# 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
The mean and RMS pressure coefficient distributions on these walls during two sampling periods are analyzed to illustrate the influence of the abrupt changes in the fluctuating pressures on the mean and RMS pressure coefficients of downstream walls. As shown in Figures 10a and 11a, period C-1 (42 s-45 s) and period C-2 (57 s-60 s) are extracted from the sampling duration (40 s-70 s). The pressure sign of point 9 is positive in period C-1, but it is negative in period C-2. As shown in Figures 10 and 11, the mean and RMS pressure coefficient distributions on the leeward wall of prism B and the windward wall of prism C are all asymmetric for these two sampling periods. These asymmetric pressure distributions on downstream prisms are not observed in previous studies for two tandem prisms with a large \(H/W\), two tandem 2D prisms, and a large group of prisms with a small \(H/W\). The characteristics of asymmetric distributions of the mean and RMS pressure coefficients in period C-1 are symmetric to those in period C-2. Hence, during the sampling duration, the asymmetric mean and RMS pressure distributions on downstream prisms are unstable at the spacing ratio \(S/W=0.7\) due to the non-stationary fluctuating pressures with irregular abrupt changes, and these pressure distributions are strongly sensitive to the number of abrupt changes during the sampling duration and the length of the sampling duration. In constrast, because the pressure distributions on the downstream prism before and after the abrupt change in fluctuating pressures are symmetric with each other (Figures 10 and 11), the overall wind loads on the downstream prism are little affected by the non-stationary fluctuating pressures on downstream prisms at \(S/W=0.7\). In addition, drag time histories of the downstream prism are stationary during the sampling duration, as shown in Figure 9. For the tandem 3D prisms and 2D square prisms at \(2<S/W\leq 3.5\) in the bistable flow regime [2; 3], a symmetric instantaneous wake and an asymmetric instantaneous wake irregular appear in the prism gap, which will induce the tap pressure time histories and wind force time histories for the downstream prism to be non-stationary. These are significantly different from the stationary wind force time histories in this present study. Figure 9: Drag coefficient time histories of three prisms at the spacing ratio of 0.7. To verify and understand the abrupt changes in the pressure coefficient time histories of downstream prisms (Figure 8c) obtained from the wind tunnel tests, a total of 30 s sampled time (about 25 min in reality) is performed by LES simulation at the spacing ratio of 0.7. Figure 12 shows the pressure coefficient time histories of taps at the same positions of wind tunnel tests in Figure 8a. As shown in Figure 12, the pressure coefficient time histories of points (3-6) and points (7-10) suddenly change at about T = 16.5 s and T = 23.5 s, respectively. This abrupt change in LES is similar to that in wind tunnel tests, but only one abrupt change occurs during the 30s sampling duration in numerical simulations. Figures 13 and 14 show the mean and RMS pressure coefficients on the leeward wall of prism B and the windward wall of prism C during the sampled period 9 s to 12 s and the sampled period 25 s to 28 s in LES. It can be seen that the asymmetric distributions and magnitudes of mean and RMS pressure coefficients in LES displayed reasonably good  
Figure 11: Pressure coefficients on walls in period C-2 (Spacing ratio: 0.7): (**a**) Period C-2 (57 s–60 s); (**b**) Mean \(C_{p}\); (**c**) RMS \(C_{p}\). Figure 12: Simulated pressure coefficient time histories in taps of prisms (Spacing ratio: 0.7). Figure 10: Pressure coefficients on walls in period C-1 (Spacing ratio: 0.7): (**a**) Period C-1 (42 s–45 s); (**b**) Mean \(C_{p}\); (**c**) RMS \(C_{p}\). agreement with those in the wind tunnel tests.