Proceedings: Indoor Air 2002
RESEARCH ON THE FLUCTUATING CHARACTERISTICS OF
AIRFLOW IN THE NATURALLY VENTILATED BUILDINGS
G Tan1,2*, L Song 1 and Y Zhu1
1
Dept. of Building Science, Tsinghua University, Beijing, China
Building Tech. Program, Dept. of Architecture, M.I.T., MA, USA
2
ABSTRACT
Natural ventilation’s fluctuating airflow has impacts on the thermal comfort and VOCs
transportation in the naturally ventilated buildings. Research about the fluctuating
characteristics of the airflow in naturally ventilated buildings was carried on in this paper. A
significant amount of data for airflow velocity of natural ventilation was recorded. Based on
these simultaneously measured velocities of the indoor and outdoor airflow, the characteristic
parameters, such as energy spectrum, turbulence intensity, and dimensional restructuring etc.,
of the airflow under naturally ventilated situation were analyzed. Not only the methods of
statistical analysis but also the methods of turbulence theory, chaos theory and fractal theory
were used to describe the structural characteristics of the fluctuating airflow of natural
ventilation. The basic rules governing how the airflow fluctuating characteristics change when
airflow goes through the building’s openings or enters the building space were summarized in
this paper.
INDEX TERMS
Airflow fluctuating characteristics, Natural ventilation, Field experiments
INTRODUCTION
Natural ventilation is a promising method to provide excellent indoor air quality in buildings.
Natural ventilation is greatly different from mechanical ventilation, due to its dynamic airflow
characteristics. These characteristics would definitely affect the movement and distribution of
particles or VOCs in naturally ventilated buildings. Furthermore, people consider natural wind
to be more acceptable than mechanical wind. For example, even in the neutral-warm
environment, mechanical wind at velocity higher than 0.8 m/s may cause unpleasantness
while natural wind at average velocity 1 m/s is still pleasantly acceptable. People can feel both
the physical and psychological comforts of natural wind, which are actually indicated by the
fluctuating characteristics. Additionally, some recent research was shown that the fluctuating
characteristics of natural ventilation could affect its mean airflow rate (Haghighat, 1991 and
Holmes, 1979) . Some studies have shown that the natural wind’s frequency energy spectrum
follows the rule of 1/f (similar to human being heartbeats) (Akia 1992 and Kobayashi, 1982).
The impacts of airflow velocity’s frequency on human thermal sensation were studied in (Xia
2000 and Jia, 1999). It was found that the frequency between 0.3Hz and 0.5 Hz has the
strongest cooling effect, and the energy of natural wind mainly lies in the low frequency
0~0.5Hz, i.e. the essence of natural wind makes humans feel cooler and more comfortable in
the neutral- warm environment. The literature (Zhu, 2000) has suggested 4 indexes including
slopes of the logarithmic energy spectrum curves to distinguish natural wind and fanned wind.
However, the fluctuating characteristics of airflow in naturally ventilated buildings are not
well understood yet.
*
Contact author email: [email protected]
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Proceedings: Indoor Air 2002
In order to explore the basic rules governing the airflow’s characteristics in natural ventilation
situation, the characteristics of natural wind, when it passes through the buildings, have been
studied in this paper.
EXPERIMENTAL DESCRIPTION
Test point 1
4.6m
Test point 2
2.0m
Test point 3
4.3m
Experiment
Room 1
0.4m
Test point 4
0.9m
Figure 1. The three experiment rooms and the test points’ locations
Figure 1 shows the experiment rooms and the four test points in one test room. During the
experiment, four hot wire anemometers were located at four different sites in every test room.
One test point was located outside, which was 1.5 meters away from the northern window,
and the other three points were located at the window, the middle of the room, and the door
respectively. All the experiments were taken in naturally ventilated situation. The air
temperature differences between the indoor and outdoor air were less than 1oC. A computer
controlled all the hot wire anemometers in the experiment. The diameter of the hot wire was
20 µm and its maximum response frequency was15 Hz. The precision of the hot -wire
anemoscope was 5%±0.02m/s. During the experiment, the hot wire anemometer recorded 10
wind velocity data every second. According to the former research (Xia, 2000 and Jia, 1999),
the energy of the natural wind mainly lies in the low frequency 0~0.5Hz. Because the data
collection frequency (10 Hz) is greater than the two times 0.5 Hz, the experimental design can
meet the requirement of the Shannon rule.
ANALYSIS METHODS
The turbulent intensity, Tu, shows information on the average magnitude of the velocity
fluctuation over an interval of time related to the mean velocity.
Tu =
v′2
v
where v is the mean airflow velocity, v′ is the velocity fluctuation.
The energy spectrum of the velocity fluctuations:
∞
∫0
E (n)dn = v ′ 2
(1)
(2)
Therefore, the time scales of turbulence comprise the integral (large) time scale, TE, and the
micro (small) -time-scale, τE. It is assumed that the turbulent motion consists of the
superposition of various sizes of eddies.
TE =
l im E (n)
n→0
(3)
4v ′ 2
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Proceedings: Indoor Air 2002
1
τE 2
1 ∂2 R
= − ( 2E ) t =0
2 ∂t
(4)
The dimensional restructuring come out as: Let us assume a time lag ∆t; and take the data
group ( v′t , v′t + ∆t ) as (x, y); then a curve for these data group can be drawn. Dimensional
restructuring, providing information for the related parts of the velocity fluctuations ( v′ ), is a
tool to study the chaos and fractal phenomenon of natural wind.
RESULTS
From the Figure 2, it can be seen that the turbulence intensities, Tu, of different locations of
the naturally ventilated buildings are similar under same average air movement velocity.
1.4
1.2
Tu
1.0
Outdoor wind
0.8
Window
0.6
Indoor air
0.4
Inner door
0.2
0.0
0
0.5
1
1.5
2
2.5
Average wind velocity(m/s)
Figure 2. The turbulent intensity of different locations
Outdoor wind
Window
0
Indoor air
Inner door
0.5
1
1.5
2
2.5
Averagewindvelcocity(m/s)
Indoor air
Window
60
50
40
30
TE
20
10
0
Outdoor wind
3
2
Average wind velocity(m/s)
1
0
0
tE
0.2 0.4 0.6 0.8
1
1.2
The energy of turbulence is mainly consumed through the small eddies. Figure 3 shows that
the τE of the test point at windows is the greatest and the outdoor wind’s τE is the smallest.
Because windows interfere with the airflow, the eddy cannot change gradually into very small
scale when its energy has been consumed while the air passes the window. Figure 3 also
shows that the large eddy scales of different locations are similar, which explains how the
airflow around the buildings is interfered with by the building’s scale, so the large eddy scales
represent similar values under the same average velocity.
Figure 3. The small eddy time scale τE and large eddy time scale TE of different locations
Figure 4 shows that the dominating energy of natural wind lies in the frequency of 0~1Hz.
According to literature (Jia, 1999), the logarithmic energy spectrum of wind can be divided
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Proceedings: Indoor Air 2002
into two parts by frequency 0.03Hz. Only the velocity data whose frequency is higher than
0.03Hz can provide the characteristics of the wind, no matter whether it is natural or fanned
wind. All these reasons let the choice of studying the data whose frequencies lie in the range
of 0.03 to 1 Hz.
Figure 4. Energy spectrum of the four different locations
The slopes of the curves between frequencies of 0.03Hz to 1 Hz are different (see Figure 4).
In order to compare the difference of the slopes, the average slope of the logarit hmic energy
spectrum curve at every location for more than 500,000 data points was calculated (shown as
Table 1).
Table 1. The average slope of logarithmic energy spectrum
Test point 1Test point Test point
Average slope of
outdoor wind
2-window 3-indoor air
logarithmic energy spectrum
-1.1
-1.7
-1.6
Test point
4-inner door
-1.7
Table 1 shows that the absolute values of slope to the airflow in the naturally ventilated
buildings are larger than 1.1. The airflow still has the natural wind’s characteristic s according
to the conclusion of literature (Zhu 2000). Obviously, because of the building’s structure, the
slopes of the logarithmic energy spectrum curves are changed when wind passes through a
building. With the absolute value of average slopes ( β), the differences can be sorted as:
Outdoor wind (β) < Window ( β) ≈ Indoor air (β)≈ Inner door ( β)
A large amount of airflow’s high frequency energy was lost greatly when the natural wind
passes through the building’s openings. That is to say, the building structure mainly impacts
on the natural wind’s high frequecy airflow.
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Proceedings: Indoor Air 2002
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1
-0.5
0
0.5
1
1.5
2
2.5
Test point 1-outdoor wind
3
3.5
Test point 2-window
y
L
B
Test point 3-indoor air
x
Test point 4-inner door
Figure 5. Dimensional restrucutring curves and definition of δ
Figure 5 shows the dimensional restructuring of the velocity flucuations for the time lag
∆t=0.1s. All the curves are spindly form, but some curves’ points are more closely. One
parameter was defined to describe the form differences of the restructuring curve as δ=B/L
(Zhu, 2000) (see Figure 5). The coefficient δ provides a method to evalutate the centralization
of the data in the dimensional restructuring curve. The bigger the δ value, the closer relations
the wind velocities have. A clear result is showed by Figure 6 that the δ values of different
locations are different. From the average, the outdoor wind’s δ is the greatest, about 0.53; the
window wind’s δ is the second, about 0.36; and the other two places (indoor air and inner
door) wind’s δ are smallest, about 0.27 and 0.25 respectively.With the differences of δ, there
iss an order as: Outdoor wind δ > Window wind δ > Indoor air δ ≈ Inner door δ.
It can be found that the airflow’s δ values decreased when natural wind passes through the
buildings. This means that the airflow velocity becomes closer to its average velocity because
of the building’s structure impacts.
1
0.8
Outdoor wind
Window
B/L
0.6
Indoor air
0.4
Inner door
0.2
0
Data Series (Here one point represents 21,600 data)
Figure 6. The dimensional restructuring δ values of wind velocity flutuations
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Proceedings: Indoor Air 2002
CONCLUSIONS
Airflow in natural ventilation has the stochastic, turbulent, chaotic and fractal characteristics.
When natural wind passes through buildings, some of its characteristics will change and some
of them won’t change greatly such as turbulent intensity, large eddy scale and micro-eddy
scale. Two coefficients, β andδ, can be used to describe the different characteristics of airflow
in naturally ventilated buildings. A large amount of natural wind’s high frequency energy will
be lost due to the building structure’s impacts when natural wind passes through buildings.
Also, the airflow’s veclocity will become closer to its average veclocity, which means some
parts of the natural wind’s information entropy will be lost due to the building structure’s
effects.
ACKNOWLEDGEMENT
Thanks to the Chinese Natural Science Fund Committee’s funding (No. 59836250).
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