## 3 Results  
### Flat-roof Building  
#### 3.1.1 Wind Velocity  
In this subsection, the velocity measurements above the flat-roof building are discussed for the four wind angles of attack, \(0^{\circ},15^{\circ},30^{\circ}\) and \(45^{\circ}\). The velocity profiles over two alignments, \(x/D=0.2\) and \(x/D=0.8\), are plotted in Figure 5, considering points 18-50 and 22-38-54 (Figure 4a). Depending on different wind angles, the positions of these points are changing. For example, at small angles, point 18 lies in the upstream half of the roof, while point 50 lies downstream. However, in the case of large wind angles, both points 18-50 are in the upstream half of the roof. In addition to velocity profiles, Figure 5 shows the longitudinal (\(I_{U}\)) and vertical (\(I_{W}\)) turbulence intensities, obtained by  
\[I_{U} =\sigma_{U}/U;\ I_{W} =\sigma_{W}/U \tag{1}\]  
where \(\sigma_{U}\) and \(\sigma_{W}\) are the standard deviations of stream-wise and vertical wind velocity components and \(U\) is the mean stream-wise wind speed. Figure 5 also illustrates the acceleration of the stream-wise wind speed as calculated with \((U-U_{ref})/U_{ref}\), where \(U_{ref}\) is the mean reference velocity measured with a Prandtl tube placed upstream of the model. Figure 5a shows the velocity profiles above points 18 and 22 for the wind angle of \(0^{\circ}\). A strong separation develops at the upstream edge, as velocity vectors above the height of z/D = 0.3 point upwards. In the vicinity of the roof, a high stream-wise turbulence intensity zone can be detected, reaching up to a value of 55%. In regions of turbulence intensities \(>\)30%, hot-wire measurements are expected to be affected by some inaccuracy and, therefore, results cannot be treated as reliable [55]. Nevertheless, these high values of turbulence intensity confirm the existence of separated flow. The separation bubble from the windward edge reattaches in the vicinity of points 50 and 54, as velocity vectors point downwards. Figure 5: Velocity vectors profiles, based on the stream-wise and vertical velocity components, showing the stream-wise turbulence intensity \(I_{\rm{II}}\), the vertical turbulence intensity \(I_{\rm{W}}\) and the percentage increase in the stream-wise wind speed. Each numerical value reported at every measurement point in the figure shows the following statistics: \(\mathbf{I_{\rm{II}}}\) [%] above on the left; \(\mathbf{I_{\rm{W}}}\)[%] below on the left; \(\mathbf{(U-U_{\rm{ref}})/U_{\rm{ref}}}\)[%] below on the right. Profiles over the flat-roof high-rise building are measured above points 18 and 50 (belonging to the marked line \(\chi\)/\(D\) = 0.2, left) and above the points 22, 38 and 54 (belonging to the marked line \(\chi\)/\(D\) = 0.8, right) for wind directions: (**a**) 0\({}^{\circ}\), (**b**) 15\({}^{\circ}\), (**c**) 30\({}^{\circ}\) and (**d**) 45\({}^{\circ}\). The arrows indicate the velocity magnitude normalised with the reference velocity (\(U_{\rm{ref}}\)), as referred to the unity vector represented in the legend. At 15\({}^{\circ}\), the velocity profiles above the upstream points 18 and 22 are similar to those found at 0\({}^{\circ}\). However, profiles above points 38 and 50 show some differences. The velocity vectors above point 38 are no longer parallel to the roof, pointing slightly upwards, suggesting a downwind shift of the average moving vertex core. Besides, vectors above point 50 at heights \(z\)/D \(\geq\) 0.3 are nearly parallel to the roof, seemingly out of the separation region, as the flow behaves analogously to points upstream of the separation bubble. As the wind angle increases to 30\({}^{\circ}\), a small separation cone generates along the upstream edge of the building, starting at its upstream corner. The down-pointing velocity vectors at the lower measuring points suggest that both points 18 and 50 are located at the tail of the separation cone where the flow tends to attach to the roof.