# Wind Forces and Flow Patterns of Three Tandem Prisms with a Small Height-Width Ratio
## 3 Results and Discussions
### Wind Pressure Time Histories on Three Tandem Prisms  
Characteristics of wind pressure time histories of the prism are influenced by the different flow patterns around prisms. At present, few studies discuss the characteristics of pressure time histories on a group of tandem prisms with a small \(H/W\) for different flow regimes, and they will be discussed in this section. The pressure coefficient time histories of twelve pressure taps on the windward wall and leeward wall of three prisms are displayed at three typical spacing ratios (0.2, 0.7, and 1.2) in Figure 8. To reflect the fluctuating characteristics of pressure signals of twelve taps clearly, only a partial sampling duration from 30s to 80s is displayed, as shown in Figure 8.  
It can be seen that the fluctuating characteristics of pressures on the windward wall of the upstream prism A are basically similar at different spacing ratios, but those of the downstream prisms B and C are significantly different at the three spacing ratios. As shown in Figure 8b, when the spacing ratio is \(S/W=0.2\), the pressure coefficients on the windward wall of downstream prisms are much smaller in magnitude than those on the upstream prism A due to shielding effects. When the spacing ratio is \(S/W=0.7\), the pressure coefficient time histories in local pressure taps of downstream walls show abrupt jump or plunge, which is non-stationary during the sampling duration, as shown in Figure 8c. The fluctuating pressures of the leeward taps of prism A and the windward taps of prism B experience abrupt change at about T = 53.5 s, and they restore to the original state after about T = 58.5 s. As shown in Figure 8c, for the leeward wall of prism B and the windward wall of prism C, there are three abrupt changes in the fluctuating pressures at about T = 34.7 s, T = 54.5 s, and T = 64 s. Compared with the windward wall of prism B, the fluctuating pressures of the windward wall of prism C undergo multiple jumps and plunges. The time moments of irregular abrupt changes in tap pressure time histories within different prism gaps are not related. When the spacing ratio is \(S/W=1.2\), the fluctuating pressures of downstream taps change from non-stationary to stationary, as shown in Figure 8d. To analyze the influence of the abrupt changes in fluctuating pressures on downstream prisms at \(S/W=0.7\), Figure 9 shows the drag coefficient time histories of three prisms at \(S/W=0.7\). Different from the non-stationary fluctuating pressures on downstream prisms, the fluctuating drag coefficients of downstream prisms are stationary. As shown in Figure 7a, the mean and RMS drag coefficients of upstream prism A are always similar  
Figure 8: Pressure coefficient time histories in taps of prisms: (**a**) Pressure taps on windward wall and leeward wall; (**b**) Spacing ratio: 0.2; (**c**) Spacing ratio: 0.7; (**d**) Spacing ratio: 1.2.  
to those of the isolated prism, which reflects that the drag coefficient of prisms mainly depends on wind loads of the windward wall. Since wind loads of the windward wall of upstream prism A are not affected by the interference effect among prisms at the small spacing ratio, the magnitude of fluctuating drag coefficients of upstream prism A is larger than that of downstream prisms B and C, as shown in Figure 9. When the spacing ratio \(S/W=0.7\), the upstream prism A has significant shielding effects on downstream prisms B and C, and wind loads of the windward wall of downstream prisms are reduced, which results in the small magnitude of fluctuating drag coefficients of downstream prisms B and C. In addition, the fluctuating drag coefficients of downstream prisms B and C are basically the same, and the reason is that the characteristics of the flow field between the downstream prisms are similar [25]. At the spacing ratio of 0.7, the non-stationary characteristics of tap pressures signals of the leeward wall of prism B and the windward wall of prism C are more obvious than those of the leeward wall of prism A and the windward wall of prism B.