## 3 Results and Discussion
### Boundary Layer Profile
#### 3.1.2 Stability Study and Wind Profile Properties  
To ensure a stable airflow within the region of interest, wind profiles at the upwind edge of the model (Line1) and the end of downwind airflow measurement region (Line 2) were measured without the scaled model. The vertical profiles of the streamwise air velocity component for Line 1 and Line 2 are compared in Figure 6. The results showed that the two profiles agreed well with each other with an average relative difference of 3.5%, indicating that a stable airflow was achieved within the region of interest. Figure 6 showed that the mean streamwise air velocity profile of the incident flow (i.e., at the upwind edge of the model) in the vertical direction followed a power law with the exponent of 0.14 and the coefficient of determination R\({}^{2}\) of 0.98:  
\[U=U_{ref}\big{(}Z/Z_{ref}\big{)}^{0.14} \tag{4}\]  
where \(U\) and \(U_{ref}\) are the mean streamwise air velocity at height \(Z\) and a reference height \(Z_{ref}\), respectively, m\(\cdot\)s\({}^{-1}\). To achieve the best fit of the model for the air velocity profile, \(U_{ref}\) = 5.55 m\(\cdot\)s\({}^{-1}\) and \(Z_{ref}\) = 0.4 m were chosen in this study. Figure 5: Dimensionless streamwise (\(U\)/\(U_{inlet}\)) (**a**) and vertical (\(W\)/\(U_{inlet}\)) (**b**) air velocity profiles of the incident flow in the vertical direction (\(Z\)) at the wind tunnel inlet wind speed (\(U_{inlet}\)) of 4, 6, 8, 10, and 12 m\(\cdot\)s\({}^{-1}\). The streamwise air velocity at the building height (\(U_{B}\)), calculated by Equation (4), was 4.77 m\(\cdot\)s\({}^{-1}\). This resulted in a building Reynolds number (\(Re_{B}\)) of 53,759, which was calculated by Equations (5) and (6). \[Re_{B}=U_{B}\cdot D/\
u \tag{5}\]  
\[D=2W_{B}\cdot Z_{B}/(W_{B}+Z_{B}) \tag{6}\]  
where \(\
u\) is the kinematic viscosity of the air, m\({}^{2}\cdot\)s\({}^{-1}\); \(D\) is the characteristic length of the scaled pig building model, calculated as the hydraulic diameter of the cross-section of the model, m; \(W_{B}\) and \(Z_{B}\) are the width and the height of the scaled model, respectively, m. The average ambient temperature and relative humidity during the whole experimental period were 22.4 \({}^{\circ}\)C and 34.2%, respectively. Therefore, \(\
u\) took the value of \(1.53\times 10^{-5}\) m\({}^{2}\cdot\)s\({}^{-1}\) at temperature of 22.4 \({}^{\circ}\)C.  
By fitting the air velocity profile of the incident flow to a logarithmic law, it gave the friction velocity (\(u^{*}\)) of 0.22 m\(\cdot\)s\({}^{-1}\) and the full-scale roughness length (z\({}_{0}\)) of \(6.4\times 10^{-3}\) m, in which the von Karman constant took the value of 0.4. This gave the roughness Reynolds number (\(Re_{20}\)) of 92, which was calculated according to Equation (7). \[Re_{20}=u^{*}\cdot z_{0}/\
u \tag{7}\]  
It is stated in the VDI-guideline [40] that a moderately rough (corresponds to grassland or farmland) turbulent boundary layer should meet the following requirements: the profile power law exponent falls within the region of 0.12-0.18, and the roughness length within 0.005-0.1 m. In this study, both the power exponent of 0.14 and \(z_{0}\) of \(6.4\times 10^{-3}\) m satisfied the aforementioned criteria, hence the generated boundary layer can be considered to represent the airflow over farmland terrain. Additionally, both the building Reynolds number (\(Re_{B}\)) of 53,759 and the roughness Reynolds number (\(Re_{z0}\)) of 92 were considerably higher than the reported critical \(Re_{B}\) of 4000 [42] and critical \(Re_{z0}\) of 2.5 [43], respectively for a Reynolds number independent flow. It further indicated that the airflow generated in our wind tunnel at \(U_{inlet}\) of 8 m\(\cdot\)s\({}^{-1}\) was fully developed turbulent. Therefore, all subsequent measurements were performed at \(U_{inlet}\) of 8 m\(\cdot\)s\({}^{-1}\). Figure 6: Comparison of vertical (\(Z\)) profiles of streamwise air velocity (\(U\)) measured at Line 1 and Line 2. \(U_{Inf}\) is the reference streamwise velocity at the reference height \(Z_{ref}\).