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Contaminant Air Propagation between Naturally Ventilated Scale Model Pig Buildings under Steady-state Conditions
ARTICLE IN PRESS Biosystems Engineering (2005) 90 (2), 217–226 doi:10.1016/j.biosystemseng.2004.10.011 SE—Structures and Environment
Contaminant Air Propagation between Naturally Ventilated Scale Model Pig Buildings under Steady-state Conditions A. Ikeguchi1; L. Okushima2; G. Zhang3; J.S. Strom3 1
National Agriculture and Bio-oriented Research Organization, Tsukuba City, Japan; e-mail of corresponding author: [email protected] 2 National Institute for Rural Engineering, Tsukuba City, Japan; e-mail: [email protected] 3 Danish Institute of Agriculture Science, Bygholm, Research Centre Bygholm, DK-8700 Horsens, Denmark; e-mail: [email protected] and [email protected] (Received 8 March 2004; accepted in revised form 27 October 2004)
Propagation of pathogens or viruses such as salmonella is a major concern. Such viruses can be spread by airborne transmission and it is important to understand the airflow between animal production buildings to limit or eradicate contamination. Wind tunnel tests were performed to examine how contaminant air transportation was affected by the distance between the buildings and the location of the contaminant source. Two identical scale models were used, one placed upwind and another downwind. Ethylene gas was used as the contaminant. The gas concentration and the airflow in and around the buildings were measured. It was found that the contaminated air reached the upwind building even if it was generated in the downwind building. When the contaminant was generated in the downward building, the concentration in the upwind building became 103 of the concentration of the contaminant source when the buildings were placed at a separation distance equal to the ridge height. However, it was diluted to a level of 105 when the separation building distance was three times of the ridge height. When the contaminant was generated in the upwind building, the contaminant concentrations in the downwind building were higher compared to the case of contaminant generated in the downwind building. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction Hygiene of animal industry has been one major concern for the public community. The spreading mechanisms of pathogens or viruses in and between livestock buildings in virus studies have been elucidated and included airborne transmission (Burt, 2002; Alexandersen et al., 2002). Alexandersen and Donaldson (2002) collected data to enhance the capability of a model that predicts the risk of airborne foot and mouth disease (FMD) virus spreading. Gerbier et al. (2002) developed a model for the prediction of the infectious potential of farms and reported that the model could be extended to include the effect of wind direction and velocity on airborne spreading of the vira. It is important to understand the airflow inside and around a livestock building in order to predict the behaviour of contaminant air. 1537-5110/$30.00
Seedorf and Hartung (1999) investigated emissions of bacteria from a duck fattening house. A mean germ concentration of approximately 105 colony forming units (cfu) was determined 25 m downwind from the building, and no airborne mesophilic bacteria at the upwind side of the building. A correlation between bacteria concentrations was also predicted by a numeric transmission model (Lagrange model) and from field measurement. Guo et al. (2001) simulated odour dispersion affected by weather conditions with an air dispersion model, INPUFF-2. Odour concentration decreased as the distance from the odour source increased. Guo et al. (2003) evaluated odour dispersions from animal production sites influenced by weather conditions. A large majority of odour events occurred at moderate or slightly stable weather conditions. Also Schauberger et al. (2002) reported that odour sensation can be expected more often for higher wind velocities 217
r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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Notation
CD CG CP E Frd G g H Iu IuR L PU Re U
dimensionless concentration (CP/CG) concentration of C2H4 at the generation point, ppm concentration of C2H4 at measurement point, ppm eave height, m Froude number with respect to density ground level gravity acceleration, m s2 representative length, m turbulence intensity in the X direction turbulence intensity in the X direction at reference height characteristic length, m evaluation parameter in Eqn (4) Reynolds number (U H/n) mean airflow velocity in the X direction, m s1
and a neutral or stable atmosphere and calculated direction-dependent separation distances for a typical agricultural site in Austria with the dynamic Austrian odour dispersion model (AODM). When a dispersion range is paid attention to a farm with several livestock buildings, more factors affecting dispersion property are added, they are building arrangement, building shape, ventilation method, etc. Ikeguchi and Okushima (2001) elucidated that dispersion properties of contaminant air from dairy buildings affected by their roof shape and wind direction. Wind tunnel was used and the steady state was assumed in order to exclude factors except wind direction and building shape. Further understanding is required virus and pathogen spread between buildings. The objective of this study was to describe contaminant transmission between two buildings placed at three separation distances apart with contaminant being emitted from either the upwind or the downwind building. The study is carried out in a wind tunnel to eliminate the effect of weather conditions with scale models of naturally ventilated pig buildings.
2. Equipment and procedure 2.1. Similarity The study was based on an assumption that airflow and contaminant generation were steady state and isothermal. The experiments on contaminant dispersion were performed in a wind tunnel, which followed three
UD UR u x, y, z zR n su r
normalised mean air velocity in the X direction (U/UR) reference mean air velocity at a reference height, m s1 instantaneous airflow velocity measured at 200 Hz in the X direction, m s1 coordinates reference height, mm kinetic viscosity, m2 s1 standard deviation of instantaneous air velocities in the X direction, m s1 density, kg m3
Subscripts a air s contaminant gas
similarity conditions: (1) geometrical similarity of the buildings; (2) similarity of the flow field; and (3) similarity with regard to contaminant dispersion. 2.1.1. Geometrical similarity Two identical 1:20 scale models of a pig production building were used, as described by Morsing et al. (2002) and Ikeguchi et al. (2003). The buildings were naturally ventilated with sidewall openings. The full-scale dimensions were 30 m (length) by 11 m (depth) by 52 m (ridge height). The ridge height was regarded as a representative length H (Fig. 1). 2.1.2. Similarity condition in flow fields There are two similarity conditions in flow fields, i.e. dynamic similarity condition and momentum similarity condition (Kimura et al., 1987). The former requires that the Reynolds number Re coincides between a wind tunnel test and the full-scale situation. However, this is generally impossible to achieve because the air velocity in a wind tunnel test to be 10–103 times as large as that in full-scale as the geometric scale is small. When a flow field is fully turbulent, the airflow in the wind tunnel and that in the full-scale building becomes similar under isothermal conditions (Kimura et al., 1987). Mochida (1988) reported that the airflow in a wind tunnel test was similar when the value of Re was more than 103. Thus, no coincidence between the value for Re in the wind tunnel test and that in the full-scale building is needed when it is larger than 103. With regard to the momentum similarity condition, the vertical profiles of the mean air velocity and the
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Vertical view of wind tunnel Total length: 68 m Diffusion section Straightener section
Testing section
Plane view of testing section 20 m
1500 mm
4m
Model
Roughness blocks
Sensor traveller 3m
Net Carpet Turntable 3.45 m
Vertical view of testing section
(a)
Y Downwind building
Upwind building
260 mm H 60 mm 70 mm 550 mm
X Axes
130 mm
Separation distance 1 × H = 260 mm, 2 × H = 520 mm, 3 × H = 780 mm
(b)
Fig. 1. (a) Wind tunnel and (b) dimensions of scale model: H, representative length ¼ ridge height, 260 mm
turbulent intensity in the wind tunnel test must coincide with that in the full-scale building. Results from wind tunnel tests are often given as ratios between the mean air velocity in a measurement point to a representative mean air velocity at a reference height. The mean air velocity ratio in the scale model is the same in the corresponding point in the full-scale building.
A half-way point between the ridge and the eaves, is generally chosen as a reference height (Japan Architecture Centre, 1996). In this study, this reference height was 019 m in the wind tunnel tests. The representative mean air velocity at the reference height was 27 m s1 in the wind tunnel test, the same as in a previous study (Ikeguchi et al., 2003). Assuming an airflow velocity U of 27 m s1, a representative height H of 026 m and a
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kinetic viscosity n of 156 105 m2 s1 , the value for Re in the wind tunnel tests was 45 104.
Table 1 Experimental design; H is representative length and equal to ridge height, 260 mm in the scale model
2.1.3. Similarity condition of contaminant dispersion Under non-isothermal condition, the similarity condition for gas dispersion is a function of the Froude number with respect to density Frd of the discharged contaminant gas and the density ratio of discharged contaminant gas to air. The Froude number with respect to density was expressed as rs ra Lg Frd ¼ (1) ra U 2
Case no
where: rs is a contaminant gas density in kg m3; ra is an air density in kg m3; L is a characteristic length in m; g is gravitational acceleration in m s2; and U is a mean air velocity in m s1. However, in this study the contaminant dispersion was approximated by a tracer gas of density similar to that of air. As isothermal conditions were assumed, mass transportation by airflow was superior to diffusion by gradient of concentration. The objective contaminants were germs, viruses and dusts on which they are attached. They are passive scalars that move with airflow. This approximation is common in a wind tunnel test with respect to discharged gas dispersion from industrial factories (Shimogata et al., 1974). Ethylene gas (C2H4) was selected as the tracer gas.
2.2. Experimental design The two identical scale model buildings were made of Plexiglas and placed along the symmetric plane in the wind tunnel. The sidewalls with inlet openings were perpendicular to the wind direction. The experimental factors were: (1) position of the contaminant emitting building; and (2) separation distance between the two buildings. The contaminant was emitted from either the upwind building or from the downwind building. The separation distances between the buildings were 1 H, 2 H and 3 H measured from sidewall-to-sidewall, where H was a representative length equal to the ridge height, i.e. 026 m in the model scale. These combinations are shown in Table 1. The building with the contaminant gas generation is referred to as the emitting building and the other, which is passively affected by the contaminant gas dispersion, is referred to as the receiving building. The term ‘separation distance’ is defined as the wall-to-wall distance between the two buildings (Fig. 1).
1 2 3 4 5 6
Contaminant-emitting building Downwind building Downwind building Downwind building Upwind building Upwind building Upwind building
Separation distance 1H 2H 3H 1H 2H 3H
2.3. Scale model buildings The buildings were 1:20 scale models of typical naturally ventilated houses for growing/finishing pigs. The models were 1500 mm in length and 550 mm in width, eaves height of 130 mm and ridge height of 260 mm. The sidewall was 70 mm high leaving a slotinlet opening height of 60 mm between the wall and the underside of the roof plate (Fig. 1). These scale models were the same as described by Zhang et al. (2003) and Ikeguchi et al. (2003).
2.4. Wind tunnel A wind tunnel at National Institution for Rural Engineering, Japan was used (Ikeguchi & Okushima, 2001; Morsing et al., 2002; Ikeguchi et al., 2003; Lee et al., 2003). The wind tunnel was 68 m long including the testing section, which was 20 m long with a 3 m by 4 m cross-section. The geometric dimensions, plane and cross-section view of the testing section are shown in Fig. 1. The wind tunnel is inside the building and the air leaving the wind tunnel is exhausted outside building from the opening of the building. The maximum average airspeed was 13 m s1 created by a fan, 6 m in diameter upstream of the testing section. The maximum airflow rate was 650 000 m3 h1 at 350 min1. Between the fan section and the testing section was a conversion section with perimeter bleedoff and a straightener section. Roughness blocks, carpet, and net were used on the floor in the testing section to develop the appropriate wind speed and turbulence intensity profiles. The models were placed on a turntable located 1255 m downstream from the front of the testing section. The turntable was used to rotate the test models through 3601 with an accuracy of 70.11 to simulate wind approaching from different directions. A sensor traveller moved a three-dimensional hot film anemometer to each measurement point with a movement accuracy of 71 mm.
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The wind profile is generally expressed using either the logarithmic or exponential equation. In the present study, the logarithmic equation was used because the roughness length appeared in the equation. The vertical mean air velocity and the vertical turbulence intensity profile in the wind tunnel were measured at the centre of the turntable without the scale models. As in a previous study (Ikeguchi et al., 2003), these were expressed as z U ¼ 119 ln (2) 056 090 z (3) I u ¼ I uR zR su (4) U where: U is the mean air velocity in m s1 in the main flow direction (X direction); z is the height above ground in m; 056 mm is a roughness length for scale model; Iu is the turbulence intensity at a given height; IuR is turbulence intensity at the reference height; zR is the reference height equal to 190 mm for the scale model; and su is the standard deviation of instantaneous air velocities in m s1. For the equations in Eqns (2) and (3), the values for the coefficients of determination R2 are 0996 and 065, respectively, and those for the standard error (SE) are 0041 and 0044. The roughness length of 056 mm corresponds to a full-scale roughness length of 112 mm, which is similar to the roughness of a typical grass field (Tsuboi, 1977). Iu ¼
2.5. Measurement 2.5.1. Gas concentration Ethylene gas (C2H4) was used as the contaminant because its specific weight is approximately equal to that of air. The C2H4 was diluted with 100% nitrogen to 1% prior to supplying to the gas-emitting building. This means that it can be considered as a passive scalar and moves with the air. The emitting rate was 417 mm3 s1. Ethylene concentration was measured by a flammable ion detector (Model SC-7501, Iwatsu electric Co. Ltd.) with an accuracy of 71%. The sampling frequency was 10 Hz and the sampling period was 40 s based on the measurement of air velocity. The measurement points are shown in Fig. 2. The numbers of measurement points outside the buildings were 7, 7, and, 11 for buildings distances of 1 H, 2 H and 3 H, respectively. The number of measurement points inside each building, including at sidewall openings, was 4; and then there was one measured point outside wind tunnel as a background.
2.5.2. Airflow The air velocity was measured by three-dimensional hot film anemometry (model IF300, TSI Inc.). A sensor traversing system carried the anemometer to each measured point. The sampling frequency was 200 Hz, and the sampling period was 419 s at each measured points. These were decided based on considering the smallest eddies of Kolmogorov and the fluctuation of fan rotation conducting larger eddies. The measurement points are shown in detail in Fig. 2. The numbers of measurement points outside the buildings were 100, 116, and 116 for buildings distances of 1 H, 2 H and 3 H, respectively. The number of measurement points inside each building was 12.
2.6. Evaluation parameters To evaluate contaminant transportation by mean of airflow, an evaluation parameter PU proposed in the previous studies by Ikeguchi and Okushima (2001) was used. It is a measure of airflow momentum between buildings and is used to explain the distribution properties of the contaminant concentration. The evaluation parameter PU is expressed as follows: UD ¼ Z
U UR
(5)
U D dz
(6)
B
PU ¼ G
where: the integration limit G is ground level and E is eaves height in m; UD is normalised air velocity in the X direction; U is measured mean air velocity in the X direction at each measuring point in m s1; and UR is a reference mean air velocity, taken as 27 m s1. This parameter means the momentum of flow in the downwind direction at a given X location in the test section from ground to ridge height. The large value expresses that a momentum in the direction of the downwind is large and the contaminant concentration is low.
3. Results and discussion The average value of background (outside wind tunnel) C2H4 concentrations during experiments was 0001 ppm and it was less than 7% of minimum value of measured concentration inside wind tunnel. So it was regarded that the effect of entry of C2H4 in the wind tunnel was small and the background concentration was approximately equal to zero.
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Windward i 3
1 2
Downwind 4
5
6
7
9
8
10
11
12
13
14
16
15
8 13 6.5 6.5 5 7 6.2 3 . 15 13 5 14 3 (a)
7 6 4
13 13.5 14
14 13.5 13
14 13.5 13
26
13
52
5 3 2 1
j
i 3
1 2
4
5
6
7
13 (b)
9
8
13
10
11 12
9
10
13
14 15 16
17
18
13
13
i 1 2
3
4
5
6
7
13
8
13
26
11
12
13
14 15 16
17
18
9
26
(c)
Fig. 2. Gas concentration and air velocity measured points for: (a) separation distance of 1 H; (b) separation distance of 2 H; and (c) separation distance of 3 H. The scale is the scale model (all units in cm): H, 26 cm, representative length ¼ ridge height; i, vertical measured line number; j, horizontal measured line number; K, air velocity measured point inside building ; , gas concentration measured point; grid intersections are air velocity measured points outside building; , gas point and concentration measured point for gas generation. Cases 1–3 at upwind building. Cases 4–6 at downwind building
The concentration of C2H4 is expressed by a dimensionless concentration CD, defined as CD ¼
CP CG
(7)
where: Cp is a concentration of C2H4 at a measured point in ppm; and CG is the concentration of C2H4 at the generation point in ppm.
3.1. Gas dispersion The dimensionless gas concentrations in the scale models 90 mm above the ground are shown in Figs 3 and 4, for gas-emitting building being downwind and upwind, respectively. The contaminant air reaches the upwind building even when the gas is emitted from the downwind building. As
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3×H
Dimensionless concentration, CD
1
2×H
Dimensionless concentration, CD
Upwind building
1×H
1 0.1 0.01 0.001
0
500
1000 1500 2000 Distance x, mm
2500
3000
Fig. 3. Dimensionless concentration (CD) at opening height (90 mm) when gas was emitted in the downwind building: H, representative length ¼ ridge height, 260 mm in the scale model; , separation distance 1 H; , separation distance 2 H; , separation distance 3 H
3×H 2×H Upwind building 1 × H Dimensionless concentration, CD
0.01 0.001 0.0001 0.00001 0
0.0001 0.00001
0.1
1.0000
0.1000
0.0100
0.0010 0
500
1000 1500 2000 Distance x, mm
2500
3000
Fig. 4. Dimensionless concentration (CD) at opening height (90 mm) when gas was emitted in the upwind building: H, representative length ¼ ridge height, 260 mm in the scale model; , separation distance 1 H; , separation distance 2 H; , separation distance 3 H
shown in Fig. 5, the average value for CD inside the upwind, receiving building is about 0001 when the separation distances are 1 H and 2 H. This means that the contaminant concentration in the upwind building is 01% of the contaminant concentration supplied to the downwind building. The average value
1 2 3 Separation distance H, H=260 mm
Fig. 5. Average dimensionless concentration (CD) in the contaminant receiving building: H, representative length ¼ ridge height, 260 mm in the scale model; , Gas was emitted in the upwind building; , Gas was emitted in the downwind building
for CD is 105 inside the receiving building when the separation distance was 3 H. Thus, the concentration inside the receiving building decreases dramatically when the separation distance between the buildings is increased sufficiently (Fig. 5). In order to avoid major transmission of contaminant from the downwind building, it is necessary to keep a distance of at least 3 H between the buildings. A two-way analysis of variance (ANOVA) was performed with respect to CD inside the receiving building using three measured points inside the building as replicate. There was significant difference (variance ratio F ¼ 728; probability P ¼ 0019) among gasgenerating location. The value of CD in the receiving building was higher with the gas-emitting upwind than with the gas emitting building being downwind. With the gas-emitting building being downwind, the one-way ANOVA gave a significant difference (F ¼ 994; P ¼ 000) for separation distance. However, there was no significant difference for separation distances with the gas-emitting building upwind of the receiving building. This shows that there was no significant relationship between the separation distance and contaminant concentration inside the receiving building when the gas was emitted from the upwind building. When the contaminant was emitted from the upwind building, the value of CD in the receiving building was about 0025, 0004 and 0004 for the separation distance 1 H, 2 H and 3 H, respectively. As the separation distance became longer than 2 H, the CD became onesixth times as low as that for 1 H. However, the CD measured with a separation distance of 3 H was almost the same as that with 2 H. These values were naturally higher than when gas was emitted from the
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Table 2 Average dimensionless concentration CD in the contaminantemitting building; H is representative length and equal to ridge height, 260 mm in the scale model
Upwind
Downwind
0153 0156 0162
0175 0195 0160
0.02
Dimensionless concentration, CD
1
0.01 0.1
0
0.01
−0.01 −0.02
0.001
−0.03 −0.04
0.0001
−0.05 −0.06
0.00001
1×H 2×H 3×H Separation distance H, H=260 mm
−0.07
Fig. 6. Average dimensionless concentration (CD) and evaluation parameter (PU) between the buildings: H, representative length ¼ ridge height, 260 mm in the scale model; , gas was emitted in the upwind building; , gas was emitted in the downwind building; , PU
downwind building. They were about 20, 3 and 264 times for 1 H, 2 H and 3 H, respectively, compared to the case of gas generated in the downwind building. The average value for CD inside the gas-emitting building is shown in Table 2. There was no significant difference among location of gas-emitting building and separation distance. The average value was 0167 for the cases in this investigation. In the area between the buildings, contaminant air dispersion followed the flow motion between buildings. The two-way ANOVA was performed with respect to CD at the middle position between the buildings using four vertical measured points as replicate. When the separation distance was 3 H, the average CD values of the vertical measured line number i of 8 and 9 (Fig. 2) were used for each height. There were significant differences for gas generated location (F ¼ 252; P ¼ 0000) and separation distance (F ¼ 538; P ¼ 0015). The average CD for each case in the middle position between the buildings is shown in Fig. 6. The trend is similar to that in the receiving building,
0.018 Dimensionless concentration, CD
1H 2H 3H
Contaminant-emitting building
Evaluation parameter PU, dimensionless
Separation distance
0.02
0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0
1×H 2×H 3×H Separation distance H, H=260 mm
Fig. 7. Dimensionless concentration (CD) at leeward side of the downwind building: H, representative length ¼ ridge height, 260 mm in the scale model; , 05 H leeward from the downwind building when gas was emitted in the downwind building; , 1 H leeward from the downwind building when gas was emitted in the downwind building; , 2 H leeward from the downwind building when gas was emitted in the downwind building; , 05 H leeward from the downwind building when gas was emitted in the upwind building; , 1 H leeward from the downwind building when gas was emitted in the upwind building; , 2 H leeward from the downwind building when gas was emitted in the upwind building
however, the values being about ten times as high as those in the receiving building. The CD in the area of the leeward side of the downwind building is shown in Fig. 7, as functions of the separation distance between the buildings. As the separation distance increased, CD had a tendency to become higher when contaminant was emitted from the downwind building. On the other hand, when the contaminant was generated in the upwind building, the CD tended to become lower as the buildings distance increased.
3.2. Airflow The CD distribution stated above is attributed to the airflow patterns, because C2H4 was transferred with airflow. The airflow patterns for normalised airflow vectors for each case are shown in Fig. 8. Contaminant reaches the upwind building as the air in the downwind building through moves in the leeward opening, out through the windward side opening towards the upwind building.
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Height, cm
: UD = 1.0 60 50 40 30 20 10 0 0
50
100
150 200 Horizontal distance, cm
250
300
100
150 200 Horizontal distance, cm
250
300
100
150 200 Horizontal distance, cm
250
300
(a)
Height, cm
: UD = 1.0 60 50 40 30 20 10 0 0
50
(b)
Height, cm
: UD = 1.0 60 50 40 30 20 10 0 0 (c)
50
Fig. 8. Airflow pattern with normalized air velocity (UD) for (a) separation distance of 1 H; (b) separation distance of 2 H; and (c) separation distance of 3 H;H, representative length ¼ ridge height, 26 cm in the scale model
Inside the upwind building, most of the air moved toward the windward sidewall for the building distance 1 H due to the eddies in and around the building, which resulted in the contaminant hardly moved out of the building [Fig. (8a)]. However, for the separation distance 2 H and 3 H, the air moved toward the upwind sidewall under the opening height and toward the downwind sidewall above the opening height [Figs 8(b) and (c)]. This shows that there was a clockwise circulation. Inside the downwind building, most air moved toward the downwind side for building distance 1 H and 3 H. On the other hand, for separation distance 2 H, the air under opening height moved toward the windward side. The air velocities inside the downwind building were smaller than those inside the upwind building. When the separation distance was 1 H, the inside air velocities in the downwind building were higher than those for the separation distance 2 H and 3 H. In the area of the leeward side of the downwind building, retouch position of the wake for the small separation distance was between 1 H and 2 H (Fig. 7). For the two larger separation distances between the buildings the retouch positions were close to 1 H and between 05 H and 1 H, respectively. This indicates that the wake became smaller as the separation
distance increased. A larger wake prevented the contaminant generated in the downwind building from leaving the building through the leeward side opening. Consequently, the CD at 05 H from the downward building (i ¼ 13 for 1 H, i ¼ 15 for 2 H and 3 H) was lower for the small building distance compared to the larger separation distance when the contaminant was emitted from the downwind building. In the area between the buildings, the air below the ridge height moved toward the upwind building for separation distance 1 H and 2 H. For the separation distance 3 H, however, the air below the opening height at the vertical measured line i ¼ 9 moved in the downwind direction. The different air velocities and flow directions affected by the building distances induced the differences of CD inside the buildings. The parameter PU between buildings may be used to express the degree of contaminant transfer between buildings. The larger value of PU means a larger momentum in the main flow direction and a smaller contaminant concentration. It can be seen in Fig. 6 that, the value of PU became larger and CD became smaller as the building distance increased. The values of PU for the separation distance 2 H and 3 H were negative. Naturally, the quantity of transferred contaminant
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became smaller as the separation distance was increased. From Figs 3 and 4, it can also be seen that the CD in this area decreased more than at the leeward side of the downwind building.
4. Conclusions Wind tunnel tests of naturally ventilated scale model pig barns were performed to investigate air transmissions of contaminants from a gas-emitted building to another contaminant receiving building. Ethylene gas was used as the contaminant. The effects of the location of the gas-emitting building and distance between the buildings, were examined under steady-state conditions. The following conclusions can be drawn. (1) The contaminant generated in the downwind building reached the upwind building because of the wake at leeward side of the downwind building. The concentration was 01% of the generated concentration in the downwind building when the separation distance was the same as the ridge height. (2) As the separation distance increased, the concentration in the receiving building was lower when the contaminant was emitted in the downwind building. In the case of a separation distance of 3 H (where H is a representative length equivalent to the ridge height), the dimensionless concentration in the receiving, upwind building became 105 for emitted concentration in the downwind building. At least a distance of 3 H between two buildings is needed to minimise the contaminant transfer from the downwind building to the upwind building. (3) When the contaminant was emitted from the upwind building, the concentrations in the receiving, downwind building were larger than those when the contaminant was emitted from the downwind building. (4) As the separation distance was increased, the wake at leeward side of the downwind building became smaller, a measure of airflow momentum between buildings, became larger and therefore the contaminant transmissions became smaller.
References Alexandersen S; Donaldson A I (2002). Further studies to quantify the dose of natural aerosols of foot-and-mouth disease virus to pigs. Epidemiology and Infection, 128(2), 313–323 Alexandersen S; Brotherhood S; Donaldson A I (2002). Natural aerosol transmission of foot-and-mouth disease virus to pigs: minimal infectious dose for strain 01 Lausanne. Epidemiology and Infection, 128(2), 301–312
Burt P J A (2002). Airborne foot and mouth virus. Weather, 57(5), 192–193 Ikeguchi A; Okushima L (2001). Airflow patterns related to polluted air dispersion on open free-stall dairy buildings with different roof shapes. Transactions of the ASAE, 44(6), 1797–1805 Ikeguchi A; Zhang G; Okushima L; Bennetsen J C (2003). Windward windbreak effects on airflow in and around a scale model of a naturally ventilated pig barn. Transactions of the ASAE, 46(3), 789–795 Gerbier G; Pouillot J N; Durand R; Moutou B; Chadoeuf J (2002). A point pattern model of the spread of foot-andmouth disease. Preventive Veterinary Medicine, 56(1), 33–49 Guo H; Jacobson L D; Schmidt D R; Janni K A (2001). Calibrating INPUFF-2 model by resident panellists for long-distance odor dispersion from animal production sites. Applied Engineering in Agriculture, 17(6), 859–868 Guo H; Jacobson L D; Schmidt D R; Nicolai R E (2003). Evaluation of the influence of atomospheric conditions on odor dispersion from animal production sites. Transactions of the ASAE, 46(2), 461–466 Japan Architecture Centre (1996). Guide Book of Wind Tunnel Tests, pp 22–26. Department of Publication in Japan Architecture Centre, Tokyo, Japan (in Japanese) Kimura K; Yoshino M; Murakami S; Moriyama M; Aratani N (1987). New Architecture, Natural Environment, Vol. 8, pp 208–212. Shokokusha, Tokyo (in Japanese) Lee I; Sase S; Okushima L; Ikeguchi A; Choi K; Yun J (2003). A wind tunnel study of natural ventilation for multi-span greenhouse scale models using two-dimensional particle image velocimetry (PIV). Transactions of the ASAE, 46(3), 763–772 Mochida A (1988). Studies on a predictive method of airflow pattern around buildings and gas dispersion. PhD Thesis, Faculty of engineering, Tokyo University, Tokyo (in Japanese) Morsing S; Ikeguchi A; Bennetsen J C; Strom J S; Ravn P; Okushima L (2002). Wind-induced isothermal airflow patterns in a scale model of a naturally ventilated swine barn with cathedral ceiling. Applied Engineering in Agriculture, 18(1), 97–101 Schauberger G; Piringer M; Petz E (2002). Calculating direction-dependent separation distance by a dispersion model to avoid livestock odour annoyance. Biosystems Engineering, 82(1), 25–27 Shimogata S; Sugawara K; Yokoyama O (1974). Wind tunnel experiment of diffusion of exhaust gas emitted from roof ventilator of long factory building. Pollution Control, 9(1–2), 62–70 (in Japanese) Seedorf J; Hartung J (1999). Measured and calculated bacteria concentrations in the vicinity of a duck fatting unit. Proceedings of International Symposium on Dust Control in Animal Production Facilities, pp 179–185. Department of Agricultural Engineering, Danish Institute of Agricultural Sciences, Research Centre Bygholm, Horsens, Denmark Tsuboi Y ed (1977). Agricultural Meteorology Handbook. Youkendo, Tokyo, Japan Zhang G; Ikeguchi A; Strom J; Morsing S; Takai H; Ravn P; Okushima L (2003). Obstacle effects on airflow and containment dispersion around a naturally ventilated livestock building. Agricultural Engineering International: CIGR Journal of Scientific Research and Development, V, BC 03 004
Contaminant Air Propagation between Naturally Ventilated Scale Model Pig Buildings under Steady-state Conditions A. Ikeguchi1; L. Okushima2; G. Zhang3; J.S. Strom3 1
National Agriculture and Bio-oriented Research Organization, Tsukuba City, Japan; e-mail of corresponding author: [email protected] 2 National Institute for Rural Engineering, Tsukuba City, Japan; e-mail: [email protected] 3 Danish Institute of Agriculture Science, Bygholm, Research Centre Bygholm, DK-8700 Horsens, Denmark; e-mail: [email protected] and [email protected] (Received 8 March 2004; accepted in revised form 27 October 2004)
Propagation of pathogens or viruses such as salmonella is a major concern. Such viruses can be spread by airborne transmission and it is important to understand the airflow between animal production buildings to limit or eradicate contamination. Wind tunnel tests were performed to examine how contaminant air transportation was affected by the distance between the buildings and the location of the contaminant source. Two identical scale models were used, one placed upwind and another downwind. Ethylene gas was used as the contaminant. The gas concentration and the airflow in and around the buildings were measured. It was found that the contaminated air reached the upwind building even if it was generated in the downwind building. When the contaminant was generated in the downward building, the concentration in the upwind building became 103 of the concentration of the contaminant source when the buildings were placed at a separation distance equal to the ridge height. However, it was diluted to a level of 105 when the separation building distance was three times of the ridge height. When the contaminant was generated in the upwind building, the contaminant concentrations in the downwind building were higher compared to the case of contaminant generated in the downwind building. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction Hygiene of animal industry has been one major concern for the public community. The spreading mechanisms of pathogens or viruses in and between livestock buildings in virus studies have been elucidated and included airborne transmission (Burt, 2002; Alexandersen et al., 2002). Alexandersen and Donaldson (2002) collected data to enhance the capability of a model that predicts the risk of airborne foot and mouth disease (FMD) virus spreading. Gerbier et al. (2002) developed a model for the prediction of the infectious potential of farms and reported that the model could be extended to include the effect of wind direction and velocity on airborne spreading of the vira. It is important to understand the airflow inside and around a livestock building in order to predict the behaviour of contaminant air. 1537-5110/$30.00
Seedorf and Hartung (1999) investigated emissions of bacteria from a duck fattening house. A mean germ concentration of approximately 105 colony forming units (cfu) was determined 25 m downwind from the building, and no airborne mesophilic bacteria at the upwind side of the building. A correlation between bacteria concentrations was also predicted by a numeric transmission model (Lagrange model) and from field measurement. Guo et al. (2001) simulated odour dispersion affected by weather conditions with an air dispersion model, INPUFF-2. Odour concentration decreased as the distance from the odour source increased. Guo et al. (2003) evaluated odour dispersions from animal production sites influenced by weather conditions. A large majority of odour events occurred at moderate or slightly stable weather conditions. Also Schauberger et al. (2002) reported that odour sensation can be expected more often for higher wind velocities 217
r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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Notation
CD CG CP E Frd G g H Iu IuR L PU Re U
dimensionless concentration (CP/CG) concentration of C2H4 at the generation point, ppm concentration of C2H4 at measurement point, ppm eave height, m Froude number with respect to density ground level gravity acceleration, m s2 representative length, m turbulence intensity in the X direction turbulence intensity in the X direction at reference height characteristic length, m evaluation parameter in Eqn (4) Reynolds number (U H/n) mean airflow velocity in the X direction, m s1
and a neutral or stable atmosphere and calculated direction-dependent separation distances for a typical agricultural site in Austria with the dynamic Austrian odour dispersion model (AODM). When a dispersion range is paid attention to a farm with several livestock buildings, more factors affecting dispersion property are added, they are building arrangement, building shape, ventilation method, etc. Ikeguchi and Okushima (2001) elucidated that dispersion properties of contaminant air from dairy buildings affected by their roof shape and wind direction. Wind tunnel was used and the steady state was assumed in order to exclude factors except wind direction and building shape. Further understanding is required virus and pathogen spread between buildings. The objective of this study was to describe contaminant transmission between two buildings placed at three separation distances apart with contaminant being emitted from either the upwind or the downwind building. The study is carried out in a wind tunnel to eliminate the effect of weather conditions with scale models of naturally ventilated pig buildings.
2. Equipment and procedure 2.1. Similarity The study was based on an assumption that airflow and contaminant generation were steady state and isothermal. The experiments on contaminant dispersion were performed in a wind tunnel, which followed three
UD UR u x, y, z zR n su r
normalised mean air velocity in the X direction (U/UR) reference mean air velocity at a reference height, m s1 instantaneous airflow velocity measured at 200 Hz in the X direction, m s1 coordinates reference height, mm kinetic viscosity, m2 s1 standard deviation of instantaneous air velocities in the X direction, m s1 density, kg m3
Subscripts a air s contaminant gas
similarity conditions: (1) geometrical similarity of the buildings; (2) similarity of the flow field; and (3) similarity with regard to contaminant dispersion. 2.1.1. Geometrical similarity Two identical 1:20 scale models of a pig production building were used, as described by Morsing et al. (2002) and Ikeguchi et al. (2003). The buildings were naturally ventilated with sidewall openings. The full-scale dimensions were 30 m (length) by 11 m (depth) by 52 m (ridge height). The ridge height was regarded as a representative length H (Fig. 1). 2.1.2. Similarity condition in flow fields There are two similarity conditions in flow fields, i.e. dynamic similarity condition and momentum similarity condition (Kimura et al., 1987). The former requires that the Reynolds number Re coincides between a wind tunnel test and the full-scale situation. However, this is generally impossible to achieve because the air velocity in a wind tunnel test to be 10–103 times as large as that in full-scale as the geometric scale is small. When a flow field is fully turbulent, the airflow in the wind tunnel and that in the full-scale building becomes similar under isothermal conditions (Kimura et al., 1987). Mochida (1988) reported that the airflow in a wind tunnel test was similar when the value of Re was more than 103. Thus, no coincidence between the value for Re in the wind tunnel test and that in the full-scale building is needed when it is larger than 103. With regard to the momentum similarity condition, the vertical profiles of the mean air velocity and the
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Vertical view of wind tunnel Total length: 68 m Diffusion section Straightener section
Testing section
Plane view of testing section 20 m
1500 mm
4m
Model
Roughness blocks
Sensor traveller 3m
Net Carpet Turntable 3.45 m
Vertical view of testing section
(a)
Y Downwind building
Upwind building
260 mm H 60 mm 70 mm 550 mm
X Axes
130 mm
Separation distance 1 × H = 260 mm, 2 × H = 520 mm, 3 × H = 780 mm
(b)
Fig. 1. (a) Wind tunnel and (b) dimensions of scale model: H, representative length ¼ ridge height, 260 mm
turbulent intensity in the wind tunnel test must coincide with that in the full-scale building. Results from wind tunnel tests are often given as ratios between the mean air velocity in a measurement point to a representative mean air velocity at a reference height. The mean air velocity ratio in the scale model is the same in the corresponding point in the full-scale building.
A half-way point between the ridge and the eaves, is generally chosen as a reference height (Japan Architecture Centre, 1996). In this study, this reference height was 019 m in the wind tunnel tests. The representative mean air velocity at the reference height was 27 m s1 in the wind tunnel test, the same as in a previous study (Ikeguchi et al., 2003). Assuming an airflow velocity U of 27 m s1, a representative height H of 026 m and a
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kinetic viscosity n of 156 105 m2 s1 , the value for Re in the wind tunnel tests was 45 104.
Table 1 Experimental design; H is representative length and equal to ridge height, 260 mm in the scale model
2.1.3. Similarity condition of contaminant dispersion Under non-isothermal condition, the similarity condition for gas dispersion is a function of the Froude number with respect to density Frd of the discharged contaminant gas and the density ratio of discharged contaminant gas to air. The Froude number with respect to density was expressed as rs ra Lg Frd ¼ (1) ra U 2
Case no
where: rs is a contaminant gas density in kg m3; ra is an air density in kg m3; L is a characteristic length in m; g is gravitational acceleration in m s2; and U is a mean air velocity in m s1. However, in this study the contaminant dispersion was approximated by a tracer gas of density similar to that of air. As isothermal conditions were assumed, mass transportation by airflow was superior to diffusion by gradient of concentration. The objective contaminants were germs, viruses and dusts on which they are attached. They are passive scalars that move with airflow. This approximation is common in a wind tunnel test with respect to discharged gas dispersion from industrial factories (Shimogata et al., 1974). Ethylene gas (C2H4) was selected as the tracer gas.
2.2. Experimental design The two identical scale model buildings were made of Plexiglas and placed along the symmetric plane in the wind tunnel. The sidewalls with inlet openings were perpendicular to the wind direction. The experimental factors were: (1) position of the contaminant emitting building; and (2) separation distance between the two buildings. The contaminant was emitted from either the upwind building or from the downwind building. The separation distances between the buildings were 1 H, 2 H and 3 H measured from sidewall-to-sidewall, where H was a representative length equal to the ridge height, i.e. 026 m in the model scale. These combinations are shown in Table 1. The building with the contaminant gas generation is referred to as the emitting building and the other, which is passively affected by the contaminant gas dispersion, is referred to as the receiving building. The term ‘separation distance’ is defined as the wall-to-wall distance between the two buildings (Fig. 1).
1 2 3 4 5 6
Contaminant-emitting building Downwind building Downwind building Downwind building Upwind building Upwind building Upwind building
Separation distance 1H 2H 3H 1H 2H 3H
2.3. Scale model buildings The buildings were 1:20 scale models of typical naturally ventilated houses for growing/finishing pigs. The models were 1500 mm in length and 550 mm in width, eaves height of 130 mm and ridge height of 260 mm. The sidewall was 70 mm high leaving a slotinlet opening height of 60 mm between the wall and the underside of the roof plate (Fig. 1). These scale models were the same as described by Zhang et al. (2003) and Ikeguchi et al. (2003).
2.4. Wind tunnel A wind tunnel at National Institution for Rural Engineering, Japan was used (Ikeguchi & Okushima, 2001; Morsing et al., 2002; Ikeguchi et al., 2003; Lee et al., 2003). The wind tunnel was 68 m long including the testing section, which was 20 m long with a 3 m by 4 m cross-section. The geometric dimensions, plane and cross-section view of the testing section are shown in Fig. 1. The wind tunnel is inside the building and the air leaving the wind tunnel is exhausted outside building from the opening of the building. The maximum average airspeed was 13 m s1 created by a fan, 6 m in diameter upstream of the testing section. The maximum airflow rate was 650 000 m3 h1 at 350 min1. Between the fan section and the testing section was a conversion section with perimeter bleedoff and a straightener section. Roughness blocks, carpet, and net were used on the floor in the testing section to develop the appropriate wind speed and turbulence intensity profiles. The models were placed on a turntable located 1255 m downstream from the front of the testing section. The turntable was used to rotate the test models through 3601 with an accuracy of 70.11 to simulate wind approaching from different directions. A sensor traveller moved a three-dimensional hot film anemometer to each measurement point with a movement accuracy of 71 mm.
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The wind profile is generally expressed using either the logarithmic or exponential equation. In the present study, the logarithmic equation was used because the roughness length appeared in the equation. The vertical mean air velocity and the vertical turbulence intensity profile in the wind tunnel were measured at the centre of the turntable without the scale models. As in a previous study (Ikeguchi et al., 2003), these were expressed as z U ¼ 119 ln (2) 056 090 z (3) I u ¼ I uR zR su (4) U where: U is the mean air velocity in m s1 in the main flow direction (X direction); z is the height above ground in m; 056 mm is a roughness length for scale model; Iu is the turbulence intensity at a given height; IuR is turbulence intensity at the reference height; zR is the reference height equal to 190 mm for the scale model; and su is the standard deviation of instantaneous air velocities in m s1. For the equations in Eqns (2) and (3), the values for the coefficients of determination R2 are 0996 and 065, respectively, and those for the standard error (SE) are 0041 and 0044. The roughness length of 056 mm corresponds to a full-scale roughness length of 112 mm, which is similar to the roughness of a typical grass field (Tsuboi, 1977). Iu ¼
2.5. Measurement 2.5.1. Gas concentration Ethylene gas (C2H4) was used as the contaminant because its specific weight is approximately equal to that of air. The C2H4 was diluted with 100% nitrogen to 1% prior to supplying to the gas-emitting building. This means that it can be considered as a passive scalar and moves with the air. The emitting rate was 417 mm3 s1. Ethylene concentration was measured by a flammable ion detector (Model SC-7501, Iwatsu electric Co. Ltd.) with an accuracy of 71%. The sampling frequency was 10 Hz and the sampling period was 40 s based on the measurement of air velocity. The measurement points are shown in Fig. 2. The numbers of measurement points outside the buildings were 7, 7, and, 11 for buildings distances of 1 H, 2 H and 3 H, respectively. The number of measurement points inside each building, including at sidewall openings, was 4; and then there was one measured point outside wind tunnel as a background.
2.5.2. Airflow The air velocity was measured by three-dimensional hot film anemometry (model IF300, TSI Inc.). A sensor traversing system carried the anemometer to each measured point. The sampling frequency was 200 Hz, and the sampling period was 419 s at each measured points. These were decided based on considering the smallest eddies of Kolmogorov and the fluctuation of fan rotation conducting larger eddies. The measurement points are shown in detail in Fig. 2. The numbers of measurement points outside the buildings were 100, 116, and 116 for buildings distances of 1 H, 2 H and 3 H, respectively. The number of measurement points inside each building was 12.
2.6. Evaluation parameters To evaluate contaminant transportation by mean of airflow, an evaluation parameter PU proposed in the previous studies by Ikeguchi and Okushima (2001) was used. It is a measure of airflow momentum between buildings and is used to explain the distribution properties of the contaminant concentration. The evaluation parameter PU is expressed as follows: UD ¼ Z
U UR
(5)
U D dz
(6)
B
PU ¼ G
where: the integration limit G is ground level and E is eaves height in m; UD is normalised air velocity in the X direction; U is measured mean air velocity in the X direction at each measuring point in m s1; and UR is a reference mean air velocity, taken as 27 m s1. This parameter means the momentum of flow in the downwind direction at a given X location in the test section from ground to ridge height. The large value expresses that a momentum in the direction of the downwind is large and the contaminant concentration is low.
3. Results and discussion The average value of background (outside wind tunnel) C2H4 concentrations during experiments was 0001 ppm and it was less than 7% of minimum value of measured concentration inside wind tunnel. So it was regarded that the effect of entry of C2H4 in the wind tunnel was small and the background concentration was approximately equal to zero.
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Windward i 3
1 2
Downwind 4
5
6
7
9
8
10
11
12
13
14
16
15
8 13 6.5 6.5 5 7 6.2 3 . 15 13 5 14 3 (a)
7 6 4
13 13.5 14
14 13.5 13
14 13.5 13
26
13
52
5 3 2 1
j
i 3
1 2
4
5
6
7
13 (b)
9
8
13
10
11 12
9
10
13
14 15 16
17
18
13
13
i 1 2
3
4
5
6
7
13
8
13
26
11
12
13
14 15 16
17
18
9
26
(c)
Fig. 2. Gas concentration and air velocity measured points for: (a) separation distance of 1 H; (b) separation distance of 2 H; and (c) separation distance of 3 H. The scale is the scale model (all units in cm): H, 26 cm, representative length ¼ ridge height; i, vertical measured line number; j, horizontal measured line number; K, air velocity measured point inside building ; , gas concentration measured point; grid intersections are air velocity measured points outside building; , gas point and concentration measured point for gas generation. Cases 1–3 at upwind building. Cases 4–6 at downwind building
The concentration of C2H4 is expressed by a dimensionless concentration CD, defined as CD ¼
CP CG
(7)
where: Cp is a concentration of C2H4 at a measured point in ppm; and CG is the concentration of C2H4 at the generation point in ppm.
3.1. Gas dispersion The dimensionless gas concentrations in the scale models 90 mm above the ground are shown in Figs 3 and 4, for gas-emitting building being downwind and upwind, respectively. The contaminant air reaches the upwind building even when the gas is emitted from the downwind building. As
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3×H
Dimensionless concentration, CD
1
2×H
Dimensionless concentration, CD
Upwind building
1×H
1 0.1 0.01 0.001
0
500
1000 1500 2000 Distance x, mm
2500
3000
Fig. 3. Dimensionless concentration (CD) at opening height (90 mm) when gas was emitted in the downwind building: H, representative length ¼ ridge height, 260 mm in the scale model; , separation distance 1 H; , separation distance 2 H; , separation distance 3 H
3×H 2×H Upwind building 1 × H Dimensionless concentration, CD
0.01 0.001 0.0001 0.00001 0
0.0001 0.00001
0.1
1.0000
0.1000
0.0100
0.0010 0
500
1000 1500 2000 Distance x, mm
2500
3000
Fig. 4. Dimensionless concentration (CD) at opening height (90 mm) when gas was emitted in the upwind building: H, representative length ¼ ridge height, 260 mm in the scale model; , separation distance 1 H; , separation distance 2 H; , separation distance 3 H
shown in Fig. 5, the average value for CD inside the upwind, receiving building is about 0001 when the separation distances are 1 H and 2 H. This means that the contaminant concentration in the upwind building is 01% of the contaminant concentration supplied to the downwind building. The average value
1 2 3 Separation distance H, H=260 mm
Fig. 5. Average dimensionless concentration (CD) in the contaminant receiving building: H, representative length ¼ ridge height, 260 mm in the scale model; , Gas was emitted in the upwind building; , Gas was emitted in the downwind building
for CD is 105 inside the receiving building when the separation distance was 3 H. Thus, the concentration inside the receiving building decreases dramatically when the separation distance between the buildings is increased sufficiently (Fig. 5). In order to avoid major transmission of contaminant from the downwind building, it is necessary to keep a distance of at least 3 H between the buildings. A two-way analysis of variance (ANOVA) was performed with respect to CD inside the receiving building using three measured points inside the building as replicate. There was significant difference (variance ratio F ¼ 728; probability P ¼ 0019) among gasgenerating location. The value of CD in the receiving building was higher with the gas-emitting upwind than with the gas emitting building being downwind. With the gas-emitting building being downwind, the one-way ANOVA gave a significant difference (F ¼ 994; P ¼ 000) for separation distance. However, there was no significant difference for separation distances with the gas-emitting building upwind of the receiving building. This shows that there was no significant relationship between the separation distance and contaminant concentration inside the receiving building when the gas was emitted from the upwind building. When the contaminant was emitted from the upwind building, the value of CD in the receiving building was about 0025, 0004 and 0004 for the separation distance 1 H, 2 H and 3 H, respectively. As the separation distance became longer than 2 H, the CD became onesixth times as low as that for 1 H. However, the CD measured with a separation distance of 3 H was almost the same as that with 2 H. These values were naturally higher than when gas was emitted from the
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Table 2 Average dimensionless concentration CD in the contaminantemitting building; H is representative length and equal to ridge height, 260 mm in the scale model
Upwind
Downwind
0153 0156 0162
0175 0195 0160
0.02
Dimensionless concentration, CD
1
0.01 0.1
0
0.01
−0.01 −0.02
0.001
−0.03 −0.04
0.0001
−0.05 −0.06
0.00001
1×H 2×H 3×H Separation distance H, H=260 mm
−0.07
Fig. 6. Average dimensionless concentration (CD) and evaluation parameter (PU) between the buildings: H, representative length ¼ ridge height, 260 mm in the scale model; , gas was emitted in the upwind building; , gas was emitted in the downwind building; , PU
downwind building. They were about 20, 3 and 264 times for 1 H, 2 H and 3 H, respectively, compared to the case of gas generated in the downwind building. The average value for CD inside the gas-emitting building is shown in Table 2. There was no significant difference among location of gas-emitting building and separation distance. The average value was 0167 for the cases in this investigation. In the area between the buildings, contaminant air dispersion followed the flow motion between buildings. The two-way ANOVA was performed with respect to CD at the middle position between the buildings using four vertical measured points as replicate. When the separation distance was 3 H, the average CD values of the vertical measured line number i of 8 and 9 (Fig. 2) were used for each height. There were significant differences for gas generated location (F ¼ 252; P ¼ 0000) and separation distance (F ¼ 538; P ¼ 0015). The average CD for each case in the middle position between the buildings is shown in Fig. 6. The trend is similar to that in the receiving building,
0.018 Dimensionless concentration, CD
1H 2H 3H
Contaminant-emitting building
Evaluation parameter PU, dimensionless
Separation distance
0.02
0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0
1×H 2×H 3×H Separation distance H, H=260 mm
Fig. 7. Dimensionless concentration (CD) at leeward side of the downwind building: H, representative length ¼ ridge height, 260 mm in the scale model; , 05 H leeward from the downwind building when gas was emitted in the downwind building; , 1 H leeward from the downwind building when gas was emitted in the downwind building; , 2 H leeward from the downwind building when gas was emitted in the downwind building; , 05 H leeward from the downwind building when gas was emitted in the upwind building; , 1 H leeward from the downwind building when gas was emitted in the upwind building; , 2 H leeward from the downwind building when gas was emitted in the upwind building
however, the values being about ten times as high as those in the receiving building. The CD in the area of the leeward side of the downwind building is shown in Fig. 7, as functions of the separation distance between the buildings. As the separation distance increased, CD had a tendency to become higher when contaminant was emitted from the downwind building. On the other hand, when the contaminant was generated in the upwind building, the CD tended to become lower as the buildings distance increased.
3.2. Airflow The CD distribution stated above is attributed to the airflow patterns, because C2H4 was transferred with airflow. The airflow patterns for normalised airflow vectors for each case are shown in Fig. 8. Contaminant reaches the upwind building as the air in the downwind building through moves in the leeward opening, out through the windward side opening towards the upwind building.
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Height, cm
: UD = 1.0 60 50 40 30 20 10 0 0
50
100
150 200 Horizontal distance, cm
250
300
100
150 200 Horizontal distance, cm
250
300
100
150 200 Horizontal distance, cm
250
300
(a)
Height, cm
: UD = 1.0 60 50 40 30 20 10 0 0
50
(b)
Height, cm
: UD = 1.0 60 50 40 30 20 10 0 0 (c)
50
Fig. 8. Airflow pattern with normalized air velocity (UD) for (a) separation distance of 1 H; (b) separation distance of 2 H; and (c) separation distance of 3 H;H, representative length ¼ ridge height, 26 cm in the scale model
Inside the upwind building, most of the air moved toward the windward sidewall for the building distance 1 H due to the eddies in and around the building, which resulted in the contaminant hardly moved out of the building [Fig. (8a)]. However, for the separation distance 2 H and 3 H, the air moved toward the upwind sidewall under the opening height and toward the downwind sidewall above the opening height [Figs 8(b) and (c)]. This shows that there was a clockwise circulation. Inside the downwind building, most air moved toward the downwind side for building distance 1 H and 3 H. On the other hand, for separation distance 2 H, the air under opening height moved toward the windward side. The air velocities inside the downwind building were smaller than those inside the upwind building. When the separation distance was 1 H, the inside air velocities in the downwind building were higher than those for the separation distance 2 H and 3 H. In the area of the leeward side of the downwind building, retouch position of the wake for the small separation distance was between 1 H and 2 H (Fig. 7). For the two larger separation distances between the buildings the retouch positions were close to 1 H and between 05 H and 1 H, respectively. This indicates that the wake became smaller as the separation
distance increased. A larger wake prevented the contaminant generated in the downwind building from leaving the building through the leeward side opening. Consequently, the CD at 05 H from the downward building (i ¼ 13 for 1 H, i ¼ 15 for 2 H and 3 H) was lower for the small building distance compared to the larger separation distance when the contaminant was emitted from the downwind building. In the area between the buildings, the air below the ridge height moved toward the upwind building for separation distance 1 H and 2 H. For the separation distance 3 H, however, the air below the opening height at the vertical measured line i ¼ 9 moved in the downwind direction. The different air velocities and flow directions affected by the building distances induced the differences of CD inside the buildings. The parameter PU between buildings may be used to express the degree of contaminant transfer between buildings. The larger value of PU means a larger momentum in the main flow direction and a smaller contaminant concentration. It can be seen in Fig. 6 that, the value of PU became larger and CD became smaller as the building distance increased. The values of PU for the separation distance 2 H and 3 H were negative. Naturally, the quantity of transferred contaminant
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became smaller as the separation distance was increased. From Figs 3 and 4, it can also be seen that the CD in this area decreased more than at the leeward side of the downwind building.
4. Conclusions Wind tunnel tests of naturally ventilated scale model pig barns were performed to investigate air transmissions of contaminants from a gas-emitted building to another contaminant receiving building. Ethylene gas was used as the contaminant. The effects of the location of the gas-emitting building and distance between the buildings, were examined under steady-state conditions. The following conclusions can be drawn. (1) The contaminant generated in the downwind building reached the upwind building because of the wake at leeward side of the downwind building. The concentration was 01% of the generated concentration in the downwind building when the separation distance was the same as the ridge height. (2) As the separation distance increased, the concentration in the receiving building was lower when the contaminant was emitted in the downwind building. In the case of a separation distance of 3 H (where H is a representative length equivalent to the ridge height), the dimensionless concentration in the receiving, upwind building became 105 for emitted concentration in the downwind building. At least a distance of 3 H between two buildings is needed to minimise the contaminant transfer from the downwind building to the upwind building. (3) When the contaminant was emitted from the upwind building, the concentrations in the receiving, downwind building were larger than those when the contaminant was emitted from the downwind building. (4) As the separation distance was increased, the wake at leeward side of the downwind building became smaller, a measure of airflow momentum between buildings, became larger and therefore the contaminant transmissions became smaller.
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