Air quality around motorway tunnels in complex terrain - computational fluid dynamics modeling and comparison to wind tunnel data

The current paper describes numerical simulations of flow and dispersion performed in a complex suburban area of Budapest using the microscale model MISKAM and accompanying wind tunnel tests which provided reference concentration data for validation of the model results. Main pollutant sources are traffic related and include a planned motorway section of 9 km length consisting of sections running in tunnel, on ground, and on viaduct. Four different route alternatives were investigated. In the paper, first a condensed review is given about problems related to air quality around motorway tunnels in complex terrain. The effect of larger scales on microscale air quality was determined using background concentrations from monitoring station time series with removal of short-term fluctuations, for which a simple method is introduced here. The validation wind tunnel tests were carried out at several wind directions on a 1:1000 scale model containing topography, buildings, and vegetation with measurement of tracer concentrations in 50 sampling locations. In the microscale CFD simulation, flow and dispersion considering topography, vegetation, and buildings were calculated threedimensional in a large domain using k–ε model, and advective diffusion equation was set up on a Cartesian grid treating air pollutants as non-reactive scalars. Results give more detailed information about the flow, for example local speedup above hills, slowdown in vegetation zones, separation regions are resolved well. Deviation of pollutant plume paths from the mean wind direction caused by the topography could be also observed. NOx concentration maps showed that air quality limit exceedances occur near motorway tunnel portals in form of large surface plumes, which can only be avoided by the application of

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