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By Tamás Régert, Tamás Lajos, Ákos Csécs, István Goricsán, Márton Balczó, Department of
Fluid Mechanics, Budapest University of Technology and Economics, Budapest, Hungary
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Air quality in the urban environment
has become one of the
most important issues of environmental
protection. The most significant
urban pollutants, such as the
emissions from vehicles, are influenced
by architectural structures, city
planning, and traffic control measures.
To examine the environmental impact
of new construction and its associated
traffic patterns, atmospheric pollutant
transport needs to be predicted,
since field measurements cannot be
made. Wind tunnel investigations and
numerical simulation are the tools that
can be applied during the planning and
licensing phases of the project.
The model of buildings prepared for FLUENT
A new Millennium City Center is being
planned in the southern part of
Budapest, close to the bank of the
Danube River, between two bridges. The
City Center will include a row of relatively
large buildings: a conference center,
museums, a concert hall, hotels, and
residences. Since these buildings will be
built adjacent to a busy roadway (carrying
60,000 vehicles/day) between the
Danube and a neighboring district of
Budapest, the possibility of an adverse
effect from the new buildings and
increased traffic on the air quality in the
neighboring district has been considered.
To respond to the concerns, the
Department of Fluid Mechanics at the
Budapest University of Technology and
Economics was commissioned to perform
experimental and numerical
analyses of the dispersion process in order
to predict the effect of the planned City
Center. A model of the site was constructed
for analysis in the test section
of a wind tunnel and FLUENT simulations
of the area were performed for
a variety of wind conditions.
Concentration distribution and the effect on the City Center in a SW-SSW
wind direction
Comparison of FLUENT and wind tunnel data: annual mean concentrations
with the City Center in place
A detailed numerical model of the
existing buildings in the area, including
the new construction, was prepared
for the FLUENT simulations. The geometry
is complicated. Several of the buildings
are characterized by critical forms
and inter-building connections. Because
of the special geometrical constraints,
an unstructured mesh of 978,000 tetrahedral
elements was created. The
wind tunnel and numerical models were
both made in 1:500 scale. The computational
domain included an empty
space around the outermost buildings
corresponding to one kilometer at full
scale to avoid the direct forcing effects
of the boundary conditions. The height
of the computational domain was 0.6m,
corresponding to 300m at full scale. The
realizable k-e model was used for turbulence
closure.
The CFD models illustrated many
well-known effects, such as street
channeling and the formation of vortices
behind buildings. The concentration
of a tracer gas at 24 sampling points,
1.5m above the street level, was measured
and calculated for five dominant
wind directions, with and without the
City Center buildings in place.
Considering the probabilities of
wind velocities and directions, a
dimensionless annual mean concentration
was calculated from the CFD
results for every sampling point and
compared with values calculated similarly
from the results of wind tunnel
measurements. While the trends are
the same, CFD predictions at only about
two-thirds of the sampling points were
in good agreement with the measured
values. The discrepancy in the low concentration
regions was suspected to
be the result of measuring errors. Most
importantly, both the numerical simulation
and the experiment showed
that because of the lifting effect of
the new buildings, a slight decrease
in pollution in the neighboring district
would occur.
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