Integrated Effects of Street Layouts and Wall Heating on Vehicular Pollutant Dispersion and their Reentry Toward Downstream Canyons

Abstract

ABSTRACTVehicle emission is becoming one of the major sources of gaseous pollutants and aerosol particles in urban air environments. Apart from pollutant source control, sustainable street design is another significant technique to reduce street air pollution. Under the validation by wind tunnel data, this paper conducts computational fluid dynamic (CFD) simulations by RNG k-e model to investigate the impacts of typical street layouts and wall heating on the dispersion of gaseous pollutants and particles (diameter d = 1 µm, 5 µm, 20 µm) in the target street canyons and their reentry toward downstream streets. The dispersion processes of gaseous pollutants and fine particles (d = 1 µm) are found similar. For uniform street layouts (aspect ratio H/W = 1) with small Froude number (Fr = 0.19–0.38), leeward-wall heating, ground heating and all-wall heating significantly enhance the primary clock-wise vortex and improve pollutant dispersion, but windward-wall heating does not. Taller upstream buildings (H1/W = 2–3) produce a clockwise vortex over the target canyon and a much weaker counter-clockwise vortex within it, seriously weakening the capacity of pollutant dispersion. For large particles (d = 20 µm), the major fraction deposits onto street ground because the gravity force dominates particle transportation. For particles of d = 5 µm, the dispersion dynamics are more complicated: In the isothermal case less particles of d = 5 µm suspend in the target canyon than d = 1 µm because the gravity force and particle deposition are more important, however, with all-wall heating more particles of d = 5 µm float in the target canyon because the upward thermal buoyancy force reduces particle deposition onto the ground. Finally for both gaseous pollutants and particles, their bulk concentrations in downstream streets decrease exponentially with increasing distance from the target canyon, whose decreasing rates are quantified. Although further investigations are still required to propose a practical framework, this paper is one of the first attempts to quantify the capacity of street particle dispersion for street design purpose.