Experimental investigation and CFD analysis of cross-ventilated flow through single room detached house model

Building and Environment 45 (2010) 2723e2734

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Experimental investigation and CFD analysis of cross-ventilated flow through single room detached house model Tomohiro Kobayashi a, *, Mats Sandberg b, Hisashi Kotani c, Leif Claesson b a

Ritsumeikan University, Shiga, Japan University of Gävle, Gävle, Sweden c Osaka University, Osaka, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 March 2010 Received in revised form 1 June 2010 Accepted 1 June 2010

Cross-ventilation is a complicated flow problem and difficult to control because wind exhibits a large degree of variation. The paper focuses on three items: a) to clarify and understand some of the basic characteristics of airflow as the influence of the opening size on the windward vortex and the leeward wake; b) to explore what information about the flow above the ground can be retrieved from pressure measurements on the ground; and c) to explore the accuracy of CFD. To meet these objectives, wind tunnel tests and CFD analyses were carried out. The studied object was a detached- house model provided with two openings. The size of these openings was changed in a wide range from narrow cracks to large openings. In the experiments, pressure measurements on the ground and PIV measurements were made. The internal flow was visualized with the sand erosion method. Pressure measurement on a floor surface is a relatively easy and an inexpensive method. In this experiment, the windward and leeward areas in particular were investigated to understand flow pattern and to confirm correspondence between flow pattern and recorded pressure on the ground. Those measurements show the difference in flow at different size openings in terms of the vortex on the windward side and the wake. When the size of the opening exceeded a certain value the near wake on the leeward side disappeared and on the windward side the vortex disappeared. The pressure distribution, flow pattern, and velocity profile are shown and compared between measurement and CFD. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Cross-ventilation Wind tunnel PIV Pressure measurement CFD Reynolds stress model

1. Introduction With the aim to conserve nonrenewable energy reserves, the use of renewable energy sources to control the internal environment has been attracting practical and academic attention. Windinduced cross-ventilation has been a beneficial method to moderate the hot and humid environment in summer in Japan. To design a well functioning cross-ventilation system, flow rate, flow pattern, and flow movement in a room must be considered in the design phase. However, a method for predicting flow rate has not yet been well established. For example, the empirical method using orifice equation cannot be applied to large openings located in series. This conventional method, based on Bernoulli’s principle where pressure loss is added, considers a single flow path. In crossventilation phenomenon, however, a flow has a “choice” (flow through or past), which is not taken into account in the orifice * Corresponding author. Department of Architecture and Urban Design, School of Science and Engineering, Ritsumeikan University, 1-1-1, Noji-Higashi, Kusatsu, Shiga, 525-8577, Japan. Tel.: þ81 77 599 4185. E-mail address: [email protected] (T. Kobayashi). 0360-1323/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.06.001

equation. Moreover, the use of a wind pressure coefficient obtained from a sealed building model is also questionable. The authors have shown that the flow rate could be about 65% of actual one when the conventional prediction method is applied to the large openings [1]. A number of studies on an improved prediction method of flow rate have been conducted. Ishihara [2] explained that the resistance of each opening cannot simply be summed up. He introduced Interference Coefficient to evaluate the resistance coefficient of a building. Kurabuchi, Ohba et al. [3] postulated that the discharge coefficient be determined based on a parameter of dimensionless internal pressure (Local Dynamic Similarity Model). Kotani and Yamanaka [4,5] proposed a prediction method which synthesize the vectors normal to the opening and parallel to a sealed building wall (Vector Addition Model). Sandberg [6,7] illustrates the concept of catchment area of the stream tube. Sandberg [6] showed that the airflow through an opening can be more than 100% of the flow far upstream through a corresponding opening. This example highlights that we do not fully understand all the phenomena involved in wind driven flows through openings. We have coupled external and internal flow and this coupling must be understood before we can fully understand wind-driven natural ventilation. Murakami

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et al. [8], Kato [9] applied a concept of energy (rate) balance for diverging and converging pipe flow junction (Guffy and Fraser [10]) to the airflow field by considering a virtual stream tube. Axley and Chung [11] formulated a multi-zone flow network model based on almost the same basic principle. These prediction concepts regarding a mechanical energy rate balance seem to be an appropriate approach because they are based on actual phenomena. The authors also aim to establish an improved prediction method based on power balance of a selected stream tube and have analyzed the stream tube by using a rectangular room model (See Kobayashi et al. [1,12]). However, microscopic details of cross-ventilation flow are quite complicated and have not been sufficiently clarified. Therefore, it is necessary to understand the fundamental phenomena first. This work involves both wind tunnel experiments and CFD simulations. Wind tunnel experiments were conducted with three different experimental techniques: static pressure measurements on the ground plane, 2-D particle image velocimetry (PIV) measurement and visualization with the sand erosion method. Static pressure measurements can be made relatively easily, and it could be beneficial to know what kind of information one can obtain from this kind of measurement. PIV is an analysis tool of high cost that enables one to see a detail of the airflow pattern (vector field) and therefore makes it possible to explore the relationship between flow pattern and pressure distribution on the ground plane. Employing the sand erosion method by using semolina powder as tracer made it possible to visualize the internal airflow pattern on the ground. CFD simulations were also conducted because these can provide complementary information such as spatial pressure/velocity distribution and turbulence statistics. Another advantage of CFD is that it makes it possible to analyze a stream tube because streamlines can be predicted. Since the simulated results need to be validated before analyzing the power transportation inside a selected stream tube which is planned in the future, the accuracy of the calculation is studied in this paper by comparing with experimental results. 2. Model and cases The basic configuration of the studied model was a pitched roof detached house without any openings. The basic configuration was then amended by introducing two openings of the same size located opposite to each other as shown in Fig. 1. The size of openings was changed and the parameters for these changes are shown in Table 1. The first two cases of small openings were only studied experimentally. The absolute porosity (4abs) indicates

Table 1 Analyzed Model Condition. Opening Width w [mm]

Height h [mm]

0 4 7 17 80 80 80 80 80 80 80 80

0 4 7 17 20 40 50 55 60 65 70 80

Absolute Porosity [%]

Net Porosity [%]

h/H []

w/W []

pffiffiffi t= A []

pffiffiffi L= A []

0 0.05 0.14 0.83 4.61 9.23 11.54 12.69 13.84 15.00 16.15 18.46

0 0.05 0.14 0.84 4.83 10.17 13.05 14.53 16.06 17.65 19.26 22.64

0 0.02 0.03 0.08 0.09 0.18 0.23 0.25 0.27 0.30 0.32 0.36

0 0.02 0.03 0.08 0.39 0.39 0.39 0.35 0.39 0.39 0.39 0.39

e 2.25 1.29 0.53 0.23 0.16 0.14 0.14 0.13 0.12 0.12 0.11

e 66.25 37.86 15.59 6.63 4.68 4.19 4.00 3.82 3.69 3.54 3.31

opening area (Ao) divided by facade area (A). The facade constitutes the blockage to the approaching flow and generates the wake whereas the opening area controls the flow through the model. The net blockage caused by the facade is (14abs) and the ratio between the inlet area for air and blockage by the facade is the net porosity 4net ¼ 4abs/(14abs). For small openings the magnitude of the net porosity is close to the absolute porosity. We can expect porosity to be an important parameter for the flow characteristics in a wake region. It is believed that the momentum of discharged flow disperses the near wake generated on the leeward side if this parameter exceeds a certain value. The model length divided by a characteristic linear dimension of the opening (L/OAo) is a parameter which in our case is between 66 and 3. It is an important parameter for the type of internal flow that will occur. If this parameter is large (>6), a jet may develop which is a boundary layer type of flow with a pressure inside the jet close to ambient pressure. The inverse, W/w, of the relative width of the opening is a parameter showing the maximum possible lateral expansion of the flow entering the house. The inverse varies from 50 to 2.6 from the smallest to the largest opening. If this parameter is large, the primary flow entering the room may be dissolved before it reaches the opening on the opposite side. A parameter for the possible maximal vertical spread is H/h which varies from 50 to 2.8. However, in the cases studied in this paper the inflow through the windward opening has a downward component, as will be shown further on. This seems to impair an upward directed vertical expansion of the flow internally. This behaviour of the flow is also confirmed by CFD predictions not shown here. Therefore this parameter seems to be of minor importance.

Fig. 1. Studied House Model.

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Fig. 2. Layout of Pressure Taps on Pressure Plate.

Wall thickness of an opening divided by a characteristic linear dimension of the opening (t/OAo) is a measure for the sharpness of the opening. A large value for this parameter implies that we have a crack in which a laminar flow might occur (Couette type of flow). 3. Methodology 3.1. Pressure measurement

3.2. PIV measurement Details of the airflow patterns can be different between crossventilation through large openings and infiltration through cracks. To investigate differences in these flow patterns due to the size of

1200

1200

1000

1000

800

800

Height [mm]

Height [mm]

A closed-circuit type of wind tunnel (measuring section: length 10 m, width 3.0 m, and height 1.5 m) belonging to University of Gävle was used for the pressure measurement on the ground plane. The test model was located on the “pressure plate”, see Fig. 2, and the wind direction was perpendicular to the openings. The pressure plate with dimensions of 800  800 mm was provided with 400 pressure taps with clearance of 37 mm between measuring points. The static pressure on the ground plane was measured for 30 s with

sampling frequency of 10 Hz. As the reference pressure, a static pressure far upstream in the wind tunnel was measured by a pitot tube. Test model was exposed to a free flow of 19.0 m/s to obtain sufficiently high pressure on the ground to make it possible to accurately record the pressure. To facilitate achieving this air speed the wind tunnel was run without any roughness elements. Although this naturally causes a discrepancy from the real boundary layer flow profile, this discrepancy is assumed not to be important for the scope of this study. Fig. 3 shows the profiles of mean velocity and turbulence intensity for approaching flow obtained by a hot wire anemometer in an empty wind tunnel.

600

600

400

400

200

200 0

0 0

5

10

15

20

25

0

10

(1) X-Component of Velocity

20

30

Turbulence Intensity [%]

Velocity [m/s]

(2) Turbulent Intensity

Fig. 3. Condition of approaching flow.

40

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For windward measurement

Leeward

Windward

1,500

Double Pulsed Nd : YAG Laser Approach flow(10m/s) Smoke generator

Model

For leeward measurement

Fig. 4. Experimental Set-up for PIV measurement on the windward/leeward side of a house model.

the opening, the velocity distributions on the windward and leeward side were measured using a PIV system. Fig. 4 illustrates the experimental set-up. A smoke generator (SAFEX, FOG 2001) was located on the leeward side of the model. The smoke could approach the model through the closed-circuit wind tunnel. A CCD camera (DANTEC, HiSense MK II) was located on the side of the test model. A double-pulsed Nd:YAG laser (New Wave Research, Solo PIV III-15) to generate light sheet was located on either the windward or leeward side, depending on the measured region. These two devices were synchronized and controlled by PC software (DANTEC FlowManagerÔ 4.0). The laser sheet was oriented along the centre plane of the model. The time interval of the laser double pulse was set as 30 ms. The sampling frequency was 5.0 Hz, and the sampling time was 24 s, i.e. 120 pairs of photographs were taken for one measurement. The Fast Fourier Transform (FFT) cross-correlation method (Willert et al. [13]) was used for the analysis. By detecting correlation peak within each interrogation window and averaging instantaneous velocity components calculated from spatial shift within a known time interval, a 2-D time-average velocity distribution was obtained. Fig. 5 shows the measured region for the windward and leeward side. For the windward measurements, the size of the measured area was 171.8 mm long by 128.8 mm high. For leeward measurements, on the other hand, it was 282.2 mm long by 211.7 mm high. Although the pressure measurements were conducted under a free flow of 19 m/s, the PIV measurements were done under a flow of 10 m/s to facilitate measurement, assuming the similarity of flow due to the high Reynolds numbers. Table 2 gives a summary of the PIV measurement.

measured profiles of velocity and turbulence intensity shown in Fig. 3, and turbulent length scale was set as 0.07 times the hydraulic diameter of the wind tunnel (L ¼ 140 mm). According to those quantities, the inlet boundary condition of turbulent energy dissipation rate (3) was given as follows;

3 ¼ CD3=4

k3=2 L

(1)

where, k is the turbulent kinetic energy given based on mean velocity (U) and turbulence intensity (I) as;

k ¼

3 ðU  IÞ2 2

(2)

The inlet boundary conditions of Reynolds normal/sheer stress are given as;

2 kði ¼ 1; 2; 3Þ 3

(3)

u0i u0j ¼ 0ði; j ¼ 1; 2; 3Þ

(4)

u0i u0i ¼

At the outlet boundary, the gauge pressure was fixed as 0 Pa. Steady state calculations were done using the SIMPLEC algorithm with a QUICK discretization scheme for advection term of the governing equations. For the turbulence model, a Reynolds stress model was used, which basically follows the LRR model (Launder et al. [14]). Here, as a turbulent diffusion term in the transport equation of Reynolds stress, the isotropic eddy viscosity approximation was adopted to keep stability the calculation. Table 4 shows the number of cells for each case.

3.3. CFD analysis Since more detailed analyses of the airflow are needed to investigate power transportation in future work, the wind tunnel test was simulated by CFD analysis. A commercial program (Fluent 6.1) was used. Fig. 6 depicts the calculated domain and mesh layout. To reduce the computational load, only half of the domain was modelled assuming a symmetry plane. Table 3 gives the summary of CFD analyses. The inlet boundary conditions were based on the

4. Result and discussion 4.1. Wind pressure coefficient As basic information, Fig. 7 shows the wind pressure coefficients on the gable walls obtained from measurement and CFD for the sealed building model case. Static pressures along a centre line on both windward and leeward walls were normalized by dynamic pressure of approaching flow. The stagnation point is located at a height of 160 mm (73% of the model height) on the windward

Table 2 Experimental condition for PIV Measurement.

Fig. 5. Measured Area on the Windward/Leeward Side by using PIV.

Camera Frame Size

1344 pixel  1024 pixel

Interrogation Area Overlap Total Number of Vectors Time Interval of Pulse Sampling Frequency Sampling Time Laser Output Energy

64 pixel  64 pixel 75% 4941 30 ms 5.0 Hz 24 s 50 mJ/Pulse

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Fig. 6. Calculated domain in CFD analysis.

facade. The wind pressure distributions are relatively well simulated by CFD, though they are slightly overestimated. 4.2. What information does recorded pressure on the ground provide? It is our hypothesis that the static pressure on the ground inside and outside the building reflects the characteristics of airflow inside a model building. One can expect that the degree of correspondence depends on the type of flow. If there is a thin boundary layer type of flow, then the pressure inside the flow is the same as in the ambient and we can expect a close correspondence. We can expect this to occur with a small opening. On the other hand if we have a flow where pressure in the flow is governed by the flow itself, we will have pressure gradients both along the direction of the flow and vertically. Then we can hope that the pressure on the ground reflects the pressure variation but the level of the pressures differs. The above hypothesis is confirmed for the cases shown in Fig. 8 which shows the static pressure distribution along a centre line of the model on/above the ground obtained from CFD analysis for the absolute porosities of 4abs ¼ 0.83% and 18.46%. The accuracy of simulated pressure on the ground plane will be shown in the following section. Obtained pressures are normalized by dynamic pressure of approaching flow. For the smaller opening, the static pressure at the height of 10 mm is lower than others because of the contraction of the flow when flowing through the opening into the model. In general, however, static pressure is almost uniform inside the model because the flow is a well-developed jet inside the model. Therefore, it seems possible to replace the spatial distributions of static pressure outside of the jet by those on the ground. In the cases with larger openings, the static pressure on the ground differs from that above the ground. The pressure above the ground is lower than that on the ground. However, the variation in the pressure above the ground is quite similar to that on the ground. Table 3 Summary of CFD analysis. Program

FLUENT 6.1

Finite difference scheme

QUICK

Algorithm

Steady state (SIMPLEC)

Boundary condition

Inlet

U : Shown in Fig. 3 k : Based on velocity and intensity 3 : Based on k and L 

The number of grids Turbulence model

 Turbulent Intensity : Shown in Figure 3 Length scale ðLÞ : 140mm Outlet Gauge Pressure : 0 [Pa] Walls Wall : Standard wall function (Generalized log law) Symmetry : Free slip from 802,874 to 1,069,544 (shown in Table 4) Reynolds Stress Model

From these results, we can conclude that the pressure recorded on the ground can reflect the variation of the pressure above the ground. Therefore, pressure measurements on the ground provide a useful method for classifying the characteristics of the flow through a building. 4.3. Pressure distributions on ground Fig. 9 presents the extracts from measured pressure distributions over the pressure plate for the cases of smallest and largest opening. The vertical axis indicates the pressure coefficient which is normalized by a dynamic pressure of approaching flow. Both pressure landscapes show a peak in front of the windward opening. This is due to the deceleration of flow which is impinging on the gable wall; i.e. dynamic pressure (kinetic energy density) is partially converted into static pressure (potential energy density) according to Bernoulli’s principle. Comparing the maximum values, a lower peak can be seen in the case of the larger opening because a larger dynamic pressure remains. On the side regions of the house model, the minimum pressure caused by acceleration of the flow and flow separation can be clearly seen. Comparing the internal pressure, small openings provide more uniform pressure distribution, while the pressure varies in the case of large openings. When the openings are small, we assume a fully developed jet is generated in the room. Here, the decrease of dynamic pressure corresponding to jet development can be interpreted as the energy (density) converted into heat. Meanwhile, in the case of large openings, both decrease and increase in pressure can be seen inside the house model corresponding to contraction and expansion of the flow. The recorded pressure coefficient distributions along the centre line are presented in Fig. 10, which are separately shown in four diagrams according to the opening size given as absolute porosity with net porosity in parentheses; the sealed building model, small openings (0.05 (0.05), 0.14 (0.14), 0.83 (0.84)), middle openings (4.16 (4.83), 9.23 (10.16), 11.54 (13.05), 12.69 (14.53)), and large openings (13.84 (16.06), 15.00 (17.65), 16.15 (19.26), 18.46 (22.64)).

Table 4 Number of cells in CFD analysis. w [mm]

h [mm]

Absolute porosity [%]

The number of cells

0 17 80 80 80 80 80 80 80 80

0 17 20 40 50 55 60 65 70 80

0 0.83 4.61 9.23 11.54 12.69 13.84 15.00 16.15 18.46

965,864 952,992 802,874 876,044 1,069,544 876,364 877,004 877,004 877,324 877,644

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240

Height [mm]

200 160

a disappearance of the vortex. Visualization by helium filled soap bubbles for some cases showed that for the opening size w ¼ 50 mm and h ¼ 50 mm (absolute porosity ¼ 11.54%) no vortex could be identified. The pressure drop across the inlet opening has decreased. The internal pressure for the largest openings shows a decay which may be due to the fact that now, when the pressure drop across the openings is small, friction becomes important. The flow is visualized in Fig. 11c.

CFD (Windward)

CFD (Leeward)

Measurement (Leeward)

120

Measurement (Windward)

80 40 0 -0.2

4.4. Visualization with the sand erosion method 0

0.2

0.4

0.6

0.8

1

Wind Pressure Coefficient [-] Fig. 7. Wind pressure coefficient obtained from a sealed building model.

In front of the sealed building model, a pressure drop (indicated by a circle) is followed by a high pressure having the same degree as the wind pressure coefficient close to the ground plane. This pressure drop is thought to be caused by a vortex shown in the following section. Although the pressure distributions change continuously depending on the size of the opening, they may be roughly classified as follows. In the cases with small openings there is still a pronounced drop in pressure in front. At both inlet and outlet openings there are large pressure drops due to acceleration of the flow through the opening, but the pressure is uniform within the building. As mentioned above, a uniform pressure is interpreted as a well-developed jet flow inside the building. The relative length of the room L/O A is from 66.25 to 11.59 which is sufficient for a jet to develop. As the size of the opening increases, the pressure drop increases on the windward side, but decreases on the leeward side. The decrease in pressure drop on the leeward side as the size of the opening increases may be due to the fact that with increasing opening size the flow has less possibility to expand (the inverse of relative width drops from 50 to 12.5). This implies that with increasing opening size there is less difference in width of the primary flow and the width of the outlet opening. In the cases with a medium-sized opening, there is still a pressure dip in front. The pressure drop on the windward side is large but very small on the leeward side. The parameter L/OAo is now between 6.63 and 4 which does not allow a jet type of flow to develop as is confirmed by visualization in Fig. 11b. Inside the house, the pressure is relatively uniform but less than zero. The pressure inside the house is relatively uniform because the pressure drop across the leeward opening is small and therefore the pressure inside the house is related to the pressure in the leeward wake. Contrasting the pressure in the wake with the pressure in the wake in the small openings case we now observe that the pressure is higher. We interpret this as an indication of that now the momentum efflux has dispersed the wake. In the cases with large openings the dip in pressure in front has disappeared or almost disappeared. This may be caused by

Fig. 11 shows pictures of the internal flow visualized with the sand erosion method, where horizontal vector plots (2.5 mm above the floor) obtained from CFD are overlapped. The pros and cons of the sand erosion method are discussed in detail in Dezi [15]. Semolina powder was spread out in an even layer on the wind tunnel floor. The semolina powder is blown away at locations where the wind speed is highest. The wind velocity is augmented stepwise, each velocity is kept constant for a fixed time period, a picture is taken and then the velocity is increased. The highest velocity occurs in the primary flow in the air stream discharged into the room through the opening on the windward side. By entrainment into the primary flow, the flow in the primary air stream increases and the flow expands in the beginning. In a room with the inlet and outlet openings located opposite to each other continuity dictates that at each cross section the net flow rate must be equal. Therefore when the flow in the primary air stream increases, a flow in the opposite direction (recirculation) is generated [16]. 4.5. Correspondence of CFD and experiment Predicted results are compared with those from experiments. Fig. 11 shows a comparison between the airflow pattern predicted by CFD and the visualization by the sand erosion method. The qualitative agreement is good, which provides confidence for the ability of both methods to provide correct qualitative information. Fig. 12 presents the pressure coefficients obtained from CFD except for the two cases of smallest openings. General tendencies are well reproduced. Concerning details, CFD results tend to underestimate the pressure drop on the windward side. This is connected to inability in some cases of CFD to predict the occurrence of the vortex on the windward side. CFD predictions show larger internal pressure for two cases of the middle opening sizes. Nevertheless, given the scope of our study, which is to see the qualitative tendencies (e.g. relationship between pressures above and on the floor) and flow patterns, the correspondence is on the whole good. 4.6. Velocity vector field on the windward and leeward sides The velocity vector field on the windward and leeward side of the building was recorded in detail with PIV and predicted in detail

Fig. 8. Static pressure on/above the ground along the centre line of a house model obtained from CFD.

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Fig. 9. Pressure distribution over the pressure plate obtained from the measurement.

with CFD. The main purpose was to explore what information about the flow field can be retrieved from pressure measurements on the ground. In addition, because both measurements and CFD predictions were carried out, it was possible to make a comparison between both methods. Fig. 13 presents extracts from velocity vectors on the windward side obtained from PIV measurement and CFD analysis with the pressure coefficient distribution on the ground plane overlapped. For a small opening, a large vortex can

clearly be seen in front of the building in both PIV and CFD results. This is due to the flow blockage discussed in the previous section. The pressure coefficient drops in front of this vortex where an upward vector appears, though this pressure drop could not simulated enough in CFD results. Seeing a detail of the highest pressure, CFD shows it is located in front of the opening, where the flow above the ground travels downward and stagnates. Afterwards, the pressure coefficient starts to decrease according to

Fig. 10. Recorded pressure along centre line of the ground plane.

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Fig. 11. Visualization of the internal flow (Free flow of 18 m/s approaches from left of the picture).

increase of dynamic pressure. This tendency is thought to be rational. In the case of middle opening, the vortex obtained from PIV becomes ambiguous, and CFD does not show it any longer. Those features in flow pattern are reflected in pressure coefficient distributions. Except that CFD results could underestimate the vortex to some extent, those simulated flow patterns agreed well with experimental ones. Comparing with a smaller opening case, the highest pressure becomes lower because of larger velocity remaining due to a decrease in opening resistance. In the case of a large opening, a moderate variation is shown in both the pressure coefficient and flow shape (stream tube passing through an opening), and the vortex cannot be seen any more. Fig. 14 also presents the flow patterns and corresponding pressure coefficient distributions on the ground for the leeward side of the model. When the openings are small, a discharged flow becomes a part of

the large wake behind the building model. This flow pattern is also well reproduced in the CFD results, though outflow momentum seems to be a little larger in the CFD predictions. As for the middle opening case, two predominant streams can be observed on the leeward side, i.e. a back-flow in the wake and a flow penetrating the wake. Thus, the wake generated behind the building is obviously affected by the outflow in this case. In the large opening case, the flow discharge blows away the wake and very little back flow can be seen. Both flow pattern and pressure distribution vary continuously of course, depending on the variation in opening size, and it was shown that the flows passing through and around the building can affect each other. Importantly, a flow field in the cross-ventilation phenomenon has two flow paths to take into account. The flow passing through a large outlet opening obviously retains considerable dynamic pressure on the leeward side, which is

Fig. 12. Calculated pressure along centre line of the ground plane by CFD analysis.

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Fig. 13. Velocity vectors on the windward side obtained from PIV measurement and CFD analysis (Pressure coefficients presented inf Figs. 10 and 11 are also overlappted on the vector field).

assumed to be negligible in the conventional method for estimating the resistance and flow rate of a building. 4.7. Vertical velocity distribution For the purpose of making a more detailed examination of accuracy of CFD, the vertical distributions of the velocity in mainstream direction are shown in Fig. 15. The velocities normalized by those at the centre of the wind tunnel. The

the the are PIV

results adjacent to the ground surface were omitted where obviously incorrect cross-correlation peak was detected due to a reflection of the laser sheet. In the measurement result for the largest opening case, unnatural velocities can be seen along the line of X ¼ 500 mm at the height around 50 mm. This seems to be an error of PIV measurement caused by the correlation peak detected at incorrect location. Except these mis-detections of peak cross-correlation, PIV results seem not to include significant error, and velocity distributions in the upper area agree well between

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Fig. 14. Velocity vectors on the leeward side obtained from PIV measurement and CFD analysis (Pressure coefficients presented in Figs. 10 and 11 are also overlappted on the vector field).

PIV and CFD. In the velocity distributions on the windward side, negative velocities due to the vortex can be seen for the small and middle opening cases in the results both PIV and CFD. However, the absolute value of the negative velocity is underestimated by CFD. On the leeward side, although the CFD results could calculate the tendency well, the velocity decay could not be precisely simulated. Since the cross-ventilation is complicated and anisotropic turbulent phenomenon, the isotropic turbulence model, such as standard k-3 model, does not seem to work well as the authors have already shown [12]. In this paper, consequently, Reynolds stress model was used to be able to evaluate each component of the Reynolds stress. Nevertheless, momentum

diffusion could not be simulated well, and as a result, a wake generated in the leeward side became large in CFD. Ooka et al. [17,18] showed the same tendency in CFD analyses of a flow around a surface-mounted cube using Reynolds stress model. This may be due to that Reynolds stress model has been developed focusing on relatively simple turbulent flow; e.g. simple turbulent shear flow and turbulent boundary layer flow on the flat plate. Therefore, it is believed that the turbulence modelling method itself including the closure coefficients was not adequate to analyze the cross-ventilation flow field quantitatively. In future work with the aim of being able to analyze the power transportation between internal and external stream tubes, there

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Fig. 15. Distributions of X-component of velocity on the windward/leeward side of the house model, which are obtained from PIV measurement and CFD analysis.

seems to be a need for more effort to improve accuracy. As mentioned above, one of the most important factor seems to be turbulence model. Although there is a possibility to improve the results by applying other modelling methods of pressure-strain term and turbulent diffusion term in the transport equation of Reynolds stress, it is believed that the calculation using Large Eddy Simulation (LES) is required as the next step.

5. Conclusions The static pressure distributions on the ground plane and flow patterns (velocity vector field) were studied when varying the opening size in the range 4abs ¼ 0.05e18.5% in absolute porosity. With the pressure taps arranged in a grid of 400 points, the measurements provide a whole-field picture of the pressure field

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which helps in understanding the flow characteristics qualitatively. The flow pattern and pressure distribution changed when the opening size was changed. The variation in pressure distribution can be explained as follows, based on the velocity vector field obtained from PIV and CFD. 1) No opening case: The vortex on the windward side was clearly identified by the pressure measurements. A characteristics feature was a pronounced dip in the pressure generated by the vortex. 2) Small opening case (4abs ¼ 0.05e0.834%): The dip in pressure on the windward side due to the vortex was of equal magnitude as for the no opening case which shows that the openings have no effect on strength of the vortex. The pressure drops are located at the openings and the internal pressure is uniform. 3) Middle opening case (4abs ¼ 4.61e12.69%): The pressure drop on the windward side before the opening is now less, which is the result of a weaker vortex. The majority of the total pressure drop is now located at the windward opening. Only a very small pressure drop occurs at the outlet opening. Therefore the level of the internal pressure, which is relatively uniform, is closely connected to the pressure in the wake which is negative. The pressure in the wake on the leeward side is now higher than in the wake when the building is provided with small-sized openings. The change in wake pressure is due to the fact that the wake is affected by the momentum flux from the opening. 4) Large opening case (4abs ¼ 13.84e16.46%): Now the pressure drop before the opening on the windward side has almost disappeared, which indicates that there is no vortex or a very weak vortex. The pressure drop across the windward opening is now small and the pressure inside the building is now gradually decreasing. This may be due to the fact that the pressure drop due to friction is of greater magnitude than the pressure drop across the openings. The wake behind the model has been dispersed by the flow discharged through the leeward opening. The use of the static pressure distribution on the ground has proven to be a valuable tool to study the flow field above the ground. Comparison between CFD predictions and pressure measurements on the ground show that the variation in the pressure field above the ground was well reproduced by the pressure field recorded on the ground. However, there may be a difference in the pressure level between the ground and above. However, when the flow inside the building was a jet type flow, the pressure level on the ground was about the same as above. The strength of measuring the pressure on the ground is that static pressure measurements on ground are much easier to carry out than velocity measurements. The sand erosion method has proven to be a valuable method for visualizing the development of the primary internal flow. Comparison with CFD predictions exhibits the same qualitative picture of the flow. A conventional approach to estimating the flow rate of a building ventilated by wind, which is based on the orifice equation, considers the pressure loss coefficient for each opening. These pressure loss coefficients are generally summed up. The wind pressure coefficients obtained from a sealed building model are adopted as driving pressure. In this procedure, only one flow path from inflow to outflow is considered, and an effect of the flow passing around the building cannot be taken into account when evaluating the overall resistance of the flow path. Indeed, this conventional approach is often adequate in many practical ventilation design situations, but still causes discrepancy from an actual flow condition for the case of large openings because it is not based on the actual flow condition of the cross-ventilation. The essential difference is that there exists an external flow which could affect

the internal flow; i.e. the flow has a “choice” whether to enter through an opening or pass around the building. In order to establish an improved method to predict flow rate based on actual flow condition, the authors focus on the power transportation between two specific stream tubes passing through/ around the building. Since the numerical simulation is necessary to analyze a selected stream tube, CFD analyses simulating the wind tunnel tests were conducted. The basic characteristics in the flow patterns on the windward/leeward side and the pressure distribution on the floor could be relatively well simulated. However, the accuracies of these calculations for the vortex and the discharged flow were not detailed enough to determine the stream tubes and estimate the transported power. As a future prospect of this work, therefore, some improvement in the turbulence model of CFD is needed. Acknowledgments The authors are deeply grateful to Professor James W. Axley for providing valuable comments. The authors would like to express our gratitude to Professor Toshio Yamanaka and Professor Kazunobu Sagara for illuminating discussions. The help from Ms. Elisabet Linden in conducting PIV measurement and visualizations is also gratefully acknowledged. A part of this work was financially supported by Grant-in-Aid for JSPS Fellows (20-912; Research representative, Tomohiro Kobayashi). References [1] Kobayashi T, Sagara K, Yamanaka T, Kotani H, Takeda S, Sandberg M. Stream tube based analysis of problems in prediction of cross-ventilation rate. The International Journal of Ventilation 2009;7(4):321e34. [2] Ishihara M. Building ventilation design. Asakura Publishing. Co. Ltd.; 1969 [in Japanese]. [3] Kurabuchi T, Ohba M, Endo T, Akamine Y, Nakayama F. Local dynamic similarity model of cross-ventilation part 1-theoretical framework. The International Journal of Ventilation 2004;2(4):371e82. [4] Kotani H, Yamanaka T. Prediction of inflow direction at large opening of cross ventilated Apartment building. Journal of Environmental Engineering (Transactions of AIJ) 2006;(609):39e45. [5] Kotani H., Yamanaka T. “Interference coefficient for discharge coefficient in prediction of cross ventilation rate through large opening”, Proceedings of 29th AIVC Conference, vol. 3, 2008. pp. 27e33. [6] Sandberg M. Airflow through large openings e a catchment problem?; 2002. Proceedings of Roomvent 2002, pp. 541e544, Copenhagen, Denmark. [7] Sandberg M. An alternative view on the theory of cross-ventilation. The International Journal of Ventilation 2004;4(4):409e18. [8] Murakami S, Kato S, Akabayashi S, Mizutani K, Kim D. Wind tunnel tests on velocityepressure field on cross-ventilation with open windows. ASHRAE Transactions 1991;97(Part 1):525e38. [9] Kato S. Flow network model based on power balance as applied to crossventilation. The International Journal of Ventilation 2004;2(4):395e408. [10] Guffy SE, Fraser DA. A power balance model of converging and diverging flow junctions. ASHRAE Transactions 1989;95(Part 2):2e9. [11] Axley JW, Chung DH. POWBAM0 mechanical power balances for multi-zone building airflow analysis. The International Journal of Ventilation 2005;4 (3):95e112. [12] Kobayashi T, Sagara K, Yamanaka T, Kotani H, Sandberg M. Wind driven flow through openings e analysis of the stream tube. The International Journal of Ventilation 2006;4(4):323e36. [13] Willert CE, Adrain RJ. Digital particle image velocimetry. Experiments in Fluids 1991;10(4):191e215. [14] Launder BE, Reece GJ, Rodi W. Progress in the development of a Reynolds stress turbulence closure. Journal of Fluid Mechanics 1975;68(Part 3):537e66. [15] Dezsö, G. On assessment of wind comfort by sand erosion, doctoral thesis at University of Eindhoven, Netherlands, 2006 [16] Guilherme Carrilho da Graça, Linden PF. Simplified modeling of cross-ventilation airflow. ASHRAE Transactions 2003;109(Part 1):65e79. [17] Ooka R, Murakami S, Mochida A. Study on modeling for fij, fijw and turbulent diffusion of - numerical simulation of flow field around cube by means of various differential stress models. Journal of Planning Engineering (Transactions of AIJ) 1998;(504):55e61 [in Japanese]. [18] Ooka R, Mochida A, Murakami S, Hayashi Y. Examining ASM by means of wind tunnel test, LES, and DSM numerical simulation of anisotropic turbulent flowfield. Journal of Planning Engineering (Transactions of AIJ) 1997; (495):61e8 [in Japanese].