DESIGN OF NATURAL VENTILATION AND
BY A TEAM OF ARCHITECTS AND ENGINEERS WITH THE CFD TECHNIQUE
Y. JIANG, H. XING, C. STRAUB,
A.M. SCOTT, L.R. GLICKSMAN, AND L.K. NORFORD
To design a comfortable indoor and outdoor environment requires
detailed information of airflow in and around buildings. The existing
simplified analyses and empirical approaches are inadequate to obtain
the needed information. This paper demonstrates how the computational
fluid dynamics (CFD) technique can be used by engineers to calculate
the airflow distributions in and around a building. Architects can
use the airflow distributions to modify their design. This design
procedure shows a model of how the architects and engineers can
work together to design a sustainable building.
Wind is a friend of a building because the wind can naturally
ventilate the building to provide a comfortable and healthy indoor
environment as well as to save energy. The conventional design approach
often ignores opportunities for innovation that can condition buildings
at lower cost or with higher air quality and acceptable thermal
comfort level, usually by means of passive cooling or natural ventilation.
Figure 1 breaks the reference weather data in Beijing into different
comfort categories. Although heating is an important issue in Beijing,
the results also indicate that buildings in Beijing require summer
cooling. However, the figure illustrates a significant reduction
of mechanical cooling if the mean air velocity in the buildings
can be increased to 2 m/s by natural ventilation. Although not shown
in the figure, air conditioning may not be necessary in Beijing,
if natural ventilation is combined with night cooling. To design
a sustainable building for Beijing, it is important to explore the
opportunity to use natural ventilation.
On the other hand, the wind can be an enemy of the building because
the wind can cause discomfort to pedestrians if the wind speed around
the building is too high. In Beijing, there are many failing buildings,
among which the wind speed on some winter days can be as high as
grade 5 or 6 (8 m/s to 14 m/s). Considering the low air temperature
in the winter, the chilling effect of the wind is so strong that
pedestrians cannot walk comfortably and safely. Therefore, it is
essential to reduce the wind speed around buildings.
For small-scale buildings, architects know something about passive
solar heating, but natural ventilation and outdoor thermal comfort
are very difficult to design even for a simple case. The purpose
of the present study is to demonstrate, with the help of the computational
fluid dynamics (CFD) technique, how architects can work with engineers
to design a sustainable building.
Traditionally, an architect predicts the airflow in
and around buildings by smart arrows as shown in Figure 2. To draw
the airflow correctly requires rich knowledge of winds. Furthermore,
the smart arrows cannot give the wind speed or, at least, the reliable
air speed, which is an important parameter for evaluating the benefits
of natural ventilation and outdoor comfort.
Figure 1 Comfort hours in Beijing
Figure 2 Smart arrows used by architects
The engineers, on the other hand, often
use a wind tunnel to simulate and measure the airflow around buildings
and an environmental chamber to determine natural ventilation. Although
the experimental approach provides reliable information concerning
airflow in and around buildings, the available data are limited
due to the expensive experimental process. Moreover, the approach
is not practical for a designer to optimize the designs because
the experimental method is very time consuming.
An alternative is to determine the airflow distributions in and
around a building by numerical simulation. There are two widely
used numerical methods. The first one is the zonal method, which
calculates inter-zonal airflow through the Bernoulli equation. The
prediction of the inter-zonal airflow relies on the external pressure
distribution caused by wind or buoyancy effect. However, the determination
of the external pressure is very complex, since the pressure distribution
depends on the incoming wind speed and direction, the building size
and shape, the size and location of the building interior opening
(Vickery and Karakatsanis, 1987). Therefore, the accuracy of the
zonal method is not good. Furthermore, the zonal model is incapable
of determining thermal comfort around a building.
The other numerical method calculates the airflow distribution
in and around buildings by the CFD technique. The CFD technique
numerically solves a set of time-averaged partial differential equations
for the conservation of mass, momentum (Navier-Stokes equations),
energy, and species concentrations. The solution provides the field
distribution of pressure, air velocity, temperature, and concentrations
of water vapor (relative humidity) and contaminants. The method,
although it has some uncertainties and requires a significant amount
of computing time, has been successfully used to predict airflow
in and around buildings (Chen, 1997 and Murakami, 1998). With the
fast development in computer speed and capacity, the CFD technique
seems to be a good approach and is therefore used in the present
Our design procedure is as follows. An architect initializes a
design. Then an engineer uses the CFD technique to calculate the
airflow in and around buildings. Based on the calculated results,
the architect modifies the design. Several iterations are necessary
until satisfactory indoor and outdoor environment for the building
is achieved. Since an architect generally does not have sufficient
fluid dynamics knowledge and numerical skills, the role of the engineer
is essential to help the architect to obtain the detailed flow information.
On the other hand, the engineer normally does not know how to design
a building. Thus, the engineer needs the help of the architect to
make the building comfortable and healthy. This paper demonstrates
this procedure through the design of a small demo building in Beijing
with natural ventilation (Vanke City Garden) and the analysis of
a building site in Beijing for the outdoor thermal comfort and natural
ventilation (Vanke Doushi Garden).
2. Natural ventilation
design in Vanke City Garden
Figure 3 shows the location of the six-story demo building (20
m high) and its surroundings in Vanke City Garden, Beijing. There
is a long, mid-rise building, 40 m high, in the north and low-rise
buildings of six stories in the east and south. A wide street is
on the west of the demo building. Thus, Vanke City Garden is a mid-
and low-rise building site, in which the outdoor thermal comfort
may not be a problem. So our design in this case focuses on natural
Figure 3 The demo building (color one) and its
site (Vanke City Garden)
Figure 4 Wind rose for Beijing
The design of natural ventilation in the demo building needs indoor
and outdoor airflow distributions. The wind rose in Beijing (Chen
et al. 1994) as shown in Figure 4 indicates that, in the summer,
the prevailing wind is from north and south. The corresponding mean
wind speed is 1.9 m/s.
With the wind information and the building site, a CFD program
was used to calculate the airflow distribution around the demo building
as shown in Figure 5. With the north wind, the air speed around
the demo building is about 0.1 m/s ~ 0.4 m/s, which is very low.
This is because the tall building in the north blocks the wind.
The demo building is in fact in a recirculation zone of the tall
building. When the wind is from the south, the air speed around
the demo building is not high either (0.3~0.8 m/s). This is mainly
attributed to the small distance between the demo building and those
buildings in the south that block the south wind.
Nevertheless, we decided to use the available low velocity wind
to design natural ventilation in the demo building. The benefits
of the natural ventilation design may be limited, but it is better
than nothing. Since the design of natural ventilation does not increase
the building construction and operation costs, it is always worthwhile
to try as long as the weather data support the use of natural ventilation.
Figure 6 shows two design schemes for a typical floor of the demo
building. The building layout has been designed to allow a free
passage of wind. This paper demonstrates how the CFD technique is
used to determine the size of the court in the south. When initializing
the design, we expect the court to increase natural lighting in
the building, to create a social space for the building residents,
as well as to scoop the wind from the south.
Figure 7 shows the airflow distribution
in and around the demo building with the wind coming from the south.
Please note that the results are not extracted from those shown
in Figure 5. A separate CFD calculation was done to obtain the detailed
airflow distribution in and around this demo building. Without separate
calculation, very fine numerical grid distribution would be needed
for the demo building. The computation would require a large capacity
and high-speed computer. The separation of the detailed airflow
information in the demo building (Figure 7) from the airflow around
Vanke City Garden (Figure 5, b) reduces the computing time significantly.
Since the wind speed around the demo building is 0.3~0.8 m/s with
the wind from the south as shown in Figure 5 (b), the second CFD
calculation for the airflow in the demo building used a uniform
south wind speed of 0.5 m/s. The calculation shows that the building
layout is good for natural ventilation because the air can flow
freely through the building. However, the court size does not have
a significant impact on the airflow pattern and flow rate. Furthermore,
the results suggest that mechanical ventilation or stack ventilation
might be needed in order to enhance the ventilation in the building.
These results are very important to help us make a final decision
on ventilation design.
Figure 5 The airflow distribution around the demo building
(section) (a) with a north wind and (b) with a south wind.
Figure 6 A typical floor plan of the demo
building, (a) with a larger court, (b) with a smaller court.
Figure 7 The air velocity
distribution in and around the demo building at 1.2 m above the
floor. (a) with a larger court, (b) with a smaller court.
3. Outdoor comfort
and site planning in Vanke Doushi Garden
The second example is to demonstrate how the architects and engineers
can work together to design a comfortable outdoor and indoor environment.
People often experience an outdoor comfort problem among high-rise
buildings where the airflow around the buildings can be accelerated.
This study uses Vanke Doushi Garden in Beijing (Figure 8) as an
example to design a comfortable outdoor environment. At the same
time, natural ventilation is also considered.
To design a comfortable outdoor environment,
we need to know the outdoor airflow distributions. Since it is mainly
the chilling effect of the wind in the winter that causes the outdoor
discomfort problem, the present design investigated the airflow
distribution on a winter day. The wind rose in Beijing (Figure
4) illustrates that, in the winter, the prevailing wind is from
the north (5o inclined to the west). In a typical year,
there are 9 days during which the wind speed is higher than 7.6
m/s in Beijing (ASHARE, 1997) and high wind days generally occur
in the winter. The present investigation studied a scenario with
a north wind of 7.6 m/s for outdoor thermal comfort consideration.
Figure 8 (a) shows the preliminary site design (Scheme I) made by
an architectural firm. Design I used 16 high-rise buildings ranging
from 33 to 90 m high. This paper presents the wind speed distribution
at the height of 1.5 m above the ground to evaluate pedestrians
comfort (Figure 9). The wind speed at section 1-1 is around 8 ~ 9m/s
(Grade 5), too high to be accepted even for a short stay in the winter.
The reason is that the wind can pass freely through the linear arrangement
of the buildings. Furthermore, the CFD calculation shows that at the
height of 30 m, the wind speed among most of the buildings is 9~10
m/s, and at the height of 70 m, the wind speed is above 12 m/s (Grade
6). The high wind speed leads to excessive high infiltration in the
winter and difficulties in using the wind for natural ventilation
in the summer. Therefore, the building site should be redesigned,
and the height of the buildings should be reduced.
Figure 8 Three designs for Vanke
Doushi Garden: (a) Original design by an architectural firm (Scheme
I), (b) our first design (Scheme II), and (c) our second design
Figure 9 Wind velocity distribution at
the height of 1.5 m above the ground around the buildings for Scheme
I with a north wind.
Since the CFD calculation shows that the
height of buildings in Design I causes a serious discomfort problem,
our architects designed Scheme II and Scheme III, both of which
have the following features:
lower building height (the building height ranges from 20 to 60
m in Scheme II, and from 20 m to 50 m in Scheme III) to reduce winter
infiltration and to provide opportunity for summer natural ventilation,
without compromising the population density
protection from the north wind in the winter by using relatively
high buildings in the north
Figure 10 shows that the discomfort problem
is greatly reduced in Scheme II, but there are still some problems.
For example, in Entrances A, B, and C, the wind speed is very high
because of the linear arrangement. Staggering the entrances can
easily solve this problem. Moreover, a number of issues need to
be carefully examined. For instance, summer natural ventilation
may not be effective in Scheme II. As shown in Figure 8 (b), more
than a half of the buildings have a long side facing east or west,
such as Buildings 1-8. Since the prevailing wind in the summer is
from the south in this site, the buildings with the long side facing
east or west may not be able to take advantage of natural ventilation,
such as cross ventilation. For example, for the buildings with the
long side facing north or south (Buildings 1-3), the
wind from the south can go through the building openings and the
cross ventilation works. In addition, the orientation is not good
for passive heating design and it is difficult to shade the building
from the strong solar radiation in the summer.
With the results for Scheme II, the architects
in our team designed Scheme III. The low-rise buildings are now
tilted 45o, thus having the long side facing south-east
In scheme III, both outdoor thermal comfort
and natural ventilation are considered. When studying the outdoor
thermal comfort, the incoming wind was set to be 7.6 m/s from the
north. Figure 11 shows the wind distribution for Scheme III for
evaluating pedestrians comfort. The high-rise buildings on
the north side can block the high wind from the north. As a result,
the wind speed in the site is small. Although at places A and B,
the wind speeds are relatively high (about 7~10 m/s). The impact
on the pedestrians comfort is small, since they are the entrances
For natural ventilation design, it is
hard to use north wind since the high-rise buildings will block
the wind. To protect cold winter wind is more important in Beijing
than to use natural ventilation in the summer. However, it is feasible
to use south wind for natural ventilation. The mean wind speed in
the summer from the south is 1.9 m/s. With such a wind speed, Figure
12 shows that the wind speed around most of the buildings at 1.5
m above the ground is above 1.0 m/s. The wind speed is sufficiently
high for natural ventilation. The tilted building arrangement helps
to introduce more wind into the site. Furthermore, the staggered
arrangement prevents the front buildings from blocking winds. Therefore,
Scheme III provides good outdoor thermal comfort and potential to
use natural ventilation. Note that Scheme III is not our final design.
The design team is studying other important issues, such as sun
availability, natural lighting, and energy in buildings, etc.
Figure 10 Wind velocity distribution
at the height of 1.5 m above the ground around the buildings for
Scheme II with a north wind.
Figure 11 Wind velocity distribution at
the height of 1.5 m above the ground around the buildings for Scheme
III with a north wind (7.6 m/s)
Figure 12 Wind velocity distribution at the height of 1.5 m
above the ground around the buildings for Scheme III with a south
wind (1.9 m/s).
This paper shows how the engineers used the computational fluid
dynamics (CFD) technique to help the architects to design natural
ventilation in buildings and thermal comfort around buildings. With
the help of the CFD technique, the engineers can calculate the airflow
distributions in and around buildings. The results were used by
the architects to modify their designs. Several iterations may be
necessary to design a building with satisfactory indoor and outdoor
comfort environment. This design procedure, which was undertaken
by a team of architects and engineers at MIT, shows that the CFD
technique is a very useful tool for building design. The cooperation
between the architects and engineers is necessary in designing a
Although the CFD technique has a great
potential for building design, it is rather time consuming. The
architects and engineers should discuss initial designs based on
their experience and knowledge. This discussion will reduce the
iteration significantly and can speed up the design process. The
CFD technique should be used only to evaluate very few final design
alternatives because the commonly used commercial CFD software still
needs a long time for the calculations in order to achieve an acceptable
accuracy, and the interface of the CFD software is not very user-friendly.
The research was supported by the MIT
Kann-Rasmussen Program. We would like to thank Christoph Ospelt
for providing the analysis result of the comfort hours in Beijing.
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