1. Introduction
Natural ventilation systems have the advantages of low capital investment, energy saving [
1], and absence of noise [
2] compared with mechanical ventilation systems. Thus, they are widely implemented in livestock buildings. Naturally ventilated pig production systems equipped with outdoor exercise yards are receiving increasing interest in Europe, the United States, and South Africa [
3,
4]. This housing system provides pigs larger living areas, enables the separation of lying and excretion areas, and allows pigs to exhibit natural behaviours [
5]. It provides pigs access to an outdoor exercise area, enables pigs to mainly excrete outdoors, and thus results in improved indoor air quality [
6], animal welfare [
3], and meat quality [
7] compared with a conventional housing system. The design of naturally ventilated pig buildings (NVPBs) with outdoor access is mainly based on the consideration of animal production performance and animal welfare [
3,
8].
However, one important issue associated with this type of buildings is the amount of air pollutants produced in the buildings and their emission to the surrounding environment. The open-type barn structure allows a direct air exchange between the indoor and outdoor environment, and consequently leads to a wide dispersion of gaseous emissions into the atmosphere [
9]. The gaseous pollutants produced from the buildings including ammonia, greenhouse gases, hydrogen sulphide, particulate matter, odours and aerosols contribute to environmental problems and causes nuisance to neighbouring residents [
10]. The transport and dispersion processes of gaseous emissions are strongly affected by airflow [
11,
12], which is influenced by the atmospheric boundary layer (ABL) such as temperature, humidity, wind speed, wind direction, and terrain [
13]. The ABL experiences periodic evolution of daytime convective [
14], transitional (occurs in the morning, late afternoon or early evening) [
15], and nocturnal stable states [
16] due to the solar diurnal cycle. This dynamic behaviour of the ABL may also affect the evolution of particulate matter [
17]. In addition to the ABL, building configurations (e.g., roof structure and openings) also affect airflow field within and around the building [
18], and thus may have an impact on the pollutant transport and dispersion processes. Therefore, a good understanding of airflow distributions including airflow pattern and mean and turbulent characteristics, particularly downwind of the buildings contributes to understanding air pollutant dispersion mechanisms.
One of the most common ways to reduce nuisance of airborne pollutants in the vicinity of pig barns is by using artificial or natural windbreaks [
19]. One function of windbreaks is acting as barriers that deflect the airflow upwards, increase the dilution of air pollutants [
20], and consequently reduce their concentration at the ground level [
21]. Ikeguchi et al. [
9] reported that using a solid wall, a screen, and another building as windbreaks placed upwind of a pig barn had different influences on air momentum and airflow patterns around the target building and might affect air dispersion patterns. Apart from the airflow redirection impact, natural windbreaks, for example shelterbelts, also contribute to the reduction of wind speed, interception/absorption of chemical compounds, particulate matter and aerosols, and therefore can dilute and mitigate airborne pollutants [
22]. It is found that the influence of natural windbreaks on the pollutant dispersion is related to the height, optical porosity and type of windbreaks [
19,
20], and the distance from the pollutant source [
20,
22].
Apart from windbreaks, it is essential to investigate the effect of building configurations e.g., the roof design on the dispersion process of gaseous pollutants. This information may help to provide building engineers guidelines for planning or design of new livestock buildings in order to reduce adverse impacts of the buildings on nearby environment and residents. For environmental reasons, outdoor exercise yards of pig buildings are often partly or totally roofed to minimise the impact of rain to remoisten the soiled areas. Additionally, the roof of a building plays an important role in air separation, airflow pattern within and around the building [
23,
24] and airflow characteristics in the wake of the building [
25], and thus is expected to affect the pollutant dispersion from the building. The effect of building roof on air pollutant transport has been investigated inside and over urban street canyons [
26]. It was found that the roof shape [
27,
28] and the roof slope [
28] significantly affected the air vortex within the canyon and the pollutant concentration and dispersion. The influences of the roof type (open-ridge, semi-monitor and mono-slope) and roof slope on air movement for a naturally ventilated dairy house were studied in a wind tunnel, and the dispersion properties were predicted from the airflow measurement results [
18]. The authors found that the open-ridge roof type tended to increase dispersion downwind from the house, and the roof slope largely affected the air movement and contaminant dispersion [
18]. However, their work only measured air velocities at the vertical symmetry plane of the building, and detailed airflow field information downwind the building was limited. Moreover, the configurations of NVPBs with roofed outdoor exercise yards are very different from street canyons and dairy houses. There is lack of airflow information about this type of buildings with roofed outdoor exercise areas. Therefore, it is required to understand in detail airflow characteristics including both mean (time-averaged) velocity and turbulent fluctuations downwind NVPBs with outdoor access. This knowledge can contribute to a better understanding of the transport and dispersion processes of airborne pollutants and of an optimised roof design.
Wind tunnel and scaled model experiments are widely applied in aerodynamics studies because of their advantages of allowing fully controlled boundary conditions, working with real airflow [
29], providing a large amount of data in a short time [
30], and the flexibility in the experimental setup [
11]. Because of the scaled-down model, similarity criteria e.g., geometric similarity, boundary similarity, Reynolds number similarity have to be met in order to make the wind tunnel experimental results be comparable to the results from full-scaled buildings [
31]. By carefully checking the similarity criteria, the wind tunnel tests method was therefore used in this paper in order to conduct the airflow characteristics research. Moreover, the data obtained from the wind tunnel measurements can also be used to validate accuracy and reliability of computational fluid dynamics (CFD) models and therefore to investigate more complex flow (e.g., airflows above emitting surfaces) and dispersion phenomena, to perform comprehensive parametric studies, and finally to mitigate the production of emissions from livestock buildings.
The objectives of this study were to investigate mean and turbulent characteristics of airflows downwind of a NVPB with an outdoor exercise yard covered with roofs with different roof slopes, to predict air pollutant distribution and dispersion properties, and to provide valuable experimental data for CFD validation. The novelties of this study are as follows:
It is the first to provide detailed airflow information around a naturally ventilated livestock building combined with a roofed outdoor area;
It predicts the potential distribution of gaseous pollutants from the building using airflow measurement results;
It provides a large amount of experimental data of both mean velocity and turbulent fluctuations downwind the building with a high resolution that can be used for CFD validation.
2. Materials and Methods
2.1. Scaled Pig Building Model
A scaled model of a naturally ventilated pig building with an outdoor exercise yard was used in this study. The prototype pig barn, situated in the state of Lower Saxony in northwest Germany, was designed for rearing around 80 fattening pigs. The scaled model was a 1:50 geometric reduction of the full-scale pig barn and was constructed at the Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB), Germany.
The scaled model was made of transparent acrylic glass and consisted of an indoor housing area and an outdoor exercise area. The external dimension of the model was 0.427 m (length) × 0.256 m (width) × 0.130 m (height). The housing area had two sidewall openings with opening heights of 0.020 m and 0.064 m, respectively. Eight pigpens with open pen fronts were placed in the housing area. The height and width of all pigpens were 0.020 m and 0.054 m, respectively. The length of the two pigpens located next to gable walls was 0.056 m, and of other pigpens was 0.050 m. Pigpen walls had a thickness of 0.002 m, and all other walls of the building model had a thickness of 0.003 m. The roof of the building housing area had a fixed slope of 15°. The free access between indoor and outdoor areas through plastic strips or rotating doors in the prototype pig barn was constructed by a 0.025 m high acrylic sheet in the scaled model. The outdoor exercise area had a flexible roof with a length of 0.108 m and with a fixed top part, which totally covered the outdoor yard. There was a 0.010 m high sidewall but were no gable walls in the outdoor area. The prototype pig building was constructed to direct the excretion behaviour of pigs to the outdoor exercise yard [
32], in which solid floors in the housing area and slatted floors with a deep pit in the outdoor area were adopted. Therefore, in this study we only considered the roof slope variations for the outdoor area, where the majority of emissions are expected to come from. Three cases with roof slopes of 5°, 15°, and 25° were studied in this paper. It has been found that the presence of animals has an insignificant impact on the airflow and pollutant dispersion within the barn [
33], therefore, pigs and other internal structures that have smaller dimensions than the group of pigs, such as feeders, drinkers, metal bars, and slatted floors are expected to have minor influences on the airflow field downwind of the building. To simplify the model construction, pigs and these internal structures were not constructed. Detailed dimensions of the scaled pig building model and the three roof slope variations are illustrated in
Figure 1.
2.2. Boundary Layer Wind Tunnel and Measurement Devices
The experiments were carried out in a large boundary layer wind tunnel (BLWT) at ATB, Germany. The BLWT was specially designed to investigate ventilation and dispersion processes in agricultural buildings [
34,
35,
36,
37], and has also been used to generate datasets for CFD validation [
24,
38,
39]. The wind tunnel is 28.5 m long, consisting of an air inlet fitted of honey combs, an air outlet equipped with an axial fan, and a 19.5 m-long test section. The cross-sectional area of the test section is 3 m (width) × 2.3 m (height). A combination of six spires and roughness elements was used to create a boundary layer flow. The spires were installed at the entrance of the test section. The roughness elements consisting of two sizes of right-angled steel brackets, with dimensions length × width × height of 0.010 m × 0.004 m × 0.004 m and 0.004 m × 0.002 m × 0.002 m for small and big brackets respectively, were arrayed in staggered rows downstream the spires. The total length of the roughness elements was 9.6 m with 0.2 m space between each row. The 1:50 scaled pig building model was placed at the symmetry line of the wind tunnel at 1.2 m downstream from the roughness elements. The model was oriented with sidewall openings perpendicular to the approaching flow and the outdoor yard at the downwind side. The blockage ratio of the scaled model to the cross-section of the wind tunnel was 0.8%, which is far less than the recommended maximum value of 5% for wind tunnel tests in VDI-guideline 3783/12 [
40], and thus the tunnel effect can be neglected.
Figure 2 shows photographs of the wind tunnel with the scaled model placed inside.
The free stream wind speed at the wind tunnel inlet (Uinlet) was measured using a Prandtl tube, connected to a pressure transducer MKS Baratron® Type 120A (MKS Instruments, Andover, USA). The Prandtl tube was located at the centre of the entrance of the test section at a height of 1.3 m from the wind tunnel floor. Air velocity and turbulence around the scaled model were measured using a 2D fibre-optic Laser Doppler Anemometer (LDA) (Dantec Dynamics, Skovlunde, Denmark) combined with the BSA Flow Software package (Dantec Dynamics, Skovlunde, Denmark). The LDA probe head was 0.06 m in diameter and 0.45 m in length and provided a focal length of 0.25 m. The LDA probe was mounted on a three-dimensional computer-controlled traverse system that allowed automated and precise probe positioning with an uncertainty of <0.1 mm. A fog generator Tour Hazer II (Smoke Factory, Burgwedel, Germany) was placed at the wind tunnel inlet to produce seeding particles for LDA measurements. The temperature, relative humidity, and static pressure of the ambient air were measured using a FHAD 46x sensor with ALMEMO® D6 plug (AHLBORN Mess- und Regelungstechnik GmbH, Ilmenau, Germany). The data were sampled approximately every hour. The mean values of temperature, relative humidity, and static pressure for each day were used to calculate the air density, and therefore to calculate the Uinlet by using together with the data from the Prandtl tube.
2.3. Measurement of Boundary Layer Profile
2.3.1. Reynolds Number Independence Study
In order to obtain a fully developed turbulent flow in the wind tunnel, a Reynolds number independence study for the approaching flow was carried out. The wind profile was measured at 20 positions along a vertical line ranging from 0.003 m to 0.6 m above the wind tunnel floor at the upstream edge of the scaled model (i.e., at Line 1 in
Figure 3). The wind profile measurements were performed without the presence of the scaled model at free stream wind speeds (
Uinlet) of 4, 6, 8, 10, and 12 m·s
−1, respectively. The streamwise (
U) and vertical (
W) velocity components were measured using the 2D LDA. The measurements at each measurement position were taken continuously until the sampling number reached 40,000 readings or the maximum sampling time reached 820 s before moving the LDA probe to another position. The average sampling rate during experiments was around 300 s
−1. A 10s pause between each measurement position was set in order to minimise the disturbance of the movement of the LDA probe to the flow field. The above LDA setup was chosen according to a preliminary experiment for the reproducibility of statistic results, in which air velocities at heights of 0.01 m, 0.03 m, 0.06 m, 0.1 m, and 0.2 m along Line 1 were measured and repeated three times at
Uinlet of 8 m·s
−1. The results showed that with this setup the measurement uncertainty for the time-averaged
U and
W velocities was 0.3%, and 5.5%, respectively (
Figure 4). The above mentioned LDA setup was used for all air velocity measurements in this study.
2.3.2. Stability Study
To establish a stable boundary layer airflow that represents a farmland terrain, the characteristics of wind profiles within the region of interest were assessed. The arrangement of the roughness elements was adjusted accordingly until reaching a desirable boundary layer flow following the VDI-guideline 3783/12 [
40]. Air velocities at two vertical lines, one at the upstream edge of the scaled model (i.e., Line 1 in
Figure 3) and the other at the downstream end of the region of interest (i.e., Line 2 in
Figure 3), from 0.003 m to 0.6 m height along the symmetry line of the model were measured. Measurements were carried out using the 2D LDA without the presence of the scaled model. Free stream wind speed in the wind tunnel was 8 m·s
−1. Parameters that define the roughness class of the boundary layer, e.g., the wind profile power exponent, roughness length, and turbulence intensity were examined.
2.4. Measurement of Airflows Downwind the Building
In order to investigate airflow characteristics and to predict air pollutant dispersion properties downwind the building, air velocities at three vertical planes (Planes 1–3,
Figure 3) downwind the 1:50 scaled pig building model were measured with the LDA. Streamwise and vertical velocity components were measured simultaneously. Mean and instantaneous air velocities were recorded to obtain average and turbulent airflow information. Plane 1 and Plane 2 were parallel to the building sidewalls at separation distances of 0.2 m (corresponded to 10 m in full-scale) and 1.0 m (corresponding to 50 m in full-scale) downwind the building model, respectively. Both Plane 1 and Plane 2 had a dimension of
Y ×
Z = 0.8 m (width) × 0.2 m (height), each consisting of 200 velocity measurement points with 10 points in each vertical line and 20 points in each horizontal line. Plane 3 was positioned along the symmetry plane of the scaled model with a dimension of
X ×
Z = 0.8 m (length) × 0.2 m (height), including 90 measurement positions distributed at 9 vertical lines. In this study,
X,
Y and
Z denoted the axes of the coordinate system, which were aligned with the streamwise, spanwise and vertical wind directions, respectively. The origin of the coordinate system was located at the centre of the downstream edge of the scaled model at the wind tunnel floor. The airflow measurement positions and the definition of the coordinate system are depicted in
Figure 3. According to the wind profile measurement results (described in
Section 3.1), a stable and fully developed turbulent flow that represents a farmland terrain boundary layer could be obtained when the free stream wind tunnel speed (
Uinlet) was at least 8 m·s
−1. Therefore, all airflow measurements downwind the scaled model were performed at
Uinlet of 8 m·s
−1.
2.5. Data Analysis
The mean and the standard deviation of the air velocity in streamwise and vertical directions at each measurement position were provided by BSA Flow Software (Dantec Dynamics, Skovlunde, Denmark). Air velocity and air turbulence characteristics were processed from 40,000 samples at each position to ensure statistically reliable results.
The velocity magnitude calculated from two-dimensional velocity components was defined as:
where
V2D is the velocity magnitude, m·s
−1;
U and
W are mean air velocities in streamwise and vertical directions, respectively, m·s
−1.
Turbulence intensity and turbulent kinetic energy of the airflow were calculated by Equations (2) and (3), respectively.
where
TI is the turbulence intensity, %;
TKE is the turbulent kinetic energy, m
2·s
−2;
V2D is the velocity magnitude, m·s
−1;
σU and
σW are standard deviations of the instantaneous air velocity in streamwise and vertical directions, respectively, m·s
−1.
It is noted that in this study V2D, TI, and TKE were calculated based on a two-component velocity analysis.