2D large eddy simulation of pollutant dispersion around a covered roadway

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In big cities, vehicle exhaust gases can be trapped inside deep and poorly ventilated urban complex geometries (street-canyons, tunnels and covered roadways). The covered roadway is a typical configuration characterized by high emissions, poor ventilation and exposure to large numbers of people. High concentration of pollutant gases may be expected with low winds, or when the wind is oriented perpendicular to the covered roadway axis. To understand the wind effects on the transport and dispersion processes close to a covered roadway, we have conducted a numerical modeling study using the 2D Large Eddy Simulation approach. The Navier–Stokes equations are integrated by a finite volume method and then solved in time using the projection method, allowing for decoupling pressure from velocity. For the turbulence, a dynamic mixed subgrid scale model is introduced to take into account the unresolved small-scale effects. The covered section is open to one side; two aspect ratios (width to height) for the covered section are tested: 3.6 and 7.2. Two wind direction cases are studied. Our results, compared with the wind-tunnel experiment of Dabberdt et al. Int. J. Vehicule Design, 20(1–4) (1998) 96, show good agreement. The numerical simulations give detailed information on pollutant transport mechanism within the covered section. The dispersion process is highly dependent on the flow turbulence and is influenced by both the geometry of covered section and the wind direction. When the open end of the covered section is to windward, a dominant anticyclonic vortex occupies the entrance of the covered section. The dispersion process is advection dominated due to the fact that the pollutant is carried outside the covered region by large unsteady structures produced in the shear region. When the open section is to leeward, the transient solutions confirm that the gas dispersion from the covered section takes place through an intermittent “vortex and purging” release, governed by the interactions between the large structures in the vicinity of the open side.