# Wind Forces and Flow Patterns of Three Tandem Prisms with a Small Height-Width Ratio
## 3 Results and Discussions
### Wind Flow among Three Tandem Prisms  
The wake characteristics among three tandem prisms at three typical spacing ratios of 0.2, 0.7, and 1.2 were simulated by LES. Figures 15 and 16 show the time-averaged velocity-magnitude contours and time-averaged streamlines of the central horizontal section (\(Z=0.5H\)) and central vertical section (\(Y=0\)) around the isolated prism and the group of prisms. The statistical duration for case \(S/W=0.2\) and case \(S/W=1.2\) is from 0 s to 12 s (about 10 min in reality), and there are two statistical durations for case \(S/W=0.7\), which are 0 s to 12 s and 24 s to 30 s. As shown in Figure 12, for the statistical duration from 0 s to 12 s, there is no abrupt change in pressure time histories in local pressure taps of downstream prisms, and for the statistical duration from 24 s to 30 s, the pressure time histories in local pressure taps of downstream prisms B and C both have undergone abrupt changes. Figure 14: RMS pressure coefficient on walls (Spacing ratio: 0.7): (**a**) Sampled period (9 s to 12 s); (**b**) Sampled period (25 s to 28 s). Figure 13: Mean pressure coefficient on walls (Spacing ratio: 0.7): (**a**) Sampled period (9 s to 12 s); (**b**) Sampled period (25 s to 28 s). Figure 16: Velocity magnitude and streamlines of vertical section (\(Y=0\)): (**a**) Isolated prism; (**b**) Spacing ratio: 0.2; (**c-1**) Spacing ratio: 0.7 (Period 0 s-12 s); (**c-2**) Spacing ratio: 0.7 (Period 24 s-30 s); (**d**) Spacing ratio: 1.2. Figure 15: Velocity magnitude and streamlines of horizontal section (\(Z=0.5\)\(H\)): (**a**) Isolated prism; (**b**) Spacing ratio: 0.2; (**c-1**) Spacing ratio: 0.7 (Period 0 s-12 s); (**c-2**) Spacing ratio: 0.7 (Period 24 s-30 s); (**d**) Spacing ratio: 1.2. As shown in Figure 15a, the incoming flow separates at the leading edge of prism side walls and forms a pair of symmetrical separated vortices around the isolated prism. A pair of symmetrical wake vortices are also observed behind the prism, which was satisfied with the symmetric time-averaged wake behind the isolated rectangular prism reported in previous studies [19,26]. It can be seen from Figure 16a that in the central vertical section of the isolated prism, three vortices are formed and respectively appear at the windward wall bottom, leading roof edge, and leeward wall bottom. When the spacing ratios \(S/W\leq 0.4\), the flow pattern around three prisms belongs to the skimming flow regime. The flow above the upstream prism roof skims over the prism gaps, and separation bubbles are not observed at the leading roof edge of downstream prisms (Figures 15b and 16b). Meanwhile, two small vortices appear in the prism gap near the two edges of the leeward wall of the upstream prism, respectively, as shown in Figure 15b. When the flow pattern transits from skimming flow to wake interference flow at spacing ratios \(0.4<S/W\leq 0.8\), shielding effects of the upstream prism gradually decrease, and the prism gap flow behind the upstream prism accelerates and strongly rolls up into the prism gaps, as shown in Figure 15c-1,c-2. Before the abrupt changes in pressure time histories of downstream prisms, the simulated time-averaged streamlines are asymmetric between prisms. A single recirculating zone at one side of the leeward wall of prisms A and B is observed in Figure 15c-1. Such asymmetric flow in the prism gap produces the asymmetric mean and RMS pressure distributions on downstream prisms, as shown in Figures 13 and 14. When the pressure time histories in local pressure taps of downstream prisms have undergone abrupt changes, the direction of wake flow between the prisms in Figure 15c-1 changes into the other side of prisms in Figure 15c-2, and these two directions of wake flow are basically symmetric along the direction of the incident flow. The flow field among prisms in the central vertical section is the same in Figure 16c-1,c-2.