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The urban heat island (UHI) phenomenon has become a major concern for
urban sustainability in the wake of global warming and rapid urbanization. This has
resulted in increased heat stress and worsened outdoor thermal comfort in urban
microclimates. Vegetation, water bodies, and cool materials are one of the most
effective strategies to alleviate the adverse effects of rising outdoor temperatures.
Computational fluid dynamics (CFD) has established itself as a valuable tool to model
various urban physics phenomena and develop climate change and UHI mitigation
and adaptation strategies. However, there exist certain numerical modeling aspects
which require further insight. In this work, CFD simulations have been performed to
analyze the effect of more realistic vegetation modeling parameters. The vegetation
modeling parameters include the actual form drag coefficient and the variable tree
transpiration rate. In addition to that, thermal comfort effectiveness of different tree
species with its various morphological characteristics, cool materials’ albedo, and
water bodies have also been studied in individual and in combination for a real urban
area. The morphological characteristics/parameters include trunk height (HT), crown
diameter (CW), crown height (CH), and leaf area density (LAD). The wind flow and
heat transfer phenomena are simulated using the unsteady Reynolds-averaged Navier–
Stokes (URANS) approach.
The simulations were performed with proposed adaptation measures for a real
urban area having hot-humid climatic conditions under heat wave conditions. It has
been found that for the studied climatic conditions, the consideration of more realistic
values of these parameters can yield significant variation in the determination of
cooling potential and flow characteristics of applied vegetation. Of all the
morphological characteristics, LAD, crown height, and trunk height are found to be
most influential in providing thermal comfort. Water bodies promotes improved
thermal conditions and urban ventilation in spatial direction. Water and vegetation
interventions promote the cooling effect by resulting in low ambient air and surface
temperature i.e. 0.9 °C and 3.5 °C; 0.3 °C and 3 °C respectively when compared with
reference case. Cool materials, when applied simultaneously on both buildings and
ground, generate a more pronounced cooling effect than when applied separately on
ground or the buildings as it results in a large reduction of air and surface temperature
i.e., of 2 °C and 6 °C respectively. Furthermore, the impact becomes more significant
for collective application of these adaptation measures. Cool materials when combined
with vegetation and water results in large reduction i.e. 2.2 °C and 1.9 °C in air
temperature; and 5.9 °C and 9 °C in surface temperature was observed respectively
compared to the reference case. For air flow velocity, it is highest for combined cool
materials with water with peak effect at the time of highest solar irradiance. The
analysis shows that the proposed interventions can effectively decrease surrounding
temperature and promote airflow; thereby promoting thermal comfort conditions.