Influence of sidewall openings on air change rate and airflow conditions inside and outside low-rise naturally ventilated buildings

Energy and Buildings 130 (2016) 453–464

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Influence of sidewall openings on air change rate and airflow conditions inside and outside low-rise naturally ventilated buildings Xiong Shen a , Ruoyu Su a , Georgios K. Ntinas b , Guoqiang Zhang c,∗ a Tianjin Key Laboratory of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, 300072 Tianjin, China b TUM School of Life Sciences Weihenstephan, Research Department Plant Sciences, Technische Universität München, 85354 Freising, Germany c Department of Engineering, Faculty Sciences and Technology, University of Aarhus, Blichers Allé 20, 8830 Tjele, Denmark

a r t i c l e

i n f o

Article history: Received 4 May 2016 Received in revised form 16 August 2016 Accepted 19 August 2016 Available online 20 August 2016 Keywords: Indoor airflow Outdoor airflow LDA measurement Wind-driven ventilation Scaled model experiment

a b s t r a c t The inside and outside velocity distribution was highly related with contaminant transport of naturally ventilated buildings. The building sidewall openings have a large impact on the velocity distribution at wind-driven conditions, but were not comprehensively investigated in previous studies. To achieve knowledge on that, a laboratory experiment was conducted using a building model in a wind channel. A laser Doppler Anemometer (LDA) was applied for the velocity and turbulence measurements. Tracer gas method (using CO2 as tracer) was applied to measure the air change rate. Eight configurations of the sidewall openings in varied sizes and locations in the sidewalls were investigated. Results showed that in the same outdoor wind conditions, the air change rate depended upon the inlet (windward opening) and outlet (leeward opening) sizes. The location of openings seems to have little impact on the air change rate. Even with the same size and location of inlet but different outlet, the velocity profiles ahead of building remain different. Inlet and indoor velocities with the same inlets were similar even in cases with varied outlet. The velocities at the outlet depended on the inlet sizes. The wake flow behind the building was highly depended on the size and location of inlet and outlet. It was recommended to consider the inlet and outlet size and location when investigate the contaminant transport from the inside of the building to the outside. © 2016 Elsevier B.V. All rights reserved.

1. Introduction As for a ventilated building, the volumetric flow rate of outside air being introduced to the building, named as the ventilation rate is correlated not only with control of indoor temperature and humidity [1], but also with removal of unpleasant odors [2], dust [3,4], airborne bacteria [5], and carbon dioxide (CO2 ) [6,7]. Natural ventilation systems are widely used in industrial buildings or agricultural buildings, such as dairy and poultry houses and greenhouses. Large openings are often constructed for dairy cattle buildings aiming at the required indoor climate and sufficient ventilation in warm period. However, during cold or windy conditions, it is necessary to maintain the ventilation rate in a certain level to achieve desired indoor climate. The velocity patterns are also very important for occupants’ requirements. To reduce heat stress in hot summer, higher air speed

∗ Corresponding author. E-mail address: [email protected] (G. Zhang). http://dx.doi.org/10.1016/j.enbuild.2016.08.056 0378-7788/© 2016 Elsevier B.V. All rights reserved.

in occupant zone may help to increase convection heat removal from occupant to improve thermal comfort. In cold winter, especially for young animals in livestock buildings such as piglets and calves, fresh-air with lower or zero air speed is preferred to reduce cold draft. In addition, in building design it should be considered that the emissions from production buildings may be affected by indoor airflow conditions. That is because different velocity patterns around the pollution source have important effect on the gas emission rate [8–10]. Therefore, it is necessary to control the indoor velocity patterns based on the specific animal requirements. Except from the indoor airflow patterns, the external airflow patterns are also very import. That is because the amount of the contaminated air out of the building and how it disperses is very important for the adjacent buildings [11–13] and the surrounding environment [14–16]. In practice, the opening size and location can be controlled in many naturally ventilated livestock buildings, which also lead to different velocity distributions [17–19] and building ventilation rate [18,20,21]. The inlet and outlet openings of the building can be adjusted by sidewall curtains or flap wall inlets. The

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experimental study to describe the effect of curtains on the indoor air quality has been reported in the pig houses [22,23]. The effect of shape and size of openings on the indoor airflow of resident buildings was reported by Shetabivash [24]. But the outdoor airflow patterns were not considered and the effect of curtains or sidewall openings on the indoor and outdoor airflow patterns was not addressed. The dispersion of the contaminant air from the building inside was also not mentioned in these studies. This study will focus on wind-driven cross-ventilated buildings, where thermal conditions are ignored because of the small temperature difference existed between indoor and outdoor. The internal and external airflow distributions, the ventilation rate in different opening size and locations were investigated. Above all, the objective of this study was to investigate effect of opening status on the indoor and outdoor airflow conditions as well as on the ventilation rate of naturally ventilated buildings. A scale model experiment was carried out in a newly constructed Wind Channel. Eight building inlet and outlet opening cases were compared for the ventilation rate and the airflow distribution ahead and behind the building.

2. Materials and methods 2.1. Wind channel In order to investigate the indoor and outdoor airflow patterns, physical experiments are often conducted as reported by many researches. Full scale experiments are more close to the reality but require excessive time and cost [25]. Although full scale experiments can be performed[18] in natural wind conditions, it is time consuming to wait for promising wind conditions, suitable for the experimental measurements. Thus, a wind tunnel can be used to create simulated wind conditions. In this study, a modified wind tunnel, namely wind channel, was developed and used to simulate the natural wind conditions as seen in Fig. 1. The wind channel did not have a solid ceiling and hence the airflow could pass through the building avoiding the extra pressure resistance around the building model that exists in a wind tunnel experiment. In this way, the wind channel was more reasonable to simulate the real wind conditions inside and outside of a building model than the wind tunnel. The wind channel was a low speed, open circuit wind tunnel without ceiling in the working section as seen in Fig. 1(a). It consisted of an axial fan, a pressure chamber and several airflow straightening plates. The axial fan with diameter 1.4 m and maximum revolution speed 450 per minute can create a maximum outlet air speed of 3.6–4.0 m/s in the front. Because air from the fan is rotating and unbalanced, the pressure chamber was used to reduce the rotation and stabilize the airflow. The pressure chamber assembled the air from the fan and released it through the outlet

consisted by several slatted openings. Along the wind direction, several horizontal plates were parallel placed to further straighten the airflow and assist to create the wind profile. The working section was 8 m from the tunnel air inlet and had a dimension of 6.00 m × 1.38 m × 1.55 m. The wind profile was further adjusted by a rough floor surface in front of the working section, together with adjustment of the vertical distance between the straightening plates. The rough surface was consisted by several small wooden blocks (0.05 × 0.05 × 0.05 m3 and 0.05 × 0.05 × 0.025 m3 ) and distributed in a distance of 3.5 m in the working section. As shown in Fig. 1, external wind directions are adjusted by rotating the circular platform below the scaled building. Air velocity in the working section was measured by a 2D Laser Doppler Anemometer [1] from Dantec Dynamics (Dantect Dynamics A/S, Denmark). The LDA is a non-contact optical instrument for the velocity measurement by detecting speed of particles in the fluid. It does not need calibration and has a unique intrinsic response to fluid velocity without any disturbance from temperature and pressure. Meanwhile, measurements with high spatial and temporal resolution can be obtained at a very small measuring volume in the fluid. Laser optics of the LDA applied the infrared laser light to measure the velocity at the measurement point. The incoming light is released from the backscatter laser optics, which also contains a receiving unit inside. It records the wind speed at the measurement point by detecting the frequency of the reflected light from the seeding particle. Based on the Doppler Effect, the frequency difference between the reflected light and incoming light is turned into speed of the particle. The measurement position was at a distance of 35 cm from the optical head unit to forbid any disturbing to airflow. The LDA was mounted on a two dimensional traverse system and moved automatically in the vertical and horizontal direction following predefined measurement positions, as seen in Fig. 1(b). The detailed setup of the wind channel experiment is presented in Fig. 1(b). A sub-section of scaled building, with external dimensions (width, length and height) of 0.55 m, 0.5 m and 0.26 m, respectively, was placed in the working section. It was mounted on the centerline of the platform with a distance of 1.5 m to the leading edge of the wind channel. The cross section plane at a distance of 4.8 m up-wind from the beginning of working section was measured for velocity calibration. Seven sampling lines in the width direction were recorded with an interval distance of 0.1 m. In the wind channel, the stream wise wind speed was evenly distributed in the cross section of the working zone before the experiment. The turbulence intensity was also well balanced while at the bottom was high due to the roughness of ground. The Reynolds number of the building was 5.37 × 104 and the reference wind speed was 3.0 m/s at the building height of 0.26 m. The Reynolds number of our experiment was higher than

Fig. 1. Experimental setup of the Wind Channel. (a) Layout of the wind channel in Air Physics Lab, Aarhus University; (b) The LDA system used for velocity measurement;.

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Table 1 Effect of sampling locations on the calculation of ventilation rate by the tracer gas method. Cr is the background CO2 concentration. Co is the outlet CO2 concentration. C is the indoor and outdoor concentration difference. Q is the ventilation rate. Case 1a

7a

Positionb

Cr (mg m−3 )

Co (mg m−3 )

C (mg m−3 )

ACR (s−1 )

1 2 3 1 2 3

1043.2 ± 37.0 1029.3 ± 36.7 1017.6 ± 35.4 1002.2 ± 49.0 1007.1 ± 52.0 1031.4 ± 53.4

1244.7 ± 35.2 1279.5 ± 12.8 1337.2 ± 18.4 1521.6 ± 29.0 1411.1 ± 25.6 1337.2 ± 19.1

201.5 ± 37.2 250.2 ± 17.4 319.6 ± 21.6 519.4 ± 32.6 404 ± 30.5 305.7 ± 25.1

1.0 ± 0.4 0.8 ± 0.1 0.6 ± 0.1 0.4 ± 0.0 0.5 ± 0.1 0.6 ± 0.1

In the cases, the wind direction was 45◦ oblique to the building sidewall. 1, 2 and 3 indicate the measurement position at the top, middle and bottom of the outlet in Case 1 and 7, respectively. a

b

inside volume of the scaled building V = 0.05 m3 . The temperature in the experiment room was remained at 20 ◦ C. Fig. 2. Simulated velocity and turbulence intensity profiles in the Wind Channel.

2.3. Opening cases

the critical Reynolds number, which equals to 4.5 × 104 , thereby we can consider the flow was independent of the Reynolds number. The measured velocity and turbulence intensity profile in front the buildings were shown in Fig. 2 and the function of the profiles can be seen in Eq. (1):

Fig. 4 shows eight cases of scaled building openings in the experiment. The configurations of inlet and outlet of the building were varying among different cases. Three heights of the inlet and outlet opening can be found for all the cases, which is in the top, bottom and whole opening of the section of the facade. The dimensionless scale of each configuration can be seen from Table 1.

U = (3.164 ± 0.024)y0.102 ± 0.0038 (R2 = 0.97)

(1) 2.4. Velocity field measurement

2.2. Scaled building model A 1:25 scaled building as seen in Fig. 3 was represented for a sub-section of the stand-alone livestock building. The roof and walls were made of a 5 mm thick clear Plexiglas sheet. The sidewall curtains were made by 0.5 m long, 50 mm wide and 2 mm thick Plexiglas strips. The outside dimensions of the models were 500 mm in length and 550 mm in width, eave height 130 mm and ridge height 260 mm. The sidewall was 30 mm high at bottom that given a fully opening size of 100 mm. The roof slope was 25◦ and the

Air velocity in the working section was measured by 2D Laser Doppler Anemometer. The measured x velocity is normalized by the reference velocity measured in the height of 0.5 m and turned into dimensionless velocity (U/Uref ) as seen in the reference[26]. The location of measurement points of velocity can be seen in Fig. 5. Those points were distributed in 14 sampling lines, including five in front of the building, two at the inlet and outlet and five behind the building. The average and standard error of each measurement point was recorded within a time interval of 150 s.

Fig. 3. Model building, CO2 injection to room space and concentration measurement setup at the outlet of the scaled building.

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Fig. 4. Definition of cases with different opening statues; the arrow is the outside wind direction; in the plots,  is the sampling point for the building air change rate.

The mean and root mean square of the velocity were calculated for a defined sampling time of 150 s for each position in this experiment. Turbulence intensity was calculated as the root mean square (RMS) of the measured velocity fluctuation divided by the mean velocity.

2.5. Measurement of air change rate A constant tracer gas release method was used to measure the air change rate [27]. In the experiment, carbon dioxide was released as tracer gas. The gas released from the liquid gas bottle was directed into an air mixing chamber (seen in Fig. 1(b)) through a gas regulator and a control valve in order to stabilize the injection rate at a constant level, q (L/min). The fresh air was pumped continuously into the air mixing chamber with a speed of 4.3 L/min to mix with the pure carbon dioxide. The two gases were mixed inside the mixing chamber, in which an axial fun was activated at the bottom to ensure the uniform mixing. As seen in Fig. 3, the chamber was connected to the building indoor space through the orifices at the building floor. The top surface of the chamber was attached to the building floor. There were 100 holes (1.5 mm diameter each) spread evenly in the joint plane to distribute the mixed air into the model building. The concentration of carbon dioxide inside the chamber, Cm (mg/m3 ), was measured at the center of the mixing chamber in order to check the mixing quality. For every case of the openings, Cm (mg/m3 ) was kept constant for at least 90 min to ensure the stability of the experiment. Detailed information of the air change rate measurement setup can be found in Shen et al. [27]. The determination of the air change rate for each setup was based on measurement data more than two hours after the source of CO2 concentration inside the mixing box was stabilized. The difference, C (mg/m3 ), between the outlet concentration, Co

(mg/m3 ), and background concentration, Cr (mg/m3 ), was monitored. C = C o −Cr

(2)

In this study, the tracer gas constant injection method was adapted to calculate the building ventilation rate. The tracer gas (CO2 ) was continually injected into the building space with a constant rate q (L/min). Assumed uniform distributed concentration field inside the building, the ventilation rate, Q (m3 /h), and the recorded difference C (mg/m3 ) were calculated by the following equations: Q = 116.3•q/C

(3)

The air change rate, ACR (1/s) of the building was calculated by the following equation: ACR = Q/(3600•V )

(4)

where V = 0.05 m3 is the volume of the indoor space. The standard error of C and Q of Eqs. (5) and (6) was calculated by the propagation of experiment error [28]: (C) = [ 2 (Co ) +  2 (Cr )]0.5

(5) 2

(ACR) = (Q )/V = 116.3•q•(C)/(C) V

(6)

As reported by Wu et al., the CO2 injection in the model and the concentration at the room outlet (leeward sidewall opening), Co (mg/m3 ), is shown in Fig. 3 [29]. All CO2 concentrations were detected by an INNOVA 1303 multi-channel gas monitor (INNOVA Air Tech, Denmark). The concentration at the outlet was measured by an air sampling tube. Each contained 11 orifices evenly distributed in the length direction. The orifices horizontally faced to the building internal space. Each orifice had a small diameter lower than 1.5 mm in order to make sure the flow rate from each orifice was equal. The diameter of the pipe was relatively small in order not to influence the airflow. Two ends of the pipe were connected to

Fig. 5. Distribution of measurement points. 1–12 is number of sampling lines. H and W are the height and width of the building, respectively.

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Table 2 Effect of CO2 injection rate, q, on the accuracy of air change rate in the opening case of Case 1 with the wind direction perpendicular to the building sidewall. Cm was the concentration of the mixed air inside the mixing box. Cr , Co and C was the background, outlet and indoor and outdoor concentration difference, respectively. The data were presented in the form (average ± standard deviation). ␴(ACR)/ACR was the relative error of air change rate(ACR). q (L min−1 )

Cm (mg m−3 )

Cr (mg m−3 )

Co (mg m−3 )

C (mg m−3 )

ACR (s−1 )

(ACR) /ACR

0.025 0.050 0.100 0.150 0.200

18389.9 ± 1527.5 22921.2 ± 965.3 51013.5 ± 273.2 54269.5 ± 168.7 54594.4 ± 118.2

961.3 ± 19.4 890.1 ± 21.5 995.7 ± 70.4 1002.9 ± 76.5 1057.7 ± 74.5

981.1 ± 19.3 921.7 ± 5.9 1075.7 ± 7.4 1090.7 ± 8.9 1144.0 ± 29.8

19.8 ± 22.3 31.6 ± 8.8 80 ± 21.7 87.8 ± 24.6 86.4 ± 41.9

1.8 ± 4.6 2.1 ± 1.2 1.7 ± 0.9 2.2 ± 1.3 3.0 ± 3.7

2.62 0.58 0.55 0.57 1.23

the air pump. The air through the orifices was sucked at a flow rate of 8 L/min and was delivered to the multiplexer of the gas monitor. The air from the tube was sent to the gas monitor for determine the CO2 concentration at the outlet. Three positions were chosen to measure the background concentration. The first position was situated beneath the wind channel nearby the air pump; the second position was located 0.6 m in front of the scale building, 0.1 m above the wind channel floor; the third position was situated 0.5 m above the floor at the center of the laboratory room. The background concentration, Cr (mg/m3 ), was the average of the measurements at the three positions. In this study, in order to determine the proper flow rate of CO2 injection, we chosen five flow rates: 0.025 L/min, 0.05 L/min, 0.10 L/min, 0.20 L/min, and 0.15 L/min in testing. In order to quantify the ability of building inlet openings on capturing the wind-driven ventilation, the effectiveness term was proposed in ASHRAE. Considering the inlets faced directly into the prevailing wind, the effectiveness of openings is defined: ␰ = Q /(Ai ·Uref )

(7)

where  is the effectiveness of the ventilation opening (dimensionless) and Uref is reference velocity (m/s) which was 3.0 m/s at the building height of 0.26 m. 3. Results and discussion 3.1. Accuracy of air change rate (ACR) measurement The accuracy of air exchange rate determination is strongly related to the tracer gas concentration measurements. In previous studies, the outlet concentration was measured at the top of the outlet [29,30], [31]. However, the measurement at the location may not be able to represent Co , because the concentration may be not uniform distributed at the opening[29]. In order to correctly determine Co , it was important to know the concentration distribution at the outlet. Table 1 shows the outlet concentrations at three measurement positions. The distribution of concentration was gradually decayed from the top to the bottom or from the bottom to the top. In general, the measured concentration at the middle of the outlet equals to the average of concentration at the top (position 1) and bottom (position 3). To deal with the uneven distribution of concentration at the outlet, the concentration difference, Cr (mg/m3 ), were revised by the following equations:



C r =

0

H

(Co − Cr )dh/H

(8)

In which, H is the height of the outlet openings. From the results in Table 1, the concentration difference at the middle of the outlet, C o,m = 0.5(C o,o + C o,H )

(9)

Co,o and Co,H were the measured concentration difference in positions 1 and 3, where h = 0 and H.

By combining Eqs. (7) and (8), the concentration difference at the outlet can be calculated by: C r = 5Co,m

(10)

The CO2 injection rate from the gas tank, q (L/min), was important for the determination of ACR as seen in Eqs. (3) and (4). In order to determine the proper q, in this study, five CO2 injection rates from 0.025 L/min to 0.200 L/min were tested. Table 2 presents the effect of the five injection rates on the uncertainty of C and ACR. Case 6 was chosen for the injection rate testing, in which the outside wind direction was perpendicular to the sidewall and inlet and outlet sizes was the largest among all cases. It may have the largest ACR under a defined wind speed. That may result in the largest measurement error for air change rate,  (ACR) in all cases. As seen in Eqs. (5) and (6), the larger ACR would lead to smaller concentration difference, C, and higher relative error,  (ACR). Following Eq. (10), we calculated the ACR by using the revised concentration difference, C (mg/m3 ); Table 2 shows the results of the tests. The data listed in Table 2 was achieved in the same wind condition and opening case. However, except for q = 0.050 L/min and q = 0.150 L/min show similar result of ACR, the results in other injection rates were different. Those cases of q show similar results and small relative error can represent the actual air change rate. Therefore, in this study, q = 0.150 L/min was applied. The data in Table 2 indicate that the injection rate may also influence the standard deviation of C and ACR. As q = 0.025 L/min, the standard deviation of ACR is even larger than the average, indicates that low rejection rate would lead to high experiment error of air change rate. As for q = 0.050 L/min, the standard deviation of C and ACR became smaller than q = 0.025 L/min but still large. The experimental error is found to be similar and small in the case of q = 0.050, 0.100 and 0.150 L/min. But for q = 0.200 L/min, the experimental error became larger than q = 0.150 L/min. This may occur because the injection rate is too fast during the measurement so that the background concentration inside the experimental room increases rapidly and thus leads to high experimental error. 3.2. Air change rate (ACR) of varied opening cases In previous research, the inlet and outlet opening size and location proved to be effective elements for cross ventilation[32]. Table 3 Air change rate (ACR) and opening effectiveness () in different cases. Ai , Ao , were the area of the inlet and outlet openings. Hi , Ho were the height of inlet and outlet, respectively. Hs , As were the height and area of sidewall full openings. Case

Ai /As

Ao /As

Hi /Hs

Ho /Hs

Ai /Ao

ACR(1 s−1 )

 = Q/(Ai ·Uref )

1 2 3 4 5 6 7 8

0.5 0.5 0.5 0.5 1 1 0.5 0.5

0.5 0.5 0.5 0.5 0.5 1 1 1

0.75 0.75 0.25 0.25 0.5 0.5 0.25 0.75

0.75 0.25 0.25 0.75 0.75 0.5 0.5 0.5

1 1 1 1 2 1 0.5 0.5

0.63 0.63 0.71 0.67 1.24 2.25 0.54 0.86

0.41 0.40 0.46 0.43 0.41 0.74 0.34 0.55

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Fig. 6. Velocity profiles in front of the building (case 1). L1–6 represents the index of the sampling lines as shown in Fig. 5. (a) Dimensionless velocity U/Uref , (b) dimensionless velocity fluctuation U’/Uref .

However, the detailed relation between varied sizes and locations of openings and the ACR still remains unknown. As seen in Table 3, Cases 1, 2, 3 and 4 had the same size of inlet and outlet opening (Ai /Ao = 1), but different in locations. The results showed almost equal ACR values among these four cases. It indicates that the location of openings had small influence on the air change rate. However, when the inlet and outlet sizes were differ/ 1), as seen in cases 5, 7 and 8, ACR values were 1.24 s−1 , ent (Ai /Ao = 0.54 s−1 , and 0.86 s−1 , shows large difference among cases. Cases 1, 2, 3, 4 shared the same outlet and inlet size (Ai /As = Ao /As = 0.5) but different outlet and inlet locations (Hi /Hs ,Ho /Hs ), the ACR was almost equal to 0.6–0.7 s−1 . While Case 5 had the same outlet size with Case 1–4, but different inlet size in terms of 100% opening air inlet (Ai /As = 1), the ACR was larger as 1.24 s−1 . This indicated that air inlet location’s influence was rather small and owed mainly to the inlet size. Cases 7, 8 shared the same 100% opening air outlet (Ao /As = 1) and inlet size (Ai /As = 0.5) but different inlet location (Ho /Hs ), the ACR varied. Thus, the inlet location has a greater impact on the ACR. Case 6 with a large air inlet (Ai /As = 1), had the maximum ACR. In general, when the outlet size is the same, the greater the air inlet is, the greater the ACR is. But, the vice versa is not necessarily true. Different values for  are given depending on the wind direction, the opening sizes etc. In this study, we can see in Table 3 that even with the same inlet sizes, the effectiveness of openings was different because of varied outlet sizes. The cases with the same outlet sizes show similar effectiveness as seen in Table 3. This indicated that the outlet size was a crucial factor to the effectiveness of inlet opening. In 50% opening outlet size, the location of opening showed little impact on the effectiveness. According to ASHRAE (2009), in perpendicular wind direction,  varies between 0.5 and 0.6 and in oblique wind directions,  falls between 0.25 and 0.35. This means that if the buildings are not placed perpendicularly to the inlet, the ACR will be smaller than that one predicted by Eq. (7). In this study, we can see that in most of cases  was around 0.4-0.6. The effectiveness was a little smaller than the one defined in ASHARE. This may occur because the wind speed ahead of the inlet opening was decreased by the existence of building sidewall. In order to see the detailed reason lead to the phenomenon, it was important to see the velocity distribution in front of building. From the results, we can conclude: 1) with 50% opening and equal opening sizes at inlet and outlet, the ACR was not substantially influenced by the location of the openings. 2) With 100% opening size at inlet and/or outlet, both the inlet and outlet opening sizes and the ratio, Ai /Ao , were important factors to influence the ACR. 3) The outlet size had large impact on the effectiveness of inlet openings.

3.3. Velocity distribution Different size and location of the inlet and outlet opening may result in varied velocity distribution inside, ahead and behind the building. By comparing the profiles of stream velocity among different cases, the effect of the building openings can be determined.

3.4. Flow field in front of building Fig. 6(a) shows the development of velocity and turbulence in front of the building model. The velocities were measured in six vertical sampling lines (L1-6), in which from L1 to L5, the lines were closer to the building inlet sidewall with the distance from 2H to 0H. From Fig. 6(a), we can see the velocity shows a gradually descend trend as closer to the building inlet. The velocity profiles turned to be similar between L1 and L2 with the distance between 1H and 2H. The velocity and turbulence between 0.125H (L4) and 0.25H (L5) shows a large change as seen in Fig. 6(a, b). There was a vortex formed at this region because of the resistance of the building sidewall. In this study, it was not possible to measure the detailed three dimensional airflow patterns at the region because the LDA was two dimensional. The measurement procedure of flow field in front of building by PIV and CFD simulation can be referred to the literature[33]. As seen in Fig. 6, the velocity values and fluctuation in L6 of Case 1 prohibited large changes cross the openings. One reason was because the inlet flow adjacent to the opening was very turbulent and it may require much longer averaging time to measure the mean velocities at the region [34]. The vortexes generated before and after the windward opening might be very unsteady and consequently affected the flow at the opening. Fig. 7 shows the velocity profiles in front of the building at different distances for all experimental setups. When the airflow approach the building model it is showed that, the profiles are affected by the air inlet openings. At sampling lines 4 and 5, the inlet sizes affect the adjacent airflow patterns ahead of the building. Different size and location of the inlet opening has different impact, e.g. cases 2, 7 and 8 as compared with cases 1, 3 and 6. However, even with the same inlet sizes (e.g. cases 5 and 6) but different outlet sizes, the velocity profiles ahead of building remain different. Even though we cannot get a conclusion about which part of opening would influence the velocity ahead of building, generally we can understand that the location in front of building that the velocity came to disparate. As seen from Fig. 7, velocity profiles in positions of 1 (2H) and 2 (1H) are very similar. As coming closer to the building from position 3 (0.5H), the velocity profiles become distinct among different cases.

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Fig. 7. Velocity profiles of cases 1–8 in front of the building between different cases of inlet and outlet openings (case 5 lacks of data). (a)–(e) represents the data at the sampling lines 1–5 as shown in Fig. 5.

For CFD simulation, a distance between the inflow boundary and the building is suggested to be at least five times of the maximum height of the building by several researches [35,36]. Other scientists reported the minimum distance can be two times of the building height for buildings with low blockage to the fluid [35]. Fig. 7 shows that the velocity profiles became identical at the distance of 2H among varied cases. This indicates that the inflow boundary in CFD simulation can be defined at least 2H in front of the building.

3.5. Inlet velocity Fig. 8 shows the velocity profiles close to the inlet of the building of different opening cases. The velocities were measured 5 mm away from the windward sidewall. As it can be seen in Fig. 8, the velocities above the building eave (0.5H) of different cases were similar. Below the height of building eave, the velocity profiles became distinguished. For cases 3, 4 and 7, the inlet was located in the lower part of the windward sidewall. Velocity profiles were very similar in cases

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Fig. 8. Velocity and velocity fluctuation profiles of cases 1–8 at the building inlet, 6 below the plot represents the sampling line at the inlet of the building as shown in Fig. 5. The horizontal line indicates the height of building eave(y/H = 0.5). (a) Dimensionless velocity U/Uref , (b) Dimensionless velocity fluctuation U’/Uref . y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet.

with different outlet sizes. Especially, cases 3 and 4, which had the same size of outlet, were very close to each other. The same state occurred also between cases 1 and 2. However, with 100% opening size of inlet opening such as in cases 5 and 6, the inlet velocity profiles differed significantly in case of different outlet openings. The discrepancy also occurred between cases 1, 2, and 8, which have the same inlet but with different outlet. In case 8 inlet velocities were different compared to cases 1 and 2, both of which shared almost similar velocity profiles. Thus, case 1 and 2 with the same 50% upper opening inlets have similar inlet velocity and the outlets show little impact. However, as shown in Fig. 7(e), the same inlet openings like case 1 and 2 could lead different velocity profiles in front of the building (measurement positions line 5). That indicates a large change of velocities within the region between position 5 (0.125H) and the inlet opening (0H). This quick change between 0.125H and 0H may also relate with the size and location of the outlet. This phenomenon also occurs between case 3 and 4. Fig. 8(b) shows the velocity fluctuation adjacent to the inlet at position 6 (0H) in cases 1–8. Wherein, cases 6, 7, and 8 had 100% opening air outlet, their velocity fluctuations were small, similarly seen also in cases 3, 4, and 7 with 50% opening inlet sizes. While in cases 1, 2 and 5 velocity fluctuations were larger. The large outlet size in cases 6, 7 and 8 (counterexample cases 1 and 2) and lower inlet in cases 3, 4 and 7 (counterexample cases 1 and 2) can lead to the small velocity fluctuations at the inlet.

3.6. Indoor velocity distribution The indoor velocity profiles of different opening cases are shown in Figs. 9 an 10. The velocity profiles of two positions inside the building (lines 7 and 8 in Fig. 5) were measured. The velocities

above the roof at the height of H in varied cases were almost equal. However, the indoor velocity patterns were varied. In general, cases 3, 4 and 7 showed similar velocity patterns in both positions inside the building, as well as cases 1 and 2. Those cases shared the same inlet, which are top-half or bottomhalf opened, respectively. Compared to cases 5 and 6 with 100% opening inlets, the velocities were notable. Case 8 was also different from cases 5 and 6, but closer to cases 3, 4 and 7. This indicated the importance of the size and location of inlet in operating the velocity distributions inside the building. In cases 1, 2 and 8, the velocities were negative close to the floor (y/H < 0.2), indicating that there existed reverse flow above the floor of the scaled building. These three cases had the same top-half inlet openings. As seen from the velocity close to the roof, cases 1 and 2 presented the same patterns and with a sudden increase along the sampling lines beneath the roof surface. The high velocity zone may result from the wall attached jet close to the roof inside the pitched-roof buildings as had been observed and reported by previous research[18]. In the three cases, the reversed velocity was formed as close to the floor by the rotation of wind inside the building space. The reversed velocity close to the floor and velocity close to the roof was higher in case 8 than in cases 1 and 2. This may result to stronger inertia effect of the incoming wind and lead to higher reversed velocity above the floor. In cases 3, 4 and 7, the velocities in position 7 were negative below the roof surface and positive above the floor, which indicates the direction of the flow rotation inside the building was opposite as compared with cases 1, 2 and 8. The velocities of the former three cases seem to be much higher than the latter in position 7. However, the velocities seem to be no difference between them in position 8. Case 6 with the 100% opening air inlet and outlet, the presented velocity patterns distinguished from other opening cases. In

Fig. 9. Velocity profiles and fluctuations of cases 1 ∼ 8 inside the building at sampling lines 7, as shown in Fig. 5; (a) dimensionless velocity U/Uref , (b) dimensionless velocity fluctuation U’/Uref . y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet.

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Fig. 10. Velocity and fluctuation of cases 1–8 inside the building at sampling line 8, as shown Fig. 5; (a) dimensionless velocity U/Uref , (b) dimensionless velocity fluctuation U’/Uref . y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet.

positions 7 and 8, the velocity profiles close to the floor were very similar, indicated that the airflow jet crossed the building space with little kinetic energy loss. Case 5 shared the same inlet with case 6, but flow patterns were quite different. It may have no rotation in the windward part of the indoor space. In the meanwhile, case 5 also showed that relatively lower floor velocity than other cases. Figs. 9 (b) and 10 (b) show the velocity fluctuation values inside the building at position 7 and 8 in cases 1–8. As seen in Fig. 9(b), in cases 3, 4 and 7 with the 50% opening and lower inlet, the velocity fluctuation was large in the lower region at the inlet (y/H = 0.1–0.3) where the inlet openings were located and gradually decreased above the eave (y/H > 0.3). On the contrary, in cases 1, 2 and 8 velocity fluctuation was large at the upper region but small in the lower region. In case 6 with 100% opening air inlet and outlet, the largest velocity fluctuation existed at the middle of the openings. The velocity fluctuation in case 6 was rather different than other cases. In cases with similar inlets, 100% opening outlet size would lead to larger velocity fluctuations. The velocity fluctuation shown in Fig. 10(b) was smaller than in Fig. 9(b) because the location of measurements was far from the inlet where high turbulence existed. As seen in Fig. 10(b), the velocity variation values at near ground of case 1 was quite big. We estimated that is due to the flow behaviors in this special opening case. High wall above floor at windward together with the leeward opening created an unsteady flow patterns before and after the opening, which resulted in the measurement challenge in limited averaging time. 3.7. Outlet velocity Fig. 11 shows the velocity profiles measured adjacent to the outlet in different cases. Case 2 shows similar velocity profiles as case

Fig. 11. Velocity profiles of cases 1–8 at the sidewall outlet of the building. y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet.

3. The velocity above the eave (y/H > 0.477) was negative, below the outlet (y/H < 0.292) was positive. This suggests a counter clock-wise flow above the outlet. In cases 1, 4 and 5 velocity profiles were very similar but different from cases 2 and 3. The velocity was close to zero below the outlet and was positive at the outlet. There were rapid changes at the outlet because it was close to the eave. The values in case 5 were slightly larger than cases 1 and 4 because of the inlet sizes were larger. Cases 6, 7 and 8 have the same fully opened outlet size, the velocity at y/H = 0.477 was negative, above the eave was positive. Thus, the velocities at the outlet were mainly determined by the size of outlet and inlet. The larger the opening

Fig. 12. Velocity developed at the weak of the scaled building (case 1). y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet. (a) U/Uref , (b)U’/Uref. y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet.

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Fig. 13. Velocity profiles behind the building (cases 5 and 8 lack of data). 10–14 below each plot represents the index of the sampling lines as shown in Fig. 5. The vertical line indicates the zero velocity (U/Uref = 0). y/H = 0.477, 0.292 and 0.104 represent the top, middle and bottom edge of the outlet.

sizes were, the larger the velocities were at the outlet. In all cases, there existed a reversed flow around the outlet close to the eave. 3.8. Velocity development behind the building Fig. 12 shows the velocity development at the weak of the building in Case 1. In the flow region between position L9 (0H) and position L12 (0.5H), there were two strong reversed flow in the flow region above the eave and below the outlet (Fig. 11(a)). The reversed flow reached a maximum velocity above the eave at the position of L10 (0.125H) and gradually turned to be opposite after L12 (0.5H) while the one below the outlet reached to the maximum at the position L12, and be opposite before L14 (2H). The

velocity from the outlet increased from L9 (0H) to L10 (0.125H) and decreased afterwards. The reversed flow may blow back the contaminant air exhausted from the outlet and lead to high contaminant concentration in the region below the outlet or above the eave. Fig. 12(b) shows the velocity fluctuation behind the scaled building. Within a distance between L9 (0H) and L10 (0.125H), the values of velocity fluctuation were small. The velocity fluctuation increased after L11 (0.5H) and decreased after L13 (1H). Large velocity fluctuations can be observed in the region of 0.5-1H below the eave and 0-1H above the eave. The profile of velocity fluctuation became stabilized at the position of L14 (2H).

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Fig. 13 shows the velocity profiles behind the scaled building under seven opening cases. In position 10, velocity was negative in cases 1, 2, 3, 4 and 7 between 0.5-1H and there was a very clear countercurrent particularly evident in case 3. Only the velocities in case 6 were positive at 0.6-1H. In Cases 1, 2 and 4 reversed flow occurred at 0-0.3H. As the position came closer to the outlet, cases 1, 2 and 4, the outlet flow maintained high similarity. Case 3 shows distinguished profiles within a distance of 1H as seen in Fig. 13. In all cases, the reversed flow disappeared at the distance of 2H. The velocity profiles behind the building of different cases turn to be similar after the distance of 1H. Even through the profiles varied between several cases, the least information given by Fig. 13 is that the size and location of outlet and inlet has large impact on the wake flow behind the building. 4. Conclusion This study mainly focused on the velocity distribution inside and outside the low-rise wind-driven ventilated buildings. The experiment was conducted in a building model on a wind channel facility. A laser Doppler Anemometer (LDA) was applied for the velocity and turbulence measurements. By comparing different cases with varied inlet and outlet in the same outdoor wind conditions, the following conclusions can be drawn: (1) The air change rate depended upon the inlet and outlet sizes, especially when the inlet was 100% opened. The location of openings seems to have little impact on the air change rate. (2) Even with the same size and location of inlet but different outlet, the velocity profiles ahead of building remain different. Inlet and indoor velocities with the same inlets were similar even in cases with varied outlet. The velocities at the outlet depended on the inlet sizes. (3) The wake flow behind the building was highly depended on the size and location of inlet and outlet. It was recommended to consider this when investigate the contaminant transport from the inside of the building to the outside. It should be mention that the air change rate determined using tracer gas is highly depended on the injection rate of tracer gas. Low injection rate leads to higher measurement error. In addition, it is showed that uneven concentration distribution at the outlet so that extra consideration is needed on where to measure and how to use the concentration data to calculate the air change rate. In this study, the inlet opening size was large and thus resulted in the high air change rate. In terms of cases with low air change rate by applying small opening sizes, we will continue to investigate in further studies. Acknowledgments The first author is grateful for the financial support of this research by National Natural Science Foundation of China (NSFC) (Grant No. 51408413). Great thanks also give to the financial support by the national key project of the Ministry of Science and Technology, China on “Green Buildings and Building Industrialization” through Grant No. 2016YFC0700500. We thank for the great supports from laboratory technician Jan Ove Johnsen. References [1] A.G. Soldatos, K.G. Arvanitis, P.I. Daskalov, G.D. Pasgianos, N.A. Sigrimis, Nonlinear robust temperature-humidity control in livestock buildings, Comput. Electron. Agric. 49 (3) (2005) 357–376. [2] G. Zhang, B. Bjerg, J.S. Storm, P. Kai, Reducing odor emission from pig production buildings by ventilation control, in: Livestock Environment VIII, 31 August −4 September, Iguassu Falls, Brazil, 2008.

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