## 4 Non-Gaussian Property of Wind Pressures
### Mean Value of Wind Pressure  
The sign of mean wind pressure coefficient, \(C_{p,\text{ mean}}\), is critical to identify whether the region is immersed in flow separation, where it is most likely to exhibit non-Gaussian properties. The experimental results of \(C_{p,\text{ mean}}\) at five representative azimuths, i.e., \(\beta=0^{\circ}\), \(90^{\circ}\), \(210^{\circ}\), \(225^{\circ}\), and \(240^{\circ}\), are presented in Figure 7. Contours are obtained using Surfer software. In comparison, the results of a single cylindrical roof presented in [30] are considered to illustrate the differences in wind pressure distribution characteristics between a combined cylindrical roof and a single one.  
As shown in Figure 7a, the suction for BCR and SCR rooftops is considerable when the model is impacted by the windward flow, i.e., \(\beta=0^{\circ}\). This is caused by the evident flow separation at the rooftops. Most of the \(C_{p,\text{ mean}}\) are negative except those on the light green regions around windward edges \(bc\) and \(fg\). This is because straightforward wind flow attacks the roof and air stagnation causes positive pressures. Such a wind pressure distribution is similar to that in [30] for a single cylindrical roof.  
Figure 7b presents the spatial distribution of \(C_{p,\text{ mean}}\) at \(\beta=90^{\circ}\). The whole roof is dominated by negative pressures except for the region around the corner \(e\). Because the blocked airflow around the BCR reverses its direction, the backflow dashes into the SCR roof, which eventually results in positive pressure. The wind flow separates at edges \(gh\) and \(cd\), which leads to a dramatic reduction in wind energy. Therefore, the values of \(C_{p,\text{ mean}}\) on the BCR are uniform and close to \(-0.3\).  
Figure 6: Normalized wind pressure coefficient \(C_{p,\text{st}}\) at tap 66. (**a**) Time history, (**b**) histogram diagram.  
Figure 7d,f plots the distribution of the \(C_{p,\,\text{mean}}\) for \(\beta=210^{\circ}\), \(225^{\circ}\), and \(240^{\circ}\). Most \(C_{p,\,\text{mean}}\) on the entire roof are negative while the positive coefficients are found in small regions, induced by the attachment of wind flow. It is noted that the wind pressure distribution on the SCR is impacted by the sheltering of the BCR. Therefore, the amplitude of wind pressure coefficients for the SCR is smaller than for the BCR.  
The above finding indicates that most of the regions on the roof are immersed in flow separation, and the wind pressures on such regions tend to be non-Gaussian. By comparison, it can be concluded that the distribution of \(C_{p,\,\text{mean}}\) for the combined cylindrical roof is similar to that of the single one, only for \(\beta=0^{\circ}\). The reserved results for the single cylindrical roof cannot be directly employed in the wind-resistant design of the combined cylindrical roof. Investigating the properties of the wind pressure on the combined cylindrical roof is essential.  
The minimal mean wind pressure coefficients \(C_{p,\,\text{mean},\,\text{min}}\) among all azimuths are investigated, whose distribution is shown in Figure 7c. It can be concluded that the entire roof is immersed in the flow separation given that all of the \(C_{p,\,\text{mean},\,\text{min}}\) are negative. In addition, values of \(C_{p,\,\text{mean},\,\text{min}}\) on edge \(ab\) and edge \(cd\) of the BCR and on edge \(ef\) of the SCR are much smaller, which indicates these parts have the largest mean wind suction.  
If the peak factors for these parts are predominant, the resulting extreme wind suction is considerable.