## 3 Results and Discussion
### Mean Air Velocities Downwind the Building  
Figure 7a shows contours of mean streamwise air velocity normalised by the air velocity at the building height (\(U_{B}\)) at Plane 1 for three roof slopes of the exercise yard of the scaled pig building model. Considerably lower air velocities were observed right behind the building with negative values in the centre (represented in dark blue in Figure 7a). The negative \(U\) velocities indicated reverse flows through Plane 1, which occurred slightly below the building height. The region of the reverse flow was affected by the roof slope in the spanwise wind direction (\(\gamma\)). It became narrower in the upper part but wider in the lower part for a steeper roof. Beyond the reverse flow region, the air flowed towards downstream with a reduced air velocity. Figure 7b compares the dimensionless mean vertical air velocity contours at Plane 1 for roof slopes of \(5^{\circ}\), \(15^{\circ}\), and \(25^{\circ}\). It was found that at both sides of the building, \(W\) velocities were negative, which indicated a downward flow direction. In contrast, within the region right behind the building upward airflows were observed. This indicated a complex three-dimensional air movement downwind the building caused by the air passing through and around the building. It is noted that the velocity contours were not strictly symmetric even though the scaled model was geometrically symmetric and the wind direction was perpendicular to the model sidewalls. The most possible reason for the asymmetrical airflow contours was the disturbance of the LDA probe to the flow field when it was positioned close to the building. The cross-sectional area of the LDA probe was 0.003 m\({}^{2}\), accounting for 5% blockage to the airflow at the position right behind the building. This was not intended but was unavoidable, restricted by the focal length of the LDA. The potential influence of the air velocity sensor to the airflow was also reported by [PERSON] et al. (2015), who observed an asymmetrical air velocity pattern at a vertical plane downstream from four aligned swine building models measured using a 3D hot-film anemometer. The results raised a potential interesting topic that is to quantify the disturbance of the LDA probe to the flow field, which could be done with CFD simulations. Figure 7: Spatial distributions of (**a**) streamwise (\(U\)) and (**b**) vertical (\(W\)) air velocities at Plane 1 for roof slopes of \(5^{\circ}\), \(15^{\circ}\), and \(25^{\circ}\). \(U_{B}\) is the streamwise air velocity at the building height. Black dash lines indicate the profile of the scaled model. Figure 8 depicts contours of dimensionless streamwise and vertical air velocities at Plane 2 for the three roof slopes. Lower air velocities at the position behind the building than around the building were still observed. This indicated that the presence of the building affected the airflow field at a distance of 1.0 m (i.e., \(7.7Z_{B}\), where \(Z_{B}\) is the building height, corresponding to 50 m in full scale) downwind the building. In contrast to Plane 1, no reverse flows occurred at Plane 2 for all three cases. It was found that \(U\) velocities right behind the building at Plane 2 were obviously higher than those at Plane 1. However, Plane 1 presented higher air velocities at the height above 0.16 m than Plane 2 (Figures 7a and 8a), which was caused by the wind shear effect that accelerated the air when the air flowed over the top of the building. Compared with Plane 1, Plane 2 presented a more homogeneous vertical air velocity pattern with slightly upward airflows occurring near the centre (Figure 8b). Only small differences among different roof slope cases for both \(U\) and \(W\) velocities at Plane 2 were observed. It demonstrated that the slope of the leeward roof did not considerably affect the airflow at Plane 2.