THE USA JOURNALS
THE AMERICAN JOURNAL OF ENGINEERING AND TECHNOLOGY (ISSN
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2689-0984)
VOLUME 06 ISSUE12
8
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PUBLISHED DATE: - 02-12-2024
PAGE NO.: - 8-12
INVESTIGATING THERMAL EFFECTS AND POLLUTANT
DISPERSION IN STREET CANYONS THROUGH CFD
MODELING
Aishah Bakar
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Ayer
Keroh, Melaka, Malaysia
INTRODUCTION
Urbanization has led to the proliferation of street
canyons
—
narrow, enclosed spaces formed by
buildings lining both sides of a street. While these
structures maximize space in densely populated
areas,
they
also
create
complex
microenvironments characterized by limited
airflow, increased heat retention, and higher
pollutant concentrations. In street canyons, natural
ventilation is often restricted, which can trap
pollutants emitted from vehicles and other sources,
leading to poor air quality and thermal discomfort
for residents and pedestrians. The thermal and
dispersion characteristics of these canyons depend
on multiple factors, including canyon geometry,
wind speed and direction, solar radiation, and the
physical properties of surrounding buildings and
surfaces.
Understanding the interplay between airflow,
thermal effects, and pollutant dispersion in street
canyons is critical for urban planners and
environmental scientists. This knowledge is
essential for developing strategies to mitigate
adverse health and environmental impacts in
urban areas. Traditional observational methods,
while valuable, are often limited in scope and fail to
capture the detailed dynamics of these complex
environments. Computational Fluid Dynamics
(CFD) modeling, however, offers a powerful tool for
simulating airflow, heat transfer, and pollutant
transport under a variety of conditions. CFD can
RESEARCH ARTICLE
Open Access
Abstract
THE USA JOURNALS
THE AMERICAN JOURNAL OF ENGINEERING AND TECHNOLOGY (ISSN
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provide insights into how different canyon
geometries
and
meteorological
conditions
influence pollutant dispersion and thermal effects,
making it an invaluable tool in urban
environmental studies.
This study employs CFD modeling to investigate
thermal effects and pollutant dispersion within
street canyons under various configurations. By
simulating different wind directions, building
heights, and aspect ratios, we aim to understand
how these factors interact to influence air quality
and thermal comfort within the canyon. The
findings from this research can inform urban
planning and policy, helping to design healthier,
more sustainable cities by mitigating the adverse
effects of pollutant accumulation and heat
retention in street canyons.
METHOD
The study utilized Computational Fluid Dynamics
(CFD) to model the airflow, thermal distribution,
and pollutant dispersion within urban street
canyons of varying geometries. Initially, a standard
urban street canyon configuration was defined,
with adjustable parameters such as building
height, aspect ratio, and street width to simulate
different canyon geometries commonly observed
in urban environments. The CFD model was set up
using the ANSYS Fluent software, incorporating a
3D domain with boundary conditions to represent
an open canyon environment, allowing for realistic
airflow and heat transfer dynamics.
Meteorological conditions, such as wind speed and
direction,
solar
radiation,
and
ambient
temperature, were varied in the simulations to
assess their impact on pollutant dispersion and
heat retention. Wind profiles were configured to
mimic urban atmospheric boundary layers, with a
focus on examining the effects of both parallel and
perpendicular wind directions relative to the
canyon orientation. Solar radiation effects were
incorporated using a radiation model within CFD,
accounting for diurnal changes in solar intensity
and heat accumulation on building surfaces. This
setup allowed us to evaluate thermal gradients and
heat islands that typically form within street
canyons due to solar exposure and limited
ventilation.
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For pollutant dispersion analysis, a continuous
source of carbon monoxide (CO) was introduced at
street level, representing typical vehicular
emissions in urban settings. The pollutant was
modeled as a passive scalar within the CFD
environment, with dispersion tracked under
varying canyon and wind conditions. Turbulence
within the canyon was modeled using the standard
k-
ε model, providing a balance between
computational efficiency and accuracy for airflow
and pollutant transport. Additionally, heat flux
from building surfaces was included to simulate
heat retention, particularly in cases of deeper
canyons, where thermal effects are often more
pronounced.
The simulation results were analyzed to assess the
spatial distribution of pollutants, temperature
gradients, and airflow patterns within each canyon
configuration. Cross-sectional and longitudinal
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data were extracted from the CFD outputs, focusing
on areas with high pollutant concentration and
heat accumulation. Comparative analyses were
conducted across simulations to identify how
different factors, such as building height, aspect
ratio, and meteorological conditions, influence
pollutant retention and thermal effects in street
canyons. Statistical methods were used to quantify
the correlation between these variables, helping to
clarify the interactions that contribute to urban
heat retention and reduced air quality in street
canyon environments.
The findings from these simulations provide
insights into the impact of street canyon geometry
and environmental conditions on thermal comfort
and air quality. These results are intended to
inform urban design practices, suggesting specific
configurations and mitigation strategies that could
reduce the adverse effects of pollutant
accumulation and heat retention in urban street
canyons.
RESULTS
The CFD simulations revealed distinct patterns of
airflow, pollutant dispersion, and thermal
gradients within the street canyons, highly
dependent on the geometry and meteorological
conditions. In deeper street canyons with high
aspect ratios, pollutant concentration was
significantly elevated, especially near ground level,
due to limited air circulation. Shallower canyons
exhibited better pollutant dispersion, with
pollutants diffusing more readily into the
surrounding environment. Thermal analysis
indicated that deeper canyons retained more heat,
particularly in cases with high solar exposure.
Wind direction played a critical role: perpendicular
winds resulted in higher pollutant accumulation
within the canyon, while parallel winds facilitated
greater pollutant removal. Additionally, surface
temperatures on building facades exposed to solar
radiation were consistently higher, contributing to
heat buildup in the canyon environment.
DISCUSSION
The findings illustrate the impact of street canyon
geometry and environmental factors on both air
quality and thermal comfort. High aspect ratio
canyons restrict natural ventilation, causing
pollutants to accumulate and posing a health risk to
pedestrians and residents. This restricted airflow,
coupled with high solar exposure, leads to
substantial heat retention, which exacerbates the
urban heat island effect. The results indicate that
wider canyons or those with lower aspect ratios
can help reduce these adverse effects by promoting
airflow and reducing pollutant retention.
Furthermore, the study highlights the role of wind
orientation; when aligned parallel to the canyon,
wind can effectively transport pollutants out of the
canyon, while perpendicular winds create
recirculation zones, trapping pollutants and
increasing exposure levels.
Thermal dynamics within the canyon were heavily
influenced by solar radiation on building surfaces,
resulting in temperature gradients that could affect
pedestrian comfort and local microclimates. These
insights into the thermal and dispersion patterns
emphasize the need for urban planning strategies
that consider canyon geometry and orientation. By
strategically designing street canyons with optimal
aspect ratios and orientations, urban planners can
mitigate pollution accumulation and heat
retention, ultimately contributing to more
sustainable
and
health-supportive
urban
environments.
CONCLUSION
This study demonstrates that the geometry and
orientation of street canyons significantly
influence pollutant dispersion and thermal effects.
Through CFD modeling, it was shown that high
aspect ratio canyons are more prone to pollutant
accumulation and heat retention, especially under
perpendicular wind conditions. Conversely,
shallower canyons and those aligned with
prevailing wind directions exhibit improved
ventilation and thermal regulation. The findings
suggest that urban design adjustments, such as
optimizing canyon width, height, and orientation,
can enhance air quality and reduce thermal
discomfort in urban street environments.
Overall, this research underscores the importance
of integrating CFD analyses into urban planning for
better environmental management in densely built
areas. Future work should explore additional
THE USA JOURNALS
THE AMERICAN JOURNAL OF ENGINEERING AND TECHNOLOGY (ISSN
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2689-0984)
VOLUME 06 ISSUE12
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meteorological
variations
and
canyon
configurations, along with potential mitigation
measures such as green facades and natural
ventilation features. By adopting these insights,
cities can reduce the health and environmental
impacts of urban street canyons, promoting
healthier and more livable urban spaces.
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