Journal of Heat Island Institute International Vol. 9-2 (2014)

Academic Article

Study on the Evaluation of Cross Ventilation Possibilities
Based on the Simulation Results for Detached Houses
Yuki Takemoto*1Hideki Takebayashi*2 Takanori Fukuoka*3Hiroshi Adachi*4
Koji Itakura*5*1 Graduate School of Engineering, Kobe University*2 Associate Professor, Graduate School of Engineering, Kobe University
3* Reserach Associate Professor, Graduate School of Engineering, Kobe University*4 Emeritus Professor, Graduate School of Engineering, Kobe University*5 Sekisui House Ltd.
Corresponding author: Yuki Takemoto, 141t042t@stu.kobe-u.ac.jpABSTRACT
In this study, CFD simulations were carried out for several typical real detached house models with various opening
patterns. A method is proposed for the improvement of the indoor thermal environment by cross ventilation at thearchitectural planning stage by using a calculation results database. The relationship between the ventilation ratio
and the opening area ratio is analyzed. The ventilation ratio was increased when a ventilation circuit was formed in
the house, and the opening area facing the prevailing wind direction was large. However, the relationship between
the ventilation ratio and the opening area facing the south side was not confirmed, because the south side opening
area was much larger in all objective typical detached house models. In typical detached houses with large southernside opening areas, the ventilation ratio is larger in the case of winds from the south.
Key Words : Detached house, Cross ventilation, CFD, Ventilation ratio

several typical real detached house models with various opening

1. Introduction

patterns. A method is proposed for the improvement of theIn mid-latitude regions such as Germany and Japan, crossindoor thermal environment by cross ventilation at theventilation is an effective method for improving the indoor

architectural planning stage by using a calculation results

thermal environment, with less energy consumption, during thedatabase. In addition, through the analysis of the calculation

daytime in autumn and spring, and throughout summer nights.

results database, the primary parameters that may be used to

Various studies on the improvement of the indoor thermal

improve the cross ventilation performance are considered.

environment using natural ventilation have been carried out. Inorder to evaluate the ventilation performance quantitatively,

2. Evaluation framework

Akabayashi et al. have proposed a method for evaluating theventilation performance using three indexes : site index, local

Cross ventilation improves the indoor thermal environment byindex, and building performance index(1). The site index is

the removal of heat and the moderation of the effective

defined as Cross Ventilation Degree Hours (CVDH), whilst thetemperature. Akabayashi et al. have pointed out that the effect of

local index is defined as a cross ventilation ratio corresponding

heat removal by ventilation is considerably larger than the effectto the building coverage ratio. The building performance index

of the moderation of effective temperature by airflow(2).

is defined as the volume fraction of CFD calculation results (atTherefore, the ventilation ratio was adopted for the evaluation

wind speeds of more than 0.3 m/s) which have been carried out

index, focusing on the effect of heat removal. The calculation

for simple house models and a real detached house model. In

method for the ventilation ratio in a detached house is shown inthis study, instead of changing the parameters such as theFigure 1, with reference to the method used in the previous

arrangement and area of openings in a simple model of a housestudy(1). The building performance index is the ventilation ratiowith no complex plan, CFD simulations were carried out forcalculated under the condition that the building coverage ratio is- 82 -

0% and inflow wind speed is 3.8 m/s. The local index is a crossTable 1 Characteristics of objective detached houses under study

ventilation ratio corresponding to the building coverage ratio.

Total floor The entrance The roof The stairs in The stairs
Plannumber area (m 2 )orientation
shape the living room position

The site index is a regional mean wind speed obtained from the1117.0

hip roof

non-existent

NE2117.0

gable roof non-existentNW

3118.8

WRF, it is possible to determine the wind direction and wind6125.5speed in the areas where these have not been measured. The

13

actual ventilation ratio is calculated by multiplying these three

1415118.5

17122.0

hip roof

22116.0

gable roof non-existent23116.0

hip roofnon-existent

N

24

116.5

hip roof

existent

Center26

122.5

gable roof

existent

Ecalculation results of the WRF (Weather Research andForecasting model). By using the calculation results from theindexes.

Sgable roof

existentE

hip roof

existentN

117.0

hip roof

non-existent

N

118.0

gable roof

existent

NEWgable roof non-existent

Nexistent

NCenterW

Fig. 1 Calculation method for the ventilation ratio in a detachedhouse
3. Calculation of the airflow distribution
3.1 Outline of the calculations
The CFD simulations were carried out for 12 detached houses,
each with two stories under the conditions that the building
Fig. 2(a) Calculation domain

coverage ratio in the surrounding area was 0 %, the inflow wind

Fig. 2(b) Detail domain

speed was 3.8 m/s and the directions were N, E, S, W, NE, NW,SE, SW. Table 1 shows the characteristics of the objective
detached houses under study. Figure 2 shows the calculation
domain. Figure 3 shows an example of the objective model usedin the study (plan number 1). Figure 4 shows the plan andperspective drawings of plan number 1. Table 2 shows theconditions for the CFD calculation. The CFD simulation uses thestandard k-ε model in the turbulence model. The calculation
domain is divided into the peripheral domain and the detailFig. 3(a) Objective model (Left to right: 1st floor, 2nd floor),domain. Grid sizes are 100 mm × 100 mm × 100 mm in theplan number 1

calculation domain. The minimum grid size is 60 mm, in orderto reproduce the shape of the opening. The grid sizes in theperipheral domain increase gradually in the horizontal andvertical directions. The opening ratio is assumed to be 70 %,with respect to the screen door. The calculation has been
performed for 96 patterns (12 plans × 8 wind directions), and themean wind speed is calculated for each respective opening.
3.2 Calculation results
Figure 5 shows an example of the wind vector distribution andthe wind speed contour of the CFD simulation results (plannumber 3). The plan of the objective house model is shown in

Fig. 3(b) Objective model (perspective), plan number 1

the center, and the calculation results corresponding to each
inflow wind direction are shown around the plan. If the windfloor where a clear ventilation circuit is formed.There are aflows out of the room through an opening, the wind speed is by acouple of large openings in different directions; the wind flows

negative value. The ventilation is particularly effective on thein and out through the openings.

- 83 -(a) Example of calculation results expression
Fig. 4(a) Plan drawing (Left to right: 1st floor, 2nd floor), plannumber 1

Fig. 4(b) Perspective drawing , plan number 1(b) FL + 1.5 m (1 F)

Table 2 CFD calculation conditions
Simulation softwareTurbulence model
Inlet
OutletLateralUpper
ground surface
WallOpening
Flat terrain categories
Convergence criterion
Wind direction

STREAM ver.8 and ver.9
Standard k-εmodelPower low
（reference height 17.2m，wind speed 3.8m/s）
Natural inflow and outflow

Boundarycondition

free-slip
Generalized log low
Pressure drop model : aperture ratio 70%
Ⅲ（power low index a=0.20)
10-4
N, E, S, W, NE, NW, SE, SW

4. Effects of the building shape and the opening area on theventilation ratio
4.1 Outline(c) FL + 1.5 m (2 F)

The ventilation ratio of each plan was calculated from the inflow

Fig. 5 Calculation results of wind speed (plan number 3)

and outflow wind speeds through the openings, and therelationship between the ventilation ratio and the building shape
was analyzed. The influence of the positions of the entrance andstairs were analyzed, as was the effect of the opening area on theventilation ratio for each wind direction.

Plan of the objective house model is shown in the center, and thecalculation results corresponding to each inflow wind direction
are shown around the plan. If the wind flows out of the room
through the opening, the wind speed is denoted by a negativevalue.

- 84 -4.2 Relationship between the ventilation ratio and theopening area
Figure 6 shows the opening area in each plan for each winddirection. The opening area on the south side was very large inall plans, except for plans 3 and 22. The opening areas on theeast, west and north sides were small in all plans.
Figure 7 shows the relationship between the effective opening
area per unit floor area and the averaged ventilation ratio. The
effective opening area was calculated using the flow rate
coefficient α=0.65. The averaged ventilation ratio was calculated
for all wind directions (N, E, S, W, NE, NW, SE, SW) for eachplan. The averaged ventilation ratio increased with the increaseof the effective opening area per unit floor area, but there wasFig. 6 Opening area in each plan for every wind direction

2

not a high correlation between them. The value of R was small
because the averaged ventilation ratio in plan 3 was very large
against the effective opening area per unit floor area.
Figure 8 shows the relationship between the opening area ratio
facing the prevailing wind direction and the ventilation ratio.
The correlation of the ventilation ratio is high with the east, west
and north opening area ratios (which are small and in the range
0 % to 7.5 %) and low with the south opening area ratio (whichis large, more than 14 %). In typical detached houses with large
southern side opening areas, the ventilation ratio is larger in thecase of winds from the south. The results also confirmed that theinfluence on the ventilation ratio of the opening area facing theprevailing wind direction is greater than that of the position ofFig. 7 Relationship between effective opening area per unit floor

the stairs and the entrance orientation.

area and averaged ventilation ratio

5. Conclusion
The relationship between the ventilation ratio and the effective
opening area was analyzed for 12 typical real detached housemodels with various opening patterns. The ventilation ratio wasincreased when a ventilation circuit was formed in the house,
and the opening area facing the prevailing wind direction waslarge. However, the relationship between the ventilation ratio
and the opening area facing the south side was not confirmed,
because the south side opening area was much larger in all
objective typical detached house models. In the typical detachedhouses with large southern side opening areas, the ventilation

Fig. 8 Relationship between opening area ratio facing to main

ratio was larger in the case of the winds from the south. It waswind direction and ventilation ratio

also confirmed that the influence on the ventilation ratio by theopening area facing the prevailing wind direction was greater(2) J. Sakaguchi, S. Akabayashi, A. Hosono, S. Kubo, Study onthan that of the position of the stairs and the entrance orientation.

the Evaluation of Cross Ventilated Detached House: Part 15-16,
Summaries of technical papers of annual meeting Architectural
Institute of Japan, 753-756 (2007)

References(1) S. Akabayashi, Y. Sasaki, J. Sakaguchi, Y. Tominaga, Studyon of the Evaluation Method of a Cross Ventilated Building(Received Dec. 8, 2014, Accepted Dec. 27, 2014)

Performance, J. Environ. Eng., AIJ 568, 49-56 (2003)- 85 -