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TECHNOLOGYfocus
Figure 3: CFD is used to simulate the impact various types and
quantities of trees have on the wind comfort profile of an area
Figure 4: Different tree species can be simulated using
CFD to evaluate their impact on the local microclimate
simultaneously, and the total simulation
time would still be 8 hours. An architect
or urban designer can simulate dozens
of urban layouts in parallel, saving time
and cost.
THE BENEFITS OF CFD
Urban areas have many environmental
factors that need accounting for when
designing buildings and cities. A city
will be subject to uneven heating and
cooling as the sun rises and sets,
depending on its built form and skyline.
Some areas will be shaded, while
others are in full receipt of the heat gain
from the sun, leading to convective air
currents that impact the local
microclimate. Wind profiles in urban
areas are artifacts of many
aerodynamic and atmospheric forces;
from surface roughness to turbulence
caused by urban canyons, these
factors tend to be transient phenomena
and are difficult to predict. They require
mathematical modeling to understand
their behavior and interaction with the
built environment.
A CFD simulation can accurately
model such phenomena, including
pedestrian wind comfort, building
aerodynamics, urban heat island effect,
and the impact of solar shading.
Specific types of urban design and
building typology also require
additional treatment. Tall buildings for
example (Figure 2) tend to be sensitive
to small changes in atmospheric
conditions, such as wind pressure and
turbulence. These atmospheric
changes create aerodynamic effects,
including downwash and corner
acceleration, that negatively impact
local pedestrian wind comfort. They
also significantly impact overall building
performance and are not captured in
standard building energy design tools
used in the industry.
Coupling these tools with CFD,
however, is a quick way to increase the
accuracy of thermal and energy models
used for design and compliance.
APPLYING CFD - THE CASE OF
GREENING STRATEGIES
A practical application of CFD can
compare urban design layouts and
greening strategies (Figure 3). Trees
and vegetation are increasingly being
used to pedestrianise areas that were
initially not intended for people. Trees
hinder the effect of wind as it passes
through their leafy canopies. A
grouping of trees as part of the
streetscape or local amenity tends to
dramatically impact an areas' character
and physical appearance. With this in
mind, we can assume varying types of
trees offer more or less resistance to
the wind flow.
For example, the Sycamore tree has
less air resistance than a Fir tree
(Figure 4), based on its calculated leaf
density. This ability to allow fluid (air) to
pass through, known as porosity, is
measured through experiments, and
the results are then defined through a
leaf area index and applied in a CFD
simulation.
SUMMARY
CFD is increasingly recognised for its
ability to capture advanced building
and urban physics and add a level of
design assurance/validation to many
projects. CFD results provide vivid
details and a deeper understanding of
how designs perform under various
circumstances, allowing more focus on
performance at the early design stages
for the construction sector and beyond.
Cloud-based engineering simulation is
unlimited by computing power or
accessibility issues, and fosters
collaborative working environments for
distributed design teams of architects,
urban designers, and engineers. This is
reflected in recent guidance from local
planning authorities. The City of London
Corporation, for example, has recently
released the wind microclimate
guidelines for developers, which
specifically mandates the use of CFD
during the design stage. It is also seen
in standards and planning jurisdictions
globally, as the construction industry is
forced to face the challenges of an
uncertain and changing climate.
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May/June 2021 27