Journal of Heat Island Institute International Vol. 12-2 (2017)

Academic Article

Use of local age of air to study the relation between ventilation
efficiency and building configuration
Franchesca G. Gonzalez O.*1 Tomohito Matsuo*1 Hikari Shimadera*1 Akira Kondo *1*1 Graduate School of Engineering, Osaka University
Corresponding author: Franchesca G. GONZALEZ O., franchesca@ea.see.eng.osaka-u.ac.jp

ABSTRACT
Since human population living in urban areas has been increasing rapidly, strategies to reduce the urban heatisland effect are becoming more crucial. It is well-known that appropriate ventilation designs where the wind is able
to blow away local heat and pollution accumulation might help with the situation. This paper uses the concept ofthe local age of air, defined as the time taken by fresh air to replace old air after it enters to a given zone, to evaluate
the relation between ventilation efficiency and building configurations. For this purpose, CFD simulations werecarried out by using OpenFOAM, the calculation domain was set to x: 1000 m, y: 1000 m, z: 180 m and the actual
building site was defined as x: 355 m y: 365 m, z: 180 m. Results showed that in complex locations with a large
number of buildings the local age of air is higher than that in contrasting locations. Additionally, it was found thatsome urban parameters, such as the ratio of the area that is not occupied by any building, improve the ventilation
efficiency.
Key Words: Urban heat island, Ventilation efficiency, Local age of air, CFD

1. Introduction

under the assumptions that the ventilation processes can be

Nowadays, with over 50% of the human population living inefficiency of the ventilation does not depend on the scale ofcities (1), urban planning is facing many challenges, such as theturbulence and vertical transport, introduced the concept of thedescribe by the same concepts in outdoors and indoors, and thereduction in the difference of the temperature observed between

local age of air and flow rate to analyze in terms of ventilation

cities and surrounding areas, known as urban heat island (2).

efficiency the city breathability and the pollutant removal within

Some studies have already identified that the natural windidealized cities. Likewise, Hang and Li (7) considered how ruralcoming from areas surrounding the cities plays an important rolewinds transport relatively clean air into high–rise urban areas to(3)have shown

dilute airborne pollutants, using air exchange efficiency and thethe benefits of the wind corridors in urban areas in summer

age of air concepts. This paper also takes into account theconsidering the influence of the temperatures according to landassumptions stated in those studies and extends the concept ofuse.

local age of air to configurations of actual cities.

in the city ventilation. For example, Hsieh et al.

In spite of some authors have stated that the heat island

The purpose of this study is to show the suitability of local age

formation could be reduced by city ventilation, the identificationof air concept to determine ventilation performance in existingof urban parameters that improves the outdoor ventilation

urban environments. Furthermore, it is aimed to discover therequires further investigations. Consequently, Wong et al. (4)relation between the complexity of actual cities and ventilationhave studied the concept of building frontal area index toefficiency, by identifying some parameters involve in the ability

describe those paths where the wind can refresh the urbanof the wind to refresh the urban thermal environment.

environment. Moreover, Gat and Unger (5) have used an urban
2. Methodology

roughness mapping method to detect potential ventilation path.
Other studies have discussed the implementation of indoor

2.1 Calculation conditionbuildings ventilation concepts such as the local age of air toevaluate the ventilation efficiency in cities. Buccolieri et al.

(6),- 12 -In order to evaluate the ventilation efficiency, computational Z U  U ref  Z ref fluid dynamics (CFD) simulations were conducted withOpenFOAM version 4.0, a general-purpose CFD toolbox. In then(1)simulations, three-dimensional incompressible steady-state flowwhere Uref is the reference wind velocity, Zref is the reference

and isothermal condition were assumed. The conservation

height, and n is the empirically known constant. In this study, 3.0

equations of mass and momentum, and the scalar transport

m/s from the west, 10 m and 0.25 were used as Uref, Zref and n,

equation were discretized by finite volume method. The standard

respectively.

k-ɛ model was employed as the turbulence model. The

In the following sections, the wind velocity expressed by thesecond-order upwind scheme was used as the discretization

normalized wind velocity Un was used and defined as,

scheme of the convection terms, and the second-order central

Un 

difference scheme was used as the discretization scheme of thediffusion terms in the governing equations. The first order Euler

UU ref(2)method was utilized as time discretization scheme for the scalar

The inlet boundary conditions of the turbulent kinetic energy k,transport equation. The SIMPLE algorithm was employed as theand the dissipation rate  were decided according to thepressure-velocity

equations 3, 4 (8) given as,

coupling

algorithm.

The

sizeofthecomputational domain was x: 1000 m, y: 1000 m, z: 180 m askshown in Figure 1, where approximately 3.4 million ofhexahedral cells were generated.

3U in  T 223
3 k2  C 4 l(3)(4)

where Uin is the inlet velocity, T is the turbulent intensity, C isthe constant value, and l is the mixing length. In this study, it
was assumed that the turbulence intensity is 0.15 and the mixing
length is 1.0 m. The turbulent Schmidt number of 0.9 was used
as mentioned in Tominaga et al. (9).
2.2 Real city configuration study
To assess the ventilation efficiency in actual cities withcontrasting configuration, Figure 2 shows Hiranomachi study
area (34.69N, 135.50E) and Maishima study area (34.66N,135.40E) both located in Osaka City, Osaka Prefecture Japan.
Fig. 1 Top view of the computational domain used in all theThe length, width, and a number of floors of every building werestudied cases.

obtained by the results of the survey on land use conducted by
Osaka City in 2005.

The grid resolution in the actual building site was 2.5 m in bothFigure 1 and Figure 2 are the top view of the actual building siteof x- and y-direction, outside this area the grid resolution had an(x: 355 m y: 365 m) and the analysis area (x: 177 m y: 183 m) inincrease of 10%.

both locations.

In z-direction, the grid resolution was 0.5 m until the height of

In Hiranomachi and Maishima urban environments, an average

20 m, and from 20 m an increase of 10% was used until reachconcentration was defined within the analysis area domain (x:the limit of the domain.

177 m y: 183 m z: 4 m). This analysis domain was defined based

For the boundary condition of the wind, the no-slip boundaryon the consideration of the lowest building height (4 m) and thecondition and the symmetry boundary condition were used at all

impact to pedestrian. In these domains, an initial concentration

the solid surfaces and at the top, north, and south boundaries ofof tracer gas was set to be 1 mg/m3. Since we want to know thethe domain, respectively. At the east (outlet) boundary, the zeroability of the surrounding wind to remove the contaminants from

gradient boundary condition was set and at the west (inlet)

the analysis area, an initial concentration of tracer gas of 0boundary, the power law boundary condition was used. By themg/m3 was assumed in the entire domain, excluding the analysis

power law condition, the inlet wind velocity U at height Z wasarea domain. As same as the wind speed, the normalized

given as,concentration Cn is used in the following sections.

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Cn dense urban environment configuration. All the calculation

CCinit(5)conditions were the same as those used in real city configuration
study, the initial concentration of tracer gas was also located in3

where the Cinit is the initial concentration 1 mg/m .

the analysis area domain (x: 177 m y : 183 m z: 4m).

The time variance of the concentration field of tracer gas in theFour arrangements with differents shapes

analysis area domain was calculated to obtain the local age ofbuildings, nine cubic buildings, four cubic buildings, and one

air.

cubic building) were used and the volume of the buildings for all(sixteen cubicfor configurations was approximated as much as possible to keep
the same volume as Hiranomachi area (496900 m3).
Scenario No. 1 consisted in the analysis area surrounded byother buildings, as shown in Figure 3; and in the scenario No. 2
the analysis area is isolated.

Fig. 3 Representation of the systematic analysis for scenario No.
1; a. sixteen buildings (every building 30 m x 30 m x 30
Fig. 2 Hiranomachi area, Osaka City (Top)m), b. nine buildings (every building 40 m x 40 m x 40 m),

Maishima area, Osaka City (Bottom)

c. four buildings (every building 50 m x 50 m x 50 m), andd. one building (80 m x 80 m x 80 m)

The local age of air LAA is defined as the time taken by fresh air
3. Results and discussion

to replace old air after it enters to a given zone and ismathematically expressed by equation 6.

 t  C p t dt
LAA  0 
0 C p t dt


3.1 Real city configuration study(6)
As we can see in Figure 4, the wind profile in Hiranomachi area
is lower in downstream; especially streets perpendicular towhere Cp is the concentration spatially averaged in the analysis

x-axis, in the center of the analysis area domain, the normalized

area domain, and t is the time. Important to notice, since invelocity Un ranges from 0.0 to 0.4.

practice we cannot integrate t from zero to infinite, we calculated

In Maishima area, on the other hand, Un is around 0.4, andthe local age of air by integrating t from 0 to 1000 s.

within the analysis area, the wind velocities are apparently
higher than 0 m/s.

2.3 Systematic analysis

From Figure 4b, the wind in Hiranomachi is much lower thanMaishima. This remarkable difference between Hiranomachi andBesides the study in Hiranomachi and Maishima, a systematicMaishima is the result of the morphology. For example, thestudy of four cases in two different scenarios was carried out toheight of the buildings ranges from 4 m to 52 m in Hiranomachi,show the parameters influencing the ventilation efficiency forand from 4 m to the highest building of 20 m at Maishima.

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Fig. 4 Actual building site wind profile Un [-] for Hiranomachi (left) and Maishima (right): a. Top views at z: 1 m, b. Lateral views at y:
500 m
Figure 5 illustrates the concentration fields in Hiranomachi andbuildings in Hiranomachi. The main reason may be the low windMaishima analysis area domain where a large amount of thespeed and the complicated building shapes. Therefore, the local

tracer gas was blown off in the first few minutes, nevertheless,age of air in Hiranomachi (147 s) is almost three times higher

some of the tracer gas has been retaining in areas surrounded bythan Maishima (49 s).

Fig. 5 Concentration field at z: 1 m for Hiranomachi analysis area (upper rows) and Maishima analysis area (bottom rows), in theinterval of time from 0 s to 700 s.

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Figure 6 shows the spatially averaged concentration in analysis

easily when the buildings are isolated, because the absent ofarea domain versus time in Hiranomachi and Maishima. The

surrounding buildings allows the wind to reach the analysis areas

average concentration in Maishima decreases much faster thanwith large velocity and less eddies. Moreover, there is a more

Hiranomachi, because of the higher wind velocity in the analysis

symmetric wind flow pattern in x-direction for cases underarea domain.

scenario No 2. This indicates that complicated wind patterncaused by complex geometry may degrade the ventilation
efficiency. The cases of sixteen buildings, however, show adifferent behaviour, in which the result of scenario No. 2
indicates the value of the local age of air to be a slightly higher.
This may be explained as follows: the wind mainly blows west
to east for scenario No.2, so that causes lower wind speed in thestreet which runs from the north to the south direction; moreover,
the turbulence of the wind is smaller in the case of sixteen
buildings in scenario No.1 than scenario No. 2, because of nosurrounding buildings; as a result, the tracer gas remains in thestreet that runs from north to south direction. In addition, sixteen
cases have the smallest street width, so that the wind is not able

Fig. 6 Spatially averaged concentration vs time in Hiranomachi

to enter into the streets.

and Maishima.

It can be said, from the comparison of the values of local age of
air for the arrangements in scenario No.1, which the ventilation

These results show that complex geometry causes lower

efficiency depends on not only the number of building because

ventilation efficiency by lower wind speed and many drifts.

local age of air in the case of sixteen buildings is lower than thecase of nine buildings.

3.2 Systematic analysis

As shown in Figure 7, though the concentration near theFigure 7 and Figure 8 show the time variance of theconcentration on the leeward of a building decrease slowly. The

concentration field and the local age of air for all the cases informer can be explained by the convection, and the latter may beboth scenarios, respectively. From the comparison of therelated with the phenomena of the turbulent flow such as eddies

scenarios, it can be said that the tracer gas was removed more

and separated flow.

perimeter of the analysis area

decreases inmediately, theFig. 7 Concentration field at z: 1 m for Case 1, Case 2, Case 3 and Case 4 in both scenarios; scenario No. 1 surrounded by buildings
and scenario No. 2 no surroundings, in the interval of time from 0 s to 300 s.

- 16 -Acknowledgements

Additionally, we defined the open space ratio ros as,ros AosAaa(7)Part of this work was financially supported by the Obayashi
Foundation. In addition, we are grateful to Osaka City

where, Aos is the free area that is not occupied by any building

Government for providing building data from their survey on

and the Aaa is analysis area (177 m x 183 m). We found thatland use conducted in Osaka City in 2005.

open space ratio has a relationship with the local age of air.
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4. Conclusions

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urban environments. With all the steps applied in this work,(Received Apr. 28, 2017, Accepted Jul. 7, 2017)

we indicated how the local age of air is used as a tool toanalyze the influence of urban morphology on the cityventilation.
In addition, it was found that irregular configurations,buildings height, surroundings, street width and open spaceratio have a relationship with the ventilation efficiency.
Through the identification of these parameters and the studyof their influence, possible modifications or suitable designsof the city spaces can be taken into consideration to help in theurban planning process to reduce the heat island effect.

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