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Introduction Measurement of contAminant concentration distribution by using image processing TECHnIQUE

Submitted for 11
th
ICWE, Lubbock, TX, June 2003

Nobuyuki Kobayashi
a
*, Kazuhide Ito
a
, Robert N Meroney
b
, Cheng-Shin Chang
c

a
Tokyo Polytechnic University, Atsugi, Kanagawa, Japan
b
Colorado State University, Fort Collins, CO, USA
c
Cheng-Shin Chang, Tamkang University, Taipei, Taiwan

Introduction
A new measurement system of contaminant concentration for environmental dispersion problem in wind
tunnel test was studied. The concentration measurement system is composed with a line scan camera (LSC),
an argon ion laser for visualization, aerosol generator for seeding particle and a computer for processing
data. This paper reports the result of the measurement of averaged concentration and concentration
fluctuation of tracer gas from a point source for two kinds of flow field (channel flow and back step flow) by
using the system.
Background
Various measurement instruments have been used for concentration measurement in wind tunnel test.
However, these concentration measurement instruments have not generally high response performance
though the result of concentration fluctuation measurement with high response hydrocarbon analyzer was
reported. Even the high response hydrocarbon analyzer cannot avoid disturbing the flow field because of the
necessity of presence in the flow. Since concentration is measured at one point in space in these instruments
in principle, it is difficult to carry out multipoint concentration measurement at once. PIV (Particle Image
Velocimetry) has been developed recently and put into practical use. This system can be used as a
concentration measurement system by using special components including fluorescence as tracer gas, and
the result of the concentration measurement with this system is reported. However, PIV system is generally
costly, therefore it is not in widespread use. Under these circumstances, the purpose of this research is to
develop the concentration measurement system, which has a high response and can measure multipoint
measurement and is comparatively simple and inexpensive.
Experimental set up
Measurements were made in wind tunnel type chamber as shown in Fig.1. Image signal value is measured
with a LSC when seeding particles as tracer gas pass through a line of a laser, and it is accumulated as image
data in a computer. Image processing used in this system has 8-bit resolution. The measurement line
corresponding with a line of laser is resolved into 512 pixels, and then deducted background value, and
transformed to concentration information by substituting for a calibration curve. DOP (dioctyl phthalate)
particle from aerosol generator is used as seeding particle. Both averaged concentration and concentration
fluctuation were measured in the flow field of the channel and the back step flow respectively. Positions of
the contaminant source and measurement lines are shown in Fig.1. Sampling frequency of the concentration
measurement is 1Hz and sampling duration is 200sec.
Experimental Results
Vertical profile of the wind velocity and turbulence intensity at the point of x=750mm are shown in Fig.2 (1)
and 2 (2). Vertical concentration distribution in the channel flow is shown in Fig.3 (1), and the one in the
back step flow is shown in Fig.3 (2). The peak value of concentration for each flow appears at the height
lower than that of the point source because tracer gas is discharged vertically downward. Since the floor and
the ceiling reflect laser, it is considered that an error of measurement arise near the wall. Time series of concentration records are shown in Fig.4 (1) and 4 (2).
Concluding remarks
There is a linear relationship between concentration and pixel value. It is confirmed that the concentration
measurement system can be used for the wind tunnel measurement with practical precision after getting an
accurate calibration curve. Concentration fluctuation of tracer gas discharged from a point source is
measured for channel flow and back step flow. As a result, the concentration distribution and the
concentration fluctuation by change of the airflow condition can be comprehended in detail.
References
Cheung, J.C.K. and Melbourne, W.H. (2001).Spectral Properties of Dispersion from a Model Plume in
Turbulent Wind Flows. Proceeding of The Fifth Asia-Pacific Conference on Wind Engineering, Kyoto,
Japan,
pp.301-304.
3
0
0
925mm
300
Line Scan Camera
Flow
x(u)
y(v)
z(w)
125 150
Meas. Line
(a)
(b)
0
50
100
150
200
250
300
0
0.5
1
1.5
2
u*
z(mm)
Channel Flow
Back Step Flow
0
50
100
150
200
250
300
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
C(g/m
3
)
z
(
m
m
)
a
b
Height of Point Source

(1) Channel Flow
(1) Wind Velocity
(1) Channel Flow

3
0
0
1000mm
1000mm
300
Ar-ion LASER
Line Scan Camera
Flow
x(u)
y(v)
z(w)
Point source (Tracer Gas)
150
0
50
100
150
200
250
300
0
0.05
0.1
0.15
0.2
T.I
z(mm)
Channel Flow
Back Step Flow 0
50
100
150
200
250
300
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
C(g/m
3
)
z
(
m
m
)
a
b
Height of Point Source

(2) Back Step Flow
(2) Turbulence Intensity
(2) Back Step Flow
Fig.1 Wind Tunnel Type Chamber
Fig.2 Vertical Profile of u*&T.I.
Fig.3 Vertical C distribution

0.0
0.1
0.2
0.3
0.4
0.5
0
25
50
75
100
125
150
175
200

Ô
(s)
C
(
g
/
m
3
)
z=225mm
z=75mm
Time (sec)
0.0
0.1
0.2
0.3
0.4
0.5
0
25
50
75
100
125
150
175
200

Ô
(s)
C
(
g
/
m
3
)
z=225mm
z=75mm
Time (sec)

Fig.4 Time series of concentration records
* Iiyama 1583, Atsugi, Kanagawa 243-0297, Japan fax +81-46-242-9923, nobuyuki@arch.t-kougei.ac.jp
(1)
Channel Flow

(2)
Back Step Flow