Car Wake Flows and Ultrafine Particle Dispersion: From Experiments to Modelling

Abstract

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...

Effects of Rear Angle on the Turbulent Wake Flow between Two in-Line Ahmed Bodies

“…The / values for DB_0 and DB_35 is comparable to The length of the reverse flow region behind the isolated square back model is L r /H = 1.36. This value is comparable to L r /H = 1.39 reported in previous studies [10,38]. In the presence of a follower, the length of the reverse flow region behind the square back model (L r /H = 1.53) increased by 13%.…”

“…Consequently, the mean reverse flow area, A/H 2 , is enhanced in the case of DB_0 (0.98), but reduced in the case of DB_25 (0.74) and DB_35 (0.81) when compared to the area of the isolated model, SB_0 (0.87). The L r /H values for DB_0 and DB_35 is comparable to values of 1.55 and 1.18, respectively reported by Murzyn et al [10]. However, the corresponding value for DB_25 is larger (53%) than that of the previous study (0.58).…”

“…Moreover, the present Ahmed bodies were completely immersed in the turbulent boundary layer (δ/H 1) but the previous study was conducted with a thin boundary layer thickness (δ/H 1). These differences in initial conditions contribute to the disparity in the L r /H values observed behind the high-drag model of the present study and that of Murzyn et al [10].…”

“…The increased level of exposure is attributed to the increased number of hours spent every day in vehicles in traffic congestion and highway platooning [6][7][8]. Under these conditions, the pollutants in the exhaust of the leading vehicle are transported into the cabin of the following vehicle due to the relatively short inter-vehicle distance and the low intake point of the ventilation systems [9][10][11][12]. Since the transport of the pollutant between the vehicles is also influenced by the wake characteristics, a good understanding of the mean flow and turbulent characteristics between two in-line vehicles is essential for developing effective flow control strategies for minimizing in-cabin air pollution.…”

“…Gnatowska and Sosnowski [27] also made similar conclusions based on results from experiments and Reynolds Averaged Navier-Stokes (RANS)-based turbulence modelling. In a recent study, Murzyn et al [10] performed detailed two-dimensional (2D) laser Doppler velocimetry (LDV) measurements of the wake structure between two Ahmed bodies using six spacing ratios ranging from 0.25 to 1.53 and three slant angles, α = 0 • , 25 • and 35 • . It was shown that the length of the recirculation region (L r ) behind the leading model is independent of spacing ratio beyond a critical value that is sensitive to the slant angle.…”