 2 ). Note that PM X refers to Particulate Matter with an aerodynamic diameter less than x µm regardless of their chemical composition or physical aspect. Among particles, UFP and nanoparticles are particularly dangerous since they can penetrate more deeply in the respiratory and circulatory systems (lung, pulmonary alveoli) and in the brain leading (but not limited) to cancers, heart and neurological diseases [5,13,15,22,23]. According to the French Environment and Energy Management Agency [24], 15% of PM 10 come from Diesel engines and dust produced by roads, brakes and tyres. Depending on weather conditions, pollution peaks can be reached above the alert levels. For instance, in the Ile de France region (Paris and suburbs, France), regulations are defined by an inter-prefectoral order (n • 2016-01383, 19 December 2016) and set as 80 µg/m 3 daily for PM 10 . In France, people are aware of that since traffic restrictions are taken when thresholds are exceeded and recommendations are provided to young and older people. As an example, limitation of outdoor activities is suggested. Being exposed to high pollution levels for a long period of time is hazardous. Recent studies pointed out the increase of cardiovascular diseases, asthma or cancer [5,15,25]. In urban environments, outside concentrations of pollutants can be enhanced by street architecture (canyon streets for instance) as well as by the increasing number of vehicles and traffic jam [2]. O...
