Authors: Walter F. Dabberdt, Walter G. Hoydysh, Michael Reads
Addresses: National Center for Atmospheric Research, Boulder, Colorado 80307-3000, USA. Environmental Science & Services Corp., Long Island City, New York 11101 USA. National Center for Atmospheric Research, Boulder, Colorado 80307-3000, USA
Abstract: Dispersion of vehicular pollutants emitted from covered roadways is particularly important in describing and controlling local air quality conditions at many complex urban roadway geometries. We have conducted a joint measurement and numerical modelling study to describe, understand and simulate the kinematics of the atmospheric transport and dispersion processes, and to assess the representativeness of various numerical model simulations. The measurement program was conducted in an environmental boundary-layer wind tunnel and involved the controlled line-source release of a tracer gas (ethane, C2 H6) and subsequent in situ sampling of concentration fields. The covered section was open to one side; two aspect ratios (width to height) for the covered section were tested 3.6 and 7.2. With the open side of the covered section to leeward of the ambient flow, pedestrian-level concentrations were observed to be fairly uniform within the covered section and were an order of magnitude greater than those downwind. An abrupt transition zone exists at the trailing edge of the covered section. A Navier-Stokes model (Fluent) realistically simulated the major transport and dispersion characteristics. A simple Gaussian line source equation was also evaluated because its numerical analogues (e.g. CAL3QHC) are often used for regulatory applications involving similarly complex geometries. As expected, this approach does not yield representative results. However, using the Gaussian model with the measured concentrations, we are able to characterise the downwind profile of the vertical dispersion (sigma) function and quantify the enhanced dispersion in the transition zone.
Keywords: air pollution; covered roadways; dispersion modelling; vehicular pollution; wind tunnel modelling.
International Journal of Vehicle Design, 1998 Vol.20 No.1/2/3/4, pp.96-104
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