Wind Pressure and Air Flow in a Full-Scale Building Model...

International Journal of Ventilation Volume 2 No 4 ________________________________________________________________________________________________________________________

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343

Wind Pressure and Air Flow in a Full-Scale Building Model under Cross Ventilation

Takao Sawachi1, Ken-ichi Narita2, Nobuyoshi Kiyota3

Hironao Seto1, Shigeki Nishizawa1 and Yuumi Ishikawa1

1Building Research Institute, Ibaraki, Japan 2Nihon Institute of Technology, Saitama, Japan

3Hiroshima Institute of Technology, Hiroshima, Japan Abstract The observation of wind pressure acting on the wall and floor of a full-scale building model under cross ventilation was carried out. The measurement of air flow was also undertaken, and the existing prediction theory of the air flow rate, namely the orifice flow equation, including the discharge coefficient, was evaluated for its accuracy. At the same time, a method of predicting the discharge coefficient has been proposed and tested. In conclusion, a tentative relationship between the discharge coefficient and the difference of wind pressure coefficient across the opening has been developed. In addition, the background concerning the difficulty of using the orifice flow equation has been described. Key words: wind pressure, cross ventilation, full scale building model, air flow rate, wind tunnel, discharge coefficient, orifice flow equation. 1. Introduction Natural ventilation is a method used to control the thermal environment (e.g. it is used for cooling) as well as to dilute pollutants. For cooling, relatively high air flow between outdoors and indoors is needed in order to dissipate the indoor heat. Openable windows are usually used for this purpose. On the other hand, for pollution control, lower rates are usually required and methods may include mechanical systems, intentional openings, which are usually smaller than windows, and/or air leakage through cracks in the building envelope. As a design tool for the pollution control objective, network simulation programs have been introduced for specialized building ventilation engineers. In network programs, the orifice flow equation provides the fundamental relationship between pressure difference and air flow rate through openings. However, for the design of cross ventilation, to achieve larger ventilation rates, there are no fully reliable practical methods to quantify the ventilation rate. This is partly because there has been doubt about the accuracy of the orifice flow equation when applied to the calculation of wind-induced ventilation through larger openings. There is a hesitance to apply the orifice flow equation, because the inaccuracy has not been well defined and possible countermeasures have not been

clarified. Aynsley (1988) reviewed, quite systematically, the mechanisms that cause inaccuracy of the orifice flow equation. In Vickery et al. (1987), Akabayashi et al. (1989), Murakami et al. (1991) and Kurabuchi et al.(1991, 2002), a discharge coefficient in the orifice flow equation was quantified and compared with the usual value. Recently, in an international research collaboration in IEA ECBCS (Energy Conservation in Building and Community Systems), the air flow through large openings has been focused upon as an important phenomenon in the development of hybrid ventilation (Sandberg, 2002). 2. Objective Among factors having an influence on the instability of the discharge coefficient is the air flow field around the building, including the wind angle and the inclination of incoming air flow into the opening. The question is whether it is possible to predict the variation of the discharge coefficient. If it is not possible, the second question is how to deal with the variation of the discharge coefficient when the orifice flow equation, incorporated into the network simulation, is used to calculate the wind induced ventilation rate. To approach the answers to these important questions, experimental data

T Sawachi, K Narita, N Kiyota, H Seto, S Nishizawa and Y Ishikawa ________________________________________________________________________________________________________________________

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obtained in a wind tunnel with a full-scale building model (Sawachi et al., 1999 and Sawachi, 2002), are analysed and a practical direction for solving the problem is searched for. 3. Methods and Experimental Plan 3.1 Description of the Main Research Facility and the Building Model For the air flow and pressure measurement, as well as for the visualization of cross ventilation in buildings, a wind tunnel was specially designed so that a full-scale experiment could be undertaken. Its section and plan are shown in Figure 1. The wind speed range in the working section is 1.0-5.0 m/s, and its distribution was checked to be close to

uniform before the construction of the building model (Sawachi et al., 1999). The turbulence intensity is less than 5% at the inlet to the working section, and the temperature can be controlled below 25 °C. The dimensions of the present building model are 5.6m x 5.6m x 3.0m, and its blockage ratio is 12% of the cross sectional area of the working section, and 34% of the inlet area. Due to the uniform air velocity in the inlet of this wind tunnel design and to the higher blockage ratio, the wind pressure on the building model should be different from that obtained in conventional wind tunnels having a vertical profile of wind speed with a much lower blockage ratio. In this project, the measurement at full-scale and the reproducibility of the wind pressure condition were critical requirements. As a future necessity for the wind

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(b) 60 measurement points on the facade

(c) 66 measurement points on the façade with anopening

Figure 2. Positions of the pressure measurement points on the roof and wall.


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345

pressure coefficient itself, it is planned to undertake conventional type wind tunnel measurements incorporating scale building models and surrounding structures. 3.2 Wind Pressure on the Wall and Floor under Cross Ventilation The wind pressure was measured using differential pressure transducers (MKS Baratoron 220C) and scanning valves (SCANIVALVE), with hard plastic tubes connecting from behind the pressure taps. The pressure taps themselves comprised short aluminium tubes buried in the surface of the wall, etc. without any protrusion, connected to the scanning valves. Figure 2 shows the positions of the pressure taps on the roof and the wall. When openings are installed on the wall, some additional pressure taps are installed in the periphery of the openings. Figure 3 (a) shows the positions of the pressure taps on the floor inside the building. When the simultaneous measurement of wind pressure at different positions is necessary, the number of taps is reduced to the number of the differential pressure transducers available as shown in Figure 3 (b). In one experiment, the ten pressure taps P1, P6, P7-P14 were used, and in another experiment the six pressure taps P1-P6 were used.

3.3 Three Dimensional Air Flow Inside the Building and its Visualisation Two types of ultra-sonic three-dimensional anemometers were used for measuring air flow; these were:- KAIJO type WA-390 (having a response time of 0.5 second) and KAIJO type DA-600 (having a response time of 0.05 second). Both types have a common type probe, with dimensions as shown in Figure 4. The horizontal location of the measurement points is shown in Figure 5 (a) for inside and near the building model. Measurements were undertaken at five different heights, i.e:- 230 mm, 710 mm, 1,190 mm, 1,670 mm and 2,100 mm above the floor. Figure 5 (b) illustrates the locations of the air flow measurement points outside the building model. Data acquisition was undertaken at 10Hz for at least three minutes. More direct observation was obtained by visualization using a SPECTRA-PHYSICS STABILITE 2017 and NEOARK CORP. FIBER HEAD laser light sheet combined with smoke generation. Two different ways of smoke generation were tried. The first was a “decay method”, in which the openings in the enclosure were closed and filming started after filling the interior space with smoke. The second was a “constant generation method”, in which the smoke was generated constantly and sourced mainly

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Figure 3. Measurement points for pressure distribution (a) and for correlation among pressures and velocity in the openings (b).


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346

in front of the opened inflow opening. In both methods, the laser light sheet was emitted through four lenses, which were installed at each corner of the room (Figure 3 (b)). 3.4 Three Dimensional Air Flow in the Openings and its Visualization The same anemometers were used for the measurement of air flow in the openings, as shown in Figure 6 (a). Figure 6 (b) represents typical air flow measurement points in the opening which is 860 mm wide and 1,740 mm high. Four anemometers attached to a pole were manually traversed to complete the three-dimensional air speed measurement at 48 points in the opening as shown in Figure 6 (b). The length of the vector along the X-axis, which is normal to the opening plane, was multiplied by its representing opening area to give a flow rate in cubic metres per hour. All flow rates for divided areas were summed to give the total flow rate through the entire single opening. Although all 48 vectors were not measured simultaneously, the average air flow vectors, which were calculated from instantaneous data at 10 Hz for three minutes, appeared to be sufficiently accurate to calculate the total flow rate through the opening.

4. Results 4.1 Wind Pressure Distribution on the Wall and the Floor under Cross Ventilation The wind pressure was measured as a pressure difference from the reference static pressure near the top of the wind tunnel’s inlet (see Figure 1), and was expressed as a wind pressure coefficient with a dynamic pressure at a reference wind speed, 3m/s, as a denominator. The wind pressure coefficients on the opening were determined by using the pressure data at six points located inside the area for the openings, as shown in Figure 2 and Figure 6 (b). Figure 7 illustrates the pressure distribution on the walls and the floor of the building model for two openings located diagonally to each other. 4.2 Air Flow Inside and Around the Building The two-dimensional vectors on the horizontal plane at the centre height of the walls (1,190 mm above the floor) for different wind angles are shown in Figure 8. The wind pressure coefficients of points adjacent to the openings are shown at the top of each figure. In addition, the correlation coefficients between the pressure of different points are shown at the bottom.

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(a) Horizontal Locations of 3D Airflow Measurement inside and near the Building Model

(b) Horizontal Locations of 3D Airflow Measurement in the Working Section of the Wind Tunnel

Figure 5. Measurement points for the 3D air flow.

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Figure 6. Measurement of air flow in openings and ventilation rate.


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0° 15° 30°

45° 60° 75°

90° 105° 120°

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Figure 7. Distribution of wind pressure coefficient on the wall and floor under cross ventilation.


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FIGURE-9 AIRFLOW INSIDE THE BUILDING AND ITS SURROUNDINGS, Figure 8. Air Flow inside the building and its surroundings


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349

4.3 Air Flow in the Openings The same three-dimensional anemometers as those used for the measurement inside and near the building, were used for the measurement of air flow through the opening, especially for the measurement of the ventilation rate. The ventilation rate for different wind angles is shown in Figure 9. It can be said that the main cause of changes in flow rate is the change of the wind pressure acting on the openings, but there is another factor, which will be discussed later. The ventilation rate was based on the sum of the flow rate measured through 48 areas in the opening. For each of the 48 areas, an instantaneous air speed at a representing point was measured for 180 seconds at 10 Hz. It was found that both inflow and outflow could coexist at a single opening. Moreover, at some points, the direction of the flow could change from time to time. Figure 10 shows such a situation for all wind angles. For example, in the inflow opening for a wind angle of 120°, in the columns Y1 to Y3, stable inflow was observed, while in column Y6, stable outflow was observed. Figure 11 shows the time change of the vectors over two seconds at the medium height of the openings. 5. Analysis 5.1 Calculation of the Discharge Coefficient by using the Difference between the Wind Pressure on the Area of the Closed Opening and on the Floor for Cross Ventilation The wind pressure coefficient is usually measured by using bluff scale models of buildings. In network simulation programs, these coefficients are used as input data for calculating air flow and internal static

pressures, etc. A similar procedure was tried in order to identify discharge coefficients by analysing the experimental data, although the adequacy of the approximation of using the exterior wind pressure in a building model without openings could be an important issue. The adequacy seems to depend

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Figure 10. Air flow directions at 48 points for 3D instantaneous air velocity measurement in the openings.

‘I’ - constant inflow, ‘O’- constant outflow and ‘B’ - points at which a bi-directional air flow was observed. See Figure

6b for locations of measurement points.

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350

upon the size of the openings that are actually present. In the present experiment, the ratio of the opening area to façade area was around 9%. When network calculations are made of air leakage through narrow cracks, i.e. representing only a very small fraction of total envelope area, the adequacy of using wind pressure coefficients based on bluff buildings has not yet been questioned. The effect of openings on the change of wind pressure on their surrounding area can be found in Figure 7. For

example, when comparing the pressure distribution on the windward façade of 0° wind angle with that on the windward façade of 90° wind angle, the information about the change can be obtained. Further comparison is possible by using pressure distribution data based on the totally bluff building model (Figure 12). Except at the vicinity of the openings, no clear change of wind pressure coefficient was found between Figures 7 and 12.

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beside the openings (for wind angles 0 to 75º).


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351

Table 1 gives the results for the determination of wind pressure coefficients and discharge coefficients for different wind angles. Figure 13 shows the average wind pressure coefficient on the exterior wall in the area of the openings and on the floor under conditions of cross ventilation. Since measurements on the floor were divided into four quadrants, the results for each quadrant are presented and are very similar to each other. Figure 14 shows the calculated discharge coefficients based

on the wind pressure results of Figure 13 and the ventilation rate given in Figure 9. The horizontal line represents the discharge coefficient of a scale model of the opening, which was measured by the conventional procedure usually used for components of ventilation systems, such as vents, ducts, terminal devices, etc. As shown in Figure 14, the discharge coefficient of the opening (300 mm x 150 mm x 19 mm), which was attached to the pressurized chamber, is 0.63. As for the inflow

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beside the openings (for wind angles 90 to 165º).


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0° 15° 30°

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Figure 12. Distribution of wind pressure coefficient on the wall without openings and on the roof. (To be compared with Figure 7 for the building with openings).

Table 1. Experimental results on wind pressure coefficient and discharge coefficient for different wind angles.

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opening, at wind angles of 0°-30°, the discharge coefficient is close to that of the reference discharge coefficient of 0.63. A minimum value of 0.26 occurs at wind angles of 90° and 105°. At 15°, the discharge coefficient exceeds the reference value by 0.05. The average in the range between 0° and 165° is 0.44. As for the outflow opening, for wind angles of 0°-45°, 150° and 165°, the discharge coefficient is close to the reference value. A maximum value of 0.74 is seen at a wind angle of 60°. The average in the range between 0° and 165° is 0.51. 5.2 Factors Influencing the Discharge Coefficient Depending on wind angle, the ventilation rate varied from 400 m3/h (90°) to 9,700 m3/h (15°), even although the outside wind speed was kept constant at 3 m/s. The relationship between the ventilation rate (expressed by the average air speed in the opening divided by the outside wind speed) and the discharge coefficient is shown in Figure 15. For the inflow opening, there is a clear linear relationship, expressed by Equation 1.

α=0.75k+0.2 (1)

where α is the discharge coefficient and k is the average air speed (m/s) in the opening divided by the outside wind speed (3 m/s). Even at the maximum value of k of 0.6 in the present experiment, the decrease seems to begin. For the outlet opening, the discharge coefficient does not decrease in the range where k is above

0.25. Below this range, the following tentative relationship, given by Equations 2 and 3 could be supposed, although there is a lower discharge coefficient found at a wind angle of 75°:

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��600,3

�600,3=== (3)

where ∆Cpi is the difference of the wind pressure coefficient across the opening, i, Q is the flow rate in m3/h, A is the opening area in m2, and υ is the reference outside wind speed in m/s. As the ratio of the average air speed in the inlet opening to the outside wind speed decreases, the discharge coefficient of the inlet opening decreases linearly. On the other hand, the discharge coefficient of the outlet opening does not decrease in the range of k > 0.25. There is no neighbourhood building around the building model in the present experiment, and the ratio of the air speeds is likely to represent the relative strength of the inflow relative to the outside air speed in the upstream region. As for the outflow opening, if it is assumed that the air speed inside the building is proportional to the ventilation rate, the ratio of the average speed of the outflow opening to the air speed in the upstream region in the building is stable. This can be the reason why the discharge coefficient for the outflow opening does not decrease when k is above 0.25. When k becomes lower than 0.25, the outside air speed in the downstream area of the outflow opening might be influential on the discharge

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

k, the ratio of the average air speed in the opening to the outside wind speed

ςDis

char

ge C

oeff

icie

nt

Inflow Opening

Outflow Opening

α=0.75k+0.2 (1)

α=1.7k+0.2 (2)

Figure 15. Discharge coefficients of the inflow and outflow openings and their relationship with “k”.

�


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354

coefficient of the opening. If the building is surrounded with other buildings, the reference outside wind speed does not necessarily represent the air speed in the upstream area of the inflow opening nor that in the downstream area of the outflow opening. In such a case, the air speed near the openings might be much lower than the reference outside wind speed. In such a situation, the discharge coefficient might stay higher even when k is less than 0.6 for the inflow opening and greater than 0.25 for the outflow opening. The relationship between the difference of wind pressure coefficient across the opening and the discharge coefficient, can be obtained by substituting Equation 3 into Equations 1 and 2, as follows:

1�75.0-1

2.0�

pC= (4)

2�.71-1

2.0�

pC= (5)

Figure 16 shows the relationship between the discharge coefficient, α, and the square root of ��pi. Equations such as Equations 4 and 5 can be useful when they are applied to network programs to calculate the air flow pattern and pressures by iteration. In the iteration process, the discharge coefficient is changeable as well as the internal pressure and the flow rate, before convergence. In Figure 8, the pattern of air flow from the upstream area of the inflow opening to the

downstream area of the outflow opening is presented. The inclination of the inflow against the façade of the building seems to be determined by the outside air flow direction. At 15°, air inflow comes through the opening most vertically, while at other wind angles the inflow is usually inclined to the plane of the opening. This means that even if the shape of the opening does not change the shape of the air flow before, through, and after the opening changes. In the present series of experiments, the shape of the air flow pattern around the opening was found to be affected by:- the wind angle, the existence of the walls including the wall with the opening itself, and by the building. More detailed information about the air flow in the opening is given in Figure 11, where fluctuating vectors of the air flow are shown for a number of two-second records. There are three types of points, namely, “constant inflow”, “constant outflow” and “bi-directional”. At the bi-directional points, the air flow direction (inflow/outflow) changes over time. It seems natural that the orifice flow equation should lose its accuracy, to some extent, when the air flow direction through the opening is not stable. In that case, the average of the air flow rate might not necessarily represent the ventilation rate due to the air exchange through a single opening. Figure 10 shows the air flow for openings at 48 locations of air flow measurement. At wind angles of 75°-105°, almost all points are bi-directional points. 5.3 Distribution of Wind Pressure on the Floor Figure 7 contains the distribution of the wind pressure on the floor in the building. There are some

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1, Squareroot of the Difference of Wind Pressure Coefficient

, Dis

char

ge C

oeff

icie

nt

Inflow Opening

Outflow Opening

)'1(C�75.0-1

2.0=�

p

)'2(C�.71-1

2.0=�

p

pC�

Figure 16. Relationship between discharge coefficient and wind pressure coefficient.

�


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355

parts with slightly higher or lower wind pressure compared with their surrounding area. For example, at a wind direction of 15°, there is an area of slightly higher wind pressure on the right hand side of the inflow opening to the end of the room opposite to

the inflow opening. Although these deviations in floor pressure from the average is almost negligible, compared to the pressure difference across the openings, it seems necessary to describe the change in floor pressure and to identify its causes. This

12

3

67

8 11 12

54

910

Wind Angle 0°

12

10

3

54

6

87

911

1312

Wind Angle 15°

12

9

3

54

6

87

1011

12

1 2 345

687 9

10111213

14

Wind Angle 165°

Wind Angle 60°

X3

X3’

0°

0.25

0.30

0.35

0.40

0 1 2 3 4 5 6 7 8 9 10 11 12 13Point No.

mm

Aq

Along the Flow Tube

Average of Overall Floor (96 Points)

15°

0.25

0.30

0.35

0.40

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14Point No.

mm

Aq

A long the Flow Tube


60°

-0.20

-0.15

-0.10

-0.05

0 1 2 3 4 5 6 7 8 9 10 11 12 13Point No.

mm

Aq

Along the Flow Tube


165°

0.05

0.10

0.15

0.20

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Point

mm

Aq

Along the Flow Tube

Average of Overall Floor (96 Points )

Figure 17. Transition of the floor wind pressure along the flow tube at wind angle 0°, 15°, 60° and 165°.


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knowledge is useful when one is choosing the location of pressure measurements in other situations such as in field measurements. Figure 17 shows the transition of the wind pressure on the floor along the flow tube compared to a horizontal line representing the average pressure on the floor. At 0°, for points 2-4, slightly higher pressure occurs and at points 7,8 and 10, slightly lower pressure occurs. At 15°, in the centre of the flow tube just after entering the opening, points 3-5 show a slightly lower pressure and around the location where the flow tube changes its direction to the left, higher pressure can be observed at points 7-9. Figure 18 shows the two-dimensional vectors for the section X3-X3’ for a wind direction of 15°. After entering the opening, very vertically, the air goes straight and strikes the end wall opposite to the opening. The flow near the wall turns back toward the inlet opening along the floor surface and meets the incoming air on the way around at points 7-8. At 60°, the flow tube hits the wall just before the outflow opening around points 9-11, where the pressure increases slightly. Before these points, slightly lower pressure is observed at points 3-6. Though these local changes of the pressure can be observed, there is no measurable consistent decline of the wind pressure on the floor along the flow tube. It seems that the total pressure does not decrease sensitively in this cross-ventilated room. If the room was narrow enough, such as a duct, there would be friction loss of the total pressure.

6. Conclusions 1) The discharge coefficient in the orifice flow

equation for openings tends to decrease depending upon the ratio of the air speed in the openings to the outside velocity. It seems natural that the air flow resistance of openings, with

identical shape, should change depending upon the shape of the air flow before and after the opening. Also, the shape of the air flow is affected considerably by the outside air speed and direction. The ratio of the air speed in the opening to the outside velocity seems to be the factor that causes the shape of the air flow through the opening to be influenced by the outside air flow. However, the mechanism has not yet been clarified and needs further investigation. The characteristics of flow at the inlet opening and outlet opening showed differences. In only a few conditions was the discharge coefficient found to be larger than 0.63 (used as a reference value) measured under static pressure difference.

2) It seems that it is natural for there to be a

limitation for the accurate prediction of the discharge coefficient in cross ventilation through relatively large openings. It is because, in a single opening, a constant coexistence of both inflow and outflow was observed. Moreover, at some points, the direction of air flow was found to change over time. This was especially observed for wind directions between 75°-105°, as shown in Figures 10 and 11.

3) However, a tentative relationship between the

discharge coefficient and the difference of wind pressure coefficient across the opening, has been drawn, as shown in Figure 16. Although the relationship seems to be promising for application in network simulations, additional data and validation is necessary.

4) The measurement result of the floor wind

pressure shows considerably even distributions except for the area near the place where the flow tube hits the wall or where the flow changes direction.

Figure 18. Two dimensional vertical vectors on section X3-X3’ (see Figure 16) at wind angle 15°.


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Acknowledgements This research is funded by Grants-in-Aid for Scientific Research of Japan Society for the Promotion of Science, as No. 14205086 of Category “A”. Building Research Institute and Ministry of Land, Infrastructure and Transportation funded for the construction of the wind tunnel used for this research, and the related research projects. These are gratefully acknowledged. References Aynsley RM: (1988) “A resistance approach to estimating air flow through buildings with large openings due to wind”, ASHRAE Transactions, pp1661-1669, Akabayashi S, Murakami S and Kato S, Hasegawa K and Kim E: (1989) “Study on pressure loss coefficient at opening set in housing wall”, Summaries of Tech. Papers of Annual Meeting, Architectural Inst. of Japan, Vol. D, pp629-632. Etheridge D and Sandberg M: (1996) “Building ventilation - theory and measurement”, John Wiley & Sons. Kurabuchi T and Kamata M: (1991) “Numerical simulation of the combined internal and external airflow for cross ventilation of buildings by means of the multi-mesh method”, J. of Architectural Plan. and Environ. Eng., AIJ, (426), pp1-11.

Kurabuchi T, Ohba M, Fugo Y and Endoh T: (2002) “Local similarity model of cross ventilation, Parts 1 and 2”, Proceedings of ROOMVENT 2002, September 8-11, Copenhagen. Melikov AK and Sawachi T: (1992) “Low velocity measurements: Comparative study of different anemometers”, Proceedings of ROOMVENT ’92, September 2-4, Aalborg. Murakami S, Kato S, Akabayashi S, Mizutani K and Kim YD: (1991) “Wind tunnel test on velocity-pressure field of cross-ventilation with open windows, ASHRAE Transactions, 97, Part.1, pp525-538. Sandberg M: (2002) “Wind induced airflow through large openings: Summary - principles of hybrid ventilation” (Edited by Per Heiselberg), Aalborg University. Sawachi T, Kiyota N and Kodama Y: (1999) “Air flow and wind pressure around a full-size cubical building model in a wind tunnel”, Proc. of the PLEA ’99 Conference, 2, pp899-904. Sawachi T: (2002) “Detailed observation of cross ventilation and airflow through large openings by full scale building model in wind tunnel”, Proceedings of ROOMVENT 2002, September 8-11, Copenhagen. Vickery BJ and Karakasanis C: (1987) “External wind pressure distributions and induced internal ventilation flow in low-rise industrial and domestic structures”, ASHRAE Transactions, pp2198-2213.


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