九州大学大学院総合理工学報告第32巻 第4号 1-8頁 平成23年2月

Engineering Sciences Reports, Kyushu University
Vol. 32, No. 4, pp. 1-8, FEB, 2011

A scale-model experiment of surface temperature
characteristics using an infrared radiation camera
Joao Tales Oliveira∗1,† , Aya Hagishima∗2, Jun Tanimoto∗3†E-mail of corresponding author: es308021@s.kyushu-u.ac.jp(Received January 31, 2011 )An outdoor measurement was performed during fine weather with a scale-model array of cubicalobstacles (size 100mm, 25% packing density, lattice-type square layout) that represent an idealized
urban tissue. Surface temperature was recorded with high-frequency (10Hz) infrared camera, alongwith wind speed and solar radiation. Complementarily, air temperature was also measured at threeheights in the near-wall region of a block surface using fine-wire thermocouples. Results showed thesimilarity of distribution between surface temperature and scalar transfer coefficient. We also found that
10s averaging period may be appropriate for the analysis of surface temperature fluctuation affectedby turbulent airflow. In the near-wall region under particularly calm wind condition, air temperaturefluctuation caused by a hot plume arising from the warmed surface was observed. The method presented
in this study seems to be a promising tool to analyze surface temperature characteristics of scale-models
in urban climate studies.
Key words : infrared thermography, field measurement, urban-like block array1.

Introduction

Micro-climate in urban areas is a very complex
phenomenon with direct implications in people’s
daily life, comfort, and health. This complexity isderived from myriad interactions of physical processes. Various types of field measurement in urban areas allow for many contributions .1) Its
merit lies in acquiring data under real atmosphericconditions – solar radiation, precipitation, atmospheric stability, large scale turbulence – which inturn give realistic features from the viewpoints ofboundary layer aerodynamics. However, field measurements posit many difficulties: they are usually
costly, require careful logistics, and can be hindered by traffic or other factors inherent to public
spaces. For these reasons, some researchers opt
to use small-scaled physical models as an alternative (Kanda, 2006) 2) . Some of these studies are
conducted in wind tunnels, others are conducted
outdoors.
Wind tunnel experiments have been conducted
to mainly grasp turbulent structures within andabove urban canopy layer under neutral condi*1 Department of Energy and Environmental Engineering*2 Department of Energy and Environmental Engineering*3 Department of Energy and Environmental Engineering

tions, whereas outdoor scale model experiments
have contributed to investigate more various aspects of urban climate in a real setting. Aida3)
examined the effect of urban surface irregularity
on albedo. Oke4) investigated nocturnal coolingrates for rural and urban environments under calmand cloudless conditions. In addition, experiments
have been conducted mainly driven by the need tounderstand energy balance of urban areas 5) ,6) air
flow within street canyon, and scalar dispersion 7)
8). Still, the effect of flow on spatial distributionof urban surface is not well investigated.
Scale-models cannot provide the data under a real
urban setting with complex geometry and mixtureof various materials like field measurements in urban areas. However they have the advantage ofallowing systematic investigation of factorial effects, such as urban geometry, material of urban
surfaces, and size, on urban thermal balance, resulting in important insights about the mechanismof urban climate.In addition, scale model experiments generally facilitate to perform multi-point
extensive measurement of various physical variables, such as temperature of air and urban surfaces, radiation flux, sensible heat flux, and turbulent statistics. It is effective to examine highly
heterogeneous urban streets.
Meanwhile, understanding of momentum and2

A scale-model experiment of surface temperature characteristics using an infrared radiation camera

scalar transfer process on a rough wall are important issues to not only urban climatology but
also various engineering fields and fluid dynamics.
For example, Cheng and Castro9) performed comprehensive measurements of profiles of mean wind
and turbulent statistics over block arrays in a windtunnel. Kovar-Panskus10) measured airflow withina two-dimensional street canyon to investigate theinfluence of solar-induced wall heating on the airflow pattern.
In the light of these facts, the aim of this study
is to challenge to observe temperature distribution and its fluctuation on a cubical array using
infrared thermography. It allows the analysis ofsurface temperature without disturbing the flow.
Although it is already used in flow visualization
11), these studies are conducted mainly indoors,
without taking into account the effect of solar radiation. Complementarily, air temperature in nearwall regions of a block surface was measured by
fast-response thermocouples to grasp the characteristics of the thermal boundary layer developed
over urban roughness.

2.

Experiment setup

2.1

Scale-model details

The experiment was done in an open space inside Chikushi Campus of Kyushu University. It
is located in the city of Kasuga, Japan, latitude
33.52°north and longitude 130.48°East. The site
was an unobstructed area between two buildings
and unshaded during measurement period. Figure
1 shows a sketch of the experiment setup, and fig.
2 shows the measurement site.
81 cubical obstacles with a size of 100 mm (hereafter, H) were arranged in lattice-type square layout with 25% packing density on a board with
100mm thickness. Both the obstacles and theboard are made of polyurethane foam , commonlyused as insulation in building envelopes. Twenty
five obstacles at the center of the array were covered with 0.3mm aluminum sheet, coated withmatte black paint. The surface area of the board
under these 25 obstacles was also covered with thesame material (gray area shown in fig. 1), thus creating an homogenous 3-dimensional array. The remaining obstacles were left uncovered. The choiceof material (thin aluminum sheet + polyurethane)and color (matte black) allows high surface emissivity (we assumed  = 1, and adjusted the camera
accordingly), low heat capacity, and a quick temperature response.

2.2

Measurement and instrumentation

Surface temperature distribution of the array
was measured with a frequency of 10Hz using a
portable infrared camera (Nippon Avionics TVS200). The camera has a temperature resolution of
0.08℃, and accuracy of ±2℃. It enables to output
the data into images with 320 x 240 pixels thatcan be processed through computation. To protect from direct sunlight, a cardboard box covered
with high-reflective tape was placed over it during all experiments. During measurements, thecamera was placed facing the board at a height ofapproximately one meter.
Simultaneously, air temperature was acquired
using fine-wire thermocouples (=0.0127mm)
placed at different heights – 1mm, 5mm, 10mm –in order to investigate the thermal profile above
a roof of a block. Three-dimensional airflow
was measured using two ultrasonic anemometers
placed at 2H (anemometer A – Kaijo WA(T)395)
and 10H (anemometer B – Young 81000), respectively. Solar radiation was measured with a pyranometer (EKO MS-602) placed in a clear area
near the site. The measurement frequency of the
anemometers and the pyranometer was synchronized with that of the camera.
2.3

Measurement period

The experiment was conducted on several days,
under different weather conditions according to table 1. The last experiment (2010/01/10), which
has no missing data from the thermocouples, wasselected for the following analysis. There were two
experiments in that day, hereafter called as exp01
and exp02.

Table 1 Measurement period. I¯
sr : mean solar radiation[W/m2 ]; ū: mean wind speed [m/s]; α: solar
angle at the time [degree].
Date
9/16

9/17

9/18
10/1

Start

End

I¯
sr

ū

α

Label

12:00
13:41
15:16
12:00
13:37
15:14
12:00
13:42
15:32
10:46
12:47

12:30
14:11
15:46
12:30
14:07
15:44
12:30
14:17
16:07
11:25
13:26

629
551
177
820
721
526
823
704
336
764
737

0.81
0.95
1.00
0.53
0.90
0.85
0.60
0.55
0.83
0.51
0.54

59.03
50.32
34.71
58.66
49.95
34.39
58.28
49.59
30.20
50.02
50.52

–
–
–
–
–
–
–
–
–
exp01
exp02

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九州大学大学院総合理工学報告 第32巻 第4号

Fig. 1 Experiment setup. Red squares represents covered
obstacles; Gray area represents aluminum sheet
laid on the insulation board. The blue dot represents the probe’s position.

Fig. 3 Weather conditions for exp01 (10:47 to 11:25 – 38min). Values averaged for a period of one minute.

Fig. 2 Experiment site of this study.

3.

Results and discussion

3.1

Climate conditions

Background climate conditions from anemometer
A are shown in figs. 3 and 4. It was a clear daywith little variation of global solar radiation except
for a brief interval of cloud cover by the end ofexp02 (fig. 4, at 13:21). In general, wind speed
was relatively calm, the mean value for the two
experiments were 0.5 m/s.

Fig. 4 Weather conditions for exp02 (12:47 to 13:26 – 39min). Values averaged for a period of one minute.

3

4

3.2

A scale-model experiment of surface temperature characteristics using an infrared radiation camera

Mean surface temperature

Figures 5 and 6 show the thermal images from theIR camera for exp01 and exp02, respectively. We
adopt surfaces with different orientations for analysis: roof and street (horizontal), wall A (exposed
to incident solar radiation) and wall B (shaded surface).
As can be inferred from table 1, in spite of thealmost same conditions of solar radiation and airtemperature, there are differences in mean surface
temperature distribution, which could be causedby the change in solar position with time.

temporal average of surface temperature of eachsurface against solar radiation and wind speed,
respectively, for the entire period of exp01 andexp02. There is no clear correlation between solar radiation and surface temperature because of
stable solar radiation during all measurement period except for several minutes. Thus, the effectof solar radiation on surface temperature for eachsurface is probably negligible.

Fig. 7 Spatial-temporal average of surface temperature
for roof, wall A, wall B and street, vs solar radiation. Averaged one minute values for the entire
period of exp01 and exp02.

Fig. 5 Distribution of mean surface temperature of thearray for the entire period of exp01. The areas
named roof, wall A, wall B, and street are delimited by black polygons.

In contrast, the negative correlation between surface temperature and wind speed is evident. The
slopes of roof, wall A, and wall B are not much different, whereas the slope of street is the smallest.
Because the street is at the bottom of the cavity,and it is less subject to the flow above the canopy.

Fig. 6 Distribution of mean surface temperature of thearray for the entire period of exp02. The areas
named roof, wall A, wall B, and street are delimited by black polygons.

Fig. 8 Spatial-temporal average of surface temperature
for roof, wall A, wall B and street, vs wind speed.
Averaged one minute values for the entire periodof exp01 and exp02.

Figures 7 and 8 show for one minute the spatial-

To narrow the period for analysis, we picked up

平成22年度

九州大学大学院総合理工学報告 第32巻 第4号

one particular minute according to the following
criteria: mostly mild wind conditions (between 0.3
and 0.6 m/s), stable global solar radiation (standard deviation of less than 3 W/m2 ), and relatively
stable wind direction perpendicular to the street(NW-SE, see fig. 1). The one-minute period at
13:04 was chosen for its range of wind conditions(mostly mild – 0.4 m/s – but with relatively high– 1.2 m/s – and low peaks – 0.2 m/s), which might
be suitable for investigation of surface temperature
reaction to fluctuation of airflow in a brief intervalof time. The subsequent sections will show results
based on this period.
3.3

Surface temperature characteristics

Figure 9 shows the distribution of mean surface
temperature for the selected time period 13:04. A
distinguishable temperature distribution for roof,
street, wall A and B can be seen. There areroughly concentric regions of high temperature on
wall A, the center starting from the lower left corner. Similarly, the region of the street closest to
wall A is also warm. These patterns might be related to the distribution of heat transfer coefficientcaused by a canopy vortex. Several wind tunnel
experiments12)13) , presented that the scalar transfer coefficient of windward wall of a 2D canopy
increases with height. It indicates that the fresh
air is introduced into the cavity along the windward wall and scalar boundary layer develops withdownward motion of the canopy vortex. The tendency observed in wall A and street is consistent
with their work.

tional gradation from the lower right corner canbe seen. In contrast, the value at the lower part ofwall A, wall B, and street are much smaller thanthat of the roof. It may be due to the differenceof wind speed close to each surface. According toOke14) , the flow regime of this array with density of 25% can be classified as skimming flow. It
indicates that the flow within canopy is relatively
week, and part of it cannot enter the cavity.
Detailed pattern of distribution of standard deviation for each period shows different tendency. Although the data of other periods are not included
here, the correspondence of the pattern and background conditions of velocity and radiation was
examined. However, we could not find a clear association between this pattern and the ambient conditions, and that remains a point to be elucidated
in future investigations.

Fig. 10 Spatially averaged standard deviation based ontime series data of each pixel for covered obstacles. One-minute average (13:04). Mean windspeed of 0.39 m/s, global solar radiation of 757
W/m2 , and air temperature of 27.7℃.

Fig. 9 Mean surface temperature distribution based on
individual pixels. One-minute average, 13:04.

Surface temperature fluctuation denoted by thestandard deviation of each pixel is depicted in fig.
10. Different patterns can be seen for each surface.
The roof area shows the highest value and a direc-

Figures 11 and 12 show the spatially-averaged
standard deviation of surface temperature for roof
against wind speed and its standard deviation, respectively. The data of three averaging periods,
1min, 10s and 2s, were compared. With 1 minaveraging, surface fluctuation is sensitive to weakwind conditions.
The standard deviation of 2s is very small regardless of the wind condition and almost same as theresolution of the infrared camera. It implies thatthe measurement setting of our study cannot detect the instantaneous fluctuation of surface temperature due to turbulence with a time scale of 2s.
In contrast, the standard deviation of 10s shows
larger values than that of 2s, and it increases with
the increase of standard deviation of wind speed,56

A scale-model experiment of surface temperature characteristics using an infrared radiation camera

although the plots are scattering. It is due to the
influence of turbulent airflow above the array. The
values of 1min are much larger than the others
and almost insensitive to both mean wind speed
and standard deviation. The reason of this tendency might be atributted to the effect of fluctuation of solar radiation. Although the data selected
for analysis was based on the criteria of standard
deviation of solar radiation lower than 3 W/m2 ,the response time of the pyranometer is 17s, thus
it is probable that the actual solar radiation fluctuated stronger compared with the recorded data.
Eventually, the time scale of 10s seems to be mostsuitable for the analysis of surface temperaturefluctuation affected by turbulent airflow. Since thesize of the cube and the whole array are 0.10m
and 1m respectively, wind speed close to the array
lower than 0.01 m/s or 0.1 m/s may be desirable tocapture the event of flow passing through a blockor the whole area of the array. Unfortunately, sucha condition is very rare in the daytime. For thisreason, in future investigations using larger obstacles might be more appropriate for capturing flow
events. Nevertheless, the fact that the infrared
camera can capture very small temperature fluctuations and give a visual output is an interesting
point that opens possibilities of further study

Fig. 12 Spatially averaged standard deviation of roof surface temperature plotted against the standard
deviation of wind speed. One minute average,
exp01 and exp02.

Fig. 13 Normalized temperature (x-axis) profile above
the roof of an obstacle for the entire period of
exp01 and exp02. Red lines represent standard
deviation.

Fig. 11 Spatially averaged standard deviation of roof surface temperature plotted against wind speed.
One minute average, exp01 and exp02.

3.4

Temperature profile above the roof

The normalized temperature profile above theroof is shown in figure 13. It is based on oneminute averaged data measured by fine-wire thermocouples at 1mm, 5mm, and 10mm for the entire
period of exp01 and exp02. The most evident fact
is the decreasing temperature with height. However, temperature at 5mm shows the longest horizontal spread, followed by 1mm and 10mm.

Figure 14 shows the instantaneous temperaturefluctuation at each measured height, defined as thedifference between instantaneous value and time
averaged value. The temperature at height of 1mm
and 10mm fluctuate approximately by ±5℃ and±8℃, respectively. In contrast, the temperature
variation at a height of 5mm is the largest among
the three. Under particularly calm wind condition,
the strongest fluctuation of temperature at 5mm ismore remarkable. At that height, we could find a
sharp increase followed by a rapid decrease during
the period from 31s to 34s. Since the wind speed
is less than 0.2m/s and the temperature difference
between the roof surface and air at 2H is about
35℃, such conditions implies that natural convection is dominant (the Grashof number is approximately 5.6 · 1010 ). Therefore, the sharp fluctuation
measured at 5mm might be related to a hot plumearising from the warmed roof surface. The rea-

平成22年度

九州大学大学院総合理工学報告 第32巻 第4号

Fig. 14 Temperature fluctuation measured at different heights by fine-wire thermocouples plotted against wind speed fora selected interval of one minute (13:04).

son why the fluctuations of other two heights are
weaker than that at 5mm is probably caused by thefollowing fact. In the near roof region, hot plumes
are intermittently generated with a subsequent descending of cool air. Hence, temperature at 1mm,
too close to the surface, might be dominantly affected by surface heating, and less sensitive to thecold downward motion. Air at 5mm is probably
easy to move because of the larger distance fromthe roof and more flexible air motion. In contrast,
at 10mm it is too far to be affected by the ascending hot plume motion.

method for analyzing surface temperature characteristics of urban-like block arrays.

4.

2) Kanda M., Progress in the scale modeling of urban
climate: Review, Theor. Appl. Climatol., 84, p.23-33(2006).

Conclusions

A method to analyze surface temperature distribution and its fluctuation due to air flow was investigated using infrared thermography. Outdoors
measurements of surface temperature of a cubicalarray showed the similarity between surface temperature and scalar transfer coefficient, although
we could not make definite conclusions about whysome patterns of standard deviation arose. This isa point to be elucidated in future investigations.
We also found that 10s averaging period may beappropriate for the analysis of surface temperature fluctuation affected by turbulent airflow, andwe suggest that a larger model is desirable if thesame materials are used.
Using high responsive thermocouples in the nearwall region of a block surface, we found that underparticularly calm wind condition, air temperaturefluctuation at 5mm has the strongest fluctuation,
which may be related to a hot plume arising fromthe warmed surface.
This work, as a first approach, shows a promising

Acknowledgments
This research was financially supported by aGrant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
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A scale-model experiment of surface temperature characteristics using an infrared radiation camera

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