Numeerisen analyysin ja laskennallisen
tieteen seminaari
20.2.2006 klo
14.15
U356
Ari Karppinen, Ilmatieteen laitos, Ilmanlaadun mallimenetelmät
Air quality
modelling in a street canyon
Exhaust gases emitted from traffic do not disperse within urban
street canyons in the same manner as in open, flat terrain
environments, for which conventional boundary-layer flow and turbulence
theory are appropriate and for which many Gaussian plume models of
contaminant dispersion exist. In street canyons, wind speeds are
suppressed, and there are frequent occurrences of separated flow
vortices, which can lead to pollutant recirculations and, consequently,
areas of higher air contaminant concentrations resulting from vehicle
emissions within these semi-confined spaces. The straight-line Gaussian
dispersion models do not apply in regions where the flow is curved or
recirculates and, thus, are not applicable within these canyons or
immediately downwind of them. Highway traffic in s urban street canyons
can have higher pollutant emission rates due to low irregular vehicle
speeds, prolonged queuing, and frequent accelerations. Consequently,
high primary pollutant concentrations can develop in these
depressions.
Although several attempts have been made to utilize
full-fledged flow models (CFD) in street canyon, nearly all
practical street canyon pollution models are still based on a
simplified description of flow and dispersion conditions in street
canyons. Concentrations of exhaust gases are computed using a
combination of a plume model for the direct contribution from street
traffic, and a box model for the recirculating part of pollutants in
the street. The simplified parametrization of the flow and dispersion
conditions in a street canyon has been deduced from extensive analysis
of experimental data and model tests.